Zones of secondary contact between closely related taxa are a common legacy of the Quaternary ice ages. Despite their abundance, the factors that keep species apart and prevent hybridization are often unknown. Here, we study a very narrow contact zone between three closely related butterfly species of the Erebia tyndarus species complex. Using genomic data, we first determined whether gene flow occurs and then assessed whether it might be hampered by differences in chromosome number between some species. We found interspecific gene flow between sibling species that differ in karyotype by one chromosome. Conversely, only F1 hybrids occurred between two species that have the same karyotype, forming a steep genomic cline. In a second step, we fitted clines to phenotypic, ecological and parasitic data to identify the factors associated with the genetic cline. We found clines for phenotypic data and the prevalence of the endosymbiont parasite Wolbachia to overlap with the genetic cline, suggesting that they might be drivers for separating the two species. Overall, our results highlight that some gene flow is possible between closely related species despite different chromosome numbers, but that other barriers restrict such gene flow.

中文翻译：

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狭窄的表型和寄生菌群形成密切相关的埃雷比亚蝴蝶的次级接触区

密切相关的分类群之间的二次接触区是第四纪冰河时代的共同遗产。尽管它们丰富，但使物种分开并阻止杂交的因素通常是未知的。在这里，我们研究了 Erebia tyndarus 物种复合体的三个密切相关的蝴蝶物种之间非常狭窄的接触区。使用基因组数据，我们首先确定是否发生基因流动，然后评估它是否可能受到某些物种之间染色体数量差异的阻碍。我们发现了染色体核型不同的同胞物种之间的种间基因流。相反，只有 F1 杂种发生在具有相同核型的两个物种之间，形成陡峭的基因组克隆。第二步，我们将基因拟合到表型，生态和寄生数据，以确定与遗传基因相关的因素。我们发现表型数据的基因和内共生寄生虫沃尔巴克氏体的流行与遗传基因重叠，表明它们可能是分离这两个物种的驱动因素。总的来说，我们的结果强调，尽管染色体数量不同，但密切相关的物种之间可能存在一些基因流动，但其他障碍限制了这种基因流动。