Ecdysone Receptor modulates EGFR signaling for timely cyst packaging in Drosophila germ cells

Abstract

Oocytes are packaged into ovarian follicles, each containing a maturing germ cell surrounded by a layer of somatic cells that secrete protective eggshell and chorion proteins. This conserved arrangement is essential for proper oocyte development and reproductive success. Continuous coordination and bi-directional signaling from somatic cells to germ cells is necessary for proper oocyte packaging. In Drosophila, although germ cell packaging was presumed to be largely controlled by somatic follicle cells enveloping passive germ cells, recent studies suggest that germ cells themselves produce motor forces that drive somatic encapsulation. Here, in support of this hypothesis, we present data suggesting that cyst encapsulation is dependent upon Ecdysone Receptor, a steroid hormone receptor known to control multiple aspects of oogenesis. Using tools to deplete EcR levels or block transcriptional activation specifically in the germline, we show that germline-autonomous EcR is necessary for the timing of cyst encapsulation. In the absence of EcR, germ cell encapsulation is slowed, resulting in increased incidence of cyst collision events in the germarium. EcR facilitates germ cell cyst encapsulation by signaling somatic cells to promote proliferation, organize escort cell projections, and intercalate stalk cells. We show that an increase in germ cell cortical contractions is sufficient to rescue cyst collisions. Finally, we conclude that EcR is likely acting with EGFR signaling to modulate escort cell activity. Overall, these data suggest that in addition to its well-characterized roles in somatic follicle cells, EcR is necessary in the germline to promote timely ovarian follicle assembly and development.