Genomic Introgression in the Hybrid zones at the Margins of the Species' Range Between Ecologically Distinct Species.

Populations in extreme environments at the margins of a species' range are often the most vulnerable to climate change, but they may also experience novel evolutionary processes, such as secondary contact and hybridization with their relatives. The range overlap resulting from secondary contact with related species that have adapted to different climatic zones may act as corridors for adaptive introgression. To test this hypothesis, we examined the hybrid zones along the altitude of two closely related species, one temperate and the other subtropical species, at their southern and northern limits on Yakushima Island, Japan. Genomic cline analysis revealed non-neutral introgression throughout the genome in both directions in the two species. Some of these genomic regions involve gene ontology terms related to the regulation of several biological processes. Our niche modeling suggests that, assuming niche conservatism, the temperate species are likely to lose their suitable habitat, and the backcrossed hybrids with the subtropical species are already expanding upslope on the island. Adaptive introgression through the hybrid zone may contribute to the persistence and expansion of the species in the southernmost and northernmost populations.