State Key Laboratory of Genetic Resources and Evolution, Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming, Yunnan 650223, PR China.
BMC Evol Biol. 2010 Feb 17;10:49. doi: 10.1186/1471-2148-10-49.
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.
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.
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.
贾第虫是一类广泛存在于多种脊椎动物肠道中的原生动物寄生虫。大量证据表明，兰氏贾第鞭毛虫可能是最原始的现存真核生物。这样一组最早分支的单细胞真核生物是如何以及何时获得成功寄生在最新分支的高等真核生物（脊椎动物）的能力，这是一个有趣的问题。基因复制长期以来一直被认为是产生进化新事物原始资源的最常见机制。为了分析贾第虫寄生生活方式的进化轨迹，我们在这里对兰氏贾第鞭毛虫的基因复制模式进行了全基因组分析。
尽管基因组比较表明，在兰氏贾第鞭毛虫中，许多基本生物途径的内容都简化了，整个基因组非常紧凑，但在我们的研究中，40%的基因被鉴定为重复基因。对这些重复基因的进化距离分析表明，兰氏贾第鞭毛虫基因组中发生了两轮大规模的复制事件。对它们的功能注释进一步表明，最近复制的基因大多数是 VSPs（变体特异性表面蛋白），这对于贾第虫在宿主中成功寄生生活是必不可少的。根据与宿主的进化比较，发现 VSPs 在兰氏贾第鞭毛虫中的快速扩张与胎盘哺乳动物的进化辐射一致。
基于兰氏贾第鞭毛虫重复基因的全基因组分析，我们发现基因复制对于贾第虫寄生生活方式的起源和进化至关重要。VSPs 的最近扩张仅发生在兰氏贾第鞭毛虫中，与它的宿主数量的增加一致。因此，我们提出了一个假设，即贾第虫宿主的增加可能是 VSPs 快速扩张的驱动力。
