Systematics of Trichostema L. (Lamiaceae) and phylogenetic relationships with its disjunct taxa in Asia

The so-called Caryopteris-Trichostema complex (Lamiaceae) consists of these two genera and several monotypic or very small genera. Trichostema is North American whereas all other taxa are Asiatic. Parsimony analyses of morphology and sequences from the nuclear ribosomal DNA internal transcribed spacer region (ITS) and the chloroplast DNA ndhF gene were performed to assess the sectional relationships within Trichostema. A cladistic analysis of the complex was carried out using ndhF sequences to reconstruct phylogenetic relationships of Trichostema with its related taxa in Asia. The monophyly of Trichostema is strongly supported in all analyses except for the morphological phylogeny. Two well-supported monophyletic sections, i.e., Chromocephalum and Trichostema, appear in all analyses. Three combinations of morphological and molecular data (ITS, ndhF, ITS plus ndhF) suggest that section Chromocephalum is sister to the rest of Trichostema, and sections Orthopodium and Paniculatum are monophyletic. Although the molecular data alone suggest that Trichostema brachiatum is not a member of section Orthopodium, morphology and all analyses of the combined morphological and molecular data suggest that T. brachiatum is sister to the rest of section Orthopodium. The ndhF phylogeny of the Caryopteris-Trichostema complex suggests that Caryopteris s.l. is polyphyletic; Caryopteris s. str., Pseudocaryopteris, Schnabelia, and Trichostema are strongly supported as monophyletic. Tripora divaricata, the sole member of the genus and characterized by its porate pollen, is sister to Pseudocaryopteris in a moderately supported clade. Rubiteucris is well supported as sister to Schnabelia. The ndhF analysis provides strong evidence to support the transfer of S. nepetifolia and S. terniflora from Caryopteris to Schnabelia by Cantino et al. Although the ndhF data suggest that the sister group of Trichostema is Caryopteris, with Amethystea the next most closely related taxon, the analyses that combined ITS and ndhF data with morphological data suggest that the sister group of Trichostema is Amethystea, a clade comprising Amethystea and Caryopteris, or a clade comprising Amethystea and Teucrium. The evolution of gynoecial morphology and the biogeographic history of the Caryopteris-Trichostema complex are discussed from a phylogenetic perspective.