The piRNA pathway in Drosophila ovarian germ and somatic cells.

RNA silencing refers to gene silencing pathways mediated by small non-coding RNAs, including microRNAs. Piwi-interacting RNAs (piRNAs) constitute the largest class of small non-coding RNAs in animal gonads, which repress transposons to protect the germline genome from the selfish invasion of transposons. Deterioration of the system causes DNA damage, leading to severe defects in gametogenesis and infertility. Studies using Drosophila ovaries show that piRNAs originate from specific genomic loci, termed piRNA clusters, and that in piRNA biogenesis, cluster transcripts are processed into mature piRNAs via three distinct pathways: initiator or responder for ping-pong piRNAs and trailing for phased piRNAs. piRNAs then assemble with PIWI members of the Argonaute family of proteins to form piRNA-induced RNA silencing complexes (piRISCs), the core engine of the piRNA-mediated silencing pathway. Upon piRISC assembly, the PIWI member, Piwi, is translocated to the nucleus and represses transposons co-transcriptionally by inducing local heterochromatin formation at target transposon loci.