How oogenesis is controlled upon nutrient challenge is a key biological question to understand the balance between reproduction and adult fitness. During Drosophila oogenesis, vitellogenic stages are highly energy consuming so their formation has to be balanced with other physiological needs. We reveal the role of the Insulin pathway and FoxO in regulating the transition from Mitotic-to-Endocycle, a critical step controlling the entry of egg chambers into vitellogenesis. We show that the M/E switch functions as a nutrient checkpoint, blocking the entry into vitellogenesis upon starvation and therefore protecting adults from energy loss. Pausing of the M/E switch involves a previously unknown crosstalk between FoxO, Cut and Notch, a fully reversible process ensuring rapid resuming of oogenesis upon re-feeding. This work reveals a FoxO-dependent nutrient checkpoint integrating metabolic cues with reproduction and protecting tissues from starvation-induced damages. In addition, we show that the Insulin pathway regulates the migration of a subset of epithelial cells to ensure oocyte fertilization. We demonstrate that Insulin signaling regulates the formation of actin-rich cellular extensions in invasive cells. During this process, FoxO represses chickadee expression, which encodes Profilin. Insulin signaling activity leads to the inhibition of FoxO and subsequent Profilin accumulation, which further allows actin polymerization, necessary for cell motility. Altogether, data reveal a crucial role for the conserved Insulin signaling pathway in regulating ovarian follicles through somatic tissues, a process which is likely to share much in common with oogenesis in mammals.
Role of the Insulin signalling pathway in coupling oogenesis rate with nutritional cues in Drosophila