【作者】 李忠虎；

【导师】 刘建全；

【作者基本信息】 兰州大学 ， 生态学， 2010， 博士

【摘要】 揭示物种自然群体遗传变异的进化力量和近缘物种间物种形成机制一直以来都是进化生物学的中心议题之一。西藏圆柏(Juniperus tibetica)及其复合体物种密枝圆柏(J. convallium)、方枝柏(J. saltuaria)和祁连圆柏(J. przewalskii)是青藏高原特有的高山植物物种,隶属于柏科(Cupressaceae),刺柏属(Juniperus),圆柏组(Sabina)。四个物种形态上鳞叶或刺叶兼有,均具一粒种子,但种子大小和生鳞叶的末次分枝形状存在较大差异。西藏圆柏、密枝圆柏和方枝柏主要分布在青藏高原的东南部边缘地区,存在同域和邻域分布现象。这三个物种和分布在高原东北部的祁连圆柏一起构成了刺柏属圆柏组物种在青藏高原现代地理分布的典型的圆柏物种复合体。本论文通过取样多个核基因座位,运用现代分子群体遗传学的分析方法研究了四个物种的核苷酸多态性和种群遗传结构,并运用隔离-迁移(IM)模型探索了物种形成与分化的分子机制。对西藏圆柏复合体物种自然分布区内53个种群共219个个体的胚乳(单倍体)DNA进行了13套核基因的扩增和测序。四个物利·在13个基因座上平均核苷酸多态性呈现低水平。方枝柏沉默位点平均多态性最高(θwsil=0.0055,πsil=0.0042),密枝圆柏最低(θwsil=0.00420,πsil=0.00211)。AMOVA分析表明四个物种存在高水平的群体间遗传分化：密枝圆柏的平均FST值最高(0.291),接着是祁连圆柏(0.266),方枝柏最低(0.173)。物种的进化历史、所受选择压力以及青藏高原复杂的地形可能一起导致了这些高山物种低水平的核苷酸多态性和较高的遗传分化。物种成对IM模拟表明祁连圆柏与密枝圆柏自起源以来,有效群体未发生剧烈的变化；西藏圆柏未得到明显的结论；但是方枝柏有效群体明显比其祖先有效群体大,说明该物种可能发生了扩张或杂交渐渗,但进一步的分析表明其迁出的基因流要远大于本身接受到的基因流,说明历史扩张是导致该物种有效群体增大的主要原因。STRUCTURE分析表明四个物种不能单独成族,只能将祁连圆柏与西藏圆柏、密枝圆柏和方枝柏分开,而后三个物种之间聚集在一起,没有明显的分化。四个物种之间共享大量的基因多态性,可能是由于这些物种分化较晚,出于不完全的谱系筛选导致物种之间共享多态性。另一原因则可能是由于它们之间的分化始终是在基因流存在的情况下进行的,或者分化后存在反复的杂交,IM分析也表明它们存在明显的基因流。分子钟标记表明,西藏圆柏和密枝圆柏之间的分化时间最短,大约为71.6万年以前,其它物种对之间的分化大约发生在第三纪末与第四纪初,约为1.6-2.2百万年以前。第四纪气候变化以及青藏高原的进一步隆升可能促进了这些物种的分化以及它们分化之后的杂交。总之,基因流和不完全的谱系筛选是导致这些物种共享多态性和分化不明显的主要原因。更多还原

【Abstract】 One of the long-standing questions in evolutionary biology is to reveal the evolutionary forces shaping genetic variations acted on the natural populations and infer the mechanisms of species divergence between closely related species. Juniperus tibetica and its closely related species, J. convallium, J. saltuaria, and J. przewalskii are the endemic alpine juniper species of the Qinghai-Tibetan Plateau (QTP), and they are subordinate to Cupressaceae family, Juniperus genera, Sabina section. All the four species are characteristic of branch with both scaly and spiny leaf and the cone with only one seed, but the size of cone and the shape of the last branches differentiated between them. J. tibetica, J. convallium, and J. saltuaria have a sympatric or parapatric distribution along the southern part of the QTP and J. przewalskii is mainly occurred at the northeastern plateau. The four junipers constitute a representative species-complex of the modern geographic distribution of juniper on the QTP. This dissertation studied the nucleotide diversity and population genetic structure of the four species inferred from the multiple nuclear loci, and revealed the process of species divergence based on the Isolation-with-Migration (IM) analysis of divergence population genetics.A total of 13 nuclear loci were amplified and sequenced for 219 individuals (megagametophyte) of 53 populations through the entire distribution ranges of J. tibetica complex. Average value of nucleotide diversity across all 13 loci was low in the four species. Juniperus saltuaria had the highest diversity (θwsil= 0.0055,πsil= 0.0042). and J. convallium maintained the lowest (θwsil= 0.00420,πsil=0.00211). These juniper species have a high between-population differentiation based on the AMOVA analysis:J. convallium had the highest average value of FST(0.291), followed by J. przewalskii (0.266), and J. saltuaria had the lowest (0.173). The evolutionary history of species, selective pressures and complex topography of the QTP may have together led to the low level nucleotide variability and significant population genetic differentiation.J.przewalskii and J. convallium have maintained the relative stable population sizes since divergence from their common ancestral species based on the 1M analysis. We got no clear signs about population size of J. tibetica. However, J. saltuaria has the largest effective population size compared to the other three species and this species may have experienced population or range expansion in the past.STRUCTURE analysis revealed two clusters, one composed of the southern QTP species, J. tibetica, J. saltuaria, and J. convallium, and the other one J. przewalskii individuals in the northeastern QTP. Two possibilties may have contributed to the large amount of shared polymorphisms between species. One may be incomplete lineage sorting due to the recently species divergence and the other one the species divergence in the face of gene flow or the introgression after speciation which is supported by IM analysis. Molecular calibration revealved the latest divergence, between J. tibetica and J. convallium, maybe 0.716 Mya while the other species diverged 1.6-2.2 Mya. The Quaternary climatic oscillations and uplift of the QTP may have together drived the speciation or the introgression between juniper species. In summary, we proposed that incomplete lineage sorting and/or species divergence in the face of gene flow have resulted in the shared polymorphisms among the J. tibetica complex.更多还原