Introgressive hybridization fuels local adaptation in oakleaf butterflies
Hybridization may have multiple impacts on speciation and adaptation. Yet the evidence of adaptive introgression events that have historically occurred between nonradiating or established species is still lacking. The oakleaf butterflies in the genus Kallima include multiple sympatric and incipient species with a known history of diversification and therefore serve as a useful system for addressing historical introgression and adaptation. By integrating population genomics, phylogenomics, CRISPR/Cas9 genome editing and de novo genome assembly, we disentangled the genome-wide pattern of admixture among multiple Kallima species and used the putative introgressed gene pale as an example to reveal its role in scale pigmentation, structure and potentially in sexual dimorphism. We further demonstrated the complex evolutionary history of the Z chromosome largely owing to historical introgression. Our study revealed genome-wide introgression that has historically occurred during butterfly dispersal and differentiation, facilitating the understanding of interspecific relationships and potential local adaptation during gradual evolution.