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.

与鸟类和哺乳动物形成鲜明对比的是，变温脊椎动物（ectothermic vertebrates）的性染色体通常未发生分化，其背后的原因仍存在争议。近期一项研究发布了来自芬诺斯坎迪亚（Fennoscandia）的东欧谱系林蛙（Rana temporaria, Linnaeus, 1758）的连锁图谱，并提出其第2连锁群（linkage group 2, LG2）可能参与性别决定。本研究对来自瑞士（西欧谱系）的低地和高地种群的连锁模式进行了分析，重点关注第2连锁群。同胞家系分析结果显示，该图谱与芬诺斯坎迪亚种群的连锁图谱在重组率和位点排列上存在显著差异，提示存在大规模倒位或易位事件。所有连锁群均表现出极强的异交换现象（雄性总图谱长度为12.2 cM，雌性则为869.8 cM）。种群内的性别决定系统存在多态性：多数家系（性比均衡）的子代表型性别与第2连锁群的父本单体型存在显著关联，而其余家系（部分家系的性比偏向雌性）则未表现出任何关联。无法确定剩余家系的性别决定因素。多种性别决定系统共存的情况，即便在雄性重组缺失的条件下，也会促使X与Y单体型发生频繁重组。据此，我们在野生捕获的成年雌雄个体中未发现第2连锁群标记的等位基因频率存在性别差异，但在一个高海拔种群中除外；该种群的非重组Y单体型表明，其性别完全由第2连锁群决定。多因子性别决定无疑是两栖动物性染色体未发生分化的重要原因。