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 ...»»»»
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.^^^^
