Y-chromosome haplotypes of varying differentiation to the X are not associated with male fitness in common frogs

The canonical model of sex-chromosome evolution assigns a key role to sexually antagonistic (SA) genes on the arrest of recombination and ensuing degeneration of Y chromosomes. This assumption cannot be tested in organisms with highly differentiated sex chromosomes, such as mammals or birds, owing to the lack of polymorphism. Fixation of SA alleles, furthermore, might be the consequence rather than the cause of recombination arrest. Here we focus on a population of common frogs (Rana temporaria) where XY males with genetically differentiated Y chromosomes (non-recombinant Y haplotypes) coexist with both XY{degrees} males with proto-Y chromosomes (only differentiated from X chromosomes in the immediate vicinity of the candidate sex-determining locus Dmrt1) and XX males with undifferentiated sex chromosomes (genetically identical to XX females). Our study shows no effect of sex-chromosome differentiation on male phenotype, mating success or fathering success. Our conclusions rejoin genomic studies that found no differences in gene expression between XY, XY{degrees} and XX males. Sexual dimorphism in common frogs seems to result from the differential expression of autosomal genes rather than sex-linked SA genes. Among-male variance in sex-chromosome differentiation is better explained by a polymorphism in the penetrance of alleles at the sex locus, resulting in variable levels of sex reversal (and thus of X-Y recombination in XY females), independent of sex-linked SA genes.\n\nImpact Summary\n\nHumans, like other mammals, present highly differentiated sex chromosomes, with a large, gene-rich X chromosome contrasting with a small, gene-poor Y chromosome. This differentiation results from a process that started approximately 160 Mya, when the Y first stopped recombining with the X. How and why this happened, however, remain controversial. According to the canonical model, the process was initiated by sexually antagonistic selection; namely, selection on the proto-Y chromosome for alleles that were beneficial to males but detrimental to females. The arrest of XY recombination then allowed such alleles to be only transmitted to sons, not to daughters. Although appealing and elegant, this model can no longer be tested in mammals, as it requires a sex-chromosome system at an incipient stage of evolution. Here we focus on a frog that displays within-population polymorphism is sex-chromosome differentiation, where XY males with differentiated chromosomes coexist with XX males lacking Y chromosomes. We find no effect of sex-chromosome differentiation on male phenotype or mating success, opposing expectations from the standard model. Sex linked genes do not seem to have a disproportionate effect on sexual dimorphism. From our results, sexually antagonistic genes show no association with sex-chromosome differentiation in frogs, which calls for alternative models of sex-chromosome evolution.