Different types of small RNAs in protozoa

The origin and evolution of small RNA molecules is a long-standing mystery. The protozoa represent the earliest eukaryotes and encompass rich diverse genetic resources. However, there are many contradictions and disputes prevalent in studies on protozoan microRNAs. In this review, we summarize the research on the analysis of small RNA transcriptome data on two representative protozoans, Trypanosoma brucei and Giardia lamblia , as generated by high throughput sequencing. The results show that these protozoans do not have canonical miRNAs. Unexpectedly, there are many types of endogenous small interfering RNAs (endo-siRNAs). In addition, stress induced tRNA derived small RNAs (sitRNAs) were observed in different life cycle stages of these parasites. In total, there are six kinds of sitRNAs in G. lamblia . These small RNAs are then found to be involved in the differentiation processes of primitive eukaryotes . These results showed that systematic analysis of small RNAs in the protozoa revealed a more complex picture than previously though