"An adaptive hypothesis for the prevalence XY over ZW sex determination when dioecy evolves from hermaphroditism"Lesaffre, ThomasThe vast majority of flowering plants are hermaphroditic, but many species have independently evolved separate sexes (dioecy). In these species, sex is typically controlled by a heterogametic sex-determining locus, with either male heterogamety where males are XY heterozygotes and females are XX homozygotes, or female heterogamety where females are ZW heterozygotes and males are ZZ homozygotes. Strikingly, XY sex determination is much more frequent than ZW among dioecious plants, but the reasons behind this asymmetry are unclear. One proposition is that separate sexes evolve through the invasion of sterility mutations at closely-linked loci, in which case XY sex determination evolves if the initial male sterility mutation is fully recessive. Alternatively, dioecy may evolve via the gradual divergence of male and female phenotypes, but the genetic basis of such divergence and its connection to XY or ZW sex determination remain poorly understood. Using mathematical modelling, we show how dioecy with XY or ZW sex determination can emerge from the joint evolution of resource allocation to male and female function with its genetic architecture. Our model reveals that whether XY or ZW sex determination evolves depends on the reproductive ecology of ancestral hermaphrodites. In particular, we find that selection to avoid self-fertilisation and inbreeding depression strongly favours XY sex determination. Taken together, our results throw new light on the possible origins of dioecy from hermaphroditism and bring forth an adaptive hypothesis for the preponderance of XY systems in dioecious plants. Beyond sex and sex determination, our model showcases how ecology can influence the way natural selection shapes the genetic architecture of polymorphic traits. |
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