Mechanistic divergence of piRNA biogenesis in Drosophila

Organisms require mechanisms to distinguish self and non-self RNA. This distinction is crucial to initiate the biogenesis of piRNAs. In Drosophila ovaries, PIWI-guided slicing and the recognition of piRNA precursor transcripts by the DEAD-box RNA helicase Yb are the two known mechanisms to initiate biogenesis in the germline and the soma, respectively. Both, the PIWI proteins and Yb are highly conserved across most Drosophila species and are thought to be essential to the piRNA pathway and for silencing TEs. However, we find that species closely related to D. melanogaster have lost the yb gene, as well as the PIWI gene Ago3. We show that neither mechanism is required to efficiently produce TE antisense piRNAs in Drosophila. Thus, there are more possible routes through which the piRNA pathway can achieve specificity than previously suggested.