Phylogeographic Evidence for a Link of Species Divergence of Ephedra in the Qinghai-Tibetan Plateau and Adjacent Regions to the Miocene Asian Aridification

The Qinghai-Tibetan Plateau (QTP) has become one of the hotspots for phylogeographical studies due to its high species diversity. However, most previous studies have focused on the effects of the Quaternary glaciations on phylogeographical structures and the locations of glacial refugia, and little is known about the effects of the aridization of interior Asia on plant population structure and speciation. Here the chloroplast DNA (cpDNA) trnT-trnF and trnS-trnfM sequences were used to investigate the differentiation and phylogeographical history of 14 Ephedra species from the QTP and northern China, based on a sampling of 107 populations. The phylogeographical analysis, together with phylogenetic reconstruction based on combined four cpDNA fragments (rbcL, rpl16, rps4, and trnS-trnfM), supports three main lineages (eastern QTP, southern QTP, and northern China) of these Ephedra species. Divergence of each lineage could be dated to the Middle or Late Miocene, and was very likely linked to the uplift of the QTP and the Asian aridification, given the high drought and/or cold tolerance of Ephedra. Most of the Ephedra species had low intraspecific variation and lacked a strong phylogeographical structure, which could be partially attributed to clonal reproduction and a relatively recent origin. In addition, ten of the detected 25 cpDNA haplotypes are shared among species, suggesting that a wide sampling of species is helpful to investigate the origin of observed haplotypes and make reliable phylogeographical inference. Moreover, the systematic positions of some Ephedra species are discussed.

青藏高原（Qinghai-Tibetan Plateau, QTP）因其极高的物种多样性，已成为系统地理学研究的热点区域之一。然而既往多数研究多聚焦于第四纪冰期对系统地理格局的影响以及冰期避难所的分布位置，对于亚洲内陆干旱化过程对植物种群结构与物种形成的影响却所知甚少。本研究以叶绿体DNA（chloroplast DNA, cpDNA）的trnT-trnF与trnS-trnfM序列为分子标记，基于对107个种群的采样，探究了分布于青藏高原及中国北方的14种麻黄属植物的分化与系统地理历史。结合基于4个叶绿体DNA片段（rbcL、rpl16、rps4及trnS-trnfM）的联合序列所构建的系统发育树开展的系统地理分析，支持该类麻黄属植物可划分为三个主要支系：青藏高原东部支系、青藏高原南部支系以及中国北方支系。各支系的分化时间可追溯至中新世中晚期，结合麻黄属植物极强的耐旱与/或耐寒特性，该分化极可能与青藏高原隆升及亚洲内陆干旱化过程密切相关。多数麻黄属物种的种内遗传变异水平较低，且未形成显著的系统地理格局，这一现象可部分归因于其克隆繁殖特性以及相对较晚的起源时间。此外，在检测到的25个叶绿体DNA单倍型中，有10个在不同物种间共享，这表明扩大物种采样范围有助于解析观测到的单倍型起源，并做出可靠的系统地理推断。本研究同时讨论了部分麻黄属物种的系统分类位置。