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Genetically modified animal
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Genetically modified animals are animals that have been genetically modified for a variety of purposes including producing drugs, enhancing yields, increasing resistance to disease, etc. The vast majority of genetically modified animals are at the research stage while the number close to entering the market remains small.[1]

Production

[edit]
Main article: Genetic engineering techniques

The process of genetically engineering mammals is a slow, tedious, and expensive process.[2] As with other genetically modified organisms (GMOs), first genetic engineers must isolate the gene they wish to insert into the host organism. This can be taken from a cell containing the gene[3] or artificially synthesised.[4] If the chosen gene or the donor organism's genome has been well studied it may already be accessible from a genetic library. The gene is then combined with other genetic elements, including a promoter and terminator region and usually a selectable marker.[5]

A number of techniques are available for inserting the isolated gene into the host genome. With animals DNA is generally inserted into using microinjection, where it can be injected through the cell's nuclear envelope directly into the nucleus, or through the use of viral vectors.[6] The first transgenic animals were produced by injecting viral DNA into embryos and then implanting the embryos in females.[7] It is necessary to ensure that the inserted DNA is present in the embryonic stem cells.[8] The embryo would develop and it would be hoped that some of the genetic material would be incorporated into the reproductive cells. Then researchers would have to wait until the animal reached breeding age and then offspring would be screened for presence of the gene in every cell, using PCR, Southern hybridization, and DNA sequencing.[9]

New technologies are making genetic modifications easier and more precise.[2] Gene targeting techniques, which creates double-stranded breaks and takes advantage on the cells natural homologous recombination repair systems, have been developed to target insertion to exact locations. Genome editing uses artificially engineered nucleases that create breaks at specific points. There are four families of engineered nucleases: meganucleases,[10][11] zinc finger nucleases,[12][13] transcription activator-like effector nucleases (TALENs),[14][15] and the Cas9-guideRNA system (adapted from CRISPR).[16][17] TALEN and CRISPR are the two most commonly used and each has its own advantages.[18] TALENs have greater target specificity, while CRISPR is easier to design and more efficient.[18] The development of the CRISPR-Cas9 gene editing system has effectively halved the amount of time needed to develop genetically modified animals.[19]

Main article: History of genetic engineering
In 1974, Rudolf Jaenisch created the first GM animal.

Humans have domesticated animals since around 12,000 BCE, using selective breeding or artificial selection (as contrasted with natural selection). The process of selective breeding, in which organisms with desired traits (and thus with the desired genes) are used to breed the next generation and organisms lacking the trait are not bred, is a precursor to the modern concept of genetic modification[20]: 1  Various advancements in genetics allowed humans to directly alter the DNA and therefore genes of organisms. In 1972, Paul Berg created the first recombinant DNA molecule when he combined DNA from a monkey virus with that of the lambda virus.[21][22]

In 1974, Rudolf Jaenisch created a transgenic mouse by introducing foreign DNA into its embryo, making it the world's first transgenic animal.[23][24] However it took another eight years before transgenic mice were developed that passed the transgene to their offspring.[25][26] Genetically modified mice were created in 1984 that carried cloned oncogenes, predisposing them to developing cancer.[27] Mice with genes knocked out (knockout mouse) were created in 1989. The first transgenic livestock were produced in 1985[28] and the first animal to synthesise transgenic proteins in their milk were mice,[29] engineered to produce human tissue plasminogen activator in 1987.[30]

The first genetically modified animal to be commercialised was the GloFish, a Zebra fish with a fluorescent gene added that allows it to glow in the dark under ultraviolet light.[31] It was released to the US market in 2003.[32] The first genetically modified animal to be approved for food use was AquAdvantage salmon in 2015.[33] The salmon were transformed with a growth hormone-regulating gene from a Pacific Chinook salmon and a promoter from an ocean pout enabling it to grow year-round instead of only during spring and summer.[34]

Mammals

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Main article: Genetically modified mammal
Some chimeras, like the blotched mouse shown, are created through genetic modification techniques like gene targeting.

GM mammals are created for research purposes, production of industrial or therapeutic products, agricultural uses or improving their health. There is also a market for creating genetically modified pets.[35]

Medicine

[edit]

Mammals are the best models for human disease, making genetic engineered ones vital to the discovery and development of cures and treatments for many serious diseases. Knocking out genes responsible for human genetic disorders allows researchers to study the mechanism of the disease and to test possible cures. Genetically modified mice have been the most common mammals used in biomedical research, as they are cheap and easy to manipulate. Examples include humanized mice created by xenotransplantation of human gene products, so as to be utilized as murine human-animal hybrids for gaining relevant insights in the in vivo context for understanding of human-specific physiology and pathologies.[36] Pigs are also a good target, because they have a similar body size, anatomical features, physiology, pathophysiological response, and diet.[37] Nonhuman primates are the most similar model organisms to humans, but there is less public acceptance toward using them as research animals.[38] In 2009, scientists announced that they had successfully transferred a gene into a primate species (marmosets) and produced a stable line of breeding transgenic primates for the first time.[39][40] Their first research target for these marmosets was Parkinson's disease, but they were also considering amyotrophic lateral sclerosis and Huntington's disease.[41]

Transgenic pig for cheese production

Human proteins expressed in mammals are more likely to be similar to their natural counterparts than those expressed in plants or microorganisms. Stable expression has been accomplished in sheep, pigs, rats, and other animals. In 2009, the first human biological drug produced from such an animal, a goat, was approved. The drug, ATryn, is an anticoagulant which reduces the probability of blood clots during surgery or childbirth was extracted from the goat's milk.[42] Human alpha-1-antitrypsin is another protein that is used in treating humans with this deficiency.[43] Another area is in creating pigs with greater capacity for human organ transplants (xenotransplantation). Pigs have been genetically modified so that their organs can no longer carry retroviruses[44] or have modifications to reduce the chance of rejection.[45][46] Pig lungs from genetically modified pigs are being considered for transplantation into humans.[47][48] There is even potential to create chimeric pigs that can carry human organs.[37][49]

Livestock

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Livestock are modified with the intention of improving economically important traits such as growth-rate, quality of meat, milk composition, disease resistance and survival. Animals have been engineered to grow faster, be healthier[50] and resist diseases.[51] Modifications have also improved the wool production of sheep and udder health of cows.[1]

Goats have been genetically engineered to produce milk with strong spiderweb-like silk proteins.[52] The goat gene sequence has been modified, using fresh umbilical cords taken from kids, in order to code for the human enzyme lysozyme. Researchers wanted to alter the milk produced by the goats, to contain lysozyme in order to fight off bacteria causing diarrhea in humans.[53]

Enviropig was a genetically enhanced line of Yorkshire pigs in Canada created with the capability of digesting plant phosphorus more efficiently than conventional Yorkshire pigs.[54][55] The A transgene construct consisting of a promoter expressed in the murine parotid gland and the Escherichia coli phytase gene was introduced into the pig embryo by pronuclear microinjection.[56] This caused the pigs to produce the enzyme phytase, which breaks down the indigestible phosphorus, in their saliva.[54][57] As a result, they excrete 30 to 70% less phosphorus in manure depending upon the age and diet.[54][57] The lower concentrations of phosphorus in surface runoff reduces algal growth, because phosphorus is the limiting nutrient for algae.[54] Because algae consume large amounts of oxygen, excessive growth can result in dead zones for fish. Funding for the Enviropig program ended in April 2012,[58] and as no new partners were found the pigs were killed.[59] However, the genetic material will be stored at the Canadian Agricultural Genetics Repository Program. In 2006, a pig was engineered to produce omega-3 fatty acids through the expression of a roundworm gene.[60]

Herman the Bull on display in Naturalis Biodiversity Center

In 1990, the world's first transgenic bovine, Herman the Bull, was developed. Herman was genetically engineered by micro-injected embryonic cells with the human gene coding for lactoferrin. The Dutch Parliament changed the law in 1992 to allow Herman to reproduce. Eight calves were born in 1994 and all calves inherited the lactoferrin gene.[61] With subsequent sirings, Herman fathered a total of 83 calves.[62] Dutch law required Herman to be slaughtered at the conclusion of the experiment. However the Dutch Agriculture Minister at the time, Jozias van Aartsen, granted him a reprieve provided he did not have more offspring after public and scientists rallied to his defence.[62] Together with cloned cows named Holly and Belle, he lived out his retirement at Naturalis, the National Museum of Natural History in Leiden.[62] On 2 April 2004, Herman was euthanised by veterinarians from the University of Utrecht because he suffered from osteoarthritis.[63][62] At the time of his death Herman was one of the oldest bulls in the Netherlands.[63] Herman's hide has been preserved and mounted by taxidermists and is permanently on display in Naturalis. They say that he represents the start of a new era in the way man deals with nature, an icon of scientific progress, and the subsequent public discussion of these issues.[63]

Researchers have developed GM dairy cattle to grow without horns (sometimes referred to as "polled") which can cause injuries to farmers and other animals. DNA was taken from the genome of Red Angus cattle, which is known to suppress horn growth, and inserted into cells taken from an elite Holstein bull called "Randy". Each of the progeny will be a clone of Randy, but without his horns, and their offspring should also be hornless.[64] In 2011, Chinese scientists generated dairy cows genetically engineered with genes from human beings to produce milk that would be the same as human breast milk.[65] This could potentially benefit mothers who cannot produce breast milk but want their children to have breast milk rather than formula.[66][67] The researchers claim these transgenic cows to be identical to regular cows.[68] Two months later, scientists from Argentina presented Rosita, a transgenic cow incorporating two human genes, to produce milk with similar properties as human breast milk.[67] In 2012, researchers from New Zealand also developed a genetically engineered cow that produced allergy-free milk.[69]

In 2016 Jayne Raper and a team announced the first trypanotolerant transgenic cow in the world. This team, spanning the International Livestock Research Institute, Scotland's Rural College, the Roslin Institute's Centre for Tropical Livestock Genetics and Health, and the City University of New York, announced that a Kenyan Boran bull had been born and had already successfully had two children. Tumaini - named for the Swahili word for "hope" - carries a trypanolytic factor from a baboon via CRISPR/Cas9.[70][71]

In October 2017, Chinese scientists announced they used CRISPR gene editing technology to create of a line of pigs with better body temperature regulation, resulting in about 24% less body fat than typical livestock.[72]

Research

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Scientists have genetically engineered several organisms, including some mammals, to include green fluorescent protein (GFP), for research purposes.[73] GFP and other similar reporting genes allow easy visualisation and localisation of the products of the genetic modification.[74] Fluorescent pigs have been bred to study human organ transplants, regenerating ocular photoreceptor cells, and other topics.[75] In 2011 green-fluorescent cats were created to find therapies for HIV/AIDS and other diseases[76] as feline immunodeficiency virus (FIV) is related to HIV.[77] Researchers from the University of Wyoming have developed a way to incorporate spiders' silk-spinning genes into goats, allowing the researchers to harvest the silk protein from the goats' milk for a variety of applications.[78]

Conservation

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Genetic modification of the myxoma virus has been proposed to conserve European wild rabbits in the Iberian peninsula and to help regulate them in Australia. To protect the Iberian species from viral diseases, the myxoma virus was genetically modified to immunize the rabbits, while in Australia the same myxoma virus was genetically modified to lower fertility in the Australian rabbit population.[79] There have also been suggestions that genetic engineering could be used to bring animals back from extinction. It involves changing the genome of a close living relative to resemble the extinct one and is currently being attempted with the passenger pigeon.[80] Genes associated with the woolly mammoth have been added to the genome of an African Elephant, although the lead researcher says he has no intention of using live elephants.[81]

Humans

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Gene therapy[82] uses genetically modified viruses to deliver genes which can cure disease in humans. Although gene therapy is still relatively new, it has had some successes. It has been used to treat genetic disorders such as severe combined immunodeficiency[83] and Leber's congenital amaurosis.[84] Treatments are also being developed for a range of other currently incurable diseases, such as cystic fibrosis,[85] sickle cell anemia,[86] Parkinson's disease,[87][88] cancer,[89][90][91] diabetes,[92] heart disease,[93] and muscular dystrophy.[94] These treatments only affect somatic cells, which means that any changes would not be inheritable. Germline gene therapy results in any change being inheritable, which has raised concerns within the scientific community.[95][96] In 2015, CRISPR was used to edit the DNA of non-viable human embryos.[97][98] In November 2018, He Jiankui announced that he had edited the genomes of two human embryos, to attempt to disable the CCR5 gene, which codes for a receptor that HIV uses to enter cells. He said that twin girls- Lulu and Nana, had been born a few weeks earlier, and that they carried functional copies of CCR5 along with disabled CCR5 (mosaicism), and were still vulnerable to HIV. The work was widely condemned as unethical, dangerous, and premature.[99]

Fish

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Main article: Genetically modified fish

Genetically modified fish are used for scientific research, as pets, and as a food source. Aquaculture is a growing industry, currently providing over half of the consumed fish worldwide.[100] Through genetic engineering, it is possible to increase growth rates, reduce food intake, remove allergenic properties, increase cold tolerance, and provide disease resistance.

