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Sex-limited genes
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Sex-limited genes
Sex-limited genes are genes that are present in both sexes of sexually reproducing species but are expressed in only one sex and have no penetrance, or are simply 'turned off' in the other. In other words, sex-limited genes cause the two sexes to show different traits or phenotypes, despite having the same genotype. This term is restricted to autosomal traits, and should not be confused with sex-linked characteristics, which have to do with genetic differences on the sex chromosomes (see sex-determination system). Sex-limited genes are also distinguished from sex-influenced genes, where the same gene will show differential expression in each sex. Sex-influenced genes commonly show a dominant/recessive relationship, where the same gene will have a dominant effect in one sex and a recessive effect in the other (for example, male pattern baldness). However, the resulting phenotypes caused by sex-limited genes are present in only one sex and can be seen prominently in various species that typically show high sexual dimorphism.
Sex-limited genes are responsible for sexual dimorphism, which is a phenotypic (directly observable) difference between males and females of the same species regardless of genotype. These differences can be reflected in size, color, behavior (ex: levels of aggression), and morphology. An example of sex-limited genes are genes which control horn development in sheep: while both males and females possess the same genes controlling horn development, they are only expressed in males. Sex-limited genes are also responsible for some female beetles' inability to grow exaggerated mandibles, research that is discussed in detail later in this article.
Sex-limited genes were first hypothesized by Charles Darwin and though he was unsuccessful in distinguishing the previously mentioned sex-linked traits, his hypothesis was the starting point for future study of the subject. His studies on sex-limited traits have been further substantiated and supported over time, distinguishing sex-limited genes and sex-linked traits. Modern study of sex-limited genes includes research on epigenetics, which is the study of inheritable phentotypic changes with no change in DNA sequence. Modern research suggests that a substantial portion of the expression of sex-limited genes and sexual dimorphism may be influenced by certain epigenetic marks.
The purpose of sex-limited genes is to resolve sexual conflict. These genes try to resolve the "push-pull" between males and females over trait values for optimal phenotype. Without these genes, organisms would be forced to settle on an average trait value, incurring costs on both sexes. With these genes, it is possible to 'turn off' the genes in one sex, allowing both sexes to attain (or at least, approach very closely) their optimal phenotypes. This phenotypic variation can play a key role in the evolution of various species and their sexual differentiation.
The idea of sex-limited genes was initially developed by Charles Darwin in 1871 in his book The Descent of Man and Selection in Relation to Sex. He did not distinguish between sex-limited, sex-linked, and sex-influenced genes, but referred to any gene that expresses differently between sexes as sex-limited. Thomas Hunt Morgan, aware of this confusing terminology, published an article in The American Naturalist in 1914 titled "Sex-Linked and Sex-Limited Inheritance," which proposed definitions of sex-linked genes and sex-limited genes (as defined in the introduction above). Morgan's paper was followed by several others involving sex-limited genes and their expression as traits. One of the more notable examples is John H. Gerould's "Inheritance of White Wing Color, a Sex-Limited (Sex-Controlled) Variation in Yellow Pierid Butterflies," published in Genetics in 1923 (and edited slightly in 1924). Gerould observed the phenotypic differences between male and female Pierid Butterflies and determined colouration to be a sex-limited trait.
The notable advancements in the early stages of the development of sex-limited genes, a brief discussion of R. A. Fisher is necessary. Commonly hailed as one of the most significant evolutionary biologists of his time, Fisher was also a talented geneticist. His book The Genetical Theory of Natural Selection, published in 1930, over 20 years before the double-helix shape of DNA was discovered, was the first attempt to explain Darwin's theories within the foundation of genetics. Chapter 6 of this book is titled "Sexual Reproduction and Sexual Selection" and includes a genetic interpretation of Darwin's initial idea of sex-limited genes. After these groundbreaking works, papers continue to be published further exploring the causes, mechanisms, evolutionary advantages, and more of sex-limited genes.
