Data from: Introgression across hybrid zones is not mediated by Large X-effects in green toads with undifferentiated sex chromosomes

Divergence between incipient species remains an incompletely understood process. Hybrid zones provide great research potential, reflecting natural organismal genomic interactions and gene evolution in a variety of recombinants over generations. While sex chromosomes are known evolutionary drivers of reproductive isolation, empirical population genetics mostly examined species with heteromorphic sex chromosomes. We recently reported restricted introgression at sex-linked markers in an amphibian system with homomorphic sex chromosomes (Hyla), consistent with a Large X-effect, designating a greater role of sex chromosomes in driving hybrid incompatibilities. Here, using a similar approach, we examined two hybrid zones of Palearctic green toads (Bufo viridis subgroup), involving several lineages that arose at different times and form secondary contacts. We find no evidence for differential introgression of sex-linked vs. autosomal markers across both zones. This absence of Large X-effects in Bufo indicates that, unlike in Hyla, hybrid incompatibilities may not result from faster-heterogametic sex and faster-male aspects of Haldane’s rule. The recent suppression of XY recombination in Hyla, but not in Bufo, may have driven greater divergence between Hyla sex chromosomes, causing stronger reproductive isolation. Alternatively, stronger linkage among Hyla’s sex-linked markers could restrict introgression. We hypothesize that the degree of sex-specific recombination may condition the importance of homomorphic sex chromosomes in speciation.

初期物种（incipient species）之间的分化仍是一个尚未完全阐明的过程。杂交带（hybrid zones）具备极高的研究价值，可反映自然生物体在多代重组体中的基因组相互作用与基因演化历程。已知性染色体是生殖隔离的进化驱动因子，但现有实证群体遗传学研究大多聚焦于具有异形性染色体（heteromorphic sex chromosomes）的物种。我们此前曾报道，在具有同形性染色体（homomorphic sex chromosomes）的两栖类系统雨蛙属（Hyla）中，性连锁标记的基因渐渗（introgression）受到限制，该结果符合大X效应（Large X-effect），表明性染色体在驱动杂交不相容性中发挥了更为重要的作用。本研究采用类似的研究方法，针对古北界绿蟾蜍（Bufo viridis亚群）的两个杂交带展开分析，这些杂交带涉及多个于不同时期分化并形成次生接触（secondary contacts）的支系。研究结果显示，两个杂交带中均未发现性连锁标记与常染色体标记（autosomal markers）间存在差异渐渗的证据。蟾蜍属（Bufo）中未观测到大X效应，这表明与雨蛙属不同，杂交不相容性可能并非源自霍尔丹法则（Haldane’s rule）中的异配性别快速演化与雄性快速演化特征。雨蛙属中新近出现的XY重组抑制现象，而蟾蜍属中并无此类情况，这或许推动了雨蛙属性染色体间更大程度的分化，进而导致更强的生殖隔离。另一种可能是，雨蛙属的性连锁标记间存在更强的连锁关系，从而限制了基因渐渗。我们据此提出假说：性别特异性重组的程度可能决定了同形性染色体在物种形成过程中的重要性。