Piwi Is Required in Multiple Cell Types to Control Germline Stem Cell Lineage Development in the Drosophila Ovary. Xing Ma^1,2. 1) Stowers Institute for Medical Research, Kansas city, MO; 2) Department of Anatomy and Cell Biology, University of Kansas, Medical Center.

   Background: The piRNA pathway plays an important role in maintaining genome stability in the germ line by silencing transposable elements (TEs). In the Drosophila ovary, escort cells (ECs) physically interact with differentiated germline stem cell (GSC) progeny and promote their differentiation by preventing BMP signaling. Although piRNAs are known to be produced in Drosophila ovarian somatic cells, their biological function remains poorly defined. Results: Using genetics and cell biology approaches, we demonstrated that Piwi, a key piRNA pathway component, functions in multiple cell types to control GSC maintenance and differentiation. EC-specific knockdown of piwi causes a reduction in EC number and accumulation of GSC-like cells in which BMP signaling activity is elevated. In the piwi knockdown ECs, TE transcripts increase significantly and consequently DNA damage is also elevated. Interestingly, simultaneous knockdown of chk2, encoding a key checkpoint regulatory kinase, can rescue the GSC lineage differentiation defect caused by piwi knockdown, indicating that DNA checkpoint activation is the cause for the germ cell differentiation defect. Although Piwi is proposed to function in the niche for maintaining GSCs, niche-specific piwi knockdown only causes moderate GSC loss phenotype. Surprisingly, germ cell-specific knockdown of piwi but not aub and armi, results in complete germ cell loss, indicating that Piwi is required intrinsically to control early germ cell development in a piRNA-independent pathway. Conclusions: Our results therefore have revealed novel functions of Piwi in ECs to promote germ cell differentiation and in early germ cells for their maintenance. We propose that Piwi is required in ECs to promote germ cell differentiation by maintaining genome stability and.