Abstract
Sex chromosomes regularly evolve suppressed recombination, distinguishing them from other chromosomes, and the reason for this has been debated for many years. It is now clear that non-recombining sex-linked regions have arisen in different ways in different organisms. A major hypothesis is that a sex-determining gene arises on a chromosome and that sexually antagonistic (SA) selection (sometimes called intra-locus sexual conflict) acting at a linked gene has led to the evolution of recombination suppression in the region, to reduce the frequency of low fitness recombinant genotypes produced. The sex chromosome system of the guppy (Poecilia reticulata) is often cited as supporting this hypothesis because SA selection has been demonstrated to act on male coloration in natural populations of this fish, and probably contributes to maintaining polymorphisms for the genetic factors involved. I review classical genetic and new molecular genetic results from the guppy, and other fish, including approaches for identifying the genome regions carrying sex-determining loci, and suggest that the guppy may exemplify a recently proposed route to sex chromosome evolution.
Highlights
Sex Chromosomes and Sex DeterminationA major question concerning sex chromosome evolution is what has led to the lack of recombination between such chromosome pairs, or between sex-linked regions of these chromosomes
One turnover model that does generate this situation, and creates selection for closer linkage, assumes that a sexually antagonistic (SA) polymorphism is maintained at an autosomal locus, such that individuals would benefit if it became linked to the sex-determining gene [10]
I outline the evidence for this trend, which argues against purely neutral processes being involved
Summary
A major question concerning sex chromosome evolution is what has led to the lack of recombination between such chromosome pairs, or between sex-linked regions of these chromosomes. Since female-sterility is very disadvantageous to females, this mutation is necessarily sexually antagonistic This two-gene model can generate polymorphisms that are maintained at both the “sex-determining” loci, especially if the genes are closely linked [4]. Transitions between XY and ZW systems have been documented in reptiles and amphibians (reviewed by [17]), and changes in the sex-determining chromosome pair in Diptera [18] In these cases, it is usually not known whether the same sex-determining genes are involved, or whether takeovers have occurred—most likely both kinds of events are possible. One turnover model that does generate this situation, and creates selection for closer linkage, assumes that a sexually antagonistic (SA) polymorphism is maintained at an autosomal locus, such that individuals would benefit if it became linked to the sex-determining gene [10]. SA polymorphisms establish only under restricted conditions [25], and empirical tests are needed to find out whether such polymorphisms occur and are maintained long enough to cause the effects proposed in theoretical models
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