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Sex chromosome
Sex chromosome
Sex chromosome
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Sex
Biological terms
Sexual reproduction
Sexuality
Sexual system

Sex chromosomes (also referred to as allosomes, heterotypical chromosome, gonosomes, heterochromosomes,[1][2] or idiochromosomes[1]) are chromosomes that carry the genes that determine the sex of an individual. The human sex chromosomes are a typical pair of mammal allosomes. They differ from autosomes in form, size, and behavior. Whereas autosomes occur in homologous pairs whose members have the same form in a diploid cell, members of an allosome pair may differ from one another.

Nettie Stevens and Edmund Beecher Wilson both independently discovered sex chromosomes in 1905. However, Stevens is credited for discovering them earlier than Wilson.[3]

Differentiation

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Human male XY chromosomes after G-banding

In humans, each cell nucleus contains 23 pairs of chromosomes, a total of 46 chromosomes. The first 22 pairs are called autosomes. Autosomes are homologous chromosomes i.e. chromosomes which contain the same genes (regions of DNA) in the same order along their chromosomal arms. The 23rd pair of chromosomes are called allosomes. These consist of two X chromosomes in females, and an X chromosome and a Y chromosome in males. Females therefore have 23 homologous chromosome pairs, while males have 22. The X and Y chromosomes have small regions of homology called pseudoautosomal regions.

An X chromosome is always present as the 23rd chromosome in the ovum, while either an X or Y chromosome may be present in an individual sperm.[4] Early in female embryonic development, in cells other than egg cells, one of the X chromosomes is randomly and permanently partially deactivated: In some cells, the X chromosome inherited from the mother deactivates; in other cells, it is the X chromosome inherited from the father. This ensures that both sexes always have exactly one functional copy of an X chromosome in each body cell. The deactivated X chromosome is silenced by repressive heterochromatin that compacts the DNA and prevents expression of most genes. This compaction is regulated by PRC2 (Polycomb Repressive Complex 2).[5]

Sex determination

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Schematic karyogram of a human, showing the sex chromosomes in green box at bottom right. The X chromosome is part of chromosome group C, and the Y chromosome is part of group G. Bands and sub-bands are annotated to the right of each chromosome (or chromosome pair), and the gene for the sex-determining region Y protein is located at Yp11.2.
Further information: Karyogram
Main article: Sex-determination system
See also: Sexual differentiation in humans

All diploid organisms with allosome-determined sex get half of their allosomes from each of their parents. In most mammals, females are XX, and can pass along either of their Xs; since males are XY they can pass along either an X or a Y. Females in such species receive an X chromosome from each parent while males receive an X chromosome from their mother and a Y chromosome from their father. It is thus the male's sperm that determines the sex of each offspring in such species.

A small percentage of humans have divergent sexual development, known as intersex. This can result from a genotype that is neither XX nor XY. It can also occur when two fertilized embryo fuse, producing a chimera that might contain two different sets of DNA, one XX and the other XY.[6] It could also result from exposure, often in utero, to chemicals that disrupt the normal influence of the allosomes on production of sex hormones, and lead to the development of either ambiguous outer genitalia or internal organs.[7]

There is a gene in the Y chromosome that has regulatory sequences that control genes that code for maleness, called the SRY gene.[8] This gene produces a testis-determining factor ("TDF"), which initiates testis development in humans and other mammals. The SRY sequence's prominence in sex determination was discovered when the genetics of sex-reversed XX men (i.e. humans who possess biological male-traits but actually have XX allosomes) were studied. After examination, it was discovered that the difference between a typical XX individual (traditional female) and a sex-reversed XX man was that the typical individuals lacked the SRY gene. It is theorized that in sex-reversed XX men, the SRY gets translocated to an X chromosome in the XX pair during meiosis.[9]

Other vertebrates

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Diverse mechanisms are involved in the determination of sex in animals.[10] For mammals, sex determination is carried by the genetic contribution of the spermatozoon. Many lower chordates, such as fish, amphibians and reptiles, have systems that are influenced by the environment. Fish and amphibians, for example, have genetic sex determination but their sex can also be influenced by externally available steroids and incubation temperature of eggs.[11][12] In some reptiles, e.g. sea turtles, only the incubation temperature determines sex (temperature-dependent sex determination).

Plants

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Many scientists argue that sex determination in plants is more complex than that in humans. This is because even flowering plants have a variety of mating systems, their sex determination primarily regulated by MADS-box genes. These genes code for proteins that form the sex organs in flowers.[13]

Plant sex chromosomes are most common in bryophytes, relatively common in vascular plants and unknown in ferns and lycophytes.[14] The diversity of plants is reflected in their sex-determination systems, which include XY and UV systems as well as many variants. Sex chromosomes have evolved independently across many plant groups. Recombination of chromosomes may lead to heterogamety before the development of sex chromosomes, or recombination may be reduced after sex chromosomes develop.[15] Only a few pseudoautosomal regions normally remain once sex chromosomes are fully differentiated. When chromosomes do not recombine, neutral sequence divergences begin to accumulate, which has been used to estimate the age of sex chromosomes in various plant lineages. Even the oldest estimated divergence, in the liverwort Marchantia polymorpha, is more recent than mammal or bird divergence. Due to this recency, most plant sex chromosomes also have relatively small sex-linked regions. Current evidence does not support the existence of plant sex chromosomes more ancient than those of M. polymorpha.[16]

The high prevalence of autopolyploidy in plants also impacts the structure of their sex chromosomes. Polyploidization can occur before and after the development of sex chromosomes. If it occurs after sex chromosomes are established, dosage should stay consistent between the sex chromosomes and autosomes, with minimal impact on sex differentiation. If it occurs before sex chromosomes become heteromorphic, as is likely in the octoploid red sorrel Rumex acetosella, sex is determined in a single XY system. In a more complicated system, the sandalwood species Viscum fischeri has X1X1X2X2 chromosomes in females, and X1X2Y chromosomes in males.[17]

Sequence composition and evolution

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Amplification of transposable elements, tandom repeats especially accumulation of long tandom repeats (LTR) retrotransposones are responsible for plant sex chromosome evolution. The insertion of retrotransposons is probably the major cause of y-chromosome expansion and plant genome size evolution. Retrotransposones contribute in size determination of sex chromosomes and its proliferation varies even in closely related species. LTR and tandom repeats play dominant role in the evolution of S. latifolia sex chromosomes.[18] Athila is new family of retroelements, discovered in Arabidopsis thaliana, present in heterochromatin region only. Athila retroelements overrepresented in X but absent in Y while tandem repeats enriched in Y-chromosome. Some chloroplast sequences have also been identified in the Y-chromosome of S. latifolia. S. vulgaris has more retroelements in their sex chromosomes compare to S. latifolia. Microsatellite data shows that there is no significant difference between X and Y-chromosome microsatellites in both Silene species. This would conclude that microsatellites do not participate in Y-chromosome evolution. The portion of Y-chromosome that never recombine with X-chromosome faces selection reduction. This reduced selection leads to insertion of transposable elements and accumulation of deleterious mutation. The Y become larger and smaller than X due to insertion of retroelement and deletion of genetic material respectively. The genus Humulus is also used as model for the study of sex chromosomes evolution. Based on the phylogenetic topology distribution there are three regions on sex chromosomes. One region that stops recombining in the ancestor of H. lupulus, second that stops recombining in modern H. lupulus and the third region called pseudoautosomal region. H. lupulus is the rare case in plants in which Y is smaller than X, while its ancestor plant has the same size of both X and Y chromosomes. This size difference should be caused by deletion of genetic material in Y but that is not the case. This is because of complex dynamics like the larger size of X than Y-chromosome may be due to duplication or retrotransposition and size of Y remains same.[19]

