A cladistic analysis of Asarum (Aristolochiaceae) and implications for the evolution of herkogamy

A cladistic analysis of Asarum was conducted to examine relationships among species within the genus and to test the monophyly of several groups of taxa that have often been treated as segregate genera. Thirty-two species were drawn from throughout the range of the genus, representing a broad sample of sections and all segregate genera. The data matrix included 37 characters derived from various aspects of vegetative and floral morphology. A strict consensus of all most parsimonious trees suggests that Asarum s.l. is monophyletic and consists of two main clades: an Asarum clade, which is characterized by connate styles and inferior ovaries, and an Asiasarum-Hexastylis-Heterotropa clade, which is characterized by ridges on the inner perianth surface, dorsal stigmas, and bifid style extensions. The latter is a large and morphologically diverse clade that includes the North American segregate Hexastylis and two Asiatic segregates. Examination of pollination mechanisms in the context of this phylogeny supports the conclusion that herkogamy, and thus obligate insect pollination, is derived from a plesiomorphic condition of autonomous self-pollination. Associated with herkogamy are characters such as glandular trichomes and other ornamentation of the surface of the calyx that probably represent increased specialization to attract insect pollinators. This study also indicates that chromosomal evolution has occurred via aneuploid decrease from an ancestral chromosome number of 2n = 26 to 2n = 24 in Heterotropa. The recognition of two subgenera, subgenus Asarum and subgenus Heterotropa, corresponding to the two clades in the cladistic analysis, is recommended.