基于贾第鞭毛虫与其近缘种的比较基因组学研究探讨有性生殖的进化

Forming gametes through meiosis and then carrying out sexual reproduction is one of the important traits that contribute to the superiority of eukaryotes over prokaryotes. This mode of reproduction can reconcile the contradiction between heredity and variation very well, and thus accelerate the evolution of organisms. However, how this important mode of reproduction evolved remains unclear. There is some evidence suggesting that Diplomonads, a group of primitive unicellular protozoans with Giardia as the representative, might be able to be taken as a valuable model to study this issue. This project is going to sequence the genomes of two free-living species, Trepomonas agilis and Heximta inflate, and then combined with the available genome databases of two parasitic species, Giardia lamblia and Spironucleus salmonicida, to perform comparative genomics studies among these four diplomonads, mainly including identification and comparison of the genes taking part in sexual reproduction -- sexual reproduction genomes, among them. Simultaneously, the sexual reproduction marker, retrotransposon, and other signs of sex such as meiosis, conjugation of cells, and ploidy changes will also be investigates and compared among them. Based on the results of the studies above, two questions will be further explored: 1) whether Diplomonads are in the primitive state of sexual reproduction evolution, if this is true, the evolution of sexual reproduction genome will be further investigated by taking them as model system; 2) whether Giardia’s simplification or even loss of sexual reproduction is the consequences of parasitic reduction, if this is true, the mechanism for this will be researched at gene and genome levels.

有性生殖是真核生物优越于原核生物的重要特征。它使遗传与变异这一矛盾很好地达成了“对立统一”，从而加速了生物进化的进程。但它自身是如何进化形成的仍不清楚。我们已有的研究提示：处在真核生物进化早期、以贾第鞭毛虫为代表的原生动物――双滴虫类有可能是探讨这一问题的很好模型。本项目拟首先测定其两种自由生活种类（活泼锥滴虫和膨六鞭虫）的基因组，再结合贾第鞭毛虫和旋核六鞭虫（均为寄生）的已有基因组，对它们的与有性生殖相关（参与减数分裂、细胞结合等）基因即“有性生殖基因亚组”进行鉴定和系统比较分析。同时还对逆转座子这一有性生殖“指示物”和其它细胞水平的性符号”如减数分裂、细胞结合、倍性变化等进行研究，探讨：1）双滴虫类是否是处在有性生殖进化的早期原始状态，若是，则可以其为模型开展有性生殖进化的研究；2）寄生是否是导致贾第鞭毛虫有性生殖简化甚至丧失的原因，若是，则探讨该现象在基因、基因组上发生机制和规律。

本项目以包括贾第虫及其近缘种在内的一些极为原始且迄今未观察到有性生殖现象的双滴虫类真核生物为研究对象，对有性生殖的进化问题进行了探讨。首先，按计划测定了自由生活的膨六鞭虫和活泼锥滴虫的基因组，在国际上首次获得了两种营自由生活的双滴虫类的基因组数据。并且还测定了敏捷贾第虫和在研究过程中发现的仓鼠贾第虫新种的基因组。其次，通过荧光标记和电子显微镜的观察，对四种双滴虫类的细胞水平的减数分裂、细胞结合、倍性变化等有性生殖活动进行大量的观察和分析，未发现它们具有相关的“性符号”。最后，我们基于基因组鉴定了共计七个双滴虫类物种的有性生殖相关基因，构建出它们的“有性生殖基因亚组”。对它们的有性生殖基因亚组进行比较分析，发现相对于典型真核生物的完整有性生殖基因亚组，所有双滴虫类物种都缺少了减数分裂特异性基因hop2和msh4以及负责细胞膜融合的HAP2基因家族；而贾第虫类因寄生生活缺失了更多一些相关基因。此外，在基因组水平也未发现这些双滴虫类具有逆转座子这一有性生殖“指示物”。综上，我们认为：双滴虫类还处于真核生物进化的早期阶段，普遍还不存在有性生殖活动，在基因组层面它们都还不具有典型真核生物的完整的有性生殖功能基因系统，这并非是某些寄生种类因寄生生活次生退化的结果，而应该是整个类群的原生特征。进一步对贾第虫不进行有性生殖但又能保持等位基因差异性很低的现象进行了研究，发现贾第虫滋养体分裂过程中存在的三种核分配方式是其重要机制。我们认为贾第虫“等双核”体制加上这种分裂过程中的三种核分配方式所形成的增殖方式可能是无性生殖和有性生殖之外的第三种生殖方式，抑或是有性生殖起源形成之前的早期形式，值得进行深入研究。.完成该项目的研究，已在BMC Genomics, Biology direct，Parasites & Vectors 等重要期刊上发表SCI论文5篇，培养毕业博士研究生2名、硕士研究生1名。