The genus Physocarpus, comprising about six species in Rosaceae, is distributed in eastern Asia and both eastern and western North America. The genus has been classified along with Stephanandra and Neillia in tribe Neillieae. Molecular data strongly support the monophyly of the tribe and the sister relationship of Physocarpus to a clade including all species of the other two genera. To elucidate phylogenetic relationships among the species of Physocarpus and to assess the pattern of historical biogeography of the genus, we determined nucleotide sequences of ITS and ETS of nrDNA, several regions of cpDNA, and the second intron of a floral homeotic gene, LEAFY, from 14 accessions of all six species of Physocarpus and four outgroups. Maximum parsimony analyses suggest that P. alternans, distributed in arid areas of western North America, is sister to the rest of the species in Physocarpus and P. amurensis, which occurs in eastern Asia, is weakly supported as sister to other remaining North American species. Dispersal-vicariance analysis optimized that the most recent common ancestor of Physocarpus might have occurred either in western North America or in both eastern Asia and western North America, which suggests that one vicariance event as well as one dispersal or extinction should have happened between eastern Asia and western North America in the evolution of Physocarpus. Two divergent types of LEAFY sequences were found in the eastern North American species, P. opulifolius; one of which was sister to P. capitatus and the other of which was nested within P. monogynus. Based on the phylogenetic frameworks, in conjunction with the fossil records and current distribution, we suggest that P. opulifolius may have originated by hybridization between P. capitatus and P. monogynus in the interior of western North America and may have expanded its distribution to eastern North America via river systems.

Key words: biogeography, ETS, LEAFY, Neillieae, phylogeny, Physocarpus