Heterogeneous evolution of sex chromosomes in the torrent frog genus Amolops

In sharp contrast with birds and mammals, sex chromosomes have been described as homomorphic in cold-blooded vertebrates. This sex-chromosome homomorphy has been suggested to result from high turnovers when they are often observed across deeply diverged clades. However, little is known about the tempo and mode of sex chromosome evolution among most closely related species. Here, we examine patterns of sex chromosome evolution among nine species of the torrent frog genus Amolops. Through the analysis of male and female GBS and RAD-seq from 182 individuals and of PCR verification for 176 individuals, we identify signatures of sex chromosomes involving two pairs of chromosomes. We find that the sex-chromosome homomorphy results from both turnover and X–Y recombination in the Amolops species, simultaneously exhibiting heterogeneous evolution on homologous and non-homologous sex chromosomes. The lower turnover rate of non-homologous sex chromosomes exists in these torrent frogs, whereas the ongoing X–Y recombination in homologous sex chromosomes would act as an indispensable force to prevent the sex chromosomes from differentiations.