Lineage admixture during postglacial range expansion is responsible for the increased gene diversity of Kalopanax septemlobus in a recently colonised territory.

We aimed to reveal the effects of range expansion and subsequent lineage admixture from separated glacial refugia on genetic diversity of Kalopanax septemlobus in Japan, by combining nuclear microsatellite data and ecological niche modelling. Allelic richness and gene diversity were compared at the population and regional level. We also statistically examined these indices as a function of population accessibility to the last glacial maximum (LGM) palaeodistribution reconstructed by ecological niche modelling to test a simple range expansion scenario from glacial refugia. Genetic diversity was highest in the populations of southern Japan and gradually decreased towards the north. However, an additional centre of genetic diversity, when measured as gene diversity, was found in northern Honshu Island, where distinct lineages were shown to be in contact. Positive effects of population accessibility to the LGM range were detected in both diversity indices at different spatial scales. The combined data support independent postglacial range expansions towards the north from the edge populations on the exposed coastal shelf of Pacific and Sea of Japan in northern Honshu during the LGM, which subsequently resulted in markedly low genetic diversity in the northernmost extant range, Hokkaido. The regional increase in gene diversity in northern Honshu is likely to be the result of postglacial lineage admixture. Relative difference in the spatial scales best relating population genetic diversity with the LGM distribution can be explained by a higher rate of allelic richness diversity loss during range expansions and stronger effects of lineage admixture on gene diversity.