Detecting pollution

[edit]

Fish can also be used to detect aquatic pollution or function as bioreactors.[101] Several groups have been developing zebrafish to detect pollution by attaching fluorescent proteins to genes activated by the presence of pollutants. The fish will then glow and can be used as environmental sensors.[102][103]

Pets

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The GloFish is a brand of genetically modified fluorescent zebrafish with bright red, green, and orange fluorescent color. It was originally developed by one of the groups to detect pollution, but is now part of the ornamental fish trade, becoming the first genetically modified animal to become publicly available as a pet when it was introduced for sale in 2003.[104]

Research

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GM fish are widely used in basic research in genetics and development. Two species of fish- zebrafish and medaka, are most commonly modified, because they have optically clear chorions (membranes in the egg), rapidly develop, and the 1-cell embryo is easy to see and microinject with transgenic DNA.[105] Zebrafish are model organisms for developmental processes, regeneration, genetics, behaviour, disease mechanisms, and toxicity testing.[106] Their transparency allows researchers to observe developmental stages, intestinal functions, and tumour growth.[107][108] The generation of transgenic protocols (whole organism, cell or tissue specific, tagged with reporter genes) has increased the level of information gained by studying these fish.[109]

Growth

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GM fish have been developed with promoters driving an over-production of "all fish" growth hormone for use in the aquaculture industry, to increase the speed of development and potentially reduce fishing pressure on wild stocks. This has resulted in dramatic growth enhancement in several species, including salmon,[110] trout,[111] and tilapia.[112]

AquaBounty Technologies have produced a salmon that can mature in half the time as wild salmon.[113] The fish is an Atlantic salmon with a Chinook salmon (Oncorhynchus tshawytscha) gene inserted. This allows the fish to produce growth hormones all year round compared to the wild-type fish that produces the hormone for only part of the year.[114] The fish also has a second gene inserted from the eel-like ocean pout that acts like an "on" switch for the hormone.[114] Pout also have antifreeze proteins in their blood, which allow the GM salmon to survive near-freezing waters and continue their development.[115] A wild-type salmon takes 24 to 30 months to reach market size (4–6 kg), whereas the producers of the GM salmon say that it requires only 18 months for the GM fish to reach that size.[115][116][117] In November 2015, the FDA of the USA approved the AquAdvantage salmon for commercial production, sale, and consumption,[118] the first non-plant GMO food to be commercialized.[119]

AquaBounty says that to prevent the genetically modified fish from inadvertently breeding with wild salmon, all of the fish will be female and reproductively sterile,[117] although a small percentage of the females may remain fertile.[114] Some opponents of the GM salmon have dubbed it the "Frankenfish".[114][120]

Insects

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See also: Genetically modified insect

Research

[edit]

In biological research, transgenic fruit flies (Drosophila melanogaster) are model organisms used to study the effects of genetic changes on development.[121] Fruit flies are often preferred over other animals due to their short life cycle and low maintenance requirements. It also has a relatively simple genome compared to many vertebrates, with typically only one copy of each gene, making phenotypic analysis easy.[122] Drosophila have been used to study genetics and inheritance, embryonic development, learning, behavior, and aging.[123] Transposons (particularly P elements) are well developed in Drosophila and provided an early method to add transgenes to their genome, although this has been taken over by more modern gene-editing techniques.[124]

Population control

[edit]

Due to their significance to human health, scientists are looking at ways to control mosquitoes through genetic engineering. Malaria-resistant mosquitoes have been developed in the laboratory.[125] by inserting a gene that reduces the development of the malaria parasite[126] and then use homing endonucleases to rapidly spread that gene throughout the male population (known as a gene drive).[127] This has been taken further by swapping it for a lethal gene.[128][129] In trials the populations of Aedes aegypti mosquitoes, the single most important carrier of dengue fever and Zika virus, were reduced by between 80% and by 90%.[130][131][129] Another approach is to use the sterile insect technique, whereby males genetically engineered to be sterile out compete viable males, to reduce population numbers.[132]

Other insect pests that make attractive targets are moths. Diamondback moths cause US$4 to $5 billion of damage a year worldwide.[133] The approach is similar to the mosquitoes, where males transformed with a gene that prevents females from reaching maturity will be released.[134] They underwent field trials in 2017.[133] Genetically modified moths have previously been released in field trials.[135] A strain of pink bollworm that were sterilised with radiation were genetically engineered to express a red fluorescent protein making it easier for researchers to monitor them.[136]

Industry

[edit]

Silkworm, the larvae stage of Bombyx mori, is an economically important insect in sericulture. Scientists are developing strategies to enhance silk quality and quantity. There is also potential to use the silk producing machinery to make other valuable proteins.[137] Proteins expressed by silkworms include; human serum albumin, human collagen α-chain, mouse monoclonal antibody and N-glycanase.[138] Silkworms have been created that produce spider silk, a stronger but extremely difficult to harvest silk,[139] and even novel silks.[140]

Birds

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Attempts to produce genetically modified birds began before 1980.[141] Chickens have been genetically modified for a variety of purposes. This includes studying embryo development,[142] preventing the transmission of bird flu[143] and providing evolutionary insights using reverse engineering to recreate dinosaur-like phenotypes.[144] A GM chicken that produces the drug Kanuma, an enzyme that treats a rare condition, in its egg passed regulatory approval in 2015.[145]

Disease control

[edit]

One potential use of GM birds could be to reduce the spread of avian disease. Researchers at Roslin Institute have produced a strain of GM chickens (Gallus gallus domesticus) that does not transmit avian flu to other birds; however, these birds are still susceptible to contracting it. The genetic modification is an RNA molecule that prevents the virus reproduction by mimicking the region of the flu virus genome that controls replication. It is referred to as a "decoy" because it diverts the flu virus enzyme, the polymerase, from functions that are required for virus replication.[146]

Evolutionary insights

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A team of geneticists led by University of Montana paleontologist Jack Horner is seeking to modify a chicken to express several features present in ancestral maniraptorans but absent in modern birds, such as teeth and a long tail,[147] creating what has been dubbed a 'chickenosaurus'.[148] Parallel projects have produced chicken embryos expressing dinosaur-like skull,[149] leg,[144] and foot[150] anatomy. In 2023, Horner claimed success in creating longer chicken tails, and expressed caution regarding future developments.[151]

In-ovo sexing

[edit]
Main article: In-ovo sexing

Gene editing is one possible tool in the laying hen breeding industry to provide an alternative to Chick culling. With this technology, breeding hens are given a genetic marker that is only passed down to male offspring. These males can then be identified during incubation and removed from the egg supply, so that only females hatch. For example, the Israeli startup eggXYt uses CRISPR to give male eggs a biomarker that makes then glow under certain conditions.[152] Importantly, the resulting laying hen and the eggs it producers are not themselves genetically edited. The European Union's Director General for Health and Food Safety has confirmed that made in this way eggs can be marketed,[153] although none are commercially available as of June 2023.[154]

Amphibians

[edit]

The first experiments that successfully developed transgenic amphibians into embryos began in the 1980s with Xenopus laevis.[155] Later, germline transgenic axolotls in Ambystoma mexicanum were produced in 2006 using a technique called I-SceI-mediated transgenesis which utilizes the I-SceI endonuclease enzyme that can break DNA at specific sites and allow for foreign DNA to be inserted into the genome.[156] Both Xenopus laevis and Ambystoma mexicanum are model organisms used to study regeneration. In addition, transgenic lines have been produced in other salamanders including the Japanese newt Pyrrhogaster and Pleurodeles watl.[157] Genetically modified frogs, in particular Xenopus laevis and Xenopus tropicalis, are used in development biology. GM frogs can also be used as pollution sensors, especially for endocrine disrupting chemicals.[158] There are proposals to use genetic engineering to control cane toads in Australia.[159][160] Many lines of transgenic X. laevis are used to study immunology to address how bacteria and viruses cause infectious disease at the University of Rochester Medical Center's X. laevis Research Resource for Immunobiology (XLRRI).[161] Amphibians can also be used to study and validate regenerative signaling pathways such as the Wnt pathway.[162][161] The wound-healing abilities of amphibians have many practical applications and can potentially provide a foundation for scar-free repair in human plastic surgery, such as treating the skin of burn patients.[163]

Amphibians like X. laevis are suitable for experimental embryology because they have large embryos that can be easily manipulated and observed during development.[164] In experiments with axolotls, mutants with white pigmented skin are often used because their semi-transparent skin provides an efficient visualization and tracking method for fluorescently tagged proteins like GFP.[156] Amphibians are not always ideal when it comes to the resources required to produce genetically modified animals; along with the one to two-year generation time, Xenopus laevis can be considered less than ideal for transgenic experiments because of its pseudotetraploid genome.[164] Due to the same genes appearing in the genome multiple times, the chance of mutagenesis experiments working is lower.[165] Current methods of freezing and thawing axolotl sperm render them nonfunctional, meaning transgenic lines must be maintained in a facility and this can get quite costly.[156][166] Producing transgenic axolotls has many challenges due to their large genome size.[166] Current methods of generating transgenic axolotls are limited to random integration of the transgene cassette into the genome, which can lead to uneven expression or silencing.[157] Gene duplicates also complicate efforts to generate efficient gene knockouts.[166]

Despite the costs, axolotls have unique regenerative abilities and ultimately provide useful information in understanding tissue regeneration because they can regenerate their limbs, spinal cord, skin, heart, lungs, and other organs.[166][167] Naturally occurring mutant axolotls like the white strain that are often used in research have a transcriptional mutation at the Edn3 gene locus.[168] Unlike other model organisms, the first fluorescently labeled cells in axolotls were differentiated muscle cells instead of embryos. In these initial experiments in the early 2000s, scientists were able to visualize muscle cell regeneration in the axolotl tail using a microinjecting technique, but cells could not be traced for the entire course of regeneration due to too harsh conditions that caused early cell death in labeled cells.[157][169] Though the process of producing transgenic axolotls was a challenge, scientists were able to label cells for longer durations using a plasmid transfection technique, which involves injecting DNA into cells using an electrical pulse in a process called electroporation. Transfecting axolotl cells is thought to be more difficult because of the composition of the extracellular matrix (ECM). This technique allows spinal cord cells to be labeled and is very important in studying limb regeneration in many other cells; it has been used to study the role of the immune system in regeneration. Using gene knockout approaches, scientists can target specific regions of DNA using techniques like CRISPR/Cas9 to understand the function of certain genes based on the absence of the gene of interest. For example, gene knockouts of the Sox2 gene confirm this region's role in neural stem cell amplification in the axolotl. The technology to do more complex conditional gene knockouts, or conditional knockouts that give the scientist spatiotemporal control of the gene is not yet suitable for axolotls.[166] However, research in this field continues to develop and is made easier by recent sequencing of the genome and resources created for scientists, including data portals that contain axolotl genome and transcriptome reference assemblies to identify orthologs.[170][171]

Nematodes

[edit]

The nematode Caenorhabditis elegans is one of the major model organisms for researching molecular biology.[172] RNA interference (RNAi) was discovered in C. elegans[173] and could be induced by simply feeding them bacteria modified to express double stranded RNA.[174] It is also relatively easy to produce stable transgenic nematodes and this along with RNAi are the major tools used in studying their genes.[175] The most common use of transgenic nematodes has been studying gene expression and localisation by attaching reporter genes. Transgenes can also be combined with RNAi to rescue phenotypes, altered to study gene function, imaged in real time as the cells develop or used to control expression for different tissues or developmental stages.[175] Transgenic nematodes have been used to study viruses,[176] toxicology,[177] and diseases[178][179] and to detect environmental pollutants.[180]

Other

[edit]

