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科学问题。杂交带（hybrid zones）具备极高的研究潜力，可反映多代重组个体中自然生物的基因组互作模式与基因演化历程。尽管性染色体已被证实是生殖隔离的重要进化驱动力，但当前的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，在一类具有同形性染色体（homomorphic sex chromosomes）的两栖动物类群——雨蛙属（Hyla）中，性连锁标记存在受限的基因渐渗（introgression）现象，该结果符合大X效应（Large X-effect）的特征，表明性染色体在驱动杂交不相容性中发挥了更为关键的作用。
本研究采用相似的研究方法，对隶属于绿蟾蜍属（Bufo）绿蟾蜍亚群的古北界绿蟾蜍的两个杂交带展开了分析，该类群包含多个于不同时期分化并形成次级接触带的演化支系。研究结果显示，在两个杂交带中均未发现性连锁标记相较于常染色体标记（autosomal markers）存在差异化渐渗的证据。这种在绿蟾蜍属中未观测到大X效应的现象表明，与雨蛙属不同，杂交不相容性可能并非源于霍尔丹法则（Haldane’s rule）中的异型配子体更快与雄性更快演化特征。
雨蛙属中新近出现的XY重组抑制现象（而绿蟾蜍属未出现该情况），可能加剧了雨蛙属性染色体之间的分化程度，进而导致更强的生殖隔离。另一种可能的解释是，雨蛙属性连锁标记之间更强的连锁关系可能限制了基因渐渗。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。