Non-vascular plants

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Ferns and lycophytes have bisexual gametophytes, so there is no evidence for sex chromosomes.[14] In the bryophytes, including liverworts, hornworts and mosses, sex chromosomes are common. The sex chromosomes in bryophytes affect what type of gamete is produced by the gametophyte, and there is wide diversity in gametophyte type. Unlike seed plants, where gametophytes are always unisexual, in bryophytes they may produce male, female, or both types of gamete.[20]

Bryophytes most commonly employ a U/V sex-determination system, where U produces female gametophytes and V produces male gametophytes. The U and V chromosomes are heteromorphic with U larger than V and are frequently both larger than the autosomes. There is variation even within this system, including UU/V and U/VV chromosome arrangements. In some bryophytes, microchromosomes have been found to co-occur with sex chromosomes and likely impact sex determination.[20]

Brown algae

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Brown algae are a group of photosynthetic multicellular eukaryotes that belong to the supergroup Stramenopila, splitting from the plant and animal lineages very early in evolution[21]. These organisms evolved a U/V sex-determination system, similar to the one found in bryophytes, with male gametophytes harboring a V chromosome and female gametophytes harboring a U chromosome[22]. Sex determination is controled by an HMG-box protein named MIN, which is present in the male V chromosome of all brown algal species [23][22]. The U/V sex chromosomes are ancestral to the brown algal lineage, arising somwhere between 450 and 224 million years ago. The closest extant relative of the brown algae, Schizocladia ischiensis, does not have U/V sex chromosomes. However, its genome does harbor the male-determinant gene MIN[23]. This, alongside ancestral state reconstruction analyses, suggest that the origin of the U/V sex chromosomes involved a recombination supression event that included the gene MIN and other six genes, which defined the ancestral sex-determining region of the U/V system[22]. Transcriptomic[24], genomic[22] and gene-editing evidence[23] suggest that the U chromosome may also contain important genes involved in female sex determination, since knock-out mutants of MIN do not lead to functional female phenotypes unless these mutants also harbor a U chromosome[23].Not all brown algal species have a U/V system. Some brown algae evolved a monoicous system (i.e., co-sexuality)[25]. These species have several V genes (including MIN) and only a few U genes, indicating that monoicous species evolved from a male genetic background that translocated putative female-determining genes[22]. Another group of species that lost the U/V system are the Fucales, an Order of brown algae that evolved a full diplont life cycle, leading to the loss of the haploid U/V sex determining system. Many species in Fucales evolved either a monoecious or an asexual sife cycle, but a few of them evolved separate sexes again[26]. From these, the species Fucus serratus seems to have evolved a putative X/Y system[27]. Male Fucus serratus show differential gene expression of the gene MIN, but the gene is also present in the genome of female individuals, suggesting the emergence of a new Y male-determining factor that has not been found yet[22].

Gymnosperms

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Dioecy is common among gymnosperms, found in an estimated 36% of species. However, heteromorphic sex chromosomes are relatively rare, with only five species known as of 2014. Five of these use an XY system, and one (Ginkgo biloba) uses a WZ system. Some gymnosperms, such as Johann's Pine (Pinus johannis), have homomorphic sex chromosomes that are almost indistinguishable through karyotyping.[17]

Angiosperms

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Cosexual angiosperms with either monoecious or hermaphroditic flowers do not have sex chromosomes. Angiosperms with separate sexes (dioecious) may use sex chromosomes or environmental flowers for sex determination. Cytogenetic data from about 100 angiosperm species showed heteromorphic sex chromosomes in approximately half, mostly taking the form of XY sex-determination systems. Their Y is typically larger, unlike in humans; however there is diversity among angiosperms. In the Poplar genus (Populus) some species have male heterogamety while others have female heterogamety.[16] Sex chromosomes have arisen independently multiple times in angiosperms, from the monoecious ancestral condition. The move from a monoecious to dioecious system requires both male and female sterility mutations to be present in the population. Male sterility likely arises first as an adaptation to prevent selfing. Once male sterility has reached a certain prevalence, then female sterility may have a chance to arise and spread.[14]

In the domesticated papaya (Carica papaya), three sex chromosomes are present, denoted as X, Y and Yh. This corresponds with three sexes: females with XX chromosomes, males with XY, and hermaphrodites with XYh. The hermaphrodite sex is estimated to have arisen only 4000 years ago, post-domestication of the plant. The genetic architecture suggests that either the Y chromosome has an X-inactivating gene, or that the Yh chromosome has an X-activating gene.[28]

Medical applications

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Allosomes not only carry the genes that determine male and female traits, but also those for some other characteristics as well. Genes that are carried by either sex chromosome are said to be sex linked. Sex linked diseases are passed down through families through one of the X or Y chromosomes. Since usually men inherit Y chromosomes, they are the only ones to inherit Y-linked traits. Men and women can get the X-linked ones since both inherit X chromosomes.[29]

An allele is either said to be dominant or recessive. Dominant inheritance occurs when an abnormal gene from one parent causes disease even though the matching gene from the other parent is normal. The abnormal allele dominates. Recessive inheritance is when both matching genes must be abnormal to cause disease. If only one gene in the pair is abnormal, the disease does not occur, or is mild. Someone who has one abnormal gene (but no symptoms) is called a carrier. A carrier can pass this abnormal gene to his or her children.[30] X chromosome carry about 1500 genes, more than any other chromosome in the human body. Most of them code for something other than female anatomical traits. Many of the non-sex determining X-linked genes are responsible for abnormal conditions. The Y chromosome carries about 78 genes. Most of the Y chromosome genes are involved with essential cell house-keeping activities and sperm production. Only one of the Y chromosome genes, the SRY gene, is responsible for male anatomical traits. When any of the 9 genes involved in sperm production are missing or defective the result is usually very low sperm counts and infertility.[31] Examples of mutations on the X chromosome include more common diseases such as the following:

  • Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under normal lighting conditions. Color blindness affects many individuals in the population. There is no actual blindness, but there is a deficiency of color vision. The most usual cause is a fault in the development of one or more sets of retinal cones that perceive color in light and transmit that information to the optic nerve. This type of color blindness is usually a sex-linked condition. The genes that produce photopigments are carried on the X chromosome; if some of these genes are missing or damaged, color blindness will be expressed in males with a higher probability than in females because males only have one X chromosome.
  • Hemophilia refers to a group of bleeding disorders in which it takes a long time for the blood to clot. This is referred to as X-Linked recessive.[32] Hemophilia is much more common in males than females because males are hemizygous. They only have one copy of the gene in question and therefore express the trait when they inherit one mutant allele. In contrast, a female must inherit two mutant alleles, a less frequent event since the mutant allele is rare in the population. X-linked traits are maternally inherited from carrier mothers or from an affected father. Each son born to a carrier mother has a 50% probability of inheriting the X chromosome carrying the mutant allele.
  • Fragile X syndrome is a genetic condition involving changes in part of the X chromosome. It is the most common form of inherited intellectual disability in males. It is caused by a change in a gene called FMR1. A small part of the gene code is repeated on a fragile area of the X chromosome. The more repeats, the more likely there is to be a problem. Males and females can both be affected, but because males have only one X chromosome, a single fragile X is likely to affect them more. Most fragile-X males have large testes, big ears, narrow faces, and sensory processing disorders that result in learning disabilities.[33]

