Divergent composition and transposon-silencing activity of small RNAs in mammalian oocytes

We found piRNAs with different lengths represented the predominant small RNA species in oocytes from the 12 explored species, except mouse. We found endo-siRNAs resulted from the truncated Dicer isoform were mouse-specific, and os-piRNAs associating with PIWIL3 in human oocytes are widespread in mammals and are typically with low levels of the 2’-3’-O-methylation. The sequences of many highly expressed piRNA clusters are fast-evolving compared with their syntenic genomic locations, and the TE families distributing in the conserved piRNA clusters are various between species.