Giardia,  as a kind of unicellular protozoa, was once thought to be the most primitive extant eukaryote because of its many so-called primitive molecular and cellular traits and branching out very early from the eukaryote trunk on many phylogenetic trees. Thus, this creature was though to be of very important significance to studying how eukaryotes evolved from  their prokaryotic ancestor. However, some authors considered the simplicity/primitiveness of many aspects of Giardia, including genomic reduction, to be the results of parasitic degeneration rather than primitive traits.

5S rRNA, its genes, and the related proteins for its transcription, transportation and assembly, which were altogether called ‘5S rRNA system’ here, exist in all organisms to perform fundamental cellular functions, which is well studied. By using this simple and explicit system, the research team “Evolutionary Genomics of Eukaryotic Cells” led by Prof. Wen Jian-fan of Kunming Institute of Zoology, CAS attempted to study whether this system is primitive or degenerative in Giardia. After collecting published data and identifying new  ones of 5S rRNA system, they compared the Giardia 5S rRNA system to those of two parasiticas well as one free-living species, which here served as the representatives with genome database in other three subgroups of Excavate, respectively, and also to those of Archaea and common eukaryotes. The results showed that the three Giardia isolates 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, and that, 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. Collectively, these results indicate clearly that 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. It is also reasonable to speculate that Giardia might be a primitive eukaryote on the aspect of basic cellular structures and functions with secondary parasitically-degenerated features closely related to its parasitic lifecycle. Therefore, they propose that Giardia is most likely a combination of eukaryotic primitiveness and secondary parasitic degeneration, and this viewpoint will be of important significance to the studies of the origin and evolution of eukaryotic cells. This work has been officially published on PLoS ONE recently (doi:10.1371/journal.pone.0036878).

This work was supported by grants from the National Natural Science Foundation of China and the Knowledge Innovation Program.