Trans-Atlantic dispersal and large-scale lack of genetic structure in the circumpolar, arctic-alpine sedge Carex bigelowii s. l. (Cyperaceae).

Paradoxically, several of the ecologically most important plant groups in the Arctic are little understood in terms of taxonomy and biogeographic history. The circumpolar Carex bigelowii s. l. (Cyperaceae) is abundant in the Arctic and is one of the most complicated arctic plant groups. While its ecology and population genetics have been extensively studied, its taxonomy is largely unexplored. We analyzed the large-scale geographical structuring of amplified fragment length polymorphisms (AFLPs) covering most of the distribution range. We detected high levels of genetic variation, most (66%) within populations, and a fairly weak genetic structure. Only the Central Asian populations, referred to as C. orbicularis, were strongly divergent. For the remaining populations, Bayesian clustering separated three distinct clusters (one European, one amphi-Atlantic, and one broadly amphi-Beringian), probably reflecting different major glacial refugia and recent transoceanic dispersal. The isolated central European populations were most closely related to those from a larger distribution area in northern Europe. Differences in genetic diversity suggest that the Alpine and Tatra populations have experienced strong bottlenecks, whereas the Krkonoše population may have been part of a continuous distribution area during the cold stages of the Pleistocene. Finally, we discuss the relevance of our results for a uniform, range-wide taxonomic concept.