We sequenced the complete ITS regions and the 5.8S gene of nuclear ribosomal DNA of 24 plants representing 20 species and two hybrids from six sections of Bidens, plus one species of Cosmos, and produced a phylogenetic tree. All Hawaiian species have identical sequences, including B. cosmoides. Bidens cosmoides is the most morphologically divergent species in the genus, and has previously been segregated into the monotypic section Degeneria and considered to represent a separate introduction of Bidens into Hawaii. Hawaiian and southeast Polynesian species represent a single monophyletic colonization of islands in the Pacific Ocean. There is a basal dichotomy in Bidens between north temperate species of wet habitats in sections Hydrocarpaea and Bidens and subtropical and tropical species of well drained habitats in sections Psilocarpaea, Campylotheca, and Greenmania, suggesting the two groups are not closely related. Segregation of the aquatic B. beckii as the monotypic genus Megalodonta is not supported. A recently discovered Bidens on Starbuck Island in the South Pacific is not closely related to Polynesian Bidens. It could represent a second colonization of the South Pacific, or it could be a recent, non-indigenous introduction. The continental sister group of Polynesian Bidens appears to be a group of Latin American herbaceous species with determinate growth in section Psilocarpaea rather than the indeterminate woody vines of section Greenmania. This suggests that woodiness and indeterminate growth are apomorphic in Polynesian Bidens.

Communicating Editor: Kathleen Kron