Molecular systematics and historical biogeography of s.s.

PREMISE OF THE STUDY

Understanding the underlying mechanisms for the evolution of intercontinental disjunct patterns has long fascinated botanists. We present a molecular phylogenetic reconstruction of s.l. (including ) with a focus on s.s. species, which have a disjunct distribution between Eurasia and North America. Within this evolutionary framework, we clarify the systematic classification and biogeographical history of s.s.

METHOD

Data from two nuclear and five chloroplast DNA regions were analyzed using the programs PAUP*, RAxML, MrBayes, BEAST, and RASP.

KEY RESULTS

Our molecular phylogeny supports s.s. as monophyletic. and are grouped according to their taxonomic species, but the accessions of are divided into two well-defined groups, i.e., one comprising western North American accessions, and the other composed of northeast Asian accessions. Molecular dating and biogeographic reconstructions suggest a northeast Asian origin for s.s. and that a complicated pattern of divergent evolution began approximately in the late Miocene. Intercontinental disjunctions of s.s. in the Northern Hemisphere appear to have occurred two times during the Pliocene.

CONCLUSIONS

Based on our results, two possible ways to treat s.s. species can be envisioned: (1) elevate Asian populations of to specific rank; or (2) combine the three s.s. species into a broadly defined single species. We recommend treatment of s.s. as a single species. The biogeographic patterns of s.s. coupled with molecular dating suggest both vicariance and long-distance dispersal events as key mechanisms for diversification of the clade.