Soroseris, Stebbinsia, and Syncalathium are three genera of the sunflower family (Asteraceae) with restricted distributions on high screes of the Tibetan Plateau. A molecular analysis to test the monophyly of the genera, evaluate the phylogenetic relationships and construct their biogeographic diversification history.
Nuclear ITS and plastid trnL-F and psbA-trnH fragments were analyzed with parsimony, Bayesian inference, and relaxed Bayesian dating for all species of Soroseris, Stebbinsia, and Syncalathium. Stebbinsia is part of a polytomy with several lineages of Soroseris. Syncalathium is biphyletic with Syn. souliei placed within subtribe Lactucinae and the remaining species close to the Soroseris-Stebbinsia clade within subtribe Crepidinae. Bayesian dating based on ITS sequences and using four fossil calibrations suggests that the stem and crown ages of the Soroseris-Stebbinsia clade and the two groups of Syncalathium are between 8.44 and 1.56 million years. Stebbinsia should be treated as a section of Soroseris and Syncalathium souliei should be excluded from Syncalathium and either placed in Lactuca s.l. or established as a new genus in Lactucinae.
The remaining species are to be treated as Syncalathium s.str. in Crepidinae. The diversification of these groups in the Tibetan Plateau is of relatively young age, and can be explained by rapid diversification and radiation of the Soroseris-Stebbinsia clade, allopatric speciation within Syncalathium s.str. and convergent evolution of Syncalathium s.str. and Syn. souliei. The speciation events correlated with climatic change and fragmentation of scree habitats during the uplift of the Tibetan Plateau. Possible migration routes in Syncalathium s.str. from the northeast to the central and southern part of the Tibetan Plateau are suggested.
This study was published in TAXON (60: 15—26) in 2011, and was supported by grants-in-aid from the National 973 Project of China (2007CB411601), the National Natural Science Foundation of China (NSFC, 30625004, 40930209, 31000101), the Yunnan Natural Science Foundation (2008CC013), Innovation Project of Chinese Academy of Sciences (KSCX2-YW-Z-1019) and the National Science Foundation of U.S.A. (DEB-0321846 to D.E. Boufford).
Selected species of Soroseris, Stebbinsia and Syncalathium in situ. A, Soroseris erysimoides; B, Sor. hirsuta; C, Sor. teres; D, Sor. glomerata; E, Sor. gillii; F, Sor. hookeriana; G, Stebbinsia umbrella; H, Syncalathium chrysocephalum; I, Syn. qinghaiense; J, Syn. kawaguchii; K, Syn. pilosum; L, Syn. roseum; M–O, Syn. souliei from three different habitats. A, D, H, J, and N photographed by Sun Hang; B courtesy of Boufford D.E.; M photographed by Yang Yang; others photographed by Zhang Jianwen.
