The internal transcribed spacer (ITS) region of nuclear ribosomal DNA was sequenced in 36 samples representing 16 of the 17 species of Lasthenia. Phylogenetic analysis supported the monophyly of only section Hologymne whereas other sections are paraphyletic or polyphyletic. Species of section Baeria together with L. leptalea (section Burrielia) form a well supported clade. Samples of L. californica (section Baeria) form two clades representing geographically separated populations in a north-south pattern. Section Burrielia is polyphyletic and the ITS data suggest that different species of this section are more closely related to taxa of either sections Baeria or Ptilomeris. The monotypic section Platycarpha shares a close affinity with some species of section Ptilomeris. Species of section Ptilomeris formed two clades. One clade includes L. burkei, L. conjugens and L. fremontii supporting a close relationship among these species as previously recognized. The remaining species of section Ptilomeris form a well supported clade together with L. microglossa (section Burrielia). The basal position of L. glaberrima of section Lasthenia suggests that the ancestor to the genus may have been a self-compatible annual species with n = 5 and confined to wet habitats, a hypothesis that contradicts previous suggestions.

Communicating Editor: Alan T. Whittemore