The genetic study of sexual dimorphism, published in Evolution, hypothesizes two methods which leads to different ornamental characteristics in male and female birds. The alleles (different versions of the same gene) responsible for sexual dimorphism can be limited to expression in only one sex when they first appear, or the alleles could begin by being expressed in both sexes then become modified (repressed or promoted) in one sex by modifier genes or regulatory elements. The concept of this study was to examine female hybrids from species where males displayed different types of ornamental traits (elongated feathers, wattles, color patches). The assumption is that different hypotheses about male-specific expression will yield different results in female hybrids. The methods and materials of the experiment are discussed in detail in the paper, but the important result that emerged was that NO female hybrids expressed any of the ornamental traits found in the parent males. Two interpretations of these results are possible: the dimorphic alleles were initially only expressed in males, or the alleles were initially expressed in both and then were suppressed in females or became limited to males by regulatory regions that are completely dominant in hybrids. The most likely genomic explanation for initial expression in both species then modification is involvement of cis-dominance, where the factors that modify the gene are located next to the gene on the chromosome. (This is in contrast to trans-dominance, where mobile products that can affect distant genes are produced.) These factors can be in the form of promoter regions, which can be either suppressed or activated by hormones. This experiment also demonstrates that these alleles come under regulatory control very quickly. This is because none of the ornamentation seen in males is seen in the very next generation. These conclusions make it likely that at least some male-specific (thus, sex-limited) genes cue their expression by hormone levels, such as threshold ratios of estrogen and testosterone.
Because sex-limited genes are present in both sexes but only expressed in one, this allows the unexpressed genes to be hidden from selection. On a short-term scale, this means that during one generation, only the sex that expresses the sex-limited trait(s) of interest will be affected by selection. The remaining half of the gene pool for these traits will be unaffected by selection because they are hidden (unexpressed) in the genes of the other sex. Since a portion of the alleles for these sex-limited traits are hidden from selection, this occurrence has been termed 'storage-effect'. On a long-term scale, this storage effect can have significant effects on selection, especially if selection is fluctuating over a long period of time. It is inarguable that selection will fluctuate over time with varying levels of environmental stability. For example, fluctuations in population density can drive selection on sex-limited traits. In less dense populations, females will have less opportunity to choose between males for reproduction. In this case, attractive males may experience both reduced reproductive success and increased predation pressure. Thus, selection on males for sex-limited traits such as increased size (elephant seals) and weaponry (claws on fiddler crabs, horns on rhinoceros beetles) will change direction with fluctuation in population density.
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Sex-limited genes
Sex-limited genes are genes that are present in both sexes of sexually reproducing species but are expressed in only one sex and have no penetrance, or are simply 'turned off' in the other. In other words, sex-limited genes cause the two sexes to show different traits or phenotypes, despite having the same genotype. This term is restricted to autosomal traits, and should not be confused with sex-linked characteristics, which have to do with genetic differences on the sex chromosomes (see sex-determination system). Sex-limited genes are also distinguished from sex-influenced genes, where the same gene will show differential expression in each sex. Sex-influenced genes commonly show a dominant/recessive relationship, where the same gene will have a dominant effect in one sex and a recessive effect in the other (for example, male pattern baldness). However, the resulting phenotypes caused by sex-limited genes are present in only one sex and can be seen prominently in various species that typically show high sexual dimorphism.
Sex-limited genes are responsible for sexual dimorphism, which is a phenotypic (directly observable) difference between males and females of the same species regardless of genotype. These differences can be reflected in size, color, behavior (ex: levels of aggression), and morphology. An example of sex-limited genes are genes which control horn development in sheep: while both males and females possess the same genes controlling horn development, they are only expressed in males. Sex-limited genes are also responsible for some female beetles' inability to grow exaggerated mandibles, research that is discussed in detail later in this article.
Sex-limited genes were first hypothesized by Charles Darwin and though he was unsuccessful in distinguishing the previously mentioned sex-linked traits, his hypothesis was the starting point for future study of the subject. His studies on sex-limited traits have been further substantiated and supported over time, distinguishing sex-limited genes and sex-linked traits. Modern study of sex-limited genes includes research on epigenetics, which is the study of inheritable phentotypic changes with no change in DNA sequence. Modern research suggests that a substantial portion of the expression of sex-limited genes and sexual dimorphism may be influenced by certain epigenetic marks.
The purpose of sex-limited genes is to resolve sexual conflict. These genes try to resolve the "push-pull" between males and females over trait values for optimal phenotype. Without these genes, organisms would be forced to settle on an average trait value, incurring costs on both sexes. With these genes, it is possible to 'turn off' the genes in one sex, allowing both sexes to attain (or at least, approach very closely) their optimal phenotypes. This phenotypic variation can play a key role in the evolution of various species and their sexual differentiation.
