Fig. 3. Activation profile of Notch during early stages of follicle cell patterning. (A-A'') During the earliest stages of oogenesis, Notch activation in follicle cells (monitored by anti-ß-gal staining of the Su(H)_m8-lacZ reporter, red) is restricted to the polar cells (marked by A101-GAL4/UAS-mCD8::GFP, green), and cannot be observed in stalks. The prominent signal in follicle cells of later egg chambers corresponds to the differentiation of main-body follicle cells and their switch from mitosis to endoreplication, following germline Dl induction of Notch signaling at stages 5-6 of oogenesis (Lopez-Schier and St Johnston, 2001; Ruohola et al., 1991). (B) Anti-ß-gal staining of the Notch reporter m7-lacZ identifies essentially the same pattern as the Su(H)_m8-lacZ reporter. (C) However, with the m7-lacZ reporter, low levels of Notch activation could also be observed in the stalk cells at early stages (arrow). (D) Immunolocalization of Dl (red) reveals relatively higher levels of Dl protein in the germ line than in the follicle cells in early egg chambers.