In sharp contrast with birds and mammals, sex-determination systems in ectothermic vertebrates are often highly dynamic and sometimes multifactorial. Both environmental and genetic effects have been documented in common frogs (Rana temporaria). One genetic linkage group, mapping to the largest pair of chromosomes and harboring the candidate sex-determining gene Dmrt1, associates with sex in several populations throughout Europe, but association varies both within and among populations. Here we show that sex association at this linkage group differs among populations along a 1500 km transect across Sweden. Genetic differentiation between sexes is strongest (FST = 0.152) in a northern-boreal population, where male-specific alleles and heterozygote excesses (FIS = -0.418 in males, +0.025 in females) testify to a male-heterogametic system and lack of X-Y recombination. In the southernmost population (nemoral climate), in contrast, sexes share the same alleles at the same frequencies (FST = 0.007 between sexes), suggesting unrestricted recombination. Other populations show intermediate levels of sex differentiation, with males falling in two categories: some cluster with females, while others display male-specific Y haplotypes. This polymorphism may result from differences among populations in the patterns of X-Y recombination, co-option of an alternative sex-chromosome pair, or a mixed sex-determination system where maleness is controlled either by genes or by environment depending on populations or families. We propose approaches to test among these alternative models, to disentangle the effects of climate and phylogeography on the latitudinal trend, and to sort out how this polymorphism relates to the ‘sexual races’ described in common frogs in the 1930s.

Table S2Microsatellite genoypes at LG2 & LG15 for males and females from six populations.Sweden LG2-15 all pops-PCAInput file for PCAgen v2.0 with all 6 Swedish populations.Sweden LG2-15 Tvedora-PCAInput file for PCAgen v2.0 with the Tvedora population.Sweden LG2-15 PCAArchive file with input files of all 6 populations and a file with the 6 populations pooled together, for PCAgen v2.0Sweden LG2-15 STRUCTUREArchive file with input files of all 6 populations and a file with the 6 populations pooled together, for Structure v2.3.3Sweden LG2-15 FSTATArchive file with input files of all 6 populations, for Fstat v2.9.3.2