Phylogenetic relationships in Asarum (Aristolochiaceae) based on morphology and ITS sequences

A cladistic analysis of Asarum was conducted using data from morphology and nuclear ribosomal internal transcribed spacer sequences (ITS) to examine the relationships among several groups of taxa that have often been treated as segregate genera. Morphological and molecular data were analyzed simultaneously for a set of 36 terminals representing the taxonomic, morphological, and geographic diversity of the genus. The two data sources are generally congruent and together provide more resolution than either by itself. The data support two main clades in the genus: an Asarum sensu stricto clade and an Asiasarum + Hexastylis + Heterotropa clade. The former consists of similar to 17 species and is distributed through North America, Europe, and Asia. Within this group, the North American species are monophyletic and derived from within the paraphyletic Asiatic species group. The Asiasarum + Hexastylis + Heterotropa clade consists of two Asiatic segregates and the North American segregate, Hexastylis. Resolution within this group supports both the monophyly of Heterotropa and a sister group relationship of Asiasarum to Hexastylis + Heterotropa. Hexastylis is paraphyletic and occurs as two separate clades on the tree; one of these is sister to Heterotropa and the other is sister to the latter plus Heterotropa. The phylogenetic data provide several clues about the biogeographic history of Asarum and suggest that: (1) the genus likely originated in Asia and underwent substantial diversification prior to colonizing North America; and (2) Asarum in North America represents at least two historically distinct groups that likely achieved their North American distributions at different times. The results of these analyses support recognition of two subgenera, Asarum and Heterotropa, each with two sections: Asarum and Geotaenium of the former, and Asiasarum and Heterotropa of the latter. Asarum sect. Ceratasarum (Hexastylis) is here treated as a synonym of Asarum sect. Heterotropa. A taxonomic conspectus of the genus is provided, and the combination Asarum sect. Geotaenium (F. Maek.) L. Kelly is made in accordance with the revised circumscription of taxa.