Department of Ecology and Evolution, University of Lausanne, Lausanne, Switzerland.
J Evol Biol. 2020 Apr;33(4):401-409. doi: 10.1111/jeb.13573. Epub 2019 Dec 20.
The canonical model of sex-chromosome evolution assigns a key role to sexually antagonistic (SA) genes on the arrest of recombination and ensuing degeneration of Y chromosomes. This assumption cannot be tested in organisms with highly differentiated sex chromosomes, such as mammals or birds, owing to the lack of polymorphism. Fixation of SA alleles, furthermore, might be the consequence rather than the cause of recombination arrest. Here we focus on a population of common frogs (Rana temporaria) where XY males with genetically differentiated Y chromosomes (nonrecombinant Y haplotypes) coexist with both XY° males with proto-Y chromosomes (only differentiated from X chromosomes in the immediate vicinity of the candidate sex-determining locus Dmrt1) and XX males with undifferentiated sex chromosomes (genetically identical to XX females). Our study finds no effect of sex-chromosome differentiation on male phenotype, mating success or fathering success. Our conclusions rejoin genomic studies that found no differences in gene expression between XY, XY° and XX males. Sexual dimorphism in common frogs might result more from the differential expression of autosomal genes than from sex-linked SA genes. Among-male variance in sex-chromosome differentiation seems better explained by a polymorphism in the penetrance of alleles at the sex locus, resulting in variable levels of sex reversal (and thus of X-Y recombination in XY females), independent of sex-linked SA genes.
性染色体进化的规范模型赋予了性拮抗（SA）基因在阻止重组和随后的 Y 染色体退化方面的关键作用。由于缺乏多态性，这种假设在具有高度分化性染色体的生物中，如哺乳动物或鸟类，无法得到验证。此外，SA 等位基因的固定可能是重组停止的结果，而不是原因。在这里，我们关注的是一个普通青蛙（Rana temporaria）种群，其中具有遗传分化的 Y 染色体（非重组 Y 单倍型）的 XY 雄性与具有原始 Y 染色体的 XY°雄性（仅在候选性别决定基因 Dmrt1 附近与 X 染色体分化）和具有未分化性染色体的 XX 雄性（在遗传上与 XX 雌性相同）共存。我们的研究没有发现性染色体分化对雄性表型、交配成功率或父系成功率有影响。我们的结论与基因组研究一致，即没有发现 XY、XY°和 XX 雄性之间的基因表达差异。普通青蛙的性别二态性可能更多地源于常染色体基因的差异表达，而不是性连锁的 SA 基因。雄性之间性染色体分化的差异似乎可以更好地用性位点等位基因的外显率多态性来解释，导致性别反转（因此 XY 雌性中的 X-Y 重组）的水平可变，而与性连锁的 SA 基因无关。
