Sex chromosomes in fish. How sex chromosomes emerge and how they end.

This paper aims to show, based on the latest studies in poeciliid fish and other fish species, how sex chromosomes appear, how they evolve and how they are supposed to end. Sex chromosomes originate from autosomes. Initially, the first differentiation appears in the form of minor genes specific to the male and female sex, scattered randomly along the length of the future sex chromosomes. Their abundance or frequency will influence the differentiation of the organism to one sex or the other. These minor genes tend to be concentrated in regions that are not predisposed to recombination through crossing-over, forming a major gene, more precisely the sex-determining locus. In this region, where genetic recombination is restricted, sexually antagonistic genes tend to accumulate. Such linkage of sexually antagonistic genes to the sex-determining locus will further promote the restriction of genetic recombination, rendering most of the sex chromosomes unable to recombine through crossing-over. The lack of genetic recombination will further promote the accumulation of non-functional DNA sequences and to the degeneration of the non-recombinant sex chromosome region. Sometimes, the decay affects the whole chromosome and the system is aborted. Therefore, some systems of sex determination have in the case of individuals of one sex, with a chromosome less than in the case of the opposite sex. In other cases, the obsolete sex-determining system is replaced by a new one, from a new pair of autosomes. We may conclude that emergence, evolution and replacement of sex chromosomes in fish is a cyclical process, which happens faster than we believed. Although mutations occur randomly, the evolution of sex chromosomes is directly correlated with the species history and ecological context. [ABSTRACT FROM AUTHOR]

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