Abstract
Frogs are ideal organisms for studying sex chromosome evolution because of their diversity in sex chromosome differentiation and sex-determination systems. We review 222 anuran frogs, spanning ~220 Myr of divergence, with characterized sex chromosomes, and discuss their evolution, phylogenetic distribution and transitions between homomorphic and heteromorphic states, as well as between sex-determination systems. Most (~75%) anurans have homomorphic sex chromosomes, with XY systems being three times more common than ZW systems. Most remaining anurans (~25%) have heteromorphic sex chromosomes, with XY and ZW systems almost equally represented. There are Y-autosome fusions in 11 species, and no W-/Z-/X-autosome fusions are known. The phylogeny represents at least 19 transitions between sex-determination systems and at least 16 cases of independent evolution of heteromorphic sex chromosomes from homomorphy, the likely ancestral state. Five lineages mostly have heteromorphic sex chromosomes, which might have evolved due to demographic and sexual selection attributes of those lineages. Males do not recombine over most of their genome, regardless of which is the heterogametic sex. Nevertheless, telomere-restricted recombination between ZW chromosomes has evolved at least once. More comparative genomic studies are needed to understand the evolutionary trajectories of sex chromosomes among frog lineages, especially in the ZW systems.
Highlights
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The canonical model of sex chromosome evolution suggests that sexually antagonistic (SA) genes play a key role in the process of sex chromosome degeneration [5,14,15,16,17,18]
There has been little empirical support to demonstrate the role of sexually antagonistic (SA) genes in sex chromosome recombination suppression, because the phenotypic effects of SA genes are difficult to demonstrate and because it is hard to show the causal effects of SA genes
Summary
Across the tree of life, species determine sex either by using environmental cues or with sex chromosomes, which are the subject of this review. The recombination arrest leads to progressive differentiation and degeneration of sex-limited chromosomes through mechanisms, such as Muller’s ratchet, genetic hitchhiking, reduced purifying selection, and stronger genetic drift [5,14,15,16,17,18]. This model has been widely accepted to account for the recombination suppression and degeneration in the highly differentiated sex chromosomes in mammals, most birds and insects [7,10]. Studies on a variety of stages of sex chromosome evolution from homomorphic to highly degenerated, the less studied early stage, are still needed to fully understand sex chromosome evolution
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