Isodon (Schrad. ex Benth.) Spach (tribe Ocimeae Dumort.; subfamily Nepetoideae) is a genus of about 100 species in Lamiaceae. The genus is distributed primarily in tropical and subtropical Asia, with a center of species diversity (ca. 70%) in the Hengduan Mountains region; southeastern corner of the Qinghai-Tibetan Plateau of south-west China. Previous studies focused mainly on the relationship between Isodon and related genera and the two African species were not involved in.
Dr. Yu Xiangqin and her teachers of Xishuangbanna Tropical Botanical Garden (XTBG) used the genus Isodon as a case study to understand better apparently rapid species radiations in the Hengduan Mountains region of south-west China and the possible mechanisms triggering Asia–Africa disjunctions. They used molecular phylogenetic approaches to reconstruct the possible historical biogeography of Isodon. The molecular phylogeny and biogeographic history of Isodon were reconstructed using sequences of three plastid markers, the nuclear ribosomal internal transcribed spacer (nrITS), and a low-copy nuclear gene (LEAFY intron II). The evolution of chromosome numbers in the genus was also investigated using probabilistic models.
The reconstructed biogeographic history of Isodon provided insights into the mechanisms triggering Asia–Africa disjunctions. Isodon including the two African endemic species was monophyletic. Isodon comprised four well-supported clades. Isodon was inferred to have originated in Asia in the late Oligocene. Isodon colonized Africa in the early Miocene via overland migration. A rapid radiation of Isodon was dated to the late Miocene, which corresponded well with one of the major uplifts of the Qinghai-Tibetan Plateau and subsequent aridification events. Combined with previous studies, their results indicated that bursts of speciation triggered by geological/climatic changes on the Qinghai-Tibetan Plateau might be a common phenomenon and of significant importance in generating the current high levels of biodiversity seen within Qinghai-Tibetan Plateau.
The study entitled “Phylogeny and historical biogeography of Isodon (Lamiaceae): rapid radiation in south-west China and Miocene overland dispersal into Africa” has been published online in Molecular Phylogenetics and Evolution.
