Fig. 8. Scheme for the spacial and temporal order of determination of follicle cell fates by the Notch and JAK/STAT pathways. (A) At stage 1 of oogenesis, follicle cells encounter high levels of Dl from the germ line. Expression of Fng in the anterior polar/stalk precursors enhances Notch signaling, leading to the induction of polar cells (PC). The relatively lower levels of Dl expressed by follicle cells are further reduced by the metalloprotease Kul, thus leading to low levels of Notch activation in those follicle cells that do not contact the germ line. This low level of activation promotes cell viability and contributes to induction of the stalk cell fate. (B) The polar cells express the Domeless ligand Upd, which diffuses to neighboring follicle cells. The main-body follicle cells, which experience high or intermediate levels of Notch activation, are refractory to JAK/STAT signaling, and repress STAT nuclear localization. By contrast, the pre-stalk cells, which are subject to low levels of Notch activation, undergo JAK/STAT activation. The combined activity of the two signaling pathways thereby facilitates proper stalk cell-fate induction.