两个温带-热带分布杨属物种复合体的生物地理学研究

Climate change is an important factor affecting species distribution, speciation and the size of populations. The Populus species are widely distributed throughout the northern temperate regions. However, there are species native to tropics regions within both two poplar species complex, which provide an excellent model for studying the biogeographic processes after the southward migration of temperate plants in response to the climatic oscillations. In this study, population genomic analysis will be employed to recover the speciation histories caused by the southward migration due to the climate change during the early Quaternary in these two species complex, respectively. Then the pattern of changes in population size during the mid-late Quaternary period will be examined and compared between the species distributed in tropical and temperate regions. The positively selected genes involved in the adaptation to tropical environment will be identified. At last, we will compare the similarities and differences of the results obtained from the independent and repeated studies of the two species complex. These results will provide important bases for revealing the biogeographic patterns of the southward migration and species divergence of temperate plants in response to the climate change.

气候变化导致物种分布变化，新物种形成和种群大小变化。杨属植物主要分布在温带地区，而两个杨属物种复合体中均有物种分别分布在热带，是阐述温带植物气候变化过程中南迁后发生生物地理学进化的优秀模式。本项目拟利用群体基因组数据和多种分析方法，研究两个物种复合体在第四纪早期由于气候变化南迁发生的物种分化，检查分布在热带、温带地区物种响应第四纪中后期气候变化导致的种群大小变化式样，以及鉴定热带气候下物种发生适应性进化的基因。通过两个物种复合体的独立重复研究，获得二者之间的差异性和共同性，为深入理解气候变化导致温带植物南迁和物种分化的生物地理学式样提供基础。

本项目整合完善了杨柳科植物的系统发育关系，并以此为基础选择胡杨组中的细叶杨和白杨组中的琼岛杨为研究对象，解析气候变化导致杨属物种复合体物种分化的生物地理学式样。这两个物种均分布于热带地区，与其近缘物种存在地理隔离，且种群规模小、生态位狭窄，我们通过对这两个物种及其近缘物种开展基因组测序、组装和群体基因组研究，基于其高精度的遗传变异图谱，发现它们在第四纪气候变化过程中均经历了长期的种群下降，近亲繁殖严重，是目前已知遗传多样性最低的杨属物种；自然选择压力检测发现它们可能通过对极端有害突变的纯化选择作用，来维持小种群的生存；我们还发现与适应热带地区高温、长日照等环境相关的基因在两个物种中存在趋同进化的现象，这些研究结果为深入理解杨树响应气候变化的物种分化和适应性进化提供了重要参考。

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