Data from: Within-population polymorphism of sex-determination systems in the common frog (Rana temporaria)

In sharp contrast with birds and mammals, the sex chromosomes of
ectothermic vertebrates are often undifferentiated, for reasons that
remain debated. A linkage map was recently published for Rana temporaria
(Linnaeus, 1758) from Fennoscandia (Eastern European lineage), with a
proposed sex-determining role for linkage group 2 (LG2). We analysed
linkage patterns in lowland and highland populations from Switzerland
(Western European lineage), with special focus on LG2. Sibship analyses
showed large differences from the Fennoscandian map in terms of
recombination rates and loci order, pointing to large-scale inversions or
translocations. All linkage groups displayed extreme heterochiasmy (total
map length was 12.2 cM in males, versus 869.8 cM in females). Sex
determination was polymorphic within populations: a majority of families
(with equal sex ratios) showed a strong correlation between offspring
phenotypic sex and LG2 paternal haplotypes, whereas other families (some
of which with female-biased sex ratios) did not show any correlation. The
factors determining sex in the latter could not be identified. This
coexistence of several sex-determination systems should induce frequent
recombination of X and Y haplotypes, even in the absence of male
recombination. Accordingly, we found no sex differences in allelic
frequencies on LG2 markers among wild-caught male and female adults,
except in one high-altitude population, where nonrecombinant Y haplotypes
suggest sex to be entirely determined by LG2. Multifactorial sex
determination certainly contributes to the lack of sex-chromosome
differentiation in amphibians.