Direct long-distance dispersal best explains the bipolar distribution of Carex arctogena (Carex sect. Capituligerae, Cyperaceae)

The bipolar disjunction, a biogeographical pattern defined by taxa with a distribution at very high latitudes in both hemispheres (> 55° N; > 52° S), is only known to occur in about 30 vascular plant species. Our aim was to use the bipolar species Carex arctogena to test the four classic hypotheses proposed to explain this exceptional disjunction: convergent evolution, vicariance, mountain-hopping and direct long-distance dispersal. Location: Arctic/boreal and temperate latitudes of both hemispheres. Methods: A combination of molecular and bioclimatic data was used to test phylogeographical hypotheses in C. arctogena. Three chloroplast markers (atpF-atpH, matK and rps16) and the nuclear ITS region were sequenced for all species in Carex sections Capituligerae and Longespicatae; Carex rupestris, C. obtusata and Uncinia triquetra were used as outrgroups. Phylogenetic relationships, divergence-time estimates and biogeographical patterns were inferred using maximum likelihood, statistical parsimony and Bayesian inference. Results: Carex sections Capituligerae and Longespicatae formed a monophyletic group that diverged during the late Miocene. Two main lineages of C. arctogena were inferred. Southern Hemisphere populations of C. arctogena shared the same haplotype as a widespread circumboreal lineage. Bioclimatic data show that Southern and Northern Hemisphere populations currently differ in their ecological regimes. Main conclusions: Two of the four hypotheses accounting for bipolar disjunctions may be rejected. Our results suggest that direct long-distance dispersal, probably southwards and mediated by birds, best explains the bipolar distribution of C. arctogena.Peer Reviewe