The phylogenetic relationships of the 16 species of Monarda (Lamiaceae) were investigated using sequences of the internal transcribed spacer regions of nuclear ribosomal DNA. Thymus and Mentha were used as outgroups, and Blephilia, Clinopodium, Conradina, Hesperozygis, Monardella, Pycnanthemum, and Ziziphora were included in the ingroup to test the monophyly of Monarda. Two parsimony searches were performed after removing redundant sequences from the analysis: one with indels scored as missing and a second with indels treated as binary characters. Both searches yielded congruent results, but the treatment of indels as binary characters resulted in considerably more resolution within Monarda. There was strong support for the monophyly of Monarda and a close relationship was found between Monarda, Blephilia, and Pycnanthemum. The molecular phylogeny was completely congruent with the infrageneric classification of the genus. Our results were consistent with hypotheses of hybridization between M. fistulosa and M. lindheimeri in Texas. Despite considerable morphological variation among many species, especially in floral characters, little molecular diversification was found in those same species groups. Intraspecific polymorphism in ITS sequence was found in over half the species examined, and may be attributable to ancestral polymorphism, hybridization, or detection of paralogous loci.

Communicating Editor: Jim Smith