Relationships of North American members of Rhodiola (Crassulaceae)

Taxa of Rhodiola L. (Crassulaceae) generally grow in arctic or alpine habitats. Some Rhodiola species are used medicinally, one taxon, Rhodiola integrifolia Raf. subsp. leedyi (Rosend. & J.W.Moore) Moran, (Leedy’s roseroot), is rare and endangered, and the group’s biogeography in North America is intriguing because of distributional disjunctions and the possibility that Rhodiola rhodantha (A.Gray) H.Jacobsen (2n = 7II) and Rhodiola rosea L. (2n = 11II) hybridized to form Rhodiola integrifolia Raf. (2n = 18II). Recent studies of the North American Rhodiola suggest that the group’s current taxonomy is misleading. We analyzed nuclear and chloroplast DNA sequences (internal transcribed spacer (ITS), trnL intron, trnL–trnF spacer, trnS–trnG spacer) from the North American Rhodiola taxa. We combined our data with GenBank sequences from Asian Rhodiola species, performed parsimony, maximum likelihood (ML), and Bayesian phylogenetic analyses, and applied a Bayesian clock model to the ITS data. Our analyses reveal two major Rhodiola clades, suggest that hybridization between R. rhodantha and R. rosea lineages was possible, show two distinct clades within R. integrifolia, and demonstrate that a Black Hills, South Dakota, Rhodiola population should be reclassified as Leedy’s roseroot. We recommend that R. integrifolia be revised, and that the Black Hills Leedy’s roseroot population be managed as part of that rare and endangered taxon.

Rhodiola integrifolia: hybrid origin and Asian relatives

We investigated the relationship among North American Rhodiola integrifolia Raf., Rhodiola rosea L., and Rhodiola rhodantha (A. Gray) H. Jacobsen by sequencing a fragment of the nuclear-encoded chloroplast-expressed glutamine synthetase (ncpGS) gene from these three species and from selected Asian representatives of the genus. We found that R. integrifolia is a hybrid descendant of R. rosea and R. rhodantha lineages. We also found that the ncpGS gene and the internal transcribed spacer regions of the nuclear ribosomal DNA of Asian Rhodiola algida Fisch. & Mey. are very similar to those of R. rhodantha. In accordance with known chromosome numbers and species morphology, we therefore propose to move R. algida into the subgenus Clementsia of the genus Rhodiola, which so far contains only two species, R. rhodantha and Asian Rhodiola semenovii (Regel & Herder) Boriss.