Abstract
Sex determination is remarkably dynamic; many taxa display shifts in the location of sex-determining loci or the evolution of entirely new sex-determining systems. Predominant theories for why we observe such transitions generally conclude that novel sex-determining systems are favoured by selection if they equalise the sex ratio or increase linkage with a locus that experiences different selection in males versus females. We use population genetic models to extend these theories in two ways: (1) We consider the dynamics of loci very tightly linked to the ancestral sex-determining loci, e.g., within the nonrecombining region of the ancestral sex chromosomes. Variation at such loci can favour the spread of new sex-determining systems in which the heterogametic sex changes (XY to ZW or ZW to XY) and the new sex-determining region is less closely linked (or even unlinked) to the locus under selection. (2) We consider selection upon haploid genotypes either during gametic competition (e.g., pollen competition) or meiosis (i.e., nonmendelian segregation), which can cause the zygotic sex ratio to become biased. Haploid selection can drive transitions between sex-determining systems without requiring selection to act differently in diploid males versus females. With haploid selection, we find that transitions between male and female heterogamety can evolve so that linkage with the sex-determining locus is either strengthened or weakened. Furthermore, we find that sex ratio biases may increase or decrease with the spread of new sex chromosomes, which implies that transitions between sex-determining systems cannot be simply predicted by selection to equalise the sex ratio. In fact, under many conditions, we find that transitions in sex determination are favoured equally strongly in cases in which the sex ratio bias increases or decreases. Overall, our models predict that sex determination systems should be highly dynamic, particularly when haploid selection is present, consistent with the evolutionary lability of this trait in many taxa.
Highlights
Animals and angiosperms exhibit extremely diverse sex-determining systems
3A: New sex-determining alleles causing cis-genetic sex determination (GSD) transitions (XY $ XY or ZW $ ZW) are favoured if they arise more closely linked with a locus that experiences selection than the ancestral sex-determining locus (R < r)
A neo-W is usually favoured when it is more closely linked to the selected locus than the ancestral sex-determining region is (R < r, e.g., Figs 3B and 4); this is true unless the last term in Eq (3) is negative and dominant over the first, which requires relatively restrictive combinations of selection and recombination parameters
Summary
Animals and angiosperms exhibit extremely diverse sex-determining systems (reviewed in [1,2,3,4,5]). The chromosome that harbours the master sex-determining locus changes, due either to translocation of the master sex-determining locus or to the evolution of a new master locus. During these transitions, the heterogametic sex can remain the same (hereafter ‘cis-GSD transitions’) as in salmonids [7, 8], Diptera [9], and Oryzias [10]. In some cases, both male and female heterogametic sex-determining systems can be found in the same species, as reported in houseflies [18], midges [19], frogs [20], cichlid fish [21], tilapia [22], sea bass [23], and lab strains of zebrafish [24, 25]. Accumulating evidence indicates that transitions between sex-determining systems are common [4]
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