State Key Laboratory of Genetic Resources and Evolution, Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming, People's Republic of China.
PLoS One. 2012;7(6):e36878. doi: 10.1371/journal.pone.0036878. Epub 2012 Jun 7.
5S rRNA is a highly conserved ribosomal component. Eukaryotic 5S rRNA and its associated proteins (5S rRNA system) have become very well understood. Giardia lamblia was thought by some researchers to be the most primitive extant eukaryote while others considered it a highly evolved parasite. Previous reports have indicated that some aspects of its 5S rRNA system are simpler than that of common eukaryotes. We here explore whether this is true to its entire system, and whether this simplicity is a primitive or parasitic feature.
METHODOLOGY/PRINCIPAL FINDINGS: By collecting and confirming pre-existing data and identifying new data, we obtained almost complete datasets of the system of three isolates of G. lamblia, two other parasitic excavates (Trichomonas vaginalis, Trypanosoma cruzi), and one free-living one (Naegleria gruberi). After comprehensively comparing each aspect of the system among these excavates and also with those of archaea and common eukaryotes, we found all the three Giardia isolates to harbor a same simplified 5S rRNA system, which is not only much simpler than that of common eukaryotes but also the simplest one among those of these excavates, and is surprisingly very similar to that of archaea; we also found among these excavates the system in parasitic species is not necessarily simpler than that in free-living species, conversely, the system of free-living species is even simpler in some respects than those of parasitic ones.
CONCLUSION/SIGNIFICANCE: The simplicity of Giardia 5S rRNA system should be considered a primitive rather than parasitically-degenerated feature. Therefore, Giardia 5S rRNA system might be a primitive system that is intermediate between that of archaea and the common eukaryotic model system, and it may reflect the evolutionary history of the eukaryotic 5S rRNA system from the archaeal form. Our results also imply G. lamblia might be a primitive eukaryote with secondary parasitically-degenerated features.
5S rRNA 是高度保守的核糖体成分。真核生物的 5S rRNA 及其相关蛋白（5S rRNA 系统）已得到深入了解。一些研究人员认为，贾第虫是现存最原始的真核生物，而另一些人则认为它是一种高度进化的寄生虫。先前的报告表明，其 5S rRNA 系统的某些方面比常见的真核生物更为简单。我们在这里探讨这种简单性是否适用于其整个系统，以及这种简单性是原始特征还是寄生特征。
方法/主要发现：通过收集和确认现有数据并确定新数据，我们获得了三种贾第虫分离株、两种其他寄生挖掘生物（阴道毛滴虫、克氏锥虫）和一种自由生活的挖掘生物（纳氏锥虫）的 5S rRNA 系统几乎完整的数据集。在综合比较这些挖掘生物以及古菌和常见真核生物的系统的各个方面后，我们发现所有三种贾第虫分离株都拥有简化的 5S rRNA 系统，该系统不仅比常见真核生物简单，而且在这些挖掘生物中也是最简单的，而且与古菌非常相似；我们还发现，在这些挖掘生物中，寄生种的系统不一定比自由生活种简单，相反，自由生活种的系统在某些方面比寄生种更简单。
结论/意义：贾第虫 5S rRNA 系统的简单性应被视为原始特征而非寄生退化特征。因此，贾第虫 5S rRNA 系统可能是介于古菌和常见真核生物模式系统之间的原始系统，它可能反映了真核生物 5S rRNA 系统从古菌形式进化的历史。我们的结果还表明，贾第虫可能是一种具有二次寄生退化特征的原始真核生物。