Other complications include:

  • 46,XX testicular disorder of sex development, also called XX male syndrome, is a condition in which individuals with two X chromosomes in each cell, the pattern normally found in females, have a male appearance. People with this disorder have male external genitalia. In most people with 46,XX testicular disorder of sex development, the condition results from an exchange of genetic material between chromosomes (translocation). This exchange occurs as a random event during the formation of sperm cells in the affected person's father. The SRY gene (normally on the Y chromosome) is misplaced in this disorder, onto an X chromosome. Any person with an X chromosome that carries the SRY gene will develop male characteristics despite not having a Y chromosome.[34]

Evolution

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Sex chromosomes evolve from standard pairs of autosomal chromosomes.[35] In a large number of organisms, the sex-determination systems presently observed are products of sex chromosome turnover. Sex chromosome turnover is a process defined as when the type of the sex chromosome changes as a product of a change in the identity of the sex-determining genes (such as by mutation) or by a change in their location.[36] In other cases, sex chromosomes may grow substantially with respect to their ancestral forms as a result of fusion events with autosomes, and autosome-sex chromosome fusions result in what are called neo-sex chromosomes. Five examples of this are now known in the songbird superfamily Sylvioidea.[37] There is one experimentally documented case of sex chromosome turnover occurring during a 30-year evolutionary experiment involving teleost fish (specifically the swordtails), in which hybridization experiments resulted in a translocation of the sex-determiner region of a sex chromosome into an autosome. This resulted in the autosome becoming a novel W sex chromosome.[38]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Sex chromosome

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Sex chromosomes are specialized chromosomes that play a primary role in determining the biological sex of an , distinguishing them from autosomes which carry genes unrelated to sex. In humans and most other mammals, the sex chromosomes consist of the X and Y chromosomes, with females typically possessing two X chromosomes (XX) and males possessing one X and one Y chromosome (XY). The presence of the in placental mammals triggers male development, while its absence results in female development. In mammals, sex determination begins with the chromosomal complement inherited from the parents, where the sperm contributes either an X or Y chromosome to the egg's X chromosome, thus establishing the zygote's sex. The key gene responsible for initiating male gonad development is the SRY (sex-determining region Y) gene, located on the short arm of the Y chromosome, which encodes a transcription factor that directs the bipotential gonad toward testis formation around six weeks of embryonic development in humans. To compensate for the dosage imbalance of X-linked genes—since females have two X chromosomes and males have one—mammals employ X-chromosome inactivation in females, randomly silencing one X chromosome in each cell via the formation of a Barr body, ensuring equivalent gene expression between sexes. Deviations from the typical XX or XY configurations, such as Turner syndrome (XO) or Klinefelter syndrome (XXY), can lead to variations in sexual development and fertility. Sex chromosome systems vary widely across species, reflecting diverse evolutionary pathways. In birds and some reptiles, the system is ZW, where males are ZZ (homogametic) and females are ZW (heterogametic), with the dosage of the Z chromosome or genes like DMRT1 influencing sex differentiation. Insects such as fruit flies (Drosophila) determine sex based on the ratio of X chromosomes to autosomes (X:A ratio), rather than the presence of a specific chromosome, while butterflies and moths use a ZW system similar to birds. Some species, including certain reptiles and fish, exhibit environmental sex determination influenced by temperature or other factors, independent of chromosomes. The X and Y chromosomes in mammals evolved from a pair of autosomes approximately 180 million years ago, with the Y chromosome undergoing significant degeneration over time, losing most of its genes except those essential for male-specific functions like . Beyond sex determination, sex chromosomes influence a range of traits through sexually dimorphic , including disease susceptibility and behavioral differences, due to the hemizygous nature of the Y chromosome in males and the escape of some X-linked genes from inactivation.

Basic Concepts

Definition and Function

Sex chromosomes are specialized chromosomes that determine the biological sex of an by carrying genes responsible for sex-specific development and . They typically evolve from a pair of autosomes when a sex-determining locus arises, leading to differentiation between the sex chromosomes, often with suppressed recombination in the heterogametic sex. In many , including mammals, the consists of X and Y chromosomes, where females are homogametic (XX) and males are heterogametic (XY); conversely, in birds and some reptiles, the ZW prevails, with males ZZ and females ZW. These chromosomes not only dictate gonadal development but also influence secondary and . The primary function of sex chromosomes is to initiate and regulate sex determination through key regulatory genes. In mammals, the SRY gene on the triggers the differentiation of bipotential gonads into testes, which produce hormones driving male development; its absence results in ovarian development and female . In other systems, such as birds, the DMRT1 gene on the Z chromosome plays a similar role in male determination. Beyond sex determination, sex chromosomes harbor genes involved in — in males and in females—often enriched in the heterogametic sex. They also carry genes unrelated to reproduction, affecting traits like (on the in humans) and contributing to . A critical function is dosage compensation, which equalizes between sexes despite differing chromosome numbers. In female mammals, one X chromosome undergoes inactivation via , forming the to match the single active X in males, though 10–15% of genes escape this, leading to higher expression in females. In , the single X in males is hyperactivated, while in birds, no full dosage compensation occurs for the Z chromosome. This mechanism prevents imbalances that could disrupt development, and failures in it underlie disorders like Turner (XO) or Klinefelter (XXY) syndromes. Overall, sex chromosomes ensure balanced sex-specific while evolving rapidly, with implications for and hybrid incompatibility.

Distribution Across Organisms

Sex chromosomes, which are specialized chromosomes that differ in number or structure between sexes to determine sexual development, are distributed unevenly across eukaryotic organisms, with prevalence varying by kingdom and clade. They are most common in animals, particularly vertebrates, where they facilitate genetic sex determination (GSD) systems, but are rarer in plants and largely absent in fungi and most protists, where alternative mechanisms like environmental cues, polygenic control, or mating types predominate. This distribution reflects independent evolutionary origins and high rates of turnover, with transitions between sex chromosome systems occurring frequently in some lineages. In animals, sex chromosomes are widespread, especially among vertebrates. Mammals typically exhibit an XY system, where males carry one X and one Y chromosome, and the Y-linked SRY gene triggers male development; this system is conserved across eutherians, marsupials, and monotremes. Birds and most reptiles use a ZW system, with females being heterogametic (ZW) and males homogametic (ZZ), as seen in chickens where the Z-linked DMRT1 gene regulates maleness. Fish display remarkable diversity, with XY or ZW systems in species like medaka (Oryzias latipes), polygenic control in zebrafish (Danio rerio), and frequent transitions between GSD and temperature-dependent sex determination (TSD) across ~30,000 species. Amphibians show similar variability, including XY systems in frogs like Xenopus laevis and TSD in some salamanders, with higher turnover rates from ZW to XY. Among invertebrates, insects often have XY (e.g., fruit flies, Drosophila melanogaster, with X-linked dosage compensation) or ZW systems (e.g., butterflies), while haplodiploidy—where males develop from unfertilized eggs—occurs in ~12% of animal species, including bees and ants, without distinct sex chromosomes. In plants, sex chromosomes are less common and primarily associated with dioecious species, which comprise about 6% of angiosperms. At least 48 land plant species across 20 families possess sex chromosomes, mostly XY systems evolved from linked sex-determining genes, often transitioning from hermaphroditic or gynodioecious ancestors. Non-vascular plants like mosses and liverworts exhibit heteromorphic sex chromosomes in some dioecious taxa, such as UV systems in certain mosses. Gymnosperms, such as Ginkgo biloba and some conifers, show heteromorphic sex chromosomes in several dioecious species. Angiosperms have both heteromorphic (e.g., Silene latifolia with differentiated XY) and homomorphic (e.g., papaya, Carica papaya, with young XY) sex chromosomes across 13 families, though many dioecious plants rely on polygenic or environmental factors instead. Fungi and protists generally lack sex chromosomes, relying on mating-type loci or cryptic genetic exchange for . In fungi, sexual compatibility is controlled by mating-type (MAT) loci—idiomorphs rather than alleles—with no dimorphic chromosomes; examples include (bipolar) and basidiomycetes (multiallelic). Protists, such as trypanosomatids (), exhibit and but without specialized sex chromosomes, often involving haploid gametes or parasexual processes in parasitic species. This absence underscores that sex chromosomes are not a universal feature of but an adaptation prominent in multicellular lineages with stable GSD.

