Systematics and biogeography of Bacopa (Plantaginaceae)

The aquatic and wetland plant genus Bacopa contains species that are found worldwide, with a center of diversity in the Americas. Although there have been regional taxonomic studies of the genus, a global synthesis is lacking. Variation in morphological features across species has complicated efforts to determine species boundaries and circumscribe infrageneric categories with confidence. Molecular data have enabled a more thorough understanding of phylogenetic relationships in Bacopa, and these can be used to develop a more stable taxonomy for the genus. Because there are approximately 60 species in the genus, it would be helpful to have a phylogenetically informed sectional classification and an improved understanding of ancestral biogeography and morphological evolution. We obtained novel molecular data for 28 species and produced phylogenetic hypotheses that include most species in the genus. The phylogenetic evidence supports at least four independent transitions to becoming fully aquatic species. We also reconstructed three separate ancestral dispersal events into Africa and beyond. Morphometric data were used to evaluate species boundaries and synonymy. We update sectional circumscriptions and propose a new section, Bacopa sect. Conobea, to accommodate species formerly in the genus Conobea that were found to be monophyletic with Bacopa.