单细胞原生动物——贾第虫（Giarida）被认为是目前已知的最原始的真核生物，然而它却能寄生在动物界中进化程度最高的脊椎动物（特别是哺乳动物）上。在生物界中这么早就分化出来的原始真核生物后来是如何成功地寄生于比它出现晚得多的高等生物上的呢？它势必经历了一个从早期的非寄生生活向寄生生活的进化，且该进化过程是一个十分成功的适应性进化过程，以使它能广泛适应地寄生于许多脊椎动物上。那么这种适应性进化的发生在基因组存在怎样的机制呢？
中国科学院昆明动物研究所真核细胞进化基因组文建凡研究员指导的博士研究生孙隽，通过对蓝氏贾第虫（Giardia lamblia）全基因组中基因重复(duplication)的系统性研究，发现该寄生原虫的基因组在进化历史上发生过两次大规模的基因重复。其中，最近的一次包括了与寄生虫逃避宿主的免疫系统的攻击有关的VSP（Variant-specific Surface Proteins，即变化的特异表面蛋白）基因的大量重复。且发现该次基因迅速扩增事件发生的时间与有胎类哺乳类动物的辐射分化时间相吻合。
该研究结果提示：贾第虫实现由自由生活到寄生到多种哺乳类生物上的成功进化与VSP基因的大量重复密切相关，从而从基因组进化的角度揭示了这一生物进化现象的内在机制。
该研究成果近期已在进化生物学领域的重要刊物BMC Evol. Biol.上发表。论文发表后引起国际上的广泛关注。(生物谷Bioon.com)
生物谷推荐原文出处：
BMC Evolutionary Biology doi:10.1186/1471-2148-10-49
Gene duplication in the genome of parasitic Giardia lamblia.
Jun Sun1,2* , Huifeng Jiang1,3* , Roberto Flores3 and Jianfan Wen1
1 State Key Laboratory of Genetic Resources and Evolution, Kunming Institute of Zoology, Chinese Academy of Sciences (CAS), Kunming, Yunnan 650223, PR China
2 Graduate School of Chinese Academy Sciences, Beijing 100039, PR China
3 Division of Nutritional Sciences, Cornell University, Ithaca, NY 14853, USA
Background
Giardia are a group of widespread intestinal protozoan parasites in a number of vertebrates. Much evidence from G. lamblia indicated they might be the most primitive extant eukaryotes. When and how such a group of the earliest branching unicellular eukaryotes developed the ability to successfully parasitize the latest branching higher eukaryotes (vertebrates) is an intriguing question. Gene duplication has long been thought to be the most common mechanism in the production of primary resources for the origin of evolutionary novelties. In order to parse the evolutionary trajectory of Giardia parasitic lifestyle, here we carried out a genome-wide analysis about gene duplication patterns in G. lamblia.
Results
Although genomic comparison showed that in G. lamblia the contents of many fundamental biologic pathways are simplified and the whole genome is very compact, in our study 40% of its genes were identified as duplicated genes. Evolutionary distance analyses of these duplicated genes indicated two rounds of large scale duplication events had occurred in G. lamblia genome. Functional annotation of them further showed that the majority of recent duplicated genes are VSPs (Variant-specific Surface Proteins), which are essential for the successful parasitic life of Giardia in hosts. Based on evolutionary comparison with their hosts, it was found that the rapid expansion of VSPs in G. lamblia is consistent with the evolutionary radiation of placental mammals.
Conclusions
Based on the genome-wide analysis of duplicated genes in G. lamblia, we found that gene duplication was essential for the origin and evolution of Giardia parasitic lifestyle. The recent expansion of VSPs uniquely occurring in G. lamblia is consistent with the increment of its hosts. Therefore we proposed a hypothesis that the increment of Giradia hosts might be the driving force for the rapid expansion of VSPs.
