Sex-chromosome differentiation and 'sex races' in the common frog (Rana temporaria).

• Published in: Proceedings of the Royal Society B: Biological Sciences
• Main Authors: Rodrigues, N., Vuille, Y., Loman, J., Perrin, N.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2015
• Subjects: Animals; Biological Evolution; Female; Genetic Linkage; Genetic Markers; Haplotypes; Male; Rana temporaria/genetics; Recombination; Genetic; Sex Characteristics; Sex Chromosomes/genetics; Sex Ratio; Sweden; Fennoscandia
• Online Access: https://serval.unil.ch/notice/serval:BIB_E0C4B51B25ED; https://doi.org/10.1098/rspb.2014.2726

Sex-chromosome differentiation was recently shown to vary among common frog populations in Fennoscandia, suggesting a trend of increased differentiation with latitude. By rearing families from two contrasted populations (respectively, from northern and southern Sweden), we show this disparity to stem from differences in sex-determination mechanisms rather than in XY-recombination patterns. Offspring from the northern population display equal sex ratios at metamorphosis, with phenotypic sexes that correlate strongly with paternal LG2 haplotypes (the sex chromosome); accordingly, Y haplotypes are markedly differentiated, with male-specific alleles and depressed diversity testifying to their smaller effective population size. In the southern population, by contrast, a majority of juveniles present ovaries at metamorphosis; only later in development do sex ratios return to equilibrium. Even at these later stages, phenotypic sexes correlate only mildly with paternal LG2 haplotypes; accordingly, there are no recognizable Y haplotypes. These distinct patterns of gonadal development fit the concept of 'sex races' proposed in the 1930s, with our two populations assigned to the 'differentiated' and 'semi-differentiated' races, respectively. Our results support the suggestion that 'sex races' differ in the genetic versus epigenetic components of sex determination. Analysing populations from the 'undifferentiated race' with high-density genetic maps should help to further test this hypothesis.