The genetic framework of the Drosophila piRNA pathway

The animal piRNA pathway is a small RNA silencing system that acts in gonads and protects the genome against the deleterious influence of transposons. A major bottleneck in the field is the lack of comprehensive knowledge of the factors and molecular processes that constitute this pathway. We conducted an RNAi screen in Drosophila and identified ~50 genes that strongly impact the ovarian somatic piRNA pathway. Many identified genes fall into functional categories that indicate essential roles for mitochondrial metabolism, RNA export, the nuclear pore, transcription elongation and chromatin regulation in the pathway. Follow-up studies on two factors demonstrate the identification of components acting at distinct hierarchical levels of the pathway. Finally, we define CG2183/Gasz as a novel primary piRNA biogenesis factor in somatic and germline cells. Based on the similarities between insect and vertebrate piRNA pathways our results have far-reaching implications for the understanding of this conserved genome defense system. Overall design: Steady-state RNA levels in wild-type ovarian somatic cells (OSC) and RNAi knock-downs of the piRNA pathway components.