Systems have been developed to create transgenic organisms in a wide variety of other animals. The gene responsible for albinism in sea cucumbers has been found, and used to engineer white sea cucumbers, a rare delicacy. The technology also opens the way to investigate the genes responsible for some of the cucumbers more unusual traits, including hibernating in summer, eviscerating their intestines, and dissolving their bodies upon death.[181] Flatworms have the ability to regenerate themselves from a single cell.[182][183] Until 2017 there was no effective way to transform them, which hampered research. By using microinjection and radiation, scientists have now created the first genetically modified flatworms.[184] The bristle worm, a marine annelid, has been modified. It is of interest due to its reproductive cycle being synchronized with lunar phases, regeneration capacity and slow evolution rate.[185] Cnidaria such as Hydra and the sea anemone Nematostella vectensis are attractive model organisms to study the evolution of immunity and certain developmental processes.[186] Other organisms that have been genetically modified include snails,[187] geckos, turtles,[188] crayfish, oysters, shrimp, clams, abalone,[189] and sponges.[190]

Food products derived from genetically modified (GM) animals have not yet entered the European market. Nonetheless, the on-going discussion about GM crops [1], and the developing debate about the safety and ethics of foods and pharmaceutical products produced by both GM animals and plants, have provoked varying views across different sectors of society[191]

Animal welfare and ethics resources

Ethics

[edit]

Genetic modification and genome editing hold potential for the future, but decisions regarding the use of these technologies must be based not only on what is possible, but also on what is ethically reasonable. Principles such as animal integrity, naturalness, risk identification and animal welfare are examples of ethically important factors that must be taken into consideration, and they also influence public perception and regulatory decisions by authorities.[192]

The utility of extrapolating animal data to humans has been questioned. This has led ethical committees to adopt the principles of the four Rs (Reduction, Refinement, Replacement, and Responsibility) as a guide for decision-making regarding animal experimentation. However, complete abandonment of laboratory animals has not yet been possible, and further research is needed to develop a roadmap for robust alternatives before their use can be fully discontinued.[193]

References

[edit]
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Genetically modified animal

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Genetically modified animals are non-human animals whose genetic material has been altered through genetic engineering techniques, such as recombinant DNA methods or genome editing tools like CRISPR-Cas9, to introduce, delete, or modify specific DNA sequences beyond what is possible via conventional selective breeding. Genetic modifications involve altering the genomes of existing animals to introduce novel traits, but do not enable the invention of entirely new animal species from scratch, which remains beyond current scientific capabilities. Examples include transgenic GloFish, derived from zebrafish with added fluorescent genes, and humanized mice engineered for research, both modifications of established species. These modifications have been applied primarily in biomedical research to create models for studying human diseases, in agriculture to develop livestock with traits like disease resistance or faster growth, and in biotechnology for producing pharmaceuticals. The first genetically modified animals were mice produced in 1974 by introducing foreign DNA into embryos, marking the beginning of transgenic animal research, with subsequent advancements in tools like CRISPR-Cas9 since the early 2010s enabling more precise and efficient edits. Significant achievements include the FDA approval of AquAdvantage salmon in 2015, engineered for rapid growth via a growth hormone gene from another fish species, and gene-edited cattle with heat-tolerant traits approved in 2022, demonstrating potential for sustainable protein production. Controversies persist over animal welfare implications from unintended health effects, environmental risks such as gene flow to wild populations potentially disrupting ecosystems, and ethical questions about patenting life forms and altering natural evolutionary processes, though empirical safety data from approved cases indicate low risk when regulated properly.

History

Pioneering Experiments (1970s-1980s)

The earliest experiments in genetic modification of animals occurred in the mid-1970s, focusing on mice as model organisms. In 1974, virologist , working at the Salk Institute, and embryologist Beatrice Mintz infected preimplantation mouse embryos with SV40 virus, resulting in the stable integration of viral DNA into the host genome and producing the first transgenic mice. This viral transduction approach demonstrated that foreign DNA could be incorporated into mammalian embryos, though initial transmission was primarily somatic. By 1976, Jaenisch reported germline transmission of the integrated SV40 DNA in offspring, confirming heritable genetic modification in mammals. These experiments laid foundational evidence for the feasibility of transgenesis, despite challenges like low efficiency and mosaicism in chimeric embryos. Concurrently, techniques for manipulating embryos advanced, with Ralph L. Brinster refining pronuclear microinjection methods originally developed in the for DNA transfer into fertilized eggs. In the early 1980s, direct DNA microinjection supplanted viral methods, enabling precise insertion of non-viral genes. In 1980, researchers and Frank Ruddle successfully incorporated viral DNA sequences into embryos via pronuclear injection, marking the first use of this technique for stable transgenesis. By 1981-1982, this approach produced the first stably inherited transgenic mice expressing foreign genes, such as human globin or viral promoters. A landmark demonstration came in 1982 when Brinster and Richard Palmiter microinjected a fusion gene combining mouse metallothionein promoter with rat into mouse eggs, yielding transgenic "supermice" that grew 1.3 to 2 times larger than controls due to elevated levels. Of 21 viable mice from injected eggs, seven integrated the , with six exhibiting the . These experiments validated functional gene expression from transgenes and spurred applications in and , such as oncomice models. Initial efforts extended beyond mice to amphibians; in the early 1980s, Xenopus laevis frogs were produced as transgenics via DNA injection into fertilized eggs, achieving expression of reporter genes like . However, mammals remained the primary focus due to their relevance to human disease modeling, with mice efficiencies improving from less than 1% integration in early trials to higher rates by decade's end through optimized and selection. These pioneering works established transgenesis as a viable tool, despite ethical and technical hurdles like variable expression and off-target effects.

Expansion into Livestock and Models (1990s-2000s)

In the 1990s, genetic modification techniques advanced beyond initial experiments to include larger species, driven by goals of improving agricultural traits such as growth rates and meat quality, as well as initiating production (""). Pronuclear remained the primary method, though was low, with success rates often below 1% for transmission in and pigs. A landmark example was Herman the Bull, the first transgenic bovine, created in 1990 by GenPharm International through of a gene into bovine embryos; the intent was for female offspring to secrete the human protein in milk for potential antimicrobial applications, though the project faced regulatory hurdles and was abandoned in the mid-1990s due to health issues in transgenics and ethical concerns. Similarly, in 1990, the produced Tracy, the first transgenic sheep expressing human alpha-1-antitrypsin (A1AT) in milk, aimed at treating hereditary ; this demonstrated feasible high-level protein expression (up to 15 g/L in milk) but highlighted challenges like variable expression and issues from promoter-driven overexpression. These efforts expanded to pigs, where 1990s research built on 1985 transgenics by inserting genes to enhance feed and lean muscle, though results often yielded animals with , lameness, and reduced , underscoring unintended physiological costs. Biopharming gained traction in the as were engineered as bioreactors for therapeutics, leveraging mammary glands for scalable superior to bacterial or cell systems in post-translational modifications. Sheep and were prioritized for their yields; for instance, 1990s projects at Pharmaceutical Proteins Ltd. (later Pharming Group) developed transgenic sheep secreting clotting factors like , with —a 1997 nuclear transfer-derived transgenic sheep—expressing up to 1% of protein as the target, paving the way for hemophilia treatments. By the early 2000s, transgenic producing recombinant antithrombin (rhAT) for reached clinical trials, with FDA approval of ATryn in 2006 marking the first animal-derived GM , though development traced to 1990s gene constructs. Agricultural applications focused on disease resistance and productivity; transgenic s resistant to porcine reproductive and respiratory syndrome (PRRS) virus emerged in lab trials by 2009, building on 1990s gene insertions, while modifications targeted resistance via genes, though field deployment lagged due to regulatory and public resistance. Overall, transgenics faced high costs—estimated at $25,000 per founder in 1992—and low transmission rates (1-5%), limiting commercial scale until aided propagation. Parallel to livestock advances, the 1990s and saw explosive growth in transgenic animal models for biomedical research, with dominating due to accessibility and short generation times. Building on 1989 knockout , conditional and tissue-specific transgenics proliferated; for example, Cre-loxP systems enabled inducible , allowing spatiotemporal control absent in constitutive models. Disease modeling expanded: the 1991 PDAPP , expressing human amyloid precursor protein (APP) with Swedish mutation, recapitulated Alzheimer's plaques, facilitating beta-amyloid hypothesis testing. Tauopathy models like JNPL3 (1998), overexpressing human P301L tau, exhibited neurofibrillary tangles and motor deficits, advancing studies. Bacterial artificial chromosome (BAC) transgenics, introduced late , improved physiological expression by incorporating large genomic loci (up to 300 kb), reducing artifacts from random integration; these were pivotal in , modeling Huntington's and Parkinson's with human BACs driving accurate pathology. By the , applications extended to and , with models (e.g., Big Blue ) quantifying mutations in vivo, and humanized bearing xenogeneic immune genes for research, though mosaicism and silencing remained limitations. This era's models accelerated causal function elucidation, with over 10,000 transgenic lines generated by 2005, underpinning target validation despite ethical debates on animal suffering from induced pathologies. Expansion to rats and larger mammals was limited but notable, with pronuclear injections yielding transgenic swine for , expressing human complement regulators to mitigate hyperacute rejection.

Genome Editing Era (2010s-Present)

The genome editing era transformed animal genetic modification by introducing programmable s for precise DNA targeting, supplanting less accurate transgenesis techniques. Zinc-finger s (ZFNs) and transcription activator-like effector s (TALENs), refined in the early 2010s, enabled site-specific double-strand breaks, but the -Cas9 system's debut in 2012 revolutionized efficiency and accessibility. Derived from bacterial adaptive immunity, uses a single to direct Cas9 cleavage, allowing multiplexed edits with reduced off-target effects compared to ZFNs and TALENs in many contexts. This precision facilitated rapid generation of knockouts, knock-ins, and allele-specific modifications in animal genomes. Early applications focused on model organisms and enhancements. In 2013, generated targeted mutations in , achieving high-efficiency transmission for developmental studies. By 2014, routine CRISPR knockouts in mice expedited disease modeling, requiring fewer animals than methods. In , TALENs produced the first hornless in 2015 via editing of the POLLED locus, addressing welfare concerns over surgical dehorning while maintaining milk production traits; however, unintended integrations of antibiotic resistance genes were later detected in some lines. advanced research, with 2015 reports of multi-gene edited pigs lacking porcine endogenous retroviruses and hyperacute rejection factors like alpha-1,3-galactosyltransferase, extending graft survival in nonhuman . Subsequent innovations enhanced editing versatility without double-strand breaks. Base editing, introduced in 2016, fuses deactivated with or deaminases for single-base conversions (e.g., C-to-T), applied in mice and pigs for modeling point mutations in diseases like sickle cell anemia. , developed in 2019, employs a reverse transcriptase- fusion and for insertions, deletions, or substitutions up to dozens of bases, demonstrated in models for precise corrections with minimal indels. These tools supported disease-resistant livestock, such as CRISPR-edited pigs resistant to porcine reproductive and respiratory syndrome (approved for U.S. food use by FDA in 2020) and with enhanced muscle yield via MSTN edits. Regulatory frameworks diverged, reflecting debates over process versus product oversight. , the FDA evaluates gene-edited animals as new animal drugs, requiring pre-market approval, but has cleared certain edits lacking foreign DNA for reduced scrutiny; approvals include GalSafe pigs (alpha-gal knockout) for human consumption and xenotransplant research. The classifies most edited animals as genetically modified organisms under Directive 2001/18/EC, mandating rigorous risk assessments regardless of foreign DNA presence, effectively stalling commercialization. Challenges persist, including mosaicism in embryos, off-target mutations (mitigated by high-fidelity Cas variants), and ethical concerns over edits, though empirical data show improved via fewer invasive procedures and targeted traits like disease resistance.

