Phylogenetic relationships in Asarum(Aristolochiaceae) based on morphology and ITSsequences.

A cladistic analysis of Asarum was conducted using data frommorphology and nuclear ribosomal internal transcribed spacer sequences(ITS) to examine the relationships among several groups of taxa thathave often been treated as segregate genera. Morphological andmolecular data were analyzed simultaneously for a set of 36 terminalsrepresenting the taxonomic, morphological, and geographic diversity ofthe genus. The two data sources are generally congruent and togetherprovide more resolution than either by itself. The data support twomain clades in the genus: an Asarum sensu stricto clade and anAsiasarum + Hexastylis +Heterotropa clade. The former consists of ∼17 species andis distributed through North America, Europe, and Asia. Within thisgroup, the North American species are monophyletic and derived fromwithin the paraphyletic Asiatic species group. The Asiasarum+ Hexastylis + Heterotropa clade consists oftwo Asiatic segregates and the North American segregate,Hexastylis. Resolution within this group supports both themonophyly of Heterotropa and a sister group relationship ofAsiasarum to Hexastylis + Heterotropa. Hexastylis is paraphyletic and occurs as two separate clades onthe tree; one of these is sister to Heterotropa and the otheris sister to the latter plus Heterotropa. The phylogeneticdata provide several clues about the biogeographic history ofAsarum and suggest that: (1) the genus likely originated inAsia and underwent substantial diversification prior to colonizing NorthAmerica; and (2) Asarum in North America represents at leasttwo historically distinct groups that likely achieved their NorthAmerican distributions at different times. The results of theseanalyses support recognition of two subgenera, Asarum andHeterotropa, each with two sections: Asarum andGeotaenium of the former, and Asiasarum andHeterotropa of the latter. Asarum sect.Ceratasarum (Hexastylis) is here treated as a synonymof Asarum sect. Heterotropa. A taxonomic conspectusof the genus is provided, and the combination Asarum sect.Geotaenium (F. Maek.) L. Kelly is made in accordance with therevised circumscription oftaxa.