Niche expansion of polyploid cytotypes shaped the phylogeographical history of the Salix retusa complex in the European Alpine System.

Background and aims: Alpine plants exhibit diverse postglacial recolonization patterns following the last glacial periods. Polyploidization may have impacted these dynamics by introducing ecological and physiological novelties that facilitate adaptation to changing environments. However, consistent trends in the recolonization, niche optima and dynamics of polyploids and their related diploids remain elusive. In this study, we investigate the biogeographical history of the Salix retusa polyploid complex in the European Alpine System. By comparing genetic patterns and their climatic and edaphic niche optima, we explore how polyploidization shaped species' geographical distributions by influencing their ecological adaptation.

Methods: RAD sequencing was used to reconstruct the biogeographical history and genetic structure of two related willow species. High-resolution edaphic and climatic data were used to compare the niche optima, breadth and dynamics between the species.

Key results: The distribution of the polyploid species overlaps with more peripheral refugial areas, which correlates with its broader geographical range in the European Alpine System. However, genetic analyses suggest more potential peripheral glacial refugia within the Alps for the diploid. Our findings indicate niche conservatism within the S. retusa complex, with the polyploid species having a broader niche but the diploid being adapted to a more extreme niche.

Conclusions: In our study species, polyploidy is associated with a higher genetic diversity and geographical structure, which might be due to a broader ecological niche and distribution. However, it did not appear to facilitate adaptation or confer a survival advantage during the last glaciation.