Identification of a novel autonomous role for Ecdysone Receptor during Drosophila ovarian germ cell differentiation
A tightly regulated network of signals coordinates the division of Drosophila germline stem cells (GSCs) and differentiation of their daughter cells to produce viable oocytes. The steroid hormone, ecdysone, is known to promote GSC self-renewal; however, due to lack of compatible genetic tools, potential autonomous roles in the germline have not been fully evaluated. Ecdysone elicits a diverse array of transcriptional responses by binding to a heterodimeric complex composed of Ecdysone Receptor (EcR) and Ultraspiracle (Usp). To elucidate whether EcR facilitates autonomous reception of ecdysone in the germline, we built germline-compatible genetic tools to manipulate EcR levels or activity. Depleting EcR mRNA or loss of EcR ligand binding in the germline caused a loss of GSCs over time, demonstrating that it is necessary for GSC self-renewal. Further, over-expressing either EcR.A or EcR.B1 isoforms resulted in undifferentiated germ cell tumors and decreased numbers of GSCs. This is a ligand-dependent function of EcR, as over-expression of EcR that cannot bind ecdysone suppressed tumor development. By restricting over-expression to later stages of the germline, we found that EcR is sufficient to induce tumors only when over-expressed in the GSCs and early daughter cells. In tumorous ovaries, stem-like cells were also identified outside of their normal stem cell niche, suggesting that these cells remain competent to respond to BMP signals. Using single-cell RNA-sequencing to compare EcR over-expressing and Tkv over-expressing ovaries, we found remarkably similar transcriptomes in heterogenous populations of germ cells. These data suggest that EcR promotes differentiation of germ cells by directly regulating components of the BMP signaling pathway and that low levels of EcR expressed in wildtype GSCs are sufficient for ligand-dependent activation of an ecdysone-responsive transcriptional program. Altogether these data reveal a novel, autonomous role of EcR in GSC maintenance and regulation of differentiating daughter cells.
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