Department of Ecology and Evolution, University of Lausanne, Lausanne, Switzerland.
J Evol Biol. 2013 Jul;26(7):1569-77. doi: 10.1111/jeb.12163. Epub 2013 May 25.
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，变温脊椎动物的性染色体通常没有分化，其原因仍存在争议。最近公布了来自芬诺斯堪的亚（东欧血统）的林蛙（Rana temporaria (Linnaeus, 1758）的连锁图谱，该图谱提出了连锁群 2（LG2）的性别决定作用。我们分析了来自瑞士（西欧血统）的低地和高地种群的连锁模式，特别关注 LG2。同系分析显示，在重组率和基因座顺序方面与芬诺斯堪的亚图谱存在很大差异，这表明存在大规模的倒位或易位。所有连锁群都表现出极端的异配性（雄性的总图谱长度为 12.2cM，而雌性为 869.8cM）。种群内的性别决定是多态的：大多数家族（性别比例相等）显示出后代表型性别与 LG2 父系单倍型之间的强烈相关性，而其他家族（其中一些表现出雌性偏性性别比例）则没有显示出任何相关性。无法确定后者中决定性别的因素。这种多种性别决定系统的共存应该会导致 X 和 Y 单倍型的频繁重组，即使在没有雄性重组的情况下也是如此。因此，除了一个高海拔种群外，我们在野生捕获的雄性和雌性成年个体的 LG2 标记中没有发现等位基因频率的性别差异，在该种群中，非重组的 Y 单倍型表明性别完全由 LG2 决定。多因素性别决定无疑导致了两栖动物性染色体分化的缺乏。
