Project description:Drosophila Piwi-family proteins have been implicated in transposon control. Here, we examine piwi-interacting RNAs (piRNAs) associated with each Drosophila Piwi protein and find that Piwi and Aubergine bind RNAs that are predominantly antisense to transposons, whereas Ago3 complexes contain predominantly sense piRNAs. As in mammals, the majority of Drosophila piRNAs are derived from discrete genomic loci. These loci comprise mainly defective transposon sequences, and some have previously been identified as master regulators of transposon activity. Our data suggest that heterochromatic piRNA loci interact with potentially active, euchromatic transposons to form an adaptive system for transposon control. Complementary relationships between sense and antisense piRNA populations suggest an amplification loop wherein each piRNA-directed cleavage event generates the 5’ end of a new piRNA. Thus, sense piRNAs, formed following cleavage of transposon mRNAs, may enhance production of antisense piRNAs, complementary to active elements, by directing cleavage of transcripts from master control loci. Keywords: small RNA libraries from Drosophila ovaries
Project description:Transposons evolve rapidly and can mobilize and trigger genetic instability. piRNAs silence these genome pathogens, but it is unclear how the piRNA pathway adapts to new transposons. In Drosophila piRNAs, encoded by heterochromatic clusters are maternally deposited in the embryo. Paternally inherited P-element transposons thus escape silencing and trigger a genetic instability and sterility. We show that this syndrome, termed P-M hybrid dysgenesis, also disrupts the piRNA biogenesis machinery and activates resident transposons. As dysgenic hybrids age, however, fertility is restored, P-elements are silenced, and P-element piRNAs are produced de novo. In addition, the piRNA biogenesis machinery is restored and resident elements are silenced. Significantly, new resident transposons insertions accumulate in piRNA clusters, and these new insertions are transmitted to progeny with high fidelity, produce novel piRNAs, and are associated with reduced transposition. P-M hybrid dysgenesis thus leads to heritable changes in chromosome structure that appear to enhance transposon silencing.