Methods of Genetic Modification

Classical Transgenesis

Classical transgenesis refers to the introduction of exogenous DNA sequences into the of an animal, typically resulting in random integration sites rather than precise targeting. This method predates site-specific technologies like CRISPR-Cas9 and relies on techniques such as pronuclear to achieve stable transmission of the . The process begins with the isolation and preparation of the transgene, often consisting of a promoter, coding sequence, and polyadenylation signal to ensure expression. Linearized DNA is then microinjected into the pronucleus of a fertilized oocyte, where it integrates into the host genome during early embryonic development, frequently as multiple concatenated copies. Success rates vary by species, with efficiencies around 10-30% for founder animals in mice, though expression levels are unpredictable due to position variegation effects and potential silencing. Implanted embryos are transferred to surrogate mothers, and offspring are screened for transgene integration via PCR or Southern blotting. Pioneering work occurred in the 1970s, with Rudolf Jaenisch and Beatrice Mintz reporting the first genetically modified mouse in 1974 by inserting a viral DNA genome into early-stage embryos, though initial attempts lacked stable germline transmission. Germline transmission was achieved by Jaenisch in 1976 using retroviral vectors. The first stable transgenic mice expressing an integrated foreign gene were produced in 1981 by John Gordon and Frank Ruddle through pronuclear injection of rabbit beta-globin DNA. In 1982, Ralph Brinster and Richard Palmiter created the "supermouse" by overexpressing a rat growth hormone gene, demonstrating phenotypic enhancement via transgenesis. These milestones established classical transgenesis as a foundational tool for animal genetic modification. Applications extended beyond rodents to livestock, where pronuclear microinjection produced transgenic pigs, sheep, and cattle by the late 1980s. For instance, transgenic pigs expressing human proteins for were generated, though random integration posed risks of oncogenic disruption. In aquaculture, classical methods yielded growth-enhanced salmon by introducing an ocean pout antifreeze protein promoter driving Chinook salmon growth hormone, approved for commercial use in 2015 after decades of development. Limitations include mosaicism in founders, requiring breeding to homozygosity, and ethical concerns over due to unintended , prompting a shift toward precise in recent years.

Site-Specific Genome Editing

Site-specific genome editing employs engineered nucleases to induce double-strand breaks at predetermined genomic loci in animals, facilitating precise modifications such as insertions, deletions, or substitutions via the cell's or pathways. This approach contrasts with random integration methods by enabling targeted changes, thereby reducing ectopic insertions and associated risks like . Zinc finger nucleases (ZFNs), among the earliest tools, pair DNA-binding domains with the endonuclease dimerization domain to cleave specific sequences; they were first demonstrated for targeted disruption in cells in 1994 and applied to animal models, including and rats, by the early 2000s for creating lines. Transcription activator-like effector nucleases (TALENs), introduced around 2010, utilize bacterial TALE proteins fused to for customizable recognition of longer DNA stretches, achieving higher specificity than ZFNs in applications like embryo editing and somatic cell targeting, with efficiencies reaching up to 10% for biallelic modifications in embryos. The CRISPR-Cas9 system, adapted from bacterial adaptive immunity and reported for eukaryotic in 2012, uses a single to direct the nuclease to protospacer adjacent motif-flanked targets, offering multiplexing capabilities and efficiencies exceeding 50% in many animal species; its first applications in animals included targeted mutations in in 2013 and rapid expansion to mammals like mice for modeling by 2014. In livestock, CRISPR-Cas9 has enabled edits such as the 2016 creation of hornless by inserting the Celtic polled allele into Holstein genomes without foreign DNA, yielding 100% transmission in edited calves, and knockout of porcine endogenous retroviruses in pigs for safety, with over 60 copies inactivated in cell lines. These techniques have transformed biomedical research by generating precise disease models, such as -edited pigs recapitulating via DMD gene disruption, mirroring human pathology more accurately than rodents due to physiological similarities. In , TALENs and have produced edited with knockouts, enhancing muscle growth by 20-30% in trials. Despite advantages in precision and speed—reducing model generation from years to months—challenges persist, including off-target effects (mitigated to below 0.1% in optimized protocols) and mosaicism in edited embryos, necessitating validation via whole-genome sequencing.

Delivery and Selection Techniques

Delivery of genetic material into animal embryos for transgenesis typically employs pronuclear microinjection, where exogenous DNA is injected directly into the pronucleus of fertilized oocytes using a micromanipulator and fine glass needle, achieving integration rates of approximately 10-30% in mammals like mice and livestock. This method, pioneered in mice in 1980 and extended to larger animals, remains standard for random transgene insertion despite low efficiency due to the large genome size and epigenetic silencing in animals compared to plants. For site-specific editing with tools like CRISPR-Cas9, cytoplasmic microinjection of Cas9 ribonucleoproteins (RNPs) or guide RNA into one-cell zygotes enables targeted modifications, with success rates exceeding 50% in species such as mice, pigs, and zebrafish by minimizing mosaicism through early embryonic delivery. Electroporation has emerged as an efficient, needle-free alternative for delivery, applying short electric pulses to permeabilize embryo membranes and introduce RNPs or plasmids, yielding editing efficiencies up to 80% in bovine and porcine zygotes while reducing physical trauma and operator skill requirements. Viral vectors, particularly lentiviruses, facilitate delivery by pseudotyping for broad and stable integration via reverse transcription, producing transgenic founders at rates of 60-70% in mice when injected into the perivitelline space of oocytes. (SCNT) complements these by editing cultured somatic cells (e.g., fibroblasts) via or lipofection before into enucleated oocytes, enabling precise knock-ins in but with efficiencies often below 5% due to reprogramming failures. Selection of modified animals relies on co-introduction of reporter or selectable markers during delivery to identify integrants. Common positive selection markers include neomycin phosphotransferase for antibiotic resistance in stages or fluorescent proteins like (GFP), detectable via in live embryos and offspring without harming viability. In transgenic pigs and sheep, GFP co-expression has facilitated sorting of edited blastocysts, streamlining founder identification. For genome-edited animals, marker-free selection predominates to evade concerns, involving PCR genotyping, sequencing, or of founders and progeny, though initial enrichment may use transient markers excised via post-integration. Negative selection markers, such as for sensitivity, aid in counterselecting random integrants during targeted editing, enhancing precision in SCNT-derived clones.

Applications in Mammals

Medical and Xenotransplantation Uses

Genetically modified animals serve as bioreactors for producing proteins, exploiting mammary glands, eggs, or blood for scalable expression of therapeutic molecules. Transgenic goats engineered to express antithrombin III in have yielded ATryn, the first FDA-approved from a GM animal, authorized in 2009 for treating hereditary antithrombin deficiency by preventing blood clots. Similarly, transgenic rabbits produce in , approved in as Ceprotin for coagulation disorders, demonstrating cost-effective yields up to 5 grams per liter compared to systems. Chickens modified via to secrete monoclonal antibodies in eggs offer advantages in and purification, with studies reporting yields of 1-3 mg per egg for anti-cancer therapeutics. These applications reduce production costs by 50-90% relative to microbial or mammalian cell lines while maintaining bioactivity, though regulatory scrutiny addresses and purity concerns. In , genetically edited provide organs compatible with recipients by mitigating hyperacute rejection through targeted gene and insertions. with homozygous alpha-1,3-galactosyltransferase (GGTA1) , first achieved in 2003, eliminate the Gal triggering immediate , enabling survival times exceeding 100 days in nonhuman models. Multigene edits incorporating transgenes like CD46, CD55, and , alongside and endothelial protein C receptor, further suppress complement activation and coagulation; a 10-gene edited transplanted into a brain-dead in 2023 functioned for 77 hours, normalizing levels without hyperacute rejection. In cardiac xenotransplants, a heart with similar edits implanted in a living in January 2022 sustained function for 60 days before failure due to non-immunologic factors, highlighting progress beyond limits of months. Liver xenografts from six-gene edited in 2025 brain-dead recipients showed bile production and metabolic activity for over 10 days, addressing size-matching and physiologic compatibility challenges. Despite advancements, persistent issues include porcine transmission risks, mitigated by additional , and chronic antibody-mediated rejection requiring immunosuppressive regimens. Clinical trials, approved by FDA in 2024 for renal xenotransplants, underscore potential to alleviate organ shortages, with over U.S. awaiting transplants annually.

Livestock Productivity Enhancements

Genetically modified livestock have been developed to enhance productivity traits such as growth rate, feed efficiency, and resource utilization, primarily through transgenesis and genome editing techniques. These modifications target physiological processes to increase meat, milk, or wool yields while potentially reducing environmental impacts from waste. For instance, transgenic pigs engineered to express the Escherichia coli phytase gene in their salivary glands, known as Enviropigs, digest plant phosphorus more effectively, reducing manure phosphorus excretion by up to 65% and allowing reduced supplemental phosphorus in feed, which lowers costs and pollution. This trait improves nutrient efficiency without altering growth performance, as demonstrated in trials where modified pigs maintained comparable weight gain to controls. In , has produced hornless variants by inserting the Celtic polled using / or TALENs, eliminating the need for surgical dehorning, which reduces labor costs, animal stress, and risks to handlers, thereby enhancing farm efficiency. These edits, achieved in cell cultures and transferred to embryos, result in offspring lacking horns while preserving other breed characteristics, with no reported off-target effects in verified lines. Similarly, (MSTN) gene knockouts via in and other ruminants promote , increasing lean meat yield by 20-40% in edited animals compared to wild-type, as evidenced in peer-reviewed studies across species. Goats modified via / to knock out the beta-lactoglobulin (BLG) produce with reduced content, potentially expanding market access and value without compromising yield, while some lines incorporate human for enhanced nutritional profiles. In sheep, transgenic expression of (IGF-1) under keratin promoters has increased wool production by up to 45% in lines, with finer fiber diameter, directly boosting output from fleece. These enhancements, however, require evaluation of long-term health and reproductive viability, as early transgenics showed accelerated maturation but higher metabolic demands. Overall, such modifications demonstrate potential for sustainable intensification, though regulatory and adoption barriers persist due to public and policy concerns.

Biomedical Research Models

Genetically modified mammals, primarily mice and rats, serve as essential tools in biomedical research for elucidating gene functions, modeling human disease pathologies, and evaluating potential therapies. These models enable precise manipulation of the genome to mimic genetic alterations associated with conditions such as cancer, neurodegeneration, and metabolic disorders, providing insights unattainable through in vitro systems alone. Transgenic approaches, involving the insertion of exogenous DNA sequences, were first achieved in mice during the early 1980s, allowing for gain-of-function studies via overexpression of specific genes. Complementing this, knockout models, developed through homologous recombination in embryonic stem cells in the late 1980s with initial reports in 1989, facilitate loss-of-function analyses by disrupting endogenous genes. This gene-targeting methodology earned Mario Capecchi, Martin Evans, and Oliver Smithies the 2007 Nobel Prize in Physiology or Medicine for its transformative impact on mammalian genetics. In disease modeling, transgenic mice expressing oncogenes, such as the pioneering "oncomouse" developed in the 1980s, replicate tumor formation and progression, aiding by revealing molecular drivers of oncogenesis. For , models like Tg2576, which overexpress mutant precursor protein (APP), exhibit amyloid plaque accumulation and cognitive deficits, enabling studies on pathogenesis and therapeutic candidates. Similarly, multi-mutant lines such as 5xFAD incorporate familial Alzheimer's mutations in APP and presenilins, accelerating and pathology for rapid assessment. Knockout models further dissect gene roles; for instance, disruptions in genes like mimic lipid disorders akin to . Over 4,600 genetically engineered mouse strains, encompassing transgenic, knockout, and humanized variants, are commercially accessible, supporting diverse research applications. Beyond basic research, these models underpin and by validating therapeutic targets and predicting compound efficacy and safety prior to testing. In target validation, conditional knockouts permit tissue-specific inactivation, clarifying causal roles in without embryonic lethality. For , transgenic reporter mice detect mutagenicity through heritable mutations at specific loci, enhancing hazard identification over traditional assays. ized mice, engineered with genes or cells, improve relevance for and , as seen in models for or research. While models dominate due to technical feasibility and rapid breeding—mice reaching maturity in 8-10 weeks—larger mammals like rabbits and pigs offer complementary insights for organ-specific studies, though their use remains limited by higher costs and longer generation times. Empirical data from these models have directly informed clinical successes, such as inhibitors derived from oncogene-driven cancer models, underscoring their causal value despite species-specific physiological divergences that can affect translatability.

