Single-cell RNA sequencing identifies regulators of differentiation and nutritional cues in Drosophila female germ cells

Drosophila ovarian germline stem cells (GSCs) are powerful model for stem cell research. However, due to the scarcity of GSCs in ovarian tissue, it is difficult to obtain the transcriptional profile of GSCs and identify novel GSC markers. In this study, we took advantage of single cell RNA sequencing (scRNA-seq) to profile the germline cells and somatic cells in wild type Drosophila ovary. We then performed an in vivo RNAi screen and network analysis to identify genes that are involved in the early stages of germ cell differentiation. We identified 33 genes with limited expression during early germ cell development and identified 19 genes that potentially regulate germ cell differentiation. Among these, an uncharacterized gene, which we named eggplant (eggpl), is highly expressed in GSC and downregulated in early daughter cells. Upon RNAi knockdown of eggpl, we observed an increase in germ cell proliferation, an accumulation of cysts in the early mitotic (2- and 4-cell) stages and an increase in overall ovary size compared to control when flies were maintained on a standard diet. In flies fed a rich yeast diet, the expression of eggpl was significantly lower and the effects of eggpl RNAi were suppressed, suggesting that downregulation of eggpl may link nutritional status to germ cell proliferation and differentiation. We also found that the matrix metalloproteases, Mmp1 and Mmp2 as well as the tissue inhibitor of metalloproteases (Timp) are additional regulators of eggpl. Collectively, this study provides new insight into the signaling networks that regulate early germ cell development and identifies eggpl as a key player in this process.