|其他摘要||Capparis spinosa L., a perennial creeping subshrub, is a type member of a relatively large genus (Capparis Tourn. ex L.) of the Capparaceae. In arid region of China, there is only one single genus with one single species belonging exclusively to this family. As a Tertiary relic originated from the arid Tethyan Flora, C. spinosa is a typical xerophyte in northwestern region. Here we study the genetic diversity and phylogeography of C. spinosa, in order to explore the genetic pattern and evolution process of desert species in response to paleogeologic and paleoclimatic changes.
1. Genetic diversity of Capparis spinosa
Genetic diversity within and among populations is essential for the long-term survival and adaptive evolution of plant species. We systematically assessed the genetic variability and diversity of Capparis spinosa widely distributed in Tianshan Mountains and adjacent desert areas, to explore the influence of vicariance on differentiation pattern of xerophytes in arid regions. A total of 300 individuals from 25 populations were sampled and 14 haplotypes were identified using two chloroplast DNA sequences (rps12-rpl20 and ndhF). A high level of total genetic diversity (HT = 0.859) was detected, attributing to extensive distribution range with numerous large-sized populations. The spatial analysis of molecular variance clustered the 25 populations into 4 major geographic units, according to the geographical distribution and genetic relationship of cpDNA haplotypes in populations. The analyses of molecular variance showed that most of the total genetic variation (66.33%) existed among geographic units. Furthermore, significant genetic differentiation (GST = 0.670) and a low level of gene flow (Nm = 0.2463) were found among populations via Permut procedure. Our results indicated that there was a considerable correlation between genetic divergence and geographic distribution. Complex mountain and desert isolations in Tianshan region limited the gene exchange between disjunct populations, thereby resulting in high genetic differentiation among geographical units.
2. Phylogeography of Capparis spinosa
The desert and semi-desert elements of the Western Himalayan and Eastern Central Asian regions have been deemed to have originated from the arid Tethyan Flora. Complex geological movements more or less affected or changed floristic structures, while the alternation of glacials and interglacials in Quaternary is presumed to have further shaped the present discontinuous genetic pattern of temperate plants. Here we consider Capparis spinosa and discuss its phylogeographic structure, and explore how it responded in a stepwise fashion to Pleistocene geologic and climatic changes. In total, 31 natural populations were sampled and 24 haplotypes were identified, based on three cpDNA intergenic spacers, trnL-trnF, rpS12-rpL20, and ndhF (329F, 927R). SAMOVA results suggested that the 31 populations were clustered into 5 major geographical groups; a similar divergence trend was found by constructing a network diagram and a BEAST phylogenetic tree. AMOVA results revealed that the main genetic differentiation existed between groups, gene flow between which might be restricted by vicariance. Molecular clock dating indicated that the intraspecific divergence began in early Pleistocene, consistent with a time of intense uplift of the Himalayas and Tianshan Mountains; and intensified in middle Pleistocene. Species distribution modeling (SDM) suggested range reduction in the high mountains during the Last Glacial Maximum (LGM) as a result of cold climates during glacial advances, while gorges at mid-elevations in the Tianshan range appear to have served as potential refugia. Populations of low-altitude desert regions on the other hand, probably experienced only marginal impacts from glaciation, according to the high levels of genetic diversity and genetic distance. The findings of allopatric divergence, local evolution, and dispersal during interglacial and postglacial periods highlight the complex influence of Pleistocene intense mountain uplift and glaciation occurrence on relic plant species, providing the basis for future studies on how xerophytes gradually responded to Quaternary historic events.|