The piRNA pathway in the Drosophila germline

Maintaining and securing genome integrity is of central importance for all organisms. Besides chemical and physical DNA damage, which are highly efficiently repaired by different DNA repair machineries, a major additional threat for genome integrity is posed by transposable elements. These parasitic DNA portions are able to integrate and to multiply in host genomes. Essentially all organisms harbor a wide variety and large quantity of transposons. In order to prevent uncontrolled transposition and DNA damage, plants, fungi, and animals have evolved efficient suppression systems, which lead to the selective silencing of transposon gene expression and hence their transposition. Amongst these defense pathways, small RNA-mediated silencing pathways have a central importance. Pathways that utilize small regulatory RNAs are able to selectively silence gene expression if the target RNA is complementary to the small regulatory RNA. Indeed, small RNAs act as sequence specific guides to recruit the silencing effectors (proteins from the Argonaute protein family) to the target RNA. In animals, transposon silencing is particularly important in germline cells, as these are the only cells that pass on the genetic information to future generations. In these cells, a specialize small RNA silencing pathwaythe Piwi/piRNA pathwayis active and it leads to the very selective and potent silencing of transposable elements. Loss of this pathway leads to transposon de-silencing, widespread genome damage, germcell loss, and sterility. In the project The piRNA pathway in the Drosophila germline, we systematically studied this genome defense system in the fruitfly Drosophila melanogaster using genetics and molecular biology approaches. The major findings of our work are: (1) Using a genetic screen, we uncovered the set of genes that that is involved in the piRNA pathway. All in all, more than 40 proteins have specific functions in this genome defense system and these findings provide many entry points into understanding the molecular makeup of this pathway. (2) We could show that piRNAs do not only guide the destruction of transposon transcripts, but that they also guide transcriptional silencing of transposons in the nucleus. This has lead to interesting links between this small RNA pathway and heterochromatin biology. (3) Finally, we have investigated the genomic source loci that encode piRNAs. Intriguingly, many of these loci are themselves embedded in heterochromatin and we found that specialized pathways allow their transcription into the piRNA precursor molecules. Considering the strong similarities between the Drosophila piRNA pathway and the counterparts in mammals including humans, our findings serve as important pioneering studies in the general understanding of transposon silencing in animals.