Repeatedly out of Beringia: Cassiope tetragona embraces the Arctic

Aim Eric Hultén hypothesized that most arctic plants initially radiated from Beringia in the Late Tertiary and persisted in this unglaciated area during the Pleistocene glaciations, while their distribution ranges were repeatedly fragmented and reformed elsewhere. Whereas taxonomic and fossil evidence suggest that Cassiope tetragona originated in Beringia and expanded into the circumarctic area before the onset of the glaciations, lack of chloroplast DNA (cpDNA) variation may suggest that colonization was more recent. We address these contradictory scenarios using high‐resolution nuclear markers. Location Circumpolar Arctic. Methods The main analysis was by amplified fragment‐length polymorphism (AFLP), while sequences of chloroplast DNA verified the use of Cassiope mertensiana as an outgroup for C. tetragona. Data were analysed using Bayesian clustering, principal coordinates analyses, parsimony and neighbour‐joining, and measures of diversity and differentiation were calculated. Results The circumpolar C. tetragona ssp. tetragona was well separated from the North American C. tetragona ssp. saximontana. The genetic structure in ssp. tetragona showed a strong east–west trend, with the Beringian populations in an intermediate position. The highest level of diversity was in Beringia, while the strongest differentiation in the data set was found between the populations from the Siberian Arctic west of Beringia and the remainder. Main conclusions The results are consistent with a Beringian origin of the species, but the levels and geographical patterns of differentiation and gene diversity suggest that the latest expansion from Beringia into the circumarctic was recent, possibly during the current interglacial. The results are in accordance with a recent leading‐edge mode of colonization, particularly towards the east throughout Canada/Greenland and across the North Atlantic into Scandinavia and Svalbard. As fossils demonstrate the presence of the species in North Greenland 2.5–2.0 Ma, as well as in the previous interglacial, we conclude that C. tetragona expanded eastwards from Beringia several times and that the earlier emigrants of this woody species became extinct. The last major westward expansion from Beringia seems older, and the data suggest a separate Siberian refugium during at least one glaciation.