Sex Determination Systems

In Mammals

In mammals, sex determination is predominantly governed by a chromosomal system where the presence of the directs male development, while its absence leads to female development. This characterizes therian mammals, including marsupials and placentals, with females possessing two X chromosomes (XX) and an X and a Y (XY). The 's role was established through genetic studies showing that XY individuals develop testes, XX individuals ovaries, and XO individuals (lacking a second sex chromosome) also develop ovaries, indicating that ovarian development is the default pathway. The key genetic trigger for male development is the SRY (sex-determining region Y) gene, located on the . Discovered in , SRY encodes a that initiates testis differentiation by activating downstream genes such as SOX9 in the bipotential around embryonic day 10.5 in mice. SRY expression is transient, peaking at embryonic day 11.5, after which SOX9 sustains its own expression and promotes differentiation, which organizes testis cord formation and hormone production (e.g., testosterone and ). In females, genes like WNT4 and RSPO1 promote ovarian development by suppressing male pathways, with WNT4 knockout in XX mice leading to partial masculinization. Disruptions in this system can result in (DSDs), affecting about 1 in 100 live births. For instance, SRY mutations or deletions cause (Swyer syndrome), where individuals develop as females despite a , while SRY translocations to the can produce XX males. Dosage-sensitive genes like DAX1 (on the ) can override SRY when duplicated, leading to XY sex reversal. These findings underscore SRY's necessity and sufficiency for male determination in therians, confirmed by transgenic experiments where SRY insertion into XX embryos induces testis formation. An exception occurs in monotremes (egg-laying mammals like the and echidna), which diverged early from other mammals and possess a multiple sex chromosome system (up to 10 X and 10 Y chromosomes in males, forming a meiotic chain). Unlike therians, monotremes lack SRY and instead rely on the Y-linked AMHY gene (a paralog of ) as the primary male sex-determining factor. AMHY expression begins in male gonads at fetal day 6 and precedes SOX9 upregulation, functioning via TGF-β signaling rather than direct transcription like SRY. This represents independent evolution of sex determination in mammals, with AMHY showing homology to bird and reptile systems but no shared origin with therian Y chromosomes.

In Other Vertebrates

In non-mammalian vertebrates, sex determination exhibits greater diversity than in mammals, encompassing both genetic systems with differentiated sex chromosomes and environmental cues such as temperature, often with high plasticity allowing . These systems have evolved independently multiple times, leading to varied chromosomal pairs like ZW (female heterogamety) or XY (male heterogamety), and in some cases, no distinct sex chromosomes. Turnover of sex-determining genes and chromosomes is frequent, particularly in poikilothermic groups, reflecting adaptations to ecological pressures. Birds primarily employ a ZW sex determination system, where males are homogametic (ZZ) and females heterogametic (ZW), contrasting with the mammalian XY system. The Z chromosome, larger and gene-rich, carries DMRT1, a key testis-determining gene conserved across vertebrates, while the W chromosome is smaller and often degenerated. Dosage compensation in birds is partial and gene-specific, without global inactivation mechanisms like in mammals, resulting in higher Z-linked gene expression in males. This system likely originated around 150 million years ago in the avian lineage and shows homology with some reptilian Z chromosomes. In chickens (Gallus gallus), estrogen signaling modulates ovarian development in ZW individuals, and aromatase inhibitors can induce female-to-male sex reversal. Reptiles display a mosaic of mechanisms, with temperature-dependent sex determination (TSD) predominant in crocodilians, many turtles, and some lizards, where incubation temperature overrides any genetic signals to direct gonadal fate—higher temperatures often producing females in turtles like the red-eared slider (Trachemys scripta). Genetic systems exist in other lineages, such as ZW in snakes and some lizards (e.g., bearded dragon, Pogona vitticeps, with XY), but sex chromosomes are frequently undifferentiated and homomorphic. Key genes include SOX9 and DMRT1 for testis differentiation, and aromatase (CYP19) for estrogen-mediated ovarian development, with temperature influencing their expression. Evolutionary lability is evident, as some species exhibit both TSD and GSD, and sex chromosome turnover occurs via shifts in sex-determining loci. Amphibians feature flexible sex determination, with genetic systems like XX/XY (male heterogamety) in many frogs (e.g., Rana rugosa) or ZZ/ZW in others, though sex chromosomes are typically homomorphic and morphologically similar. Hormonal and environmental factors, including temperature and steroids, can override genetic cues, leading to frequent ; for instance, in African clawed frogs ( laevis), the DM-W on the W acts as an ovary-determining factor. This plasticity highlights an evolutionary continuum between genetic and environmental control, with rapid turnover of sex chromosomes and genes across species. Dosage compensation mechanisms remain poorly understood but appear absent or minimal. Fish exhibit the most varied sex chromosome systems among vertebrates, including XY, ZW, and even trigenic (WXZ) setups in about 10% of species with identifiable chromosomes, though most are homomorphic and undifferentiated. Genetic sex determination predominates in model species like medaka (Oryzias latipes), where the DMY gene (a DMRT1 paralog) on the Y chromosome triggers male development, while environmental factors such as , pH, or social cues influence others, as in the rainbow trout (Oncorhynchus mykiss) with SDY as a male-determiner. High evolutionary turnover is characteristic, with sex chromosomes evolving independently over 60 times, often from autosomes, and genes like AMHY, GSDF, and AMHR2 serving as master regulators in different lineages. This diversity enables adaptive sex ratios in response to environmental variability.

