Giardia lamblia sRNA transcriptome

Small RNAs (sRNAs), including microRNAs and endo-siRNAs regulate most important biological processes in eukaryotes, such as cell division and differentiation. Although sRNAs have been extensively studied in various eukaryotes, the role of sRNAs in the early emergence of eukaryotes is unclear. To address these questions, we deep-sequenced the sRNA transcriptome of four different stages in the differentiation of Giardia lamblia, one of the most primitive eukaryotes. We identified a large number of endo-siRNAs in this fascinating parasitic protozoan, and found that they were produced from alive telomeric retrotransposons and three genomic regions i.e. eSGR regions. eSGR-derived endo-siRNAs were proved to target mRNAs in trans. Gradual upregulation of endo-siRNAs in the differentiation of Giardia suggested that they might be involved in the regulation of this process. This was demonstrated by the impairment of the differentiation ability of Giardia when GlDicer, essential for the biogenesis of endo-siRNAs, was knocked down. Endo-siRNAs are not the only sRNA regulators in Giardia differentiation, because a great number of tRNAs-derived sRNAs showed more dramatic expression changes than endo-siRNAs in this process. We totally identified five novel kinds of tRNAs-derived sRNAs, and found that the biogenesis in four of them might be correlated with that of sitRNA, discovered in our previous studies. Our studies reveal an unexpected complex panorama of sRNA in G. lamblia, and shed light on the origin and functional evolution of eukaryotic sRNAs.