高山蓍异源四倍体复合体后物种形成期的演化

Hybridization and polyploidy can lead to instant speciation by inducing genomic and transcriptomic shocks. Speciation and evolution of a polyploid species usually include three phases: formation, establishment and post-speciation evolution. Hybridization and polyploidy are ubiquitous in plants, but most of them have not left “leaves” in the Plant Tree of Life. This is because they could not have passed through the long-term post-speciation processes of evolution, including adaptation. Changes of genomes, transcriptomes, chromosomes, and physiology at the time of origins of polyploids have received considerable attentions. Theories about population genetics on the earliest stages of polyploid evolution are also supported by many empirical data. Nevertheless, we still know little about long-term post-speciation evolution of lots of the relatively old polyploid species. The knowledge about genetic and morphological differentiation during this process, and mechanisms underlining these changes is important for studying the development and lasting of biodiversity. Previous studies have shown that the Achillea alpina species complex (Asteraceae) is of allotetraploid origin(s). Their diploid progenitors correspond to two extant diploid species, Achillea acuminata and A. asiatica. The parental species are phylogenetically distant and genetically distinct, and their tetraploid progeny species thus have disomic inheritance and have kept their parental genomes relatively intact. Such genomic feature will facilitate a research strategy, i.e., to diploidize the complex polyploid problem, and consequently help us more accurately recover the polyploid evolutionary history. The present study will investigate the Achillea alpina complex using approaches of population genetics, evo-devo and ecological. We aim to answer questions about (i) population demographic history of the polyploidy species, (ii) evolutionary mechanisms of genetic and morphological differentiation between populations, and (iii) the genic and environmental roles on the morphological variation.

杂交和多倍化可以迅速创造出新特征，导致物种形成。多倍体物种的形成和演化通常包括形成期、建立期和之后的长期演化。自然界中杂交和多倍化事件很普遍，但多数并未在生命之树上留下枝叶，现存的多倍体物种大多是在前两阶段的基础上经历长期演化、适应的结果。对于多倍体物种形成之初从基因到表型的变化，前人已作了大量工作，然而对很多相对古老的多倍体物种，人们对其长期演化仍知之甚少。探知该过程中所发生的遗传和形态变化及其原因机制，对研究生物多样性的发展和维持具有重要意义。高山蓍复合体是一个分布广、生境多样、形态变异大的异源四倍体类群，其双亲基因组差异较大，形成了严格二体遗传模式，这为更准确地研究多倍体的起源、分化提供了理想材料。本项目拟综合群体遗传、进化-发育和生态学的方法，利用高通量测序、基因表达模式和形态变异分析，探究高山蓍四倍体复合体的（1）居群动态历史；（2）居群的分化机制和（3）表型分化的基因作用机制。

本项目基本按原计划完成了对高山蓍异源四倍体复合体谱系的形成过程、群体动态和分化式样的探讨；分析了多倍体物种与亲本在叶片表型上的分化及其遗传发育基础。由于异源多倍体具有复杂的遗传背景，按常规二倍体物种的研究方法难于获取其完整可靠的群体遗传数据。在本项目研究过程中，我们首先利用RNA-seq数据，设计了异源四倍体的亲本特异引物，然后利用二代测序（扩增子测序）手段，得到了四倍体物种两套完整的亲本特异DNA序列数据，解决了复杂遗传数据获取的问题。在此基础上的分析发现，异源四倍体高山蓍（A. alpina）和云南蓍（A. wilsoniana）各自为独立起源，一个在中国东北地区附近，另一个在秦岭区域。通过ABC溯祖模型的模拟分析，发现两次异源多倍化事件均发生于距今约5万年前末次盛冰期（LGM）前的末次间冰阶。利用四个蓍属物种的分布信息，结合生物气候因子，我们模拟了四个物种在LGM时期的地理分布，发现当时的气候变化可能对于新形成的异源四倍体物种向低纬度地区的扩张有着重要影响。相同亲本物种间发生多次独立的杂交，其结果产生不同的物种，这样的异源多倍体形成模式有别于更为普遍的recurrent polyploidy，在植物中鲜有报道，今后可以借助这样的体系，研究多倍体物种表型和生态分化的机制。.关于四倍体物种的叶片形态演化及其遗传基础，本项目利用演化-发育生物学手段，统计分析了各研究物种内和物种间的叶形变异和分化，进而对多个叶齿/裂相关调控基因的表达模式进行了检测，结果发现，四倍体物种中双亲拷贝的表达模式保持了在亲本中的式样；从双亲拷贝的总表达量来看，独立起源的两个四倍体物种在各调控基因上均表现出介于双亲之间的表达量；从对双亲拷贝的表达偏好来看，各基因之间不尽相同。总之，异源四倍体物种叶裂介于双亲之间的表型可能与多个叶裂基因缺乏超亲表达的总表达量有一定关联。