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，其背后的原因至今仍存在争议。近期一项研究针对来自芬诺斯坎迪亚地区的欧洲林蛙（Rana temporaria Linnaeus, 1758）的东欧谱系发布了连锁图谱（linkage map），并提出其第2连锁群（linkage group 2, LG2）具有性别决定作用。我们对来自瑞士的低地与高地种群（西欧谱系）的连锁模式展开分析，重点聚焦于第2连锁群。家系分析结果显示，与芬诺斯坎迪亚地区的图谱相比，二者在重组率与基因座排序上存在显著差异，提示存在大规模倒位或易位事件。所有连锁群均表现出极端的异交叉现象（heterochiasmy）：雄性的总图谱长度仅为12.2 cM，而雌性则达到869.8 cM。种群内的性别决定方式存在多态性：多数家系（性别比例均衡）的子代表型性别与第2连锁群的父本单倍型存在强相关性；而其余家系（部分家系的子代性别比例偏雌性化）未表现出任何相关性，此类家系中的性别决定因子尚未被探明。这类多种性别决定系统共存的现象，即便在雄性重组缺失的情况下，也可能引发X与Y单倍型的频繁重组。据此，我们在野生捕获的雌雄成体中未发现第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单倍型提示其性别完全由第2连锁群决定。多因子性别决定机制无疑是导致两栖动物性染色体未发生分化的重要原因。