青藏高原落叶松属物种形成、分化及分子亲缘地理学研究

Species of the genus Larix are relatively cold-tolerant among the coniferous species, and their vertical distributions could reach the tree line of the forest distribution. The Qinghai-Tibet Plateau (QTP) is the largest among all six major distributs of Larix taxa in China, in where the species are intensive, and is widely recognized as the present center of geographical distribution and origin. The uplift of the QTP has generated numerous isolated habitats, which have rendered the possible speciation of Larix through isolation. However, the habitats of some Larix taxa are shrinking in the QTP. Besides, much work has not yet to be done about the status quo of the germplsm, the mechanism of speciation and diversification, the relationship between the historical dynamic change process and the changes of the QTP geology and climate. These have extremely hindered the effective conservation and sustainable utilization of the germplsm. The present project aims to investigate the status quo of Larix taxa in the QTP through an integrated application of botanical, molecular biology and ecological niche modeling approaches. The major objectives are to: (1) analyze genetic diversity of the germplasm by using nuclear simple sequence repeat (SSR) markers; (2) analyze the diversification history and formation mechanism of phylogeography by using the complete chloroplast genome sequences; and (3) predict their suitable habitats and low impact areas within the QTP under climatic change scenarios using maximum entropy (MaxEnt) modeling. The findings would illustrate the genetic diversity and species formation mechanism of phylogeography of Larix, and lay a solid theoretical foundation for its conservation and utilization in the QTP.

落叶松属植物在针叶树种中较抗寒，垂直分布可达到森林分布的林线。落叶松属植物在我国青藏高原的分布面积最大，物种组成种类多，被认为是现代地理分布和起源的中心。青藏高原的隆升形成众多隔离生境，为落叶松属通过异域物种形成提供了先决条件。然而，其个别类群在青藏高原不断缩小，且其资源现状、物种形成和分化及群体历史动态变化过程与青藏高原地质及气候变迁的关系均不清楚，这极大的阻碍了对落叶松属资源的保护与利用。本研究拟通过植物学、分子生物学及生态位模型等手段对青藏高原落叶松属植物资源进行系统研究，主要内容包括：（1）运用核微卫星（SSR）分子标记分析资源遗传多样性；（2）运用叶绿体全基因组序列分析分化历史及分子亲缘地理学形成机制；（3）运用最大熵模型预测气候变化背景下物种的适生区分布。本研究结果将揭示青藏高原落叶松属资源多样性及分子亲缘地理学形成机制，以期为保护利用青藏高原落叶松属资源奠定理论基础。

本研究对分布于青藏高原的落叶松属（Larix）5个种包括贡布红杉（L. kongboensis）、怒江红杉（L. speciosa）、大果红杉（L. potaninii var. australis）、藏红杉（L. griffithii）、喜马拉雅红杉（L. himalaica）的13个居群的272个代表样本进行了研究分析。结果显示：（1）青藏落叶松属植物资源在不同居群的7个表型性状均值均存在显著性差异;（2）基于19个多态SSR位点的遗传分析表明，青藏高原落叶松群体遗传多样性较低，群体遗传变异主要存在于居群间，且不同居群间存在较弱基因流。遗传分析显示落叶松属种类主要分为2个大类群：1个大类群包括贡布红杉、怒江红杉，大果红杉，且贡布红杉应并入怒江红杉。另一个大类群包括藏红杉和喜马拉雅红杉。青藏高原落叶松属资源的遗传距离与地理距离显著相关，说明遗传分化受到了地理因素的影响;（3）研究还进一步构建了青藏高原落叶松5个种叶松叶绿体完整基因组，5个种的叶绿体基因组在基因组大小、基因含量和排列、边界变异等方面均保守;（4）对青藏高原落叶松适生分布区预测结果表明，高度适生区在末次盛冰期向雅鲁藏布江下游延伸，推测该地区可能是落叶松在末次盛冰期的避难所。然而，青藏高原其它地区也提供了避难所。落叶松在此经历了强冰期后，由避难所向西沿喜马拉雅山脉迁移，向东沿横断山迁移。这一事实也证明了从喜马拉雅山脉延伸出来的一系列山脉可能促进了落叶松物种的传播和多样性。未来气候情景下，青藏高原落叶松总量、低度和中度适宜面积呈减少趋势，而高度适宜面积呈显著增加趋势。主要原因是在全球变暖的背景下，高山冰川的融化为高原落叶松提供了适宜的生长环境，导致其向高海拔地区迁移，即气候波动导致青藏高原落叶松地理分布格局变化，这可能继续导致落叶松的遗传分化。本研究结果为青藏高原落叶松资源保护和深入开发奠定了坚实基础。