Phylogenetic relationships, character evolution and biogeographic diversification of Pogostemon s.l. (Lamiaceae).

Pogostemon (Lamiaceae; Lamioideae) sensu lato is a large genus consisting of about 80 species with a disjunct African/Asian distribution. The infrageneric taxonomy of the genus has historically been troublesome due to morphological variability and putative convergent evolution within the genus. Notably, some species of Pogostemon are obligately aquatic, perhaps the only Lamiaceae taxa which exhibit this trait. Phylogenetic analyses using the nuclear ribosomal internal transcribed spacer (ITS) and five plastid regions (matK, rbcL, rps16, trnH-psbA, trnL-F), confirmed the monophyly of Pogostemon and its sister relationship with the genus Anisomeles. Pogostemon was resolved into two major clades, and none of the three morphologically defined subgenera of Pogostemon were supported as monophyletic. Inflorescence type (spikes with more than two lateral branches vs. a single terminal spike, or rarely with two lateral branches) is phylogenetically informative and consistent with the two main clades we recovered. Accordingly, a new infrageneric classification of Pogostemon consisting of two subgenera is proposed. Molecular dating and biogeographic diversification analyses suggest that Pogostemon split from its sister genus in southern and southeast Asia in the early Miocene. The early strengthening of the Asia monsoon system that was triggered by the uplifting of the Qinghai-Tibetan Plateau may have played an important role in the subsequent diversification of the genus. In addition, our results suggest that transoceanic long-distance dispersal of Pogostemon from Asia to Africa occurred at least twice, once in the late Miocene and again during the late-Miocene/early-Pliocene.