Abstract

Pleistocene climate oscillations influenced the biogeographical history of most species. In the European Alps, mountain plants were restricted to refugial areas during cold phases of glacial cycles and recolonized newly available habitats during warm periods. The current ranges of alpine plants represent a transient stage of a continuous and dynamic recolonization process that started after the last glaciations. Differences in recolonization rate and range filling are observed between different mountain plant species, but the reasons remain insufficiently explored. Here, we investigated the effects of secondary contact hybridization on range expansion between two related willow species pairs that came into secondary contact. RAD sequencing data was used to identify potential refugial areas and characterize the secondary contact zones. Leaf phenotypes were measured using morphometrics. Distribution modeling was used to find current suitable habitats. Results suggests that peripheral glacial refugia played a major role in the history of the species. For both species' pairs, the secondary contact zones showed homoploid hybridization between parents, which is also supported by the morphometric analyses. The hybrid zones are broader than expected and characterized by introgression. Current projections of species distribution identified suitable habitats beyond the secondary contact zone. We suggest that the parents' range expansion is blocked by the hybrid zones. Indeed, due to the high genetic similarity, each dispersal beyond the secondary contact zone results in the long term in hybridization and introgression with the already established parent. Therefore, hybridization acts here as a barrier to further recolonization.