Characterization and parental contribution of piRNAs and endo-siRNAs to mouse zygotes.
Transposable elements (TEs) are widely represented in eukaryotic genomes. Recently, a set of small RNAs known as rasRNAs (repeat-associated small RNAs) have been related to the down-regulation of TEs conferring a means to safeguard genome integrity. Two key members of the rasRNAs group are piRNAs and endo-siRNAs. In this study, we have performed a comparative analysis of piRNAs and endo-siRNAs present in mouse oocytes, spermatozoa and zygotes, identified by deep sequencing and bioinformatic analysis. Both piRNAs and endo-siRNAs regulate TEs in addition to other repetitive elements such as tRNAs and rRNAs, suggesting an alternative role of rasRNAs with regard to translation regulation. The detection of piRNAs and endo-siRNAs in sperm cells and revealed also in zygotes, hints to their potential delivery to oocytes during fertilization. However, a comparative assessment of the three cell types indicates that both piRNAs and endo-siRNAs are mainly maternally inherited. Finally, we have assessed the role of the different rasRNA molecules in connection with amplification processes by way of the “ping-pong cycle”. Our results suggest that the ping-pong cycle can act on other rasRNAs, such as tRNA- and rRNA-derived fragments, thus not only being restricted to TEs during gametogenesis, as was evidenced in spermatozoa, oocytes and zygotes. Overall design: Comparative analysis from deep sequencing of piRNAs and endo-siRNAs in mouse oocytes, spermatozoa and zygotes