Conservation and Ecological Interventions

Genetic modification of mammals has been proposed and preliminarily tested for conservation purposes, primarily to address , disease susceptibility, and loss of ecological functions in endangered or extinct species. Techniques such as enable targeted edits to restore from ancient or preserved samples or to introduce adaptive traits, potentially aiding reintroduction into wild habitats. However, field applications remain limited due to regulatory hurdles, ethical debates, and uncertainties about long-term effects, with most efforts confined to laboratory models. A prominent example involves initiatives to recreate ecological proxies for extinct mammals. , founded in 2021, is engineering embryos by inserting approximately 50 genes via to produce cold-adapted hybrids capable of surviving Arctic conditions. In March 2025, the company demonstrated feasibility by editing seven genes in mice, resulting in the "Colossal Woolly Mouse" with mammoth-like traits such as denser, golden fur and enhanced cold tolerance, serving as a multiplex editing platform for validating edits before scaling to elephants. The intended ecological intervention includes reintroducing these proxies to Siberian tundra by 2028 to restore mammoth-mediated processes: trampling snow to insulate , reducing from thawing soils, and grazing to maintain carbon-sequestering grasslands over shrub-dominated landscapes degraded by . Proponents argue this could counteract and enhance ecosystem resilience, though critics highlight risks of hybrid maladaptation or unintended into wild elephant populations. For extant endangered mammals, gene editing targets genetic rescue to combat . The (Mustela nigripes), critically endangered with populations descended from just seven founders, has seen cloning successes like in 2021 to boost diversity, but proposals focus on conferring resistance to (), which causes 90-100% mortality in outbreaks. Editing ferret genomes to mimic plague-resistant domestic ferret alleles could eliminate reliance on individual vaccinations, enabling scalable wild releases and reducing human intervention in habitats across . Similar approaches are under consideration for canids, such as editing Mexican gray wolves or red wolves to excise deleterious mutations or infuse hybrid vigor from coyote-derived red wolf alleles preserved in wild populations. A July 2025 study demonstrated proof-of-principle by editing mammalian cells with historical DNA from museum specimens to recover lost alleles, potentially applicable to species like the facing diversity bottlenecks. Broader ecological interventions include editing for , such as heat or in herbivores to sustain migration corridors. In October 2025, the International Union for Conservation of Nature (IUCN) passed a motion endorsing case-by-case use of , including gene editing, for wild mammals when benefits like disease resistance outweigh risks, marking a shift from prior opposition. Empirical data from lab-edited models support trait enhancements, but causal uncertainties persist: edited traits may not propagate naturally in mammals without gene drives (which are ethically contentious and technically challenging for large, low-reproduction species), and releases could disrupt food webs or hybridize with non-target populations. Prioritizing verifiable outcomes, ongoing trials emphasize containment and monitoring to assess real-world efficacy.

Applications in Fish

Aquaculture and Growth Modifications

The AquAdvantage salmon represents the primary commercial application of genetic modification for growth enhancement in aquaculture, consisting of a transgenic line of Atlantic salmon (Salmo salar) engineered to express a Chinook salmon (Oncorhynchus tshawytscha) growth hormone gene under control of an ocean pout (Zoarces americanus) antifreeze protein promoter. This construct enables year-round growth hormone production independent of environmental temperature, allowing the fish to reach harvest weight under controlled conditions in approximately half the time of conventional Atlantic salmon—typically 16-18 months versus 24-36 months for non-transgenic counterparts grown to 4-5 kg. Growth acceleration occurs primarily in the first year, with overall biomass yield potentially up to 70% higher annually in land-based systems compared to sea-cage farmed conventional salmon over equivalent periods. The U.S. Food and Drug Administration completed its review and approved the salmon for aquaculture production and human consumption on November 19, 2015, determining it substantially equivalent in nutritional profile and safety to non-modified salmon, with no unique allergens or toxins detected. Commercialization of has proceeded in contained, land-based recirculating systems to mitigate escape risks, with initial egg production in , , and grow-out facilities operational in , , since 2017. The first commercial-scale harvest of 133 metric tons occurred in early 2021, fully sold to U.S. distributors, though subsequent production has faced economic challenges including high operational costs and limited due to labeling requirements and consumer preferences for non-GM seafood. As of 2025, output remains modest, with ongoing efforts to expand sterile triploid stocks—all females—to further contain reproduction, though regulatory approvals in major markets like the remain withheld pending additional ecological risk assessments. Beyond , experimental growth modifications target other species, such as transgenic (Oreochromis niloticus) incorporating piscine transgenes, which have demonstrated 2- to 3-fold weight increases by seven months post-hatch in trials. Similarly, transgenic common carp (Cyprinus carpio) expressing constructs exhibit approximately 1.5 times higher weight gain relative to non-transgenic controls under identical feeding regimes. These modifications aim to boost protein deposition and reduce feed conversion ratios, but remain confined to research settings without regulatory approval for commercial farming, primarily due to concerns over potential with wild populations and unverified long-term fitness effects. Recent advances in site-specific editing, such as myostatin gene knockouts in species like red sea bream (Pagrus major), yield 1.2-fold body weight increases on equivalent feed intake, signaling potential for future precision enhancements, though no additional commercial approvals exist as of 2025.

Environmental and Pollution Detection

Transgenic fish, particularly zebrafish (Danio rerio) and medaka (Oryzias latipes), have been engineered as whole-organism biosensors to detect aquatic by linking pollutant-responsive promoters to reporter genes such as (GFP) or enhanced GFP (eGFP), which induce visible upon exposure. These systems exploit the fish's ability to bioaccumulate and respond biologically to contaminants, providing real-time indicators of and that surpass static chemical sampling in capturing ecological relevance. In , metal-responsive promoters fused to fluorescent reporters enable detection of including (Cd²⁺) and (Zn²⁺) at ultra-low thresholds, such as 4 ppb for Cd²⁺, allowing for rapid, non-invasive monitoring in and potentially field settings. Similarly, transgenic medaka lines employing the 70 () promoter drive differential GFP expression patterns in response to various , distinguishing between contaminants like , , and based on expression intensity and timing, which supports targeted environmental assessment in freshwater and . A marine medaka variant established in 2021 demonstrates sensitivity to heavy metal pollution in saline conditions, offering a tool for coastal and oceanic surveillance where traditional assays falter due to matrix complexity. Beyond , these models detect endocrine-disrupting compounds; for instance, the SR4G transgenic line, featuring a synthetic estrogen-responsive promoter driving GFP, quantifies xenoestrogens and other hormonal pollutants through dose-dependent , aiding of chemicals like in wastewater effluents. Early applications, dating to 2000, positioned transgenic as sentinels for broader hazardous substances, including mutagens and organic toxins, via promoter-reporter constructs that amplify subtle exposure signals into measurable phenotypes. Such innovations, validated in controlled exposures, underscore the potential for cost-effective, sensitive screening, though field deployment remains limited by regulatory and ecological containment challenges.

Research and Ornamental Varieties

have been engineered for research purposes primarily using species like (Danio rerio) and medaka (Oryzias latipes), which serve as models due to their transparent embryos, rapid development, and genetic tractability. Transgenic lines, often incorporating fluorescent proteins such as (GFP) from , enable real-time visualization of , cellular processes, and disease progression . These models have facilitated studies on developmental disorders, metabolic diseases, and neural pathologies, with over 25 years of forward and reverse genetic screenings yielding insights into cardiac conditions and neurodegenerative pathways. For instance, biosensor transgenic have been applied to assess endocrine-disrupting chemical interactions via fluorescent reporters. Medaka fish support similar research, with transgenic strains developed for whole-brain using genetically encoded calcium indicators (GECIs) to monitor neural activity fluctuations in larvae. Knock-in techniques via have achieved high efficiency in generating precise genetic modifications, aiding investigations into , environmental contaminants, and evolutionary social behaviors. Collections of transgenic medaka with attP motifs integrated via transposon systems enable site-directed integrations for . These modifications have advanced understanding of fish biology applicable to and human biomedicine, though ecological risks from potential escapes remain a concern in peer-reviewed assessments. Ornamental varieties stem from these research origins, with GloFish—fluorescent zebrafish—commercialized after initial development in the late 1990s at the National University of Singapore to detect environmental pollutants via GFP expression. The U.S. Food and Drug Administration approved their sale as the first genetically modified pets in 2003, determining no increased risk to human health compared to unmodified zebrafish. Commercial GloFish are bred from transgenic offspring, inheriting stable fluorescence without ongoing modification, and include varieties expressing red, green, blue, or ultraviolet hues under specific lighting. However, regulations vary; possession remains illegal in California since January 2007 under broad restrictions on genetically modified fish. Unauthorized releases have occurred, such as glowing GM fish detected in Brazilian creeks in 2022, highlighting containment challenges.

Applications in Insects

Vector Control for Disease Reduction

Genetically modified mosquitoes have been developed primarily to suppress populations of disease-vector species such as Aedes aegypti (transmitting dengue, Zika, and chikungunya) and Anopheles gambiae (transmitting malaria) through the release of engineered males that produce non-viable female offspring. These self-limiting systems typically incorporate a dominant lethal gene, such as a tetracycline-repressible effector, which causes female progeny to die before reaching adulthood in the absence of tetracycline, thereby reducing the reproductive capacity of wild populations without persistent genetic alteration. Field trials of Oxitec's OX513A strain in the Cayman Islands (2009–2010) achieved over 80% suppression of target A. aegypti populations, with subsequent releases in Brazil's Jacobina municipality (2013–2015) yielding sustained reductions exceeding 90% in monitored sites. In urban pilots conducted in 2021–2022, Oxitec's updated Friendly™ Aedes aegypti (OX5034) strain demonstrated 96% suppression of local A. aegypti populations after 27 months of releases, outperforming traditional insecticide-based methods in scale and persistence. Peer-reviewed models indicate such reductions fall below thresholds required for interrupting dengue transmission, though direct correlations with disease incidence remain variable and require further longitudinal data. Regulatory approvals, including U.S. EPA experimental use permits for Florida Keys releases starting in 2021, have enabled targeted deployments, with over 100 million engineered mosquitoes released globally by 2020 without reported ecological disruptions to non-target species. Population replacement strategies employ gene drives, such as /Cas9-based systems, to propagate traits that render mosquitoes refractory to pathogens, like anti-Plasmodium effectors that halt malaria parasite development. Laboratory and semi-field trials have shown gene drives spreading at rates up to 99% in caged populations, potentially enabling rapid modification of wild vectors. The Target Malaria consortium, funded by non-profit and governmental sources, advanced to small-scale releases of non-drive GM in in August 2025 to assess containment and performance, though the project faced suspension later that year amid public and regulatory concerns. Efficacy evidence for gene drives remains confined to contained settings, with WHO recommending phased risk assessments before open releases due to potential for unintended spread. While population suppression has demonstrated verifiable reductions in vector densities, causal links to decreased disease transmission are supported by modeling but lack large-scale, randomized field validation, partly due to confounding factors like variable human behavior and climate. Critics, including independent analyses, note inconsistencies in post-release monitoring, such as incomplete suppression in non-target areas, underscoring the need for integrated surveillance. Ongoing refinements, including sex-ratio distortion for enhanced male bias, aim to improve scalability, but deployment success hinges on community acceptance and regulatory frameworks prioritizing empirical containment data.

Pest Management in Agriculture

Genetically modified represent an emerging tool in agricultural pest management, primarily through enhancements to the (SIT). In conventional SIT, mass-reared pests are sterilized via radiation and released to mate with wild females, producing non-viable offspring and gradually suppressing populations; however, radiation can impair fitness, reducing mating competitiveness. Genetic modifications address this by incorporating self-limiting lethal genes, such as those causing female-specific lethality in progeny (known as RIDL, or release of carrying a dominant lethal genetic ), which maintain insect viability without environmental persistence beyond the target generation. This approach targets lepidopteran pests like moths, offering species-specific control that minimizes non-target impacts and reduces reliance on broad-spectrum insecticides, which (Plutella xylostella) has resisted in over 90 active ingredients globally. A prominent example is the genetically engineered developed by , a pest devastating crops like and , causing annual losses exceeding $4 billion worldwide. The strain expresses a tetracycline-repressible lethal , allowing lab rearing on antibiotics but ensuring female offspring die in the field; only males are released, mating with wild females to yield mostly male progeny that further dilute the population. Field trials in the (2015) and the (ongoing since 2017) demonstrated survival rates comparable to wild moths and significant suppression, with releases correlating to over 90% reduction in local populations in contained tests, potentially cutting applications by up to 50% in integrated systems. These self-limiting traits prevent spread, addressing ecological concerns, though scalability depends on regulatory approval and public acceptance. Research on genetically modified pink bollworm (Pectinophora gossypiella), a pest, has tested similar repressible lethality systems to bolster SIT efficacy. While U.S. eradication by 2018 relied on conventional radiation-sterilized releases synergized with —reducing bollworm damage by over 99% in treated areas without establishing populations—GM strains show improved field mating success (up to 80% competitiveness) in trials, offering potential for radiation-free alternatives in resistant hotspots. Emerging applications include bioengineered (spotted-wing drosophila), a invader; genetic disruptions to in lab models promise targeted suppression, outperforming chemical sprays in precision but remain pre-commercial as of 2024. Overall, these technologies have proven effective in localized suppression during trials, with empirical indicating 70-95% pest reductions in monitored sites, though widespread adoption lags due to containment verification and integration with existing .