In Invertebrates

Invertebrates exhibit remarkable diversity in sex determination systems, encompassing genetic mechanisms involving sex chromosomes, haplodiploidy, environmental influences, and polygenic controls, often without the pronounced differentiation seen in vertebrates. Unlike the conserved XX/XY or ZW systems in many vertebrates, invertebrate sex chromosomes frequently undergo turnover and vary across phyla, reflecting evolutionary lability driven by factors such as infections and genetic conflicts. Among arthropods, insects display the most studied chromosomal systems. In Diptera, such as Drosophila melanogaster, males are heterogametic with an XY system where the X-to-autosome ratio governs sex, regulated by the Sex-lethal gene that initiates a cascade leading to female development in XX individuals. In contrast, Lepidoptera like the silkworm Bombyx mori employ a ZW system with female heterogamety, where a W-linked piRNA cluster silences the feminizer gene in ZZ males, promoting maleness; this system is conserved for over 230 million years. Hymenoptera, including bees and ants, utilize haplodiploidy, where unfertilized eggs develop into haploid males (arrhenotoky) and fertilized eggs into diploid females, a mechanism that evolved at least six times independently and influences eusocial behaviors. Crustaceans often lack distinct sex chromosomes, relying instead on polygenic sex determination (PSD), as in the copepod Tigriopus californicus, or environmental sex determination (ESD), where factors like photoperiod or crowding determine sex in Daphnia magna via epigenetic regulation of the doublesex gene. In nematodes, the Caenorhabditis elegans features an XO/XX system, with hermaphrodites (XX) and males (XO) determined by X chromosome dosage; genes like tra-1 and fem orchestrate , though many nematode species show environmental modulation or PSD. Mollusks vary widely, with some bivalves and gastropods exhibiting XY or ZW systems, such as the red coral Corallium rubrum (XY), while others like the apple snail use PSD involving multiple loci; ESD also occurs, influenced by temperature or population density. Annelids and flatworms often combine genetic and environmental cues, with few differentiated sex chromosomes; for instance, some polychaetes show ZW heterogamety, but hermaphroditism predominates, complicating chromosomal identification. Evolutionary dynamics in invertebrates highlight frequent transitions between systems, with sex chromosomes prone to fusions, losses, or neo-sex chromosome formation, as evidenced by over 770 species with XO sex chromosomes derived from XY ancestors in beetles alone, involving at least 70 independent Y chromosome losses. Endosymbionts like Wolbachia further drive shifts by manipulating host reproduction, leading to parthenogenesis or male-killing, which can stabilize or destabilize chromosomal systems. This plasticity underscores the role of sexually antagonistic selection in shaping invertebrate sex chromosomes, contrasting with the relative stability in vertebrates.

Sex Chromosomes in Plants

Sequence Composition and Evolution

Sex chromosomes in plants exhibit distinct sequence compositions characterized by elevated levels of repetitive DNA in non-recombining regions, particularly on the Y or male-determining chromosome, which contrasts with the more gene-dense autosomal regions. This accumulation of transposable elements (TEs) and tandem repeats is a hallmark of suppressed recombination, often leading to heterochromatin formation and chromosome expansion. For instance, in Silene latifolia, the Y chromosome is approximately 40% larger than the X, with about 50-60% of its sequence comprising repetitive elements such as LINE-like TRAYC and Copia retrotransposons, compared to lower densities on the X. Similarly, in Carica papaya, the male-specific region of the Y (MSY) spans roughly 8 Mb and contains approximately 72% repetitive sequences, including segmental duplications and TEs, versus about 60% in the corresponding X region. These patterns are not universal; in younger systems like Asparagus officinalis, the non-recombining region is smaller (~1-2 Mb) with moderate repeat enrichment, reflecting early stages of differentiation. Gene content on plant sex chromosomes shows reduced density and evidence of degeneration in non-recombining areas, though less severe than in many animals. In Silene latifolia, the Y harbors around 16 genes per Mb compared to 34 on the X, with many Y-linked genes retaining functionality but exhibiting lower expression levels due to TE insertions and epigenetic silencing. The Carica papaya MSY contains 78 protein-coding genes, but with pseudogene accumulation indicating ongoing loss, estimated at 14-25% compared to the X. In contrast, systems like Rumex acetosa display satellite DNA repeats (e.g., RAE180) dominating the Y, contributing to its size. Overall, plant sex chromosomes maintain higher gene retention than animal counterparts, possibly due to flexible dosage compensation mechanisms and recurrent evolutionary turnover. Evolutionarily, plant sex chromosomes originate from autosomes through the suppression of recombination around sex-determining loci, often involving two tightly linked mutations that establish male or female heterogamety. This process, dated to 5-10 million years ago in Silene latifolia, initiates repeat accumulation as an early event, preceding substantial gene loss and driven by mechanisms like inversions or epigenetic modifications that stabilize the non-recombining region. In Carica papaya, two evolutionary strata reflect sequential recombination cessations, with the older stratum showing greater TE expansion and divergence (7.3 Mya), while the younger one retains more synteny with autosomes. Degeneration proceeds via TE-mediated insertions that disrupt genes or induce silencing, but plants exhibit reversals and turnover; for example, in the Salicaceae family (Salix spp.), XY systems have independently evolved into ZW or reverted to hermaphroditism multiple times, with palindromic repeats facilitating structural stability. Such dynamics highlight plants' labile sex chromosome evolution compared to more stabilized animal systems, influenced by polyploidy and environmental pressures.

In Non-vascular Plants

Non-vascular plants, collectively known as and comprising es, liverworts, and s, exhibit sex determination primarily in their dominant haploid generation, where sex chromosomes—termed U (female-determining) and V (male-determining)—play a key role, contrasting with the diploid-dominant systems in vascular plants. , or separate sexes, is prevalent across bryophytes, occurring in approximately 57% of , 68% of liverwort , and 40% of , with sex chromosomes often heteromorphic and larger than due to repetitive DNA accumulation. These chromosomes likely evolved independently in each bryophyte lineage through mechanisms such as autosome fusions following whole-genome duplications, leading to recombination suppression and non-recombining regions that harbor sex-specific genes. Unlike in diploid organisms, both U and V chromosomes show comparable degeneration patterns driven by haploid selection, with no evidence of dosage compensation. In liverworts, the model species has provided the most detailed insights, featuring U and V s where the U carries a feminizer (an ortholog of MpBPCU) that promotes development, while its absence allows default male differentiation on the V . The V , fully sequenced, spans about 25 Mb with 43% repetitive content and encodes 64 protein-coding , many of which have homologs on the U , indicating early divergence during haploid evolution estimated at 5–11 million years ago. Transitions to (bisexual gametophytes) in liverworts, such as in Ricciocarpos natans, involve fragmentation of the U and fusion of its (including the feminizer) to autosomes, while the V often persists as a micro; this pattern repeats across multiple independent monoecious lineages, preserving core sex determination networks through regulatory rewiring. Other liverworts like Frullania dilatata show size dimorphism, with s possessing two U (n=9) and s one V (n=8), resulting in a 1.36-fold higher DNA content in s. Moss sex chromosomes, as exemplified by , are notably large—U at 110.5 Mb and V at 112.2 Mb, roughly five times the size of autosomes—and contain over 3,000 transcripts each, including sex-specific and sporophyte-expressed , with about 80% repetitive sequences. Sequencing in Syntrichia caninervis further reveals high retention and minimal degeneration, challenging expectations of rapid decay in haploid systems. These chromosomes likely arose from two evolutionary events dated 0.6–3.5 million years ago, with possible DNA insertions contributing to size differences. Hornworts display in 40% of species, typically with 4–5 large autosomes plus one small sex chromosome (U in females, V in males), while species possess 5–6 chromosomes including 1–2 accessory chromosomes derived from ancestral sex chromosomes. The FGMYB is proposed as an ancestral sex-determining gene, analogous to its role in liverworts, with frequent shifts between and mirroring patterns in and liverworts. Overall, sex chromosomes remain underexplored, with open questions surrounding heterochromatin's regulatory role and precise evolutionary timelines, best addressed through integrated cytogenetic and genomic approaches like high-throughput sequencing.

