Phylogeography, more than elevation, accounts for sex-chromosome differentiation in Swiss populations of the common frog (Rana temporaria)

Sex chromosomes in vertebrates range from highly heteromorphic (as in most birds and mammals) to strictly homomorphic (as in many fishes, amphibians, and non-avian reptiles). Reasons for these contrasted evolutionary trajectories remain unclear, but species such as common frogs with polymorphism in the extent of sex-chromosome differentiation may potentially deliver important clues. By investigating 92 common-frog populations from a wide range of elevations throughout Switzerland, we show that sex-chromosome differentiation strongly correlates with alleles at the candidate sex-determining gene Dmrt1. Y-specific Dmrt1 haplotypes cluster into two main haplogroups, YA and YB, with a phylogeographic signal that parallels mtDNA haplotypes: YA populations, with mostly well-differentiated sex chromosomes, occur primarily south of the main alpine ridge that bisects Switzerland, while YB populations, with mostly undifferentiated (proto-)sex chromosomes, occur north of this ridge. Elevation has o...