网眼瓦韦复合体(蕨类植物)对青藏高原的适应机制—近交与多倍化

Recently, several studies support the hypothesis that polyploidy goes on along with self-breeding. The progress of polyploidy leads to the transition of reproductive mode to inbreeding and allows the rapid formation of genotypes enable to form persistent populations in isolated areas with extreme climatic conditions. The Qinghai-Tibetan Plateau forms a formidable challenge to plant life by its enormous altitudes, range of climatic conditions, and topographic structure. Thus, the region is well suited to study evolutionary processes in which plants adapt to new extreme climatic conditions, e.g. long cold winters, exposure to high UV light, and the formation of small geographic isolated populations. Thus, both inbreeding and polyploidy are expected to be common in plants adapted to the conditions at the Tibetan plateau compared with their relatives in other areas such as the Hengduan mountains. Here, we want to test this hypothesis by studying. Lepisorus clathratus complex, a group of derived ferns with its diversity centre in Qinghai-Tibetan Plateau, includes specimens with regular, dehiscent sporangia and once with irregular, indehiscent sporangia. The indehiscent sporangia might be the mechanism promoting inbreeding. This complex is the exact material to study the relationship between polyploidy and inbreeding. Building up on the existing research, we propose to compare populations to enable us testing the hypothesis of inbreeding and polyploidy as adaptive strategies in Himalayan plants. These patterns will be further studied via (1) genetic characterization of these and previously collected populations using low copy nuclear genes, SSR and AFLP, (2) determination of the ploidy levels, and (3) controlled crossing experiments. We aim to discuss the relationship of ploidy level and habitat, mating system of each population to explore the adaptive mode of ferns in this region, and test if reproductive isolation via transformation of the reproductive mode and/ or ploidy level is the driven factor in the diversification and local persistence in these ferns.

青藏高原－横断山分布的网眼瓦韦复合体存在不同的染色体倍性，并且具有宽、窄两种孢子囊环带类型，宽环带是导致近交的关键性状。其中的宽环带类型主要分布于青藏高原，是研究近交与多倍化的关系及其适应胁迫环境的理想材料。本项目将在已有工作基础上以网眼瓦韦复合体为研究背景，以其中的宽环带类群为重点研究对象，利用居群样品，通过多项研究手段获取染色体倍性资料，构建居群谱系关系，分析居群遗传结构和交配系统，揭示网眼瓦韦复合体的关键性状孢子囊环带和不同染色体倍性的地理分布式样，分析不同海拔及生境中居群的交配系统，阐明交配系统和倍性变化的相关性，验证孢子囊环带与交配系统的关系，检验近交与多倍化共同作用是是植物适应青藏高原极端环境重要机制的假说，并试图阐明近交、多倍化和关键性状孢子囊环带之间的因果关系。本研究对理解多倍化程度极高的蕨类植物在青藏高原胁迫环境下生物多样性的起源和维持以及新性状的起源具有重要意义。

网眼瓦韦复合体中存在宽环带和窄环带两种类型孢子囊，宽环带类型分布于青藏高原和横断山海拔4000米以上地区，是适应极端环境的创新性性状；而窄环带类型广布于中国西南、北方等海拔2000-3500米的山地。多倍化现象被认为是蕨类植物演化的主要驱动力之一，我们观察到宽环带类型的网眼瓦韦复合体中多种倍性共存。因此，该类群是研究特化性状与多倍化关系及探讨生物对极端生境适应机制的良好材料。本研究利用居群细胞学方法，结合形态与地理分布，揭示多倍体的分布及其与环带类型的关系；进而开发了四个高分辨的叶绿体分子标记，两个核基因和六对SSR位点对青藏高原和横断山区的网眼瓦韦该类群植物进行群体遗传多样性、谱系生物地理、多倍体遗传组成分析，探讨多倍体居群遗传多样性的地理格局，起源模式，探讨多倍体物种的形成和适应机制。.本研究结果显示：1.多倍体广泛存在于该复合体中，其中四倍体占绝对优势，六倍体仅发现少数个体与四倍体混生；2. 多倍体的分布呈现明显的地理格局，四倍体主要分布于横断山区、喜马拉雅南部及中国北部山区；3. 青藏高原多倍体分布数量极少，仅有极少数个体的四倍体和六倍体，二者都是零星分布于少数地点；4.宽环带特征与多倍化没有因果关系，窄环带中也有多倍体，并且数量更多、分布更广；5. 网眼瓦韦复合体同时存在同源多倍体和异源多倍体，同源多倍体来源于复合体内部居群间二倍体异交后染色体加倍产生，比亲本拥有更高的遗传多样性；6. 分布于横断山区的单倍型更倾向于作为母本参与多倍体的形成，显示出一定的偏好性；7. 异源四倍体耳基宽带蕨，是由网眼瓦韦复合体中的宽带蕨（父本）与本属中的印度瓦韦（母本）杂交多倍化产生，遗传多样性极低，很可能是近期的单起源事件；8. 耳基宽带蕨中存在宽、窄环带及过渡类型，它们别对耳基宽带蕨的维持与扩散起到重要作用。我们由此推论：多倍化的产生是由多样而又相对隔离的生境、生物本身的生物学特征、繁殖系统共同作用的产物，单一的极端环境并不能促进多倍化的维持与扩张，多倍化不是适应青藏高原极端环境的主要机制。