In Gymnosperms

Gymnosperms display a predominance of sexual systems, with separate male and female individuals occurring in approximately 64.6% of the roughly 1,033 known species across the four main groups: cycads, ginkgo, , and gnetophytes. This high incidence of contrasts sharply with angiosperms, where it appears in only about 5-6% of species, and likely evolved independently 10-13 times within gymnosperms, often from monoecious ancestors to promote in the absence of mechanisms. Despite the prevalence of , genetically determined are rare, having been cytologically or molecularly identified in just 0.6% of species (around six to ten documented cases) from three families: (cycads), , and (conifers), as well as some gnetophytes. These systems are typically XX/XY or ZW/ZZ, with morphological heteromorphism evident in most cases, though degeneration of the heteromorphic remains limited compared to many animal or angiosperm systems. In cycads, which are entirely dioecious, heteromorphic sex chromosomes follow an XX/XY pattern. For instance, in Cycas revoluta, the Y chromosome is notably smaller than the X, with distinct size differences observable during meiosis, providing early evidence of sex linkage. Similar heteromorphism, often involving satellite structures or rDNA signals, has been reported in other cycads such as Cycas pectinata, Stangeria eriopus, and Bowenia species, suggesting conserved mechanisms within the group. In gnetophytes, another fully dioecious lineage, examples include Gnetum ula with a heteromorphic eighth chromosome pair in the XX/XY system and Ephedra foliata showing satellite heteromorphism in a median pair, indicating potential independent evolution of sex determination in this clade. The relict species , the sole member of and also fully dioecious, possesses a sex-determining (SDR) on spanning about 50 Mb, with a non-recombining Y-specific region under 5 Mb; this system may represent either XX/XY male heterogamety or ZW/ZZ female heterogamety, based on conflicting cytological reports. Molecular analyses reveal low synonymous divergence, pointing to a relatively recent origin in the (60-125 million years ago), with minimal Y-chromosome degeneration and recombination suppression in both sexes, differing from more advanced differentiation in other taxa. A distinctive multiple-chromosome system occurs in coniferous species, such as P. macrophyllus, P. laticrura (syn. P. longifoliolatus), and P. elatus, where females carry four X chromosomes (X₁X₁X₂X₂) and males three (X₁X₂Y); the Y likely arose from fusion of two telocentric autosomes, highlighting chromosomal rearrangements in sex evolution. Overall, the scarcity of identified sex chromosomes in gymnosperms, despite widespread , suggests that environmental or polygenic factors often control sex , with genetic systems emerging sporadically and showing less stratification than in vertebrates or angiosperms. Evolutionary turnover appears frequent, potentially driven by the ancient diversification of gymnosperms, and ongoing genomic studies may uncover additional SDRs in this understudied group.

In Angiosperms

In angiosperms, sex chromosomes have evolved independently numerous times, with occurring in approximately 5–6% of across more than 50% of families, often arising from hermaphroditic ancestors through mechanisms promoting and . These chromosomes typically follow an XY system in males (heterogametic) and XX in females, though ZW systems exist in some lineages; they can be heteromorphic, with morphologically distinct X and Y chromosomes, or homomorphic, appearing similar but differentiated by non-recombining regions. Sex often involves small sex-determining regions (SDRs) on autosomes that suppress recombination, leading to the accumulation of sex-specific genes, such as those regulating floral organ identity or hormonal pathways like cytokinins. At least 39 angiosperm across 17 families possess identified sex chromosomes, with 19 in 4 families showing heteromorphic pairs and 20 in 13 families exhibiting homomorphic ones. Heteromorphic sex chromosomes are exemplified by Silene latifolia (white campion), where the Y chromosome is larger and differentiated, carrying male-promoting genes and showing degeneration with about 40% gene loss compared to the X; this system, approximately 5–25 million years old, features 2–3 evolutionary strata from successive recombination suppressions. Another case is Rumex acetosa (sorrel), with a prominent Y chromosome enriched in repetitive sequences and transposable elements, facilitating male sterility in XX individuals. In contrast, homomorphic systems predominate in many species, such as Carica papaya (papaya), where the SDR spans about 8–10 Mb on chromosome 1, with two strata and Y-specific genes like those for male fertility, yet chromosomes remain morphologically similar despite ~9 million years of divergence. Asparagus officinalis displays a ~1 Mb male-specific Y region (MSY) containing genes like SOFF and TDF1 that trigger male development, while Vitis vinifera (grapevine) has a compact ~150 kb MSY with genes such as VviINP1 influencing floral sex. These examples highlight how angiosperm sex chromosomes often evolve rapidly on small genomic scales, contrasting with the larger, more degenerated systems in animals. Evolutionarily, angiosperm sex chromosomes originate from autosomes via mutations causing male or female sterility, followed by linkage of sex-determining alleles and recombination suppression, a process estimated to have occurred 871 to 5,000 times since the angiosperm radiation around 158–179 million years ago. Y degeneration proceeds through deleterious mutations, gene loss, and proliferation, as seen in Silene latifolia where the Y accumulates repetitive DNA but retains essential genes; however, unlike many animal systems, plant Y chromosomes can remain homomorphic for extended periods without full degeneration. Turnover is frequent, with shifts in SDR location or heterogamety (e.g., XY to ZW in some yams) driven by translocations or new mutations, as observed in genera like Salix (willows) and (strawberries). Dosage compensation, the equalization of X-linked between sexes, is partial or absent in most studied angiosperms; in Silene latifolia, some X-hemizygous genes achieve near-complete compensation (expression ratio ~1), but the mechanism remains unclear and is not globally upregulated like in mammals. Recent genomic advances, including long-read sequencing, have revealed the dynamic nature of these systems, identifying master sex-determining genes like ARR17 in (European aspen) within a ~1.5 Mb MSY, and highlighting polyploidy's potential role in stabilizing or inhibiting further differentiation. In (date palm), an XY system on involves neo-Y accumulation of gypsy retrotransposons, underscoring repetitive elements' role in early . Overall, angiosperm sex chromosomes emphasize lability and convergence, with small, gene-poor SDRs enabling repeated evolutionary innovations without the extensive suppression seen elsewhere.

