Geographical origins of North American Rhodiola (Crassulaceae) and phylogeography of the western roseroot, Rhodiola integrifolia
Complex migration histories with repeated range shifts during the Pleistocene characterize many arctic–alpine plants. Identifying these patterns provides insight into the causes of current distributions and possible responses to climate warming. We investigated patterns of genetic variation in North American species of a widespread Northern Hemisphere plant group to test different hypotheses of origin and refugial persistence.
We used a phylogeographical approach to investigate the geographical origins of North American Rhodiola, especially the widespread western species R. integrifolia. Populations were sampled over much of the North American range (66 of R. integrifolia, 6 of R. rhodantha and 4 of R. rosea). We performed maximum likelihood phylogenetic analyses on sequences of the nuclear internal transcribed spacer (ITS) region and the plastid trnH–psbA intergenic spacer, and analysed geographical patterns of haplotype distribution and genetic diversity using plastid restriction-site and sequence data from all populations.
Separate lineages of Rhodiola dispersed into North America via the Bering Strait (R. integrifolia and R. rhodantha) and amphi-Atlantic regions (R. rosea). Genetic patterns within R. integrifolia indicate southward spread from Beringia, with subsequent persistence in both northern and southern refugia. Rhodiola integrifolia and the regional endemic R. rhodantha show evidence of past hybridization (resulting in chloroplast capture) where their ranges overlap in the southern Rocky Mountains. However, phylogenetic evidence suggests that both species are closely related to Asian taxa and probably migrated independently into North America.
The current geographical distributions and genetic structure of North American Rhodiola are the result of a complex history including multiple migrations into North America, persistence in multiple refugia during glaciations, and past hybridization. This complexity of processes within one group underscores the diverse history of arctic–alpine plants and the likelihood of divergent responses to changing environments.