Phylogeographical insight into the Aleutian flora inferred from the historical range shifts of the alpine shrub Therorhodion camtschaticum (Pall.) Small (Ericaceae)

Aim: The Aleutian Islands, one of the southern margins of Beringia, were covered with ice caps during the Last Glacial Maximum (LGM). Although insular refugia for animal species have been hypothesized in this region, phylogeographical studies have never explored the biogeographical history of plant species. We aim to assess the population history of the flora of the Aleutian Islands and its biogeographical relevance to East Asia. Location: The Aleutian Islands, Kamchatka, the Japanese Archipelago. Methods: We sequenced seven nuclear loci from the alpine shrub Therorhodion camtschaticum (Pall.) Small (Ericaceae) and elucidated the geographical distribution of genetic variation. The divergence history among populations from the Aleutian Islands, Kamchatka and northern Japan was inferred by analyses based on coalescent simulations. Results: Populations were genetically distinct between northern Japan and Beringia. In spite of the genetic differentiation, the divergence between Beringia and northern Japan occurred during or after the last glacial period [maximum-likelihood estimates (MLEs) = 10.6–15.5 ka with 95% highest posterior densities (HPDs): 1.7–43.5 ka]. The latter region has a larger population size than the former region. In contrast, widespread genetic similarity was revealed among the geographically separated populations in the Aleutian Islands and southern Kamchatka. Among these populations, a model assuming population decline fits better than a model that assumes either constant population size or population growth. Main conclusions: Our study suggests that insular refugia were unlikely for T. camtschaticum in the Aleutian Islands. Instead, T. camtschaticum may have had a wider range during LGM, encompassing both the Aleutian Islands and southern Kamchatka, or have colonized the Aleutian Islands from a LGM refugium that was the source of colonization into southern Kamchatka as well or encompassed the latter region. Our study provides novel insight into the biogeographical history of terrestrial biota in the Aleutian Islands.