Structure and Differentiation

Chromosomal Features

Sex chromosomes are characterized by their heteromorphic nature in many taxa, where the two chromosomes differ in size, shape, and genetic content due to suppressed recombination in specific regions. This suppression often leads to the evolution of a differentiated sex-determining region (SDR), flanked by pseudoautosomal regions (PARs) that permit limited recombination to ensure proper meiotic pairing. In heterogametic individuals (XY males or ZW females), the single active copy of the smaller chromosome (Y or W) typically exhibits degeneration, resulting in reduced gene content and accumulation of repetitive, heterochromatic sequences. For instance, across vertebrates and plants, sex chromosomes often originate from autosomes, with structural features like inversions, translocations, and expansions of transposable elements driving differentiation. In mammals, the X chromosome is relatively large and gene-rich, comprising about 5% of the (~155 Mb) with approximately 900–1,100 protein-coding genes, many of which are housekeeping or escapees. In contrast, the Y chromosome is markedly smaller (~60 Mb total, with ~23 Mb ), containing fewer than 200 genes, predominantly multicopy and testis-specific, and enriched with (up to 95% of the male-specific region). The human Y features two small PARs (~2.7 Mb and ~0.33 Mb) that facilitate X-Y during , while the non-recombining portion shows stratified evolutionary layers from sequential recombination arrests over ~166 million years. Similar patterns occur in other mammals, such as mice (Y ~90 Mb with 674 genes) and (Y ~51 Mb with 1,274 genes), highlighting conserved structural dimorphism despite species-specific variations in size and repeat content. Birds exhibit a ZW , with the Z chromosome larger and more gene-dense (~75–80 Mb in , ~1,200 genes) than the gene-poor, heterochromatic W (~10–20% of Z size, <100 genes in many species). The W often appears as a microchromosome with extensive repetitive DNA and suppressed recombination outside a small PAR, leading to pronounced heteromorphism; for example, in ostriches, the non-recombining region covers one-third of the Z, while in emus, it is confined to the W centromere and Z short arm. This contrasts with mammalian XY systems by featuring female heterogamety and slower W degeneration in basal birds like ratites, where large PARs maintain homology over >130 million years. Invertebrate sex chromosomes, such as Drosophila's XY, show even greater variability, with the neo-Y degenerating rapidly through transposon invasions and gene loss. In plants, sex chromosomes display diverse structures across non-vascular, , and angiosperm lineages, often starting as homomorphic pairs before differentiating via recombination suppression. Bryophytes like mosses () feature UV systems with large U/V chromosomes (>100 Mb, >3,400 s), showing minimal heteromorphism and high gene retention due to haploid selection. Gymnosperms, such as (XY), have moderately sized Y chromosomes (~27 Mb, 241 s) with inversions promoting differentiation. Angiosperms vary widely: papaya's XY system includes a ~10 Mb Y with heterochromatic expansions, while latifolia's Y is 1.5 times larger than X due to repeats, and ZW systems in species like exhibit small SDRs (~1–27 Mb) with early gene loss. Overall, plant sex chromosomes tend to evolve slower degeneration than counterparts, retaining more functional genes in non-recombining regions through mechanisms like occasional recombination or haploid purifying selection.

Mechanisms of Differentiation

Sex chromosome differentiation typically begins when a pair of autosomes acquires a sex-determining locus, leading to the suppression of recombination between the proto-X and proto-Y (or proto-Z and proto-W) chromosomes in the region surrounding this locus. This suppression prevents the exchange of alleles, allowing sexually antagonistic mutations—those beneficial to one sex but deleterious to the other—to accumulate on the respective chromosomes. In many systems, such as mammalian XY pairs, this process is facilitated by chromosomal inversions or other structural changes that further restrict recombination, promoting the linkage of sex-determining genes. Once recombination is halted, the non-recombining chromosome (often the Y or W) undergoes progressive degeneration due to the accumulation of deleterious mutations, repetitive elements, and transposable elements, a phenomenon explained by and background selection. For instance, in vertebrates like mammals, the has lost most of its genes over evolutionary time, retaining primarily male-specific functions, while the preserves ancestral gene content. In contrast, some systems exhibit slower degeneration; in birds' ZW system, the W chromosome shows variable decay depending on the lineage, with some retaining substantial gene content despite lacking recombination. This heteromorphism—morphological and between sex chromosomes—varies widely, from nearly homomorphic pairs in young systems (e.g., certain ) to highly differentiated ones in older lineages. To counteract the gene dosage imbalance resulting from degeneration, dosage compensation mechanisms evolve to equalize expression between sexes and with autosomes. In mammals, this is achieved through X-chromosome inactivation, where one X in females is largely silenced via epigenetic modifications like Xist RNA coating. In Drosophila, males hyperactivate their single X chromosome through the male-specific lethal (MSL) complex, which modifies chromatin to upregulate transcription. Avian and some reptilian systems often lack global dosage compensation, relying instead on sex-specific regulation of individual genes, highlighting the diversity of evolutionary solutions to this challenge. These mechanisms underscore the dynamic interplay between differentiation and functional adaptation in sex chromosome evolution.

Evolution

Origins and Turnover

Sex chromosomes have evolved independently multiple times across eukaryotic lineages, typically originating from a pair of homologous autosomes that acquire a locus conferring a major effect on determination. This process begins when a or regulatory change establishes a -determining on one homolog, creating a proto- chromosome pair; subsequent suppression of recombination between the homologs prevents the loss of sexually antagonistic alleles beneficial to one sex but deleterious to the other. The foundational theory for this evolutionary pathway was proposed by Ohno in , who argued that sex chromosomes arise from autosomes through the co-option of existing genes into sex determination roles, a mechanism supported by comparative genomic analyses across vertebrates and . In many cases, the initial sex-determining is a , such as DMRT1 in birds and reptiles or SRY in mammals, highlighting despite diverse genetic starting points. Recombination suppression, often initiated by chromosomal inversions or epigenetic modifiers, marks the early stages of differentiation and sets the stage for Y (or W) chromosome degeneration through mutation accumulation and gene loss, as theorized by Muller in 1914. This suppression evolves in response to selective pressures like sexual antagonism, where genes advantageous for one sex are linked to the sex-determining region to avoid recombination with harmful variants on the other homolog ( 1987). Empirical evidence from model organisms, such as the fruit fly Drosophila melanogaster, demonstrates how proto-sex chromosomes form strata of decreasing age, reflecting sequential recombination arrest events over millions of years. In plants, similar origins occur, with examples like the (Salix) where sex determination co-opted autosomal genes involved in floral development. Turnover of sex chromosomes, the replacement of one sex-determining system by another, is a dynamic process observed in taxa with labile sex determination, driven by mechanisms such as the emergence of novel sex-determining alleles on s or translocations of existing ones. This turnover maintains evolutionary flexibility, preventing the fixation of deleterious mutations on degenerating sex chromosomes and allowing to changing environments, as modeled by Bull and Charnov (1977) through and selection. In poikilothermic vertebrates like and reptiles, turnover rates are high, with independent origins and replacements occurring within the last 10-50 million years; for instance, in the threespine stickleback (Gasterosteus aculeatus), the XY sex chromosomes differentiated from an on less than 26 million years ago. also exhibit turnover, as seen in bryophytes where UV systems have shifted multiple times via new autosomal recruiters. Factors promoting turnover include polygenic sex determination thresholds that can shift to monogenic systems or vice versa, often influenced by or environmental cues (van Doorn and Kirkpatrick 2007). In amphibians, such as the (Rana temporaria), genomic studies reveal intraspecific variation in sex chromosomes, indicating ongoing turnover facilitated by temperature-dependent . Despite these dynamics, some lineages like mammals maintain ancient systems (e.g., XY originating ~180 million years ago), underscoring that turnover frequency correlates with the stability of environmental sex cues and genetic architecture. Overall, turnover underscores the evolutionary lability of sex chromosomes, contrasting with their apparent stability in certain clades.

