| Summary: | The roseroot genus Rhodiola is widely distributed in arctic and alpine areas of the Northern Hemisphere. It is most speciose in the high mountain ranges of central Asia. Rhodiola integrifolia occurs at high altitudes and high latitudes in western North America and northeastern Asia. During the Pleistocene glaciations the region between Asia and North America known as Beringia was ice free and acted as a glacial refugium for coldadapted taxa. I surveyed variation in a nuclear (ITS) and chloroplast (psbA-trnH spacer) DNA region in R. integrifolia and its North American relatives, R. rosea and R. rhodantha. Phylogenetic analyses based on ITS showed that (i) the western North American species R. integrifolia and R. rhodantha are distinct but closely related sister taxa; and (ii) these two species and the eastern North American R. rosea belong to separate clades within Rhodiola. Analyses of the plastid region showed that although the sister species R. integrifolia and R. rhodantha are distinct, some populations sampled in the southern Rocky Mountains (where the two species overlap) share psbA-trnH haplotypes, suggesting that they hybridized at some time in the past. Within R. integrifolia, both nuclear and plastid DNA regions showed strong north-south patterns of differentiation, a pattern consistent with western North America’s glacial history. Restriction site analysis and sequencing of the plastid psbA-trnH spacer region from samples from 66 populations of R. integrifolia revealed 12 restriction-site haplotypes and 28 sequence haplotypes. A few of the sequence haplotypes were widely distributed, but most were relatively localized. Of the localized haplotypes, 10 were exclusively Beringian and an additional four were found along the northern boundary of glaciation (at the last glacial maximum) in the Yukon and Alaska; two haplotypes were found in northern coastal BC (Queen Charlotte Islands and adjacent mainland), in the vicinity of possible glacial refugia on the Queen Charlotte Islands. Only five haplotypes ...
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