Phylogenetic and Biogeographic Diversification in Osmorhiza (Apiaceae)

Osmorhiza Raf. (Apiaceae) consists of 10 species disjunctly distributed in temperate Asia (1 sp.) and the Americas (9 spp.). Osmorhiza berteroi and 0. depauperata show an American antitropical disjunction. Within North America, these two species are also disjunctly distributed in eastern and western North America and the Great Lakes regions. A phylogenetic analysis was conducted to clarify inter- and intraspecific relationships based on sequences of the ITS and 5.85 regions of nrDNA. With Anthriscus, Geocaryum, and Myrrhis as outgroups, the monophyly of Osmorhiza is strongly supported. The ITS phylogeny suggests the basal position of the Asiatic 0. aristata and the monophyly of the nine New World species. The ITS sequence of Osmorhiza aristata is relatively divergent from those of all other species even though it is morphologically similar to the eastern North American 0. claytonii and 0. longistylis (which form a Glade), suggesting early divergence followed by morphological stasis. Osmorhiza berteroi, 0. brachypoda, 0. depauperata, O. mexicana, O. occidentalis, and 0. purpurea constitute a monophyletic group (= western North American clade). The morphologically distinct 0. glabrata from the central Andes forms a trichotomy with the eastern North American Glade (O. claytonii and 0. longistylis) and the western North American clade in parsimony and maximum likelihood analyses. The 11 populations studied of the widespread 0. berteroi form a clade, and showed little sequence divergence, suggesting recent establishment of the widely disjunct populations following long-distance dispersal. Disjunct populations of 0. depauperata from the Rocky Mountains and eastern North America have an identical ITS profile. Osmorhiza occidentalis, however, shows a high level of infraspecific sequence divergence. The ITS phylogeny and the low sequence divergence values suggest rapid diversification of Osmorhiza in western North America.