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.