Degeneration Processes

Degeneration of the heterogametic sex chromosome ( or ) is a key consequence of recombination suppression in sex chromosome , leading to reduced content, pseudogenization, and accumulation of repetitive elements due to weakened selection in non-recombining regions. This process is explained by mechanisms such as , where deleterious mutations accumulate irreversibly, and the Hill-Robertson effect, which impairs efficient selection against linked harmful variants. In animals, degeneration is pronounced; for example, the mammalian , originating from autosomes ~180 million years ago, has lost ~97% of its ancestral genes, retaining ~70 functional genes mostly involved in , such as those in the azoospermia factor (AZF) regions. A lecture by geneticist David Page provides an overview of the evolutionary history of human X and Y chromosomes, including their origins from autosomes around 200 million years ago and the Y chromosome's degeneration to retaining only about 17 genes. Similarly, in , the has degenerated extensively, losing nearly all protein-coding genes present on the ancestral autosome and evolving into a heterochromatic structure with ampliconic regions for genes. In birds, the W chromosome shows comparable decay, with gene loss rates influenced by sexual antagonism and dosage sensitivity. These patterns highlight how non-recombination exposes the heterogametic chromosome to mutational bias and , often resulting in size reduction and functional specialization. In , degeneration tends to proceed more slowly than in animals, largely due to the prolonged haploid phase, which subjects recessive mutations to direct selection and limits loss to ~4-8% per million generations in some . However, suppression of recombination still promotes proliferation and relocation to autosomes, as detailed in the plants-specific evolution section. Overall, while animal systems often exhibit rapid, progressive decay, plant sex chromosomes frequently retain greater functionality through haploid purging and structural adaptations.

Clinical Relevance

Disorders of Sex Development

Disorders of Sex Development (DSD) encompass congenital conditions in which the development of chromosomal, gonadal, or anatomical sex is atypical. Sex chromosome DSD specifically arise from numerical or structural abnormalities in the X and Y chromosomes, leading to variations in sex determination and differentiation. These disorders are classified under the Chicago Consensus as including 45,X (Turner syndrome), 47,XXY (Klinefelter syndrome), 47,XYY, and mosaics such as 45,X/46,XY (mixed gonadal dysgenesis). They affect approximately 1 in 4,500 live births overall for DSD, with sex chromosome variants representing a significant subset. Turner syndrome, characterized by a 45,X karyotype or mosaics thereof, occurs in about 1 in 2,500 live female births. Clinically, it presents with short stature, primary amenorrhea, ovarian dysgenesis leading to infertility in over 95% of cases, and associated features such as webbed neck, lymphedema, and cardiac anomalies like coarctation of the aorta. Diagnosis typically involves karyotyping to confirm the monosomy X, often supplemented by fluorescence in situ hybridization (FISH) for mosaicism detection. Management includes growth hormone therapy to address short stature, estrogen-progesterone replacement for puberty induction, and monitoring for comorbidities like thyroiditis or diabetes; fertility options are limited to oocyte donation due to streak gonads. Klinefelter syndrome, defined by a 47,XXY , affects roughly 1 in 500 to 1,000 male births. Key features include tall stature, small testes, with low testosterone levels, in up to 40% of cases, due to , and potential learning or behavioral challenges. is confirmed via analysis, frequently identified postnatally through evaluations or prenatal screening. Treatment focuses on testosterone replacement starting in to promote secondary and , alongside psychological support; fertility preservation may involve testicular extraction, successful in about 40-50% of attempts. The 47,XYY , known as Jacobs syndrome, occurs in approximately 1 in 1,000 male births and often presents with subtle phenotypes. Common manifestations include tall stature, , , mild , and increased risk for developmental delays, attention-deficit hyperactivity disorder, or autism spectrum traits, though many individuals are asymptomatic. Fertility is generally preserved, but some cases show or genitourinary anomalies. establishes the , typically prompted by physical or neurodevelopmental concerns. is supportive, emphasizing early intervention for learning or behavioral issues, with no routine hormonal therapy required. Triple X syndrome, or 47,XXX, affects approximately 1 in 1,000 births. It is often mild or , but may present with tall stature, developmental delays, learning disabilities, , and menstrual irregularities; fertility is usually preserved, though premature ovarian insufficiency occurs in some cases. Diagnosis is via karyotyping, often identified through or evaluation for developmental concerns. Management is supportive, including early educational interventions, speech therapy, and monitoring for associated health issues like seizures or renal anomalies; hormonal therapy may be needed for ovarian dysfunction. Mixed gonadal dysgenesis, often associated with a 45,X/46,XY mosaic karyotype, results in asymmetric gonadal development, such as a streak gonad on one side and a dysgenetic testis on the other. It manifests with variable phenotypes ranging from Turner-like features in females to ambiguous genitalia or male-typical external traits, alongside risks of gonadal malignancy up to 30% due to Y chromosome material. Prevalence is rare, estimated at less than 1 in 20,000 births. Diagnosis requires karyotyping, hormonal assays (e.g., low anti-Müllerian hormone), pelvic imaging, and sometimes gonadal biopsy. Management is individualized, often involving gonadectomy to mitigate cancer risk, hormone replacement tailored to the desired sex, and multidisciplinary care including psychological support. Across these disorders, prenatal diagnosis via or has increased detection rates, enabling early intervention. Long-term care emphasizes multidisciplinary approaches to address physical, reproductive, and needs, with ongoing into genetic mechanisms improving outcomes.

Genetic Applications

Sex chromosomes play a pivotal role in genetic applications, particularly in diagnostic testing, forensic analysis, and biotechnological interventions. In , (NIPT) utilizes to detect sex chromosome aneuploidies, such as (45,X), (47,XXY), and triple X syndrome (47,XXX), with high sensitivity for conditions involving extra or missing X or Y chromosomes. This approach analyzes maternal plasma for chromosomal imbalances, enabling early identification of fetal sex chromosome abnormalities without invasive procedures like . For instance, NIPT achieves positive predictive values ranging from 36.9% to higher in specific age groups for sex chromosome issues, though confirmation via karyotyping is recommended due to potential false positives from confined placental mosaicism. In forensic genetics, the Y chromosome's male-specific markers, particularly short tandem repeats (STRs), are widely applied in sexual assault investigations and paternity testing. Y-STR profiling allows identification of male DNA in mixed samples where female DNA predominates, such as in rape cases, by targeting the non-recombining region of the Y chromosome that passes unchanged from father to son. This technique has proven effective in resolving complex cases, including cold cases, with databases like the YHRD (Y-chromosome Haplotype Reference Database) facilitating matches across populations. Additionally, Y-chromosome analysis supports lineage tracing in forensics, distinguishing paternal ancestry and aiding in disaster victim identification. Genetic association studies increasingly incorporate sex chromosomes to uncover sex-biased disease risks, adjusting for X and Y chromosome dosage in genome-wide analyses. Traditional autosomal-focused GWAS often overlook sex chromosomes, but specialized statistical models now account for hemizygosity on the X in males and pseudoautosomal regions, revealing links to conditions like autism spectrum disorder associated with extra Y chromosomes. In , Y-chromosome haplogroups provide insights into and diversity, with reduced variability compared to autosomes due to its patrilineal inheritance. Biotechnological applications leverage sex chromosome engineering for species management and agriculture. In aquaculture, genetic modifications targeting sex chromosomes enable production of single-sex populations, such as all-female for faster growth, using techniques like to disrupt Y-linked genes or create sex-reversed individuals. Similarly, in , sterile insect techniques (SIT) employ sex chromosome manipulations to generate non-transgenic, sex-sorted males for release, reducing populations of species like mosquitoes without ecological harm. These methods exploit the dimorphic nature of XY systems, ensuring targeted interventions while preserving genetic integrity.

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