Homology-dependent silencing by an exogenous sequence in the Drosophila germline.

The study of P transposable element repression in Drosophila melanogaster led to the discovery of the trans-silencing effect (TSE), a homology-dependent repression mechanism by which a P-transgene inserted in subtelomeric heterochromatin (Telomeric Associated Sequences) represses in trans, in the female germline, a homologous P-lacZ transgene inserted in euchromatin. TSE shows variegation in ovaries and displays a maternal effect as well as epigenetic transmission through meiosis. In addition, TSE is highly sensitive to mutations affecting heterochromatin components (including HP1) and the Piwi-interacting RNA silencing pathway (piRNA), a homology-dependent silencing mechanism that functions in the germline. TSE appears thus to involve the piRNA-based silencing proposed to play a major role in P repression. Under this hypothesis, TSE may also be established when homology between the telomeric and target loci involves sequences other than P elements, including sequences exogenous to the D. melanogaster genome. We have tested whether TSE can be induced via lacZ sequence homology. We generated a piggyBac-otu-lacZ transgene in which lacZ is under the control of the germline ovarian tumor promoter, resulting in strong expression in nurse cells and the oocyte. We show that all piggyBac-otu-lacZ transgene insertions are strongly repressed by maternally inherited telomeric P-lacZ transgenes. This repression shows variegation between egg chambers when it is incomplete and presents a maternal effect, two of the signatures of TSE. Finally, this repression is sensitive to mutations affecting aubergine, a key player of the piRNA pathway. These data show that TSE can occur when silencer and target loci share solely a sequence exogenous to the D. melanogaster genome. This functionally supports the hypothesis that TSE represents a general repression mechanism which can be co-opted by new transposable elements to regulate their activity after a transfer to the D. melanogaster genome.