Identifying the mechanisms involved in the formation and maintenance of species is a central question in evolutionary biology, and distinguishing the selective drivers of populations’ divergence from demographic processes is of particular interest to better understand the speciation process. Hybrid zones are recognized to provide ideal places to investigate the genetic architecture of speciation and to identify the mechanisms allowing diverging species to maintain their integrity in the face of gene flow. Here, we studied two alpine butterfly species, Coenonympha macromma and C. gardetta, which can be found flying together and hybridizing in narrow contact zones in the southern French Alps. We characterized the genomic composition of individuals, their morphology and their local habitat requirements, within and around a hybrid zone. Genetic diversity analysis at 794 SNPs revealed that all individuals within the hybrid zone were highly admixed, which was not the case outside the hybrid zone. Cline analysis showed that, despite ongoing hybridization, 56 out of 122 loci differentially fixed or nearly so between the two species were impermeable to introgression across the sharp hybrid zone (9 km wide). We also found concordance in cline position and width among genetic, morphological and environmental variation, suggesting a coupling of different reproductive barriers. Habitat characteristics such as the presence of trees and shrubs and the start of the growing season were strongly associated with the genetic variation, and we found evidence of divergence at genetic markers associated with morphology and physiology, putatively involved in visual or environmental reproductive isolation. We discuss the various behavioural and ecological factors that might interplay to maintain current levels of divergence and gene flow between this species pair.

中文翻译：

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两种高山蝶类之间的尖锐杂种区的遗传，形态和生态变异。

确定物种形成和维持所涉及的机制是进化生物学的中心问题，特别是要更好地理解物种形成过程，将种群差异与人口过程的选择性驱动因素区分开来是特别有意义的。杂种区被认为是调查物种形成的遗传结构和确定机制的理想场所，以使物种在面对基因流的情况下保持其完整性。在这里，我们研究了两种高山蝶类，Coenonympha macromma和C. gardetta，它们可以一起飞翔并在法国南部阿尔卑斯山的狭窄接触区内杂交。我们对杂种区内和周围个体的基因组组成，形态和当地栖息地需求进行了表征。794个SNP的遗传多样性分析表明，杂种区域内的所有个体高度混合，杂种区域外并非如此。克莱因分析显示，尽管正在进行杂交，但在两个物种之间122个差异固定或接近固定的基因座中，有56个无法渗透到整个尖锐的杂交区（9公里宽）。我们还发现遗传，形态和环境变异之间的株系位置和宽度一致，表明了不同生殖障碍的耦合。生境特征（例如树木和灌木的存在以及生长期的开始）与遗传变异密切相关，我们发现证据表明与形态和生理相关的遗传标记存在差异，推测涉及视觉或环境生殖隔离。我们讨论了可能相互作用的各种行为和生态因素，以维持当前物种对之间的差异和基因流动水平。