Industrial and Basic Research Uses

Genetically modified have been engineered for industrial production of biomaterials and recombinant proteins, leveraging their silk glands or other tissues as bioreactors. Transgenic silkworms (Bombyx mori) expressing spider dragline silk genes, such as MaSp1 and MaSp2, produce hybrid fibers with tensile strengths up to 1.3 GPa and toughness exceeding native silkworm by 50%, enabling scalable of high-performance materials for textiles and composites. Similarly, silkworms modified to express the Drosophila dumpy gene yield with enhanced (up to 20 GPa) and breaking strength, demonstrating potential for industrial fibers rivaling without requiring complex spinning processes. These modifications, achieved via piggyBac transposon-mediated transgenesis, allow cocoon-based harvesting of proteins at yields of several grams per kilogram of cocoons, reducing costs compared to bacterial or mammalian systems. Beyond silk, transgenic silkworms serve as platforms for industrial-scale recombinant protein synthesis, including human collagen type III at levels of 10-20% of total cocoon protein, facilitating production of biomaterials for medical applications like tissue scaffolds. Emerging efforts involve black soldier flies () genetically engineered to secrete human insulin and growth factors in their larval , aiming for cost-effective from waste substrates, though scalability remains under validation in pilot systems as of 2024. These applications exploit insects' rapid reproduction and low rearing costs, with silkworm systems achieving protein purities over 90% post-extraction, though challenges include differences from mammalian hosts affecting bioactivity. In , genetically modified fruit flies function as premier model organisms for dissecting functions, developmental pathways, and disease mechanisms due to their short 10-day , polytene chromosomes amenable to visualization, and 75% genetic homology to humans. Transgenic tools like GAL4/UAS binary systems enable tissue-specific overexpression or knockdown of genes, revealing roles in processes such as and circadian rhythms; for instance, targeted mutations in period genes confirmed molecular clocks in 1984, foundational to . CRISPR-Cas9 editing in has accelerated , with over 10,000 lines generated by 2015 for high-throughput screens, modeling human conditions like Parkinson's via α-synuclein expression. These modifications, often via site-specific integrases like ΦC31, support precise replacement without off-target effects, underpinning discoveries in conserved signaling like and Notch pathways. Beyond flies, transgenic beetles and moths aid studies in and immunity, but dominates with millions of mutants archived in stock centers, enabling unattainable in vertebrates.

Applications in Other Taxa

Avian Modifications

Genetically modified avian species, predominantly chickens, have been developed primarily through engineering techniques involving primordial germ cells (PGCs) and CRISPR/ to achieve targeted insertions, knockouts, or modifications. These approaches enable stable transmission of alterations to offspring, bypassing challenges posed by the avian reproductive system, such as the impermeability of the to direct injection. Early transgenic efforts, dating to the and , produced birds expressing reporter genes like (GFP) ubiquitously in embryos, validating transmission via PGC culture and chimera systems. A primary application focuses on conferring resistance to virus (IAV), a major economic and zoonotic threat to . In a 2023 study, researchers used / to generate transgenic chickens expressing an decoy targeting the viral polymerase, rendering the birds resistant to infection from high-dose challenges of H5N1 and H7N9 strains; while not fully preventing in all cases, the modifications significantly blocked onward transmission to contact birds in contained trials. Independent verification confirmed partial immunity, with edited chickens showing no clinical disease symptoms upon exposure, though full population-level resistance requires further refinement to eliminate residual shedding risks. Similar editing has targeted avian leukosis virus subgroup J (ALV-J), where a specific receptor in a commercial line prevented and tumor formation following experimental inoculation, as demonstrated in 2021 trials. Chickens have also been engineered as bioreactors for pharmaceutical production, leveraging their egg-laying capacity to secrete human therapeutic proteins. Modifications inserting genes for humanized monoclonal antibodies or interferon beta into the oviduct-specific ovalbumin promoter have yielded eggs containing up to 1-3 mg/mL of target proteins, scalable for industrial extraction; a 2025 review highlighted successful expression of antibodies against cancer and viral pathogens, with purification efficiencies exceeding 90% in pilot processes. These transgenic lines maintain normal and , producing viable offspring that inherit the trait, though regulatory hurdles limit . In , avian models facilitate studies of and . For instance, CRISPR-mediated knockout of the in chickens, reported in October 2024, decoupled testosterone's effects on aggression and muscle growth, revealing nuanced sex-specific roles in without altering baseline physiology. Such modifications, often combined with tissue-specific knockouts via primordial electroporation, have advanced understanding of avian immunology and , with efficiencies reaching 20-50% transmission in optimized protocols. No genetically modified avian species are approved for consumption as of 2025, with applications confined to research and potential veterinary or biopharma contexts.

Amphibians, Reptiles, and Nematodes

Genetic modification in amphibians has primarily supported and regeneration research, utilizing species such as the (Xenopus laevis) and (Ambystoma mexicanum). Emerging techniques, including viral vectors for , have enabled targeted editing in neural circuits of frogs, newts, and axolotls, facilitating studies of development as demonstrated in preprints from 2024. These tools adapt methods proven in mammals, allowing optogenetic and chemogenetic manipulation without permanent transgenesis in some cases. In reptiles, CRISPR-Cas9 has been applied to create the first gene-edited specimens for studying pigmentation and morphological development. In April 2019, researchers at the microinjected unfertilized eggs of the lizard (Anolis sagrei) to knock out the Tyr gene, producing albino offspring and establishing reptiles as viable models for . Similar protocols have been adapted for s, enabling targeted mutations in species like the ground gecko (Paroedura picta) to investigate limb development and regeneration. In June 2023, CRISPR editing of corn snakes (Pantherophis guttatus) disrupted genes involved in scale patterning, revealing mechanisms underlying Turing-like hexagonal formations through altered sonic hedgehog signaling. These modifications remain confined to laboratory research, with no documented agricultural or commercial releases. Nematodes, particularly Caenorhabditis elegans, represent one of the most extensively genetically modified non-vertebrate taxa, serving as a model for genetic dissection since the 1970s and with CRISPR/Cas9 integration accelerating precise edits from 2016 onward. Applications encompass aging research, where strains expressing human disease-associated proteins model neurodegeneration, as in Parkinson's via alpha-synuclein expression, yielding insights into proteotoxicity validated across 1,000+ mutant lines. In toxicology, genome-modified C. elegans expressing human cytochrome P450 enzymes (e.g., CYP1A1) metabolize xenobiotics comparably to mammals, enabling high-throughput screening of 10,000+ compounds for toxicity. Drug discovery leverages mutant screens for mechanisms, identifying nematicide targets and repurposed therapies, with over 500 screens conducted by 2025. Extensions to parasitic nematodes, such as Panagrolaimus species, use CRISPR for gene knockouts to study anhydrobiosis and host interactions. These efforts prioritize basic and translational research, with C. elegans' transparent body and 959-cell hermaphroditic genome enabling causal inference in gene function unavailable in vertebrates.

Empirical Benefits and Verified Outcomes

Agricultural Yield and Efficiency Gains

The , a genetically modified approved for commercial production by the U.S. in 2015, achieves market size in about 18 months, roughly half the time required by non-modified counterparts, which typically take 30-36 months under similar conditions. This accelerated growth phase, driven by the insertion of a regulated by an promoter, enables up to twice the production speed during early development without altering final adult size or flesh quality. Consequently, facilities can harvest more cycles annually, boosting yield per production unit while reducing holding periods and associated operational costs. Feed efficiency represents another verified gain, with requiring approximately 25% less feed to reach harvest weight than conventional , as demonstrated in controlled rearing trials. This stems from the modified prioritizing somatic growth over gonadal development in early stages, optimizing for accumulation rather than . In practice, such reductions translate to lower input costs and decreased environmental pressure from uneaten feed in systems, supporting higher net yields without expanded . In swine production, the Enviropig, a transgenic line developed by researchers at the and tested through the early , exemplifies nutrient utilization improvements by expressing a bacterial in to break down plant-based . Trial data indicated these pigs excreted 24-44% less and up to 24% less in compared to unmodified pigs on identical phytate-rich diets, eliminating the need for synthetic supplements or exogenous additives. This enhanced feed conversion maintained equivalent growth rates and , potentially cutting supplementation costs by 30-65% while sustaining output per animal. Broader applications, including gene-edited and pigs targeting or pathways, have shown in experimental settings improved feed conversion ratios and reduced time to market weight by 10-20%, though commercial deployment remains limited by regulatory hurdles. These modifications causally link genetic interventions to higher protein deposition and energy efficiency, yielding more edible product per caloric input, as evidenced by peer-reviewed growth assays. Overall, such traits empirically support intensified production with fewer resources, countering land and feed constraints in global animal .

Human Health Advancements

Genetically modified animals have facilitated human advancements primarily through biopharmaceutical production, , and disease modeling. In biopharming, transgenic such as and cows are engineered to secrete recombinant human proteins in their , serving as bioreactors for therapeutics that are difficult or costly to produce via microbial or cell systems. For instance, transgenic goats producing human antithrombin III in milk yielded ATryn, the first FDA-approved from such animals in 2009, used to prevent blood clots in hereditary deficiency patients by providing a safer alternative to plasma-derived versions with lower viral contamination risks. Similarly, transgenic sows have expressed human , , and in milk, enabling scalable production of clotting factors for hemophilia treatment. These approaches leverage mammalian for proteins requiring complex post-translational modifications, potentially reducing production costs compared to traditional methods. Xenotransplantation represents another key advancement, with gene-edited pigs developed to mitigate immune rejection and viral risks in recipients, addressing chronic organ shortages. Pigs are modified via /Cas9 to inactivate porcine endogenous retroviruses and genes like alpha-1,3-galactosyltransferase causing hyperacute rejection, while inserting complement regulators and anticoagulants for compatibility. In 2022, genetically modified hearts were transplanted into brain-dead s, sustaining function for weeks, paving the way for clinical trials. By 2025, the FDA authorized trials for xenotransplants by companies like eGenesis, using multi-gene edited pigs to test long-term viability in end-stage renal disease patients. A milestone occurred in October 2025 with the first successful auxiliary liver xenotransplant from a genetically modified into a living , demonstrating short-term functionality without immediate rejection. These developments could alleviate the global waitlist exceeding 100,000 for kidneys alone, though long-term outcomes remain under evaluation in ongoing trials. Genetically modified animals also serve as precise models for studying human diseases, accelerating therapeutic development. Knock-in or knock-out rodents and pigs recapitulate genetic conditions like cystic fibrosis or Huntington's, enabling causal mechanistic insights unattainable in vitro. Porcine models, due to physiological similarities with humans, have advanced cardiovascular and cancer research; for example, gene-edited pigs mimic human myocardial infarction for drug testing. Such models have contributed to empirical progress in understanding disease pathways, with verifiable outcomes including refined protocols for gene therapies validated across species. Despite these benefits, adoption requires rigorous safety data, as preclinical xenotransplant studies in non-human primates confirm reduced rejection but highlight needs for immunosuppression optimization. Overall, these applications underscore causal links between genetic engineering and tangible health gains, supported by regulatory approvals and trial data.

Environmental and Conservation Impacts

Genetically modified insects, particularly mosquitoes engineered to suppress disease-vector populations, have shown verifiable environmental benefits by reducing reliance on chemical insecticides. Field trials of Oxitec's transgenic Aedes aegypti mosquitoes in Brazil and the Cayman Islands achieved 80-95% suppression of target populations, with associated reductions in dengue transmission up to 91%, and monitoring indicated no significant non-target ecological effects or spread of the modification beyond intended areas. The U.S. Environmental Protection Agency's approval of OX5034 male mosquitoes for release determined they cause no unreasonable adverse environmental impacts, as the self-limiting trait ensures population declines without persistence in ecosystems. In , the , genetically modified for rapid growth and cold-water tolerance, supports conservation by diminishing harvest pressure on wild stocks, which have declined due to and loss. U.S. environmental assessments concluded that facility-contained production of sterile, all-female presents negligible risk of establishment or to wild populations, even in escape scenarios, based on modeling of survival, reproduction, and fitness disadvantages. Land-based farming further minimizes discharge and disruption compared to ocean net-pens, potentially lowering the overall of salmon production. Transgenic livestock modifications aimed at enhancing feed efficiency and disease resistance offer indirect conservation advantages by optimizing resource use in animal agriculture, which occupies 77% of global agricultural land. Empirical data indicate low environmental risks from such animals, with transgenic cattle and pigs unlikely to establish feral populations or disrupt biodiversity due to reduced fitness in wild conditions. For endangered species conservation, genetic engineering techniques, including CRISPR-based edits, have been proposed to restore genetic diversity and confer pathogen resistance, as in efforts to bolster black-footed ferret populations against plague; while field-verified outcomes are emerging, lab successes demonstrate feasibility without broad ecosystem alteration.