The idea of sex-limited genes was initially developed by Charles Darwin in 1871 in his book The Descent of Man and Selection in Relation to Sex. He did not distinguish between sex-limited, sex-linked, and sex-influenced genes, but referred to any gene that expresses differently between sexes as sex-limited. Thomas Hunt Morgan, aware of this confusing terminology, published an article in The American Naturalist in 1914 titled "Sex-Linked and Sex-Limited Inheritance," which proposed definitions of sex-linked genes and sex-limited genes (as defined in the introduction above). Morgan's paper was followed by several others involving sex-limited genes and their expression as traits. One of the more notable examples is John H. Gerould's "Inheritance of White Wing Color, a Sex-Limited (Sex-Controlled) Variation in Yellow Pierid Butterflies," published in Genetics in 1923 (and edited slightly in 1924). Gerould observed the phenotypic differences between male and female Pierid Butterflies and determined colouration to be a sex-limited trait.
The notable advancements in the early stages of the development of sex-limited genes, a brief discussion of R. A. Fisher is necessary. Commonly hailed as one of the most significant evolutionary biologists of his time, Fisher was also a talented geneticist. His book The Genetical Theory of Natural Selection, published in 1930, over 20 years before the double-helix shape of DNA was discovered, was the first attempt to explain Darwin's theories within the foundation of genetics. Chapter 6 of this book is titled "Sexual Reproduction and Sexual Selection" and includes a genetic interpretation of Darwin's initial idea of sex-limited genes. After these groundbreaking works, papers continue to be published further exploring the causes, mechanisms, evolutionary advantages, and more of sex-limited genes.
The genetic study of sexual dimorphism, published in Evolution, hypothesizes two methods which leads to different ornamental characteristics in male and female birds. The alleles (different versions of the same gene) responsible for sexual dimorphism can be limited to expression in only one sex when they first appear, or the alleles could begin by being expressed in both sexes then become modified (repressed or promoted) in one sex by modifier genes or regulatory elements. The concept of this study was to examine female hybrids from species where males displayed different types of ornamental traits (elongated feathers, wattles, color patches). The assumption is that different hypotheses about male-specific expression will yield different results in female hybrids. The methods and materials of the experiment are discussed in detail in the paper, but the important result that emerged was that NO female hybrids expressed any of the ornamental traits found in the parent males. Two interpretations of these results are possible: the dimorphic alleles were initially only expressed in males, or the alleles were initially expressed in both and then were suppressed in females or became limited to males by regulatory regions that are completely dominant in hybrids. The most likely genomic explanation for initial expression in both species then modification is involvement of cis-dominance, where the factors that modify the gene are located next to the gene on the chromosome. (This is in contrast to trans-dominance, where mobile products that can affect distant genes are produced.) These factors can be in the form of promoter regions, which can be either suppressed or activated by hormones. This experiment also demonstrates that these alleles come under regulatory control very quickly. This is because none of the ornamentation seen in males is seen in the very next generation. These conclusions make it likely that at least some male-specific (thus, sex-limited) genes cue their expression by hormone levels, such as threshold ratios of estrogen and testosterone.
Because sex-limited genes are present in both sexes but only expressed in one, this allows the unexpressed genes to be hidden from selection. On a short-term scale, this means that during one generation, only the sex that expresses the sex-limited trait(s) of interest will be affected by selection. The remaining half of the gene pool for these traits will be unaffected by selection because they are hidden (unexpressed) in the genes of the other sex. Since a portion of the alleles for these sex-limited traits are hidden from selection, this occurrence has been termed 'storage-effect'. On a long-term scale, this storage effect can have significant effects on selection, especially if selection is fluctuating over a long period of time. It is inarguable that selection will fluctuate over time with varying levels of environmental stability. For example, fluctuations in population density can drive selection on sex-limited traits. In less dense populations, females will have less opportunity to choose between males for reproduction. In this case, attractive males may experience both reduced reproductive success and increased predation pressure. Thus, selection on males for sex-limited traits such as increased size (elephant seals) and weaponry (claws on fiddler crabs, horns on rhinoceros beetles) will change direction with fluctuation in population density.