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Sex-limited

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Sex-limited

Sex-limited genes are male pattern baldness).

Sex-limited genes are responsible for sexual dimorphism, which is a phenotypic (directly observable) difference between males and females of the same species. These differences can be reflected in size, color, behavior (ex: levels of aggression), and morphology. An example of sex-limited genes are genes which instruct the male elephant seals to grow big and fight, at the same time instructing female seals to grow small and avoid fights.[1] These genes are also responsible for some female beetles' inability to grow exaggerated mandibles,[2] research that is discussed in detail later in this article.

The overall point of sex-limited genes is to resolve intralocus sexual conflict. In other words, 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.

A Brief History

Genetics

Many studies have been published exploring the genetic basis of sex-limited genes. One paper, published in Evolution, evaluates the hypothesis that sex-limited traits can arise in two ways.[3] The alleles 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 (enlongated 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 (when the factors that modify the gene are located next to the gene on the chromosome). 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 male-specific (thus, sex-limited) genes cue their expression by hormone levels - the absence of estrogen or the presence of testosterone.

Fitness Consequences

Storage Effect[4]

Rapid Evolution[5]

Effects of Sexual Antagonism[6]

Sexual antagonism occurs when two species have conflicting optimal fitness strategies concerning reproduction (see link in introduction paragraph).

In 2010, Hosken et al. completed an important study exploring the effects of sexually antagonistic selection on sex-limited trait expression.[2] They asked if sex-specific trait selection always resolved intralocus conflict, as it was believed to do. By using a species of flour beetle, Gnatocerus cornutus, exhibiting sex-limited traits in the form of exaggerated mandible size, they were able to test this hypothesis. Exaggerated mandibles are only developed in males; females never develop exaggerated mandibles. The point of this experiment was to determine how mandibles affect fitness. If these sex-limited genes are truly quelling intralocus sexual conflict, male mandible size should have no effect on female fitness. After selecting for males with exaggerated mandibles (full materials and methods can be found within the paper), it was experimentally determined that males with exaggerated mandibles had a higher fitness - they experienced increased fighting and mating success. It was also found, however, that females found in the populations of males with exaggerated mandibles had lower fitness (as determined by lifetime reproductive success, LRS) relative to the fitness of females in populations with males with smaller mandibles. Since this male sex-limited trait has an impact on female fitness, intralocus sexual conflict has not been resolved. This highlights the importance of sexual conflict to evolution, because it cannot simply be defused by sex-limited trait expression.

Effects on Animal Behavior

References


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