Scientific Risks and Evidence-Based Assessments

Physiological and Health Effects on Animals

In growth-enhanced , such as AquAdvantage Atlantic salmon engineered with a and an promoter, individuals reach market size in 16-18 months versus 36 months for conventional strains, with regulatory assessments confirming no significant adverse physiological effects on the fish themselves, including normal in fertile lines and absence of from the introduced . However, certain experimental lines with high expression have exhibited skeletal deformities, increased disease susceptibility, and reduced fitness, though commercial strains are selected to minimize these outcomes. In , transgenic pigs overexpressing in early experiments (1980s-1990s) displayed chronic health detriments from sustained elevation, including gastric ulcers, , , , and lameness, linked causally to metabolic imbalances like excessive insulin-like growth factor-1. More recent gene-edited pigs, such as six-gene-modified Bama miniature swine for (involving knockouts of alpha-1,3-galactosyltransferase, CMAH, and B4GALNT2, plus human transgenes for complement regulation and thrombosis control), exhibit normal physiological , with histopathological and functional evaluations revealing no abnormalities in vital organs like the heart, liver, spleen, lungs, or kidneys, and comparable growth, , and serum biochemistry to wild-type controls. Transgenic pigs engineered for enhanced retention via a bacterial demonstrate accelerated growth rates and improved feed efficiency without reported physiological impairments, though long-term studies on reproductive fitness and skeletal integrity remain limited. In disease-resistant models, such as pigs edited for resistance to porcine reproductive and respiratory syndrome virus (PRRSV), modified animals show reduced viral loads, lower , and preserved function compared to non-edited counterparts, thereby enhancing overall and welfare under exposure. For biomedical research models, genetically modified rodents and pigs recapitulating human pathologies—e.g., Duchenne muscular dystrophy pigs with gene disruptions—intentionally induce muscle degeneration, cardiac dysfunction, and reduced lifespan, but these effects stem from the targeted disease simulation rather than off-target genetic instability, with welfare mitigated through early protocols and analgesics. Empirical reviews of gene-edited farm animals indicate that unintended physiological risks, such as mosaicism or off-target mutations, occur but are infrequent in optimized CRISPR-Cas9 applications, with no systemic evidence of heightened cancer incidence or immune dysregulation beyond baseline breed variations. Overall, peer-reviewed data underscore that adverse health outcomes in GM animals arise primarily from overexpression artifacts or incomplete phenotypic characterization in nascent technologies, whereas refined edits yield animals physiologically equivalent or superior to conventional ones in targeted traits like disease tolerance.

Ecological and Biodiversity Considerations

Genetically modified animals raise ecological concerns primarily through potential to wild populations, altered competitive dynamics, and indirect effects on food webs or non-target , though of widespread remains limited due to practices in most applications. For instance, escaped GM organisms could hybridize with wild relatives, potentially introducing transgenes that confer fitness advantages or disadvantages, but animal-specific barriers such as and sterility measures mitigate this risk compared to plants. Regulatory assessments emphasize that without escape, ecological impacts are negligible, as seen in peer-reviewed evaluations of contained systems. In the case of , engineered for rapid growth via gene insertion, the U.S. Food and Drug Administration's 2012 environmental assessment concluded no significant ecological risk, citing production in land-based, secure facilities with triploid, all-female sterility rendering them reproductively inviable even if escaped. Field studies simulating escape showed GM salmon neither outcompeted nor displaced wild in shared habitats, with survival rates comparable or lower due to the absence of wild-adapted traits. Updated 2024 assessments for expanded facilities reaffirmed these findings, noting minimal probability of establishment in wild ecosystems. Genetically modified mosquitoes, such as Oxitec's strains with lethal tetracyline-suppressible genes, have been deployed in field trials to suppress vector populations for disease control, with environmental monitoring in and the detecting no adverse effects or gene persistence beyond target generations. The U.S. Environmental Protection Agency's 2020-2021 evaluations found negligible risk to non-target or ecosystems, as self-limiting designs prevent long-term population suppression or . Potential food web disruptions from reduced numbers were deemed unlikely, given natural predators' adaptability and the localized scale of releases. Broader biodiversity considerations include potential benefits from GM animals in conservation, such as gene drives in to eradicate on islands, which could restore native and without chemical interventions. However, critics from environmental advocacy groups argue for precautionary restrictions, citing unproven long-term trophic cascades, though these claims often lack empirical backing and overlook data from over a decade of trials showing stability. Overall, evidence-based assessments indicate that with physical containment and genetic safeguards, GM animals pose contained ecological risks, outweighed by verified applications in pest management that indirectly preserve .

Human Consumption Safety Data

The U.S. Food and Drug Administration (FDA) regulates genetically modified (GM) animals intended for human consumption as new animal drugs, requiring demonstrations of substantial equivalence in nutritional composition, absence of toxins, and lack of allergenicity compared to non-GM counterparts. In 2015, the FDA approved , engineered for faster growth via a gene from and an promoter, after reviewing data showing no significant differences in nutrient levels, fatty acids, or from conventional fillets; the agency concluded the fish posed no greater risk of food allergies or toxicity. Scientific assessments of GM animal products for human consumption rely on compositional analysis, targeted , and 90-day feeding studies to detect unintended effects, as longer-term human trials are not required when equivalence is established. Peer-reviewed reviews of animal feeding trials with GM feeds or products, including those derived from modified , report no verified adverse effects on parameters such as organ function, , or , aligning with outcomes from over 28 years of GM consumption where no causal links to human harm have been substantiated. The (EFSA) guidance for GM animal food and feed mandates case-by-case evaluation of molecular changes, digestibility, processing effects, and potential post-market monitoring, with assessments concluding that techniques like gene editing do not introduce novel safety risks beyond those of conventional breeding when unintended alterations are absent. Limited commercialization of GM animals—primarily the in select markets—has yielded no post-approval reports of human health issues attributable to consumption, though systematic surveillance remains ongoing due to low . Claims of risks, such as increased susceptibility in transgenic , originate from advocacy organizations rather than peer-reviewed data and have not influenced regulatory approvals, which prioritize over speculative concerns. Overall, the absence of documented harms in approved cases and analogous GM products supports the determination that GM animal-derived foods are safe for human consumption when rigorously assessed.

Controversies and Ideological Opposition

Debunking Precautionary Principle Excesses

The , when invoked to impose indefinite moratoriums or heightened scrutiny on genetically modified animals absent of harm, exemplifies regulatory excess by subordinating verifiable data to speculative uncertainties. This approach often equates the precision of targeted genetic modifications—such as inserting a single for enhanced growth—with the uncontrolled mutations from traditional breeding methods, despite the latter's history of unintended traits like accumulation in some crops. For instance, regulatory frameworks treating GM animals as presumptively riskier have delayed innovations without commensurate gains, as long-term studies on animals consuming GM feed demonstrate no elevated risks compared to conventional diets. A prominent case is the , engineered with a regulated by an promoter to achieve faster growth; the U.S. approved it on November 19, 2015, after determining it poses no greater to human health or the environment than non-GM salmon, with nutritional profiles equivalent and allergenicity tests negative. Despite this, precautionary concerns over potential escape and interbreeding—despite physical containment in land-based facilities and sterility measures—prolonged development from initial trials in the , illustrating how the principle amplifies hypothetical ecological disruptions while overlooking that farmed salmon already exert selective pressures on wild stocks through conventional escapes. Empirical monitoring post-approval has confirmed containment efficacy, with no verified instances of adverse impacts. Similarly, the Enviropig, developed in the to express a bacterial in for breaking down and reducing manure pollution by up to 75%, was abandoned in 2012 amid regulatory barriers and opposition rooted in precautionary aversion to novelty, despite lab demonstrations of efficacy without health detriments to the pigs. This forwent tangible environmental gains, as runoff from conventional swine operations contributes to algal blooms in waterways like the Gulf of Mexico's dead zone, which spans over 5,000 square miles annually. Such outcomes highlight the principle's bias toward inaction, where accepted practices like chemical fertilizer use—linked to comparable —evade equivalent scrutiny. Critics argue that precautionary excesses foster asymmetric , demanding exhaustive proof of absolute safety for GM animals while tolerating probabilistic harms from status-quo alternatives, such as disease outbreaks in non-engineered costing U.S. $3.4 billion yearly. Over three decades of GM crop deployment, involving trillions of animal meals derived from them, has yielded no corroborated patterns of or ecological collapse attributable to the technology, underscoring that targeted modifications often yield more predictable outcomes than , which has produced varieties with elevated acrylamide precursors without analogous regulatory halts. By prioritizing unproven doomsday scenarios over causal evidence—like the absence of dominance in contained animal systems—the principle impedes advancements in areas such as , where GM pigs edited for human organ compatibility could address the 17-person daily mortality from transplant waitlists.

Public Misinformation and Anti-Innovation Campaigns

Public opposition to genetically modified animals frequently arises from unsubstantiated claims that they introduce novel toxins, allergens, or uncontrollable into wild populations, assertions contradicted by regulatory data showing equivalence to conventional counterparts in safety profiles. For example, fears surrounding the —engineered for faster growth—included predictions of heightened allergenicity and ecological dominance, yet U.S. assessments in 2015 found no increased risks compared to non-GM , with fact-checks confirming the absence of for such harms. These misconceptions persist despite animal feeding studies demonstrating no adverse effects on or productivity from GM feeds or products. Anti-innovation campaigns by environmental NGOs, such as , have amplified these narratives through direct actions like the 2000 destruction of a GM maize trial in the UK—acquitted on grounds—and broader labeling GM animals as "Frankenfoods" unfit for consumption. In response, 107 Nobel laureates in 2016 urged to cease its opposition to GM technologies, arguing that such tactics prioritize ideology over empirical evidence of benefits like reduced via efficient . These efforts foster distrust, with global polls revealing a median 48% viewing GM foods as unsafe despite on their equivalence to non-GM varieties. The resultant public skepticism creates regulatory and market barriers, as seen in mandatory labeling for GM salmon under the U.S. National Bioengineered Food Disclosure Standard, which stigmatizes approved products and deters . Media amplification exacerbates this, with analyses of 2019–2021 coverage finding nearly 10% containing misinformation on risks, often sourced from activist groups rather than peer-reviewed data. Consequently, innovations like hornless via gene editing—reducing injury risks without altering other traits—face adoption hurdles, prolonging reliance on less precise breeding methods. Such campaigns, while framing opposition as precautionary, empirically delay verifiable gains in and , as evidenced by stalled commercialization of GM leading to AquaBounty Technologies' operational halt in 2024.

Economic and Regulatory Barriers to Adoption

The regulatory treatment of genetically modified (GM) animals as novel veterinary drugs in jurisdictions like the imposes substantial barriers to commercialization, requiring extensive preclinical and environmental risk assessments that can span decades. In the U.S., the (FDA) evaluates GM animals under the Federal Food, Drug, and Cosmetic Act, demanding proof of safety for the animal, human consumers, and the environment, which contrasts with lighter-touch frameworks for GM crops regulated primarily by the USDA. This drug-like scrutiny, including mandatory containment measures to prevent interbreeding with wild populations, has resulted in only a handful of approvals, such as the in 2015—after initial development in the early 1990s and formal applications over two decades prior. Such protracted timelines amplify economic hurdles, as development costs for transgenic often exceed those for due to lower success rates in gene integration and higher animal-specific testing demands, with alone deterring from smaller firms. For instance, the AquAdvantage salmon's path to market involved over 25 years of iteration and submission, culminating in restrictive production conditions that limited initial commercialization to land-based facilities in and , contributing to ongoing financial strains for developer AquaBounty Technologies. Broader economic analyses highlight that these barriers reduce , as the high upfront R&D expenses—potentially in the hundreds of millions—coupled with uncertain market entry, discourage private sector pursuit of GM animal traits like resistance or enhanced growth. In the and other regions with process-based regulations, de facto moratoriums on GM animals for food use further entrench barriers, mandating case-by-case authorizations under frameworks like Directive 2001/18/EC that prioritize potential long-term risks over empirical safety data from contained trials. This has stalled projects like enzyme-producing GM pigs, abandoned partly due to insurmountable approval costs and trade incompatibilities, while varying global standards—such as outright bans in countries like —fragment markets and elevate premiums for producers. Even in approving jurisdictions, mandatory labeling requirements, as imposed on , can provoke consumer backlash and segregation costs, undermining economic viability despite evidence of nutritional equivalence to conventional counterparts.

