Roles of piRNA Pathway Components Piwi and Aubergine In Drosophila melanogaster Female Germline Development

Abstract

Recent studies have identified a new class of small RNAs with 24-30 nucleotides in length. This class of small RNAs is associated with the PIWI clade Argonaute proteins, thus named PIWI-interacting RNAs (piRNAs). piRNAs mediate silencing of transposable elements (TEs) by transcriptional silencing and transcript destruction. In this study, we investigated the roles of two piRNA pathway components Piwi and Aubergine (Aub) in Drosophila melanogaster female germline development. Our results demonstrate that Piwi functions in the differentiation niche, the escort cells (ECs), of the Drosophila ovary to repress the germline stem cell (GSC) self-renewal molecule dpp transcripts, thus ensuring proper germline differentiation. It has been proposed that Piwi functions in the somatic niche to control GSC self-renewal. Our study demonstrates the intrinsic role of Piwi in the germline to maintain primordial germ cells (PGCs) before adulthood and GSCs during adulthood. We have also revealed a new intrinsic role of Piwi in promoting germline differentiation. Our study demonstrates the requirement of Piwi in multiple cell types of the Drosophila ovary for both GSC maintenance and germ cell differentiation. Different from the expression of Piwi in all cell types in the ovary, Aub is specifically expressed in the germline. Our genetic results demonstrate that Aub is only required in the germline for GSC maintenance and germ cell differentiation, similar to the function of Piwi in the germline. A mutation of lok, the homolog of chk2 in Drosophila, is sufficient to rescue the aub mutant GSCs, demonstrating that the aub mutant GSC loss is caused by DNA damage checkpoint activation. Removing one copy of aub significantly enhances the germ cell differentiation defect of bam heterozygous deletion. Aub is also capable of forming a protein complex with Bam in Drosophila S2 cells and co-localizes with Bam in the nuage, a perinuclear structure important for piRNA processing and TE silencing. These data together suggest that Aub and Bam function cooperatively to promote germ cell differentiation. TE transcripts are upregulated in bam mutants as they are in aub mutants, further supporting the notion that Bam may interact with the piRNA pathway to repress TEs.