The growing interest in the lability of sex determination in non‐model vertebrates such as amphibians and fishes has revealed high rates of sex chromosome turnovers among closely related species of the same clade. Can such lineages hybridize and admix with different sex‐determining systems, or could the changes have precipitated their speciation? We addressed these questions in incipient species of toads (Bufonidae), where we identified a heterogametic transition and characterized their hybrid zone with genome‐wide markers (RADseq). Adult and sibship data confirmed that the common toad B. bufo is female heterogametic (ZW), while its sister species the spined toad B. spinosus is male heterogametic (XY). Analysis of a fine scale transect across their parapatric ranges in southeastern France unveiled a narrow tension zone (∼10 km), with asymmetric mitochondrial and nuclear admixture over hundreds of kilometers southward and northward, respectively. The geographic extent of introgression is consistent with an expansion of B. spinosus across B. bufo’s former ranges in Mediterranean France, as also suggested by species distribution models. However, widespread cyto‐nuclear discordances (B. spinosus backrosses carrying B. bufo mtDNA) run against predictions from the dominance effects of Haldane's rule, perhaps because Y and W heterogametologs are not degenerated. Common and spined toads can thus successfully cross‐breed despite fundamental differences in their sex determination mechanisms, but remain partially separated by reproductive barriers. Whether and how the interactions of their XY and ZW genes contribute to these barriers shall provide novel insights on the debated role of labile sex chromosomes in speciation.

中文翻译：

---

具有不同性染色体系统的蟾蜍之间的杂交和渗入

在两栖类和鱼类等非模式脊椎动物中，人们对性别决定的不确定性越来越感兴趣，这表明同一进化枝的密切相关物种之间的性染色体更新率很高。这样的血统是否可以与不同的性别决定系统杂交和混合，或者这些变化是否会加剧它们的物种形成？我们在蟾蜍的初生物种（Bufonidae）中解决了这些问题，我们在其中鉴定了异配过渡，并用全基因组标记（RADseq）表征了它们的杂种区。成人及亲缘关系的数据证实，蟾蜍B.蟾蜍是雌性异形（ZW），而它的姊妹种脊蟾蜍B.刺是男性异性恋（XY）。对法国东南部沿其父系分布的一个细尺度样条进行的分析揭示了一个狭窄的张力带（约10 km），线形和核掺合物的不对称线形分别位于南北数百公里。物种分布模型也表明，渗入的地理范围与刺孢小球藻在整个法国地中海地区的蟾蜍的先前范围的扩展一致。但是，广泛的细胞核不一致（携带B. bufo的B. spinosus倒影mtDNA）与Haldane规则的优势效应所产生的预测背道而驰，这可能是因为Y和W异源配体没有退化。尽管它们的性别决定机制存在根本差异，但普通蟾蜍和纺蟾蜍仍可成功杂交，但仍被生殖障碍部分隔离。它们的XY和ZW基因的相互作用是否以及如何促进这些障碍，将为不稳定的性染色体在物种形成中的争议作用提供新颖的见解。