Dufresnes Christophe, Stöck Matthias, Brelsford Alan, Perrin Nicolas
Department of Ecology and evolution, Biophore Building, University of Lausanne, Lausanne, Switzerland.
Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany.
PLoS One. 2014 Jun 3;9(6):e97959. doi: 10.1371/journal.pone.0097959. eCollection 2014.
In contrast with mammals and birds, most poikilothermic vertebrates feature structurally undifferentiated sex chromosomes, which may result either from frequent turnovers, or from occasional events of XY recombination. The latter mechanism was recently suggested to be responsible for sex-chromosome homomorphy in European tree frogs (Hyla arborea). However, no single case of male recombination has been identified in large-scale laboratory crosses, and populations from NW Europe consistently display sex-specific allelic frequencies with male-diagnostic alleles, suggesting the absence of recombination in their recent history. To address this apparent paradox, we extended the phylogeographic scope of investigations, by analyzing the sequences of three sex-linked markers throughout the whole species distribution. Refugial populations (southern Balkans and Adriatic coast) show a mix of X and Y alleles in haplotypic networks, and no more within-individual pairwise nucleotide differences in males than in females, testifying to recurrent XY recombination. In contrast, populations of NW Europe, which originated from a recent postglacial expansion, show a clear pattern of XY differentiation; the X and Y gametologs of the sex-linked gene Med15 present different alleles, likely fixed by drift on the front wave of expansions, and kept differentiated since. Our results support the view that sex-chromosome homomorphy in H. arborea is maintained by occasional or historical events of recombination; whether the frequency of these events indeed differs between populations remains to be clarified.
与哺乳动物和鸟类不同，大多数变温脊椎动物的性染色体在结构上没有分化，这可能是由于频繁的更替，或者是由于XY重组的偶然事件。最近有人提出，后一种机制是欧洲树蛙（雨蛙）性染色体同形的原因。然而，在大规模的实验室杂交中尚未发现一例雄性重组的情况，西北欧的种群一直表现出具有雄性诊断等位基因的性别特异性等位基因频率，这表明在它们最近的历史中不存在重组。为了解决这一明显的矛盾，我们通过分析整个物种分布范围内三个性连锁标记的序列，扩大了系统地理学的研究范围。避难种群（巴尔干半岛南部和亚得里亚海沿岸）在单倍型网络中显示出X和Y等位基因的混合，并且雄性个体内的成对核苷酸差异并不比雌性多，这证明了XY重组的反复发生。相比之下，起源于最近一次冰期后扩张的西北欧种群则表现出明显的XY分化模式；性连锁基因Med15的X和Y配子同源基因呈现不同的等位基因，可能是在扩张的前沿波上通过漂变固定下来的，并且此后一直保持分化。我们的结果支持这样一种观点，即雨蛙的性染色体同形是由偶然的或历史上的重组事件维持的；这些事件的频率在不同种群之间是否确实存在差异还有待阐明。
