Piwi-interacting RNA (piRNA) biogenesis pathway in the Drosophila ovary. (A) Primary piRNA pathway in somatic support cells (cream region surrounding the central egg). The transposon sequence in piRNA clusters (the majority are unistrand clusters; see Figure 5 below) in somatic support cells is in an exclusively anti-sense orientation with regard to the direction of transcription. The resultant transcripts are transported to the cytoplasm, recognized, and processed by several factors, including Zucchini, Armi, and Yb. Finally, they are loaded onto the PIWI protein. (B) The ping-pong amplification loop in germ cells (light blue region). Transcripts from piRNA clusters (mainly dual-strand clusters; see Figure 5 below) and active transposons are processed into piRNAs by Aub and Ago3. piRNAs from sense transposon transcript are preferentially loaded onto Ago3, and those from anti-sense transposon transcript are preferentially loaded onto Aub.