Yb body assembly on the flamenco piRNA precursor transcripts reduces genic piRNA production

In Drosophila ovarian somatic cells, piRNAs against transposable elements are mainly produced from the ~180-kb flamenco (flam) locus. flam transcripts are gathered into one-two foci, located close to nuclear envelope, and processed into piRNAs in the cytoplasmic Yb bodies. The mechanism of Yb body formation remains unknown. Using RNA FISH, we found that in the follicle cells of ovaries the 5'-ends of flam transcripts are usually located in close proximity to the nuclear envelope and outside of Yb bodies, while their extended downstream regions mostly overlap with Yb bodies. In flamKG mutant ovaries, flam transcripts containing the 1st and, partially, 2nd exons but lacking downstream regions are gathered into foci at the nuclear envelope, but Yb bodies are not assembled. Strikingly, piRNA production from the protein-coding gene transcripts is enhanced in flamKG ovaries. We propose that normally in ovarian follicle cells flam downstream transcript regions function not only as a substrate for generation of piRNAs but also as a scaffold for Yb body assembly, which is necessary to minimize piRNA production from protein-coding gene transcripts. By contrast, in ovarian somatic cap and escort cells Yb body assembly does not require flam transcription. Overall design: The fractions of small RNAs (19-29 nt) from ovaries of flam heterozygous flies (flamKG/FM7 control) and flamKG/Df mutants were sequenced using Illumina NovaSeq 6000