Post-glacial East Asian origin of the alpine shrub Phyllodoce aleutica (Ericaceae) in Beringia

Aim: The ice-free area around the Bering Land Bridge, Beringia, has been noted as an important refugium of organisms preferring cold environments such as arctic-alpine plants throughout the Pleistocene climate oscillations. Although numerous phylogeographical studies have supported this refugium, recent studies have challenged the idea of a homogeneous refugium in Beringia. We aim to examine a novel scenario of the post-glacial colonization history of an alpine plant Phyllodoce aleutica in Beringia. Location: The Japanese Archipelago, the Kamchatka Peninsula, the Aleutian Islands, Alaska. Method: The range-wide genetic structure of P. aleutica was elucidated by sequencing 13 nuclear loci. The phylogeographical history was inferred using model-based approaches based on coalescent simulations together with the potential distributions predicted by ecological niche modelling. Results: Bayesian clustering and phylogenetic networks revealed that P. aleutica was divided into three geographically structured groups: the main island of the Japanese Archipelago, the northern island of the archipelago (Hokkaido) and Beringia, including eastern Hokkaido. The demographic history underlying the genetic structure and the potential distributions at present and during Last Glacial Maximum suggest that their divergence likely predated the last glacial period. Except for genetic admixture with the sister species Phyllodoce glanduliflora, populations in Alaska exhibited lower genetic diversity than those in East Asia and exclusively shared two widespread genotypes. Approximate Bayesian computation showed that a demographic model postulating post-glacial expansion into Alaska fit better than alternative models. Main conclusions: Our study suggests that P. aleutica in eastern Beringia originated through post-glacial colonization from East Asia, providing novel insight into the biogeographical history of alpine flora in Beringia.