Identification of Blattella germanica piRNAs

Abstract

The Piwi-interacting RNA (piRNA) system is an evolutionarily conserved mechanism involved in the control of transposable elements and maintenance of genomic stability, especially in germ line cells and in early embryo stages. However, relevant particularities, both in mechanism and function, exist across species among metazoans and even within the insect class. As a member of the scarcely studied hemimetabolan group, the cockroach Blattella germanica can be a suitable reference model to study insect evolution. The results presented in this work have been obtained applying three different processes of identification of piRNAs, first using a global analysis grouping all libraries of RNA-seq, second performing an individual analysis for each library and third applying an additional filtering step to the second process. These three methodologies have allowed us to identify piRNAs in a robust way, followed by the identification of piRNAs clusters. Finally, piRNAs expression of B. germanica has been studied across 11 developmental stages, ranging from unfertilized egg to nymphs and adult female. Our results confirm the dual origin of piRNA in this species, with a majority of them being generated from the primary pathway, and a smaller but highly expressed set of sequences participating in the secondary (“ping-pong”) reamplification pathway. The study of piRNAs expression across the developmental stages shows an intriguing partial complementarity in expression is observed between the piRNA from the two biogenesis pathways, with those generated in the secondary pathway being quite restricted to early embryo stages. In addition, many piRNAs are exclusively expressed in late embryo and nymphal stages. These results point at piRNA functions beyond the role of transposon control in early embryogenesis, suggesting a more complex scenario, with different sets of piRNAs acting in different times and having a range of functions wider than previously thought.