7 EGF Receptor Signaling in Drosophila Oogenesis

This chapter discusses the spatial regulation of epidermal growth factor receptor (Egfr) activation in oogenesis and considers possible mechanisms for localization of its ligand. The spatial regulation of Egfr activity in the follicular epithelium of the ovary is achieved by the localization of its ligand, gurken, within the germline. The final distribution of gurken within the oocyte appears to be specified both by the localization of the gurken RNA and by regulation of gurken protein accumulation, possibly at the level of translation. Localized activation of the Egfr distinguishes certain subpopulations of follicle cells, thereby generating asymmetry within the follicular epithelium. In early oogenesis, Egfr activation in posterior follicle cells defines the anterior–posterior polarity of the egg chamber, and in midoogenesis restriction of Egfr activity to dorsal follicle cells determines dorsal–ventral polarity. A number of factors that required downstream of the Egfr have been identified but the mechanism by which the observed patterning of the follicular epithelium is achieved remains unclear. The dynamic expression patterns of some of these targets suggest that the initial gurken-Egfr signal at the dorsal side of the follicular epithelium mediates an initial distinction between dorsal and ventral follicle cells and also initiates subsequent refinement processes that determine the final pattern of cell fates.