Beyond gene flow: (non)-parallelism of secondary contact in a pair of highly differentiated sibling species

Replicated secondary contact zones can provide insights on the barriers to gene flow that are important during speciation and can reveal to which degree secondary contact may result in similar evolutionary outcomes. Here, we studied two secondary contact zones between highly differentiated Alpine butterflies of the genus Erebia using whole-genome re-sequencing data. We assessed the genomic relationships between populations and species and find hybridization to be rare, with no to little current or historical introgression in either contact zone. There are large similarities between the contact zones, consistent with an allopatric origin of interspecific differentiation, with no indications for ongoing reinforcing selection. Consistent with expected reduced effective population size, we further find that scaffolds related to the Z-chromosome show increased differentiation compared to the already high levels across the entire genome, which could also hint towards a contribution of the Z chromosome to species divergence in this system. Finally, we detected the presence of the endosymbiont Wolbachia, which can cause reproductive isolation between its hosts, in all E. cassioides, while it appears to be fully or largely absent in contact zone populations of E. tyndarus. We discuss how this rare pattern may have arisen and how it may have affected the dynamics of speciation upon secondary contact.复现型次生接触带可为物种形成过程中至关重要的基因流障碍提供研究视角，并可揭示次生接触在多大程度上会产生趋同的演化结果。本研究依托全基因组重测序数据，对两个高度分化的阿尔卑斯眼蝶属（Erebia）物种间的次生接触带展开研究。我们对种群与物种间的基因组亲缘关系进行了评估，发现杂交事件较为罕见，且两个接触带均未检测到当前或历史时期的基因渐渗，或仅存在极微量的渐渗。两个接触带间存在高度相似性，符合物种间分化的异域起源假说，且未发现存在持续强化选择的迹象。与预期的有效种群大小降低效应一致，我们进一步发现，相较于全基因组本已较高的分化水平，Z染色体相关的支架序列（scaffold）表现出更高的分化程度，这也暗示Z染色体可能对该系统中的物种分化存在贡献。最后，我们在所有卡西奥眼蝶（E. cassioides）个体中均检测到了内共生菌沃尔巴克氏体（Wolbachia）——该类共生菌可引发宿主间的生殖隔离，而在廷达罗斯眼蝶（E. tyndarus）的次生接触带种群中，该共生菌则完全或近乎完全缺失。我们还讨论了这一罕见分布模式的可能成因，以及其对次生接触过程中物种形成动态的潜在影响。