Ethical Considerations

Animal Welfare from First-Principles View

Animal welfare fundamentally concerns the biological and behavioral states that enable animals to thrive without unnecessary suffering, encompassing physical health, absence of chronic pain or disease, and opportunities for species-typical behaviors. From causal mechanisms, genetic modifications in animals alter specific traits via targeted DNA changes, allowing evaluation of welfare impacts through measurable outcomes like morbidity rates, stress indicators (e.g., cortisol levels), and longevity, rather than presumptions of inherent harm from intervention. Empirical assessments prioritize direct observation over speculative risks, recognizing that conventional breeding has long induced welfare trade-offs, such as mastitis-prone high-yield dairy cows, without invoking "unnaturalness" as a disqualifier. Targeted genetic edits can mitigate established welfare deficits. For instance, CRISPR-edited lacking the Pc (polled) variant produce hornless , eliminating the need for dehorning—a procedure involving tissue or that inflicts acute , as evidenced by elevated nociceptive responses and behavioral distress in calves subjected to it. In 2019, a genome-edited bull sired hornless calves with no reported health anomalies beyond those typical of the breed, demonstrating that such modifications avoid the procedural trauma while preserving viability. Similarly, engineering disease resistance, as in modified for PRRS tolerance, reduces incidence of respiratory syndromes that cause prolonged suffering, with studies showing lower mortality and clinical signs compared to non-edited controls. AquAdvantage salmon, engineered with a Chinook under an promoter, reach market size in 18 months versus 30 for conventional strains, shortening exposure to aquaculture stressors like crowding and water quality fluctuations. Veterinary assessments found no deviations in metrics or welfare indicators from non-GM triploid salmon, with triploidy itself—a standard sterility measure—already employed to enhance without inherent welfare costs. While early-generation GM animals in research may exhibit artifacts like mosaicism-induced phenotypes, refined techniques like minimize off-target effects, yielding animals physiologically comparable to or superior to selectively bred counterparts in welfare parameters. Opposition often conflates welfare with teleological notions of "," yet first-principles scrutiny demands evidence of net , which case-specific data frequently refute. For example, beta-lactoglobulin knockout goats for hypoallergenic showed no or health impairments, averting inflammatory conditions tied to stasis in overproducing lines. Regulatory and peer-reviewed evaluations consistently affirm that well-designed GM traits do not exacerbate suffering when vetted against baselines, underscoring potential for welfare gains over status quo breeding practices that amplify vulnerabilities for productivity. This approach privileges verifiable physiological endpoints, enabling innovations that causally reduce disease burdens or procedural pains inherent in unmanaged traits.

Balancing Human Utility Against Speculative Harms

Genetically modified animals offer substantial human utility through enhanced agricultural efficiency, medical advancements, and reduced resource demands. For instance, the , approved by the U.S. (FDA) in 2015, incorporates genes from and an from , enabling it to reach market size in approximately 18 months—half the time required for conventional —while consuming 25% less feed. This modification addresses pressures and improves protein availability without altering nutritional composition or introducing allergens, as confirmed by FDA safety assessments showing equivalence to non-GM salmon. Similarly, gene-edited livestock, such as engineered for resistance to bovine via insertion of the SP110 , demonstrate potential to mitigate disease-related losses, which account for up to 20% of global livestock mortality and exacerbate food insecurity in developing regions. In , GM pigs modified to eliminate porcine endogenous retroviruses and other immunogenic factors have enabled historic xenotransplants, including the first genetically edited pig implanted into a in March 2024, functioning for over two months post-procedure and offering a viable bridge to alleviate the chronic organ shortage affecting over 100,000 U.S. waitlist patients annually. These applications extend to producing therapeutic proteins in animal or creating precise disease models in large mammals, surpassing limitations for studies, as evidenced by porcine models replicating complex conditions like . Empirical data from regulatory reviews and long-term feeding studies affirm no detectable risks from consuming GM animal products, with systematic analyses of over 1,700 studies finding no substantiated adverse effects on animal or . Speculative harms, such as hypothetical to wild populations or unforeseen ecological disruptions, lack empirical validation in contained, approved systems like land-based facilities designed to prevent escape. Claims of novel risks from often stem from precautionary extrapolations rather than data, contrasting with the absence of tangible harms in decades of GM crop deployment, where billions of meals have been consumed without incident. Ethical reasoning grounded in causal evidence prioritizes verifiable benefits—such as averting through resilient or saving lives via xenografts—over indeterminate fears, particularly when mitigation strategies like sterility in GM (via triploidy) neutralize propagation risks. Regulatory bodies, applying risk-based frameworks, have approved these technologies only after demonstrating that utilities exceed any identified hazards, underscoring a rational balance favoring human welfare.

Regulatory Frameworks

Approval Processes in Key Jurisdictions

, the () regulates genetically modified animals through its Center for , treating heritable intentional genomic alterations (IGAs) as new animal drugs under a risk-based framework outlined in Guidance for Industry (GFI) #187A, issued , , and GFI #187B, issued 2025. This approach evaluates safety for the animal, human consumers, and the environment based on the specific alteration's characteristics, rather than the method of production, requiring sponsors to submit investigational new animal drug (INAD) files and, if applicable, new animal drug applications (NADAs) demonstrating no increased risks compared to conventional counterparts. The first commercial approval was for on November 19, 2015, engineered for faster growth via an inserted gene from and an promoter, after assessments confirmed nutritional equivalence and allergenicity profiles similar to non-modified . In the , the (EFSA) conducts risk assessments for genetically modified animals under Regulation (EC) No 1829/2003, focusing on /feed safety, molecular characterization, and environmental risks via a case-by-case process that includes a six-month opinion timeline post-completeness check. Applications must detail the genetic modification process, intended use, and potential unintended effects, with EFSA's GMO Panel evaluating , allergenicity, and ecological impacts, often invoking precautionary elements despite empirical data requirements. No genetically modified animals have received EU authorization for commercial or feed use as of 2025, reflecting stringent hurdles; however, EFSA issued guidance in 2013 for environmental risk assessments of GM animals, emphasizing long-term monitoring and containment to mitigate gene flow risks unsubstantiated by U.S. approvals. Canada employs a product-based safety assessment for novel traits in genetically modified animals under Health Canada's Novel Foods Regulations and the Canadian Food Inspection Agency (CFIA) for feed and environmental aspects, requiring pre-market notifications only if the modification introduces novel traits posing potential risks. The process verifies compositional, nutritional, and toxicological equivalence to conventional animals, with approvals granted for in March 2016 following reviews confirming no unique hazards. Unlike mandatory approvals for all modifications, Canada's framework allows lower oversight for gene-edited animals without foreign DNA, aligning with empirical safety data over process-based scrutiny. In , the National Technical Commission on Biosafety (CTNBio) oversees approvals for genetically modified animals under the (Law No. 11.105/2005), granting commercial and environmental release authorizations after technical evaluations of molecular stability, health, and ecological impacts. CTNBio issued its first approval for environmental release of a GM animal in 2014 via Normative Resolution No. 7 (2009), and by 2023 had approved commercial use of approximately 15 GM animal lines, including gene-edited varieties, facilitating faster adoption compared to precautionary regimes elsewhere due to reliance on demonstrated trait benefits like disease resistance.

International Harmonization Challenges

Divergent national regulatory frameworks pose significant barriers to the international commercialization and trade of genetically modified (GM) animals. In the United States, the (FDA) regulates GM animals as new animal drugs under a product-based approach, focusing on the safety and efficacy of the final modified organism rather than the modification technique; this facilitated the approval of —a GM engineered for faster growth—on November 19, 2015. In contrast, the employs a process-based precautionary framework, requiring comprehensive risk assessments by the (EFSA) for any GM animal or derived product, with no such approvals granted to date due to stringent environmental, health, and traceability requirements. approved the same in May 2016 following a assessment, highlighting bilateral alignments but underscoring broader inconsistencies, as only these two jurisdictions have authorized GM animals for food use globally. Many other countries, including those in and Asia, adapt crop-centric regulations to animals inadequately, leading to de facto bans or prolonged approval delays. International bodies like the Commission offer voluntary guidelines to bridge these gaps, including principles adopted in 2001 for assessing the safety of foods derived from recombinant-DNA animals, emphasizing comparative analysis of composition, toxicity, and allergenicity against non-GM counterparts. These standards aim to promote science-based harmonization, yet their non-binding nature allows persistent divergence; for instance, the EU's emphasis on process-derived risks often exceeds Codex recommendations, influenced by public opposition and precautionary mandates rather than empirical evidence of harm. The (WTO) further complicates resolution through its Sanitary and Phytosanitary (SPS) Agreement, which encourages reliance on Codex but permits measures based on precaution if scientifically justified—though disputes, primarily over GM crops, reveal how such flexibilities create non-tariff barriers, as exporting nations like the argue that import restrictions lack sufficient risk data. Animal-specific challenges exacerbate this, including difficulties in standardizing environmental release protocols and evaluations across jurisdictions. These inconsistencies hinder global trade and innovation, with GM animal products facing export rejections or mandatory segregation in precautionary markets, potentially violating WTO rules if not substantiated by risk assessments. For example, while US-approved could theoretically enter international supply chains, import bans effectively limit , deterring investment in animal estimated to yield benefits like disease-resistant . Efforts toward , such as those by the International Council for Laboratory Animal Science (ICLAS), focus on research animals but falter on commercial applications due to entrenched cultural and political divides, where empirical safety data from approvals like AquAdvantage—showing no nutritional or toxicological differences from conventional —are discounted in favor of speculative concerns. Overall, without stronger alignment on product-safety criteria over process scrutiny, the field risks fragmented progress, with only niche approvals amid widespread regulatory uncertainty.

Recent Policy Shifts Toward Innovation

In the United Kingdom, the Genetic Technology (Precision Breeding) Act 2023, receiving royal assent on March 23, 2023, marked a significant departure from European Union-era regulations by creating a distinct category for "precision-bred" organisms, encompassing gene-edited vertebrate animals excluding humans. This framework exempts qualifying animals—those with targeted edits mimicking natural mutations or traditional breeding, without foreign DNA integration—from the rigorous environmental risk assessments and containment requirements imposed on conventional genetically modified organisms under the Environmental Protection Act 1990. The Act facilitates faster market entry for innovations such as disease-resistant livestock or animals with enhanced feed efficiency, with oversight shifting to a notification-based system administered by the Advisory Committee on Releases to the Environment, supplemented by mandatory animal welfare evaluations. Implementation advanced with the publication of draft guidance on March 5, 2025, clarifying eligibility criteria, and associated regulations entering force progressively throughout 2025, positioning the UK as a post-Brexit hub for agricultural biotechnology. Parallel developments in other jurisdictions reflect a broader trend toward product-based rather than process-based regulation for gene-edited animals lacking transgenes. In , gene-edited animals produced via techniques like knockouts—without insertion of non-native DNA—are exempt from oversight, enabling case-by-case biosafety reviews focused on phenotypic traits rather than editing method, though no commercial approvals had occurred as of 2025. similarly treats certain gene edits, such as targeted deletions, as non-genetically modified if they align with conventional breeding outcomes, streamlining approvals through the National Technical Biosafety Commission without mandatory GMO labeling or extended field trials. These policies, refined through bilateral cooperation like the October 20, 2022, Argentina- agreement on , prioritize empirical risk assessment over precautionary defaults, fostering applications in for traits like hornlessness or heat tolerance. Such shifts contrast with persistent hurdles in major markets like the and , where gene-edited animals remain subject to veterinary drug-equivalent scrutiny, often delaying commercialization. Proponents argue these innovations enhance and , as evidenced by the UK's emphasis on reducing emissions and use in precision-bred livestock, though critics highlight potential undetected off-target effects absent process-triggered scrutiny. Overall, these policy evolutions, effective from 2023 onward, signal growing recognition of gene editing's precision over traditional transgenesis, with regulatory focus narrowing to verifiable hazards rather than per se.

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