WANG Yu-Jin, LI Xiao-Juan, HAO Gang, LIU Jian-Quan
2004, 42 (6): 481–499
Abstract Androsace L. consists of more than 100 species distributed mainly in northern temperate areas. This genus, typically characterized by having small flowers with a constricted corolla throat, was subdivided into six sections: sect. Samuelia Schlechtd., sect. Mirabiles (Hand.Mazz.) Yang & Huang, sect. Androsace, sect. Chamaejasme Koch., sect. Aizoidium Hand.Mazz., and sect. Orthocaulon Hand.Mazz. In this genus, the species with cushion-like growth belong to sect. Chamaejasme occurring in alpine habitats. There are other three small genera in the tribe Androsaceae. Pomatosace Maxim., with only one species, P. filicula Maxim., endemic to Qinghai-Tibetan Plateau, is traditionally placed in the subtribe Soldanellinae because of its circumscissile capsules, but other morphological characters and the recently reported evidence from cytology indicate that its affinity is closer with Androsace. Douglasia Lindley has eight homogeneous species in Northern American mountains. Vitaliana Sesler, with only one species in the European Alps, is usually considered a congener of Douglasia. These four genera, morphologically similar in having small flowers with a constricted corolla throat and “Androsace pollen”, referred here as “Androsace group”, are disjunctly distributed from the Qinghai-Tibetan Plateau, to Central- and Northern-Asia, Europe and North America. It is an ideal group to elucidate the origin and dispersal patterns of the Northern Hemisphere flora, which has a distribution center in the eastern Qinghai-Tibetan Plateau and the Hengduan Mountains. In addition, the habits of this group range from annuals to rosette perennials and cushion-like perennials. The cushion-like species are disjunctly distributed in the central Qinghai-Tibetan Plateau and European Alps mountains. Further studies are needed to determine whether they are closely related to each other or it is only the result of convergent evolution under the habitat pressure selection in separate sites. We firstly reported cpDNA trnL-F, nrDNA ITS sequences respectively or both for 29 populations of 14 species in Androsace and Pomatosace mainly from the central Qinghai-Tibetan Plateau. A phylogeny was further constructed for these species, and those distributed in Europe and North America in “Androsace group” and related genera in Primulaceae based on newly reported sequences and those downloaded from GenBank. Phylogenetic inferences indicated that the four genera in “Androsace group” form a well-supported monophyletic clade. Two main clades were discovered in the “Androsace group”: a wholly Androsace clade, and the other comprising Pomatosace, Douglasia, Vitaliana and nine Androsace species in all analyses. The grouping and position of three species of sect. Samuelia and two species of sect. Androsace varied according to the analyses of the different datasets, trnL-F, ITS or a combination of them. The biogeographical mapping of species distribution revealed that the basal species of each clade sited in southeastern China and the eastern Qinghai-Tibetan Plateau, which obviously supports the origin area hypotheses for “Androsace group” based on morphological studies. From the eastern Qinghai-Tibetan Plateau multiple lineages spread to and colonized Europe in different ages. The crude timing based on ITS sequence differentiation indicated a recent diversification of “Androsace group” within Miocene of the Tertiary. The cushion-like species evolved independently in Asia and Europe. The origin and diversification of cushion-like species in the Qinghai-Tibetan Plateau might be earlier than those in Europe. Their occurrence in the Qinghai-Tibetan Plateau area might be correlated with the large-scale uprising of the plateau and the mountains building since Miocene. But the appearance of the cushion-like species in Europe might result from the fast habitat isolation due to the climate oscillation and the cycles of developing and retreating of the ice sheet in the Northern Hemisphere during the Quaternary. The large-scale colonization of cushion-like Androsace species in the Qinghai-Tibetan Plateau probably occurred in the late Holocene when the global climate re-cooled after the end of glacial cycles. The intraspecific genetic differentiation in some species might partly reflect their retreat and re-colonization in the Qinghai-Tibetan Plateau area during the climate oscillation.