piRNA-mediated nuclear accumulation of retrotransposon transcripts in the Drosophila female germline

Germline silencing of transposable elements is essential for the maintenance of genome integrity. Recent results indicate that this repression is largely achieved through a RNA silencing pathway that involves Piwi-interacting RNAs (piRNAs). However the repressive mechanisms are not well understood. To address this question, we used the possibility to disrupt the repression of the Drosophila I element retrotransposon by hybrid dysgenesis. We show here that the repression of the functional I elements that are located in euchromatin requires proteins of the piRNA pathway, and that the amount of ovarian I element piRNAs correlates with the strength of the repression in the female germline. Antisense RNAs, which are likely used to produce antisense piRNAs, are transcribed by heterochromatic defective I elements, but efficient production of these antisense small RNAs requires the presence in the genome of euchromatic functional I elements. Finally, we demonstrate that the piRNA-induced silencing of the functional I elements is at least partially posttranscriptional. In a repressive background, these elements are still transcribed, but some of their sense transcripts are kept in nurse cell nuclear foci together with those of the Doc retrotransposon. In the absence of I element piRNAs, either in dysgenic females or in mutants of the piRNA silencing pathway, sense I element transcripts are transported toward the oocyte where retrotransposition occurs. Our results indicate that piRNAs are involved in a posttranscriptional gene-silencing mechanism resulting in RNA nuclear accumulation.