東亞及東南亞紅豆杉之親緣地理

Abstract

East and Southeast Asia host many gymnosperm species whose historical distributions are thought to have been largely shaped by glacial cycles over time. The long-lived, slow-growing temperate yew tree (Taxus) has survived at high elevations, in rare, widely scattered relict populations, which may serve as indicators to reconstruct the phylogeographic history of gymnosperms in this region. In this study, four cpDNA (trnL-trnF, trnH-psbA, petG-trnP, atpI-atpH) and nDNA (ITS) markers were first screened for variation using 110 Taxus individuals from 18 populations distributed throughout southeast China, Taiwan and the Philippines. All markers were used to help clarify the phylogeny and ancestral ranges of Taxus in this region. The trnL-trnF and ITS markers were selected for their higher variability to further examine the phylogeography and historical demography with a more extensive sampling of 287 Taxus individuals. Haplotype networks and distributions together with AMOVA analysis indicate significant differentiation between mainland China and insular populations and high genetic structure of Taxus groups among all three regions. However, a rare lineage preserved in Huangshan, Zhejiang Province of eastern China appears more closely related to the haplogroup of Taiwan compared to the major Chinese lineage present today. Haplotype networks, ancestral state reconstruction and dispersal-vicariance suggest a colonization route from China into Taiwan and from Taiwan to the Philippines. The pattern of differentiation seems associated with significant isolation of respective colonist populations by oceanic barriers (Taiwan and Luzon Straits) to dispersal. Molecular clock estimations indicated a mean divergence time between the major Asian mainland and insular clades of 1.8-2.0 mya, between the more closely related Chinese (Huangshan) lineage and insular clades of 1.1-1.7 mya, and between the Taiwan and Philippines lineages 1.0-1.4 mya. Haplotype networks, distributions and gene flow analysis indicate a pattern of rare, asymmetric gene flow after initial colonization, flowing out from SE China to insular populations as well as from Taiwan to the Philippines. Potential refugial areas for Taxus include Huangshan and perhaps Lin’an, China and north Taiwan, based on their apparent preservation of more ancient haplotypes and higher number of rare, derived haplotypes compared to other populations. Mismatch analyses and Bayesian skyline plots indicate that Taxus populations of mainland China have recently contracted after expansion during the last glacial period while Taxus in Taiwan has likely been recovering from a past bottleneck, though seems to have a more stable history compared to SE China. Taxus in the Philippines has had insufficient time to accumulate enough variation to reliably detect demographic changes.