Delta signaling from the germ line controls the proliferation and differentiation of the somatic follicle cells during Drosophila oogenesis

The body axes of Drosophila are established during oogenesis through recipr ocal interactions between the germ line cells and the somatic follicle cell s that surround them. The Notch pathway is required at two stages in this p rocess: first, for the migration of the follicle cells around the germ line cyst and, later, for the polarization of the anterior-posterior (A-P) axis of the oocyte. Its function in these events, however, has remained controv ersial. Using clonal analysis, we show that Notch signaling controls eel pr oliferation and differentiation in the whole follicular epithelium. Notch m utant follicle cells remain in a precursor state and fail to switch from th e mitotic cell cycle to the endocycle. Furthermore, removal of Delta from t he germ line produces an identical phenotype, showing that Delta signals fr om the germ cells to control the timing of follicle cell differentiation. T his explains the axis formation defects in Notch mutants, which arise becau se undifferentiated posterior follicle cells cannot signal to polarize the oocyte. Delta also signals from the germ line to Notch in the soma earlier in oogenesis to control the differentiation of the polar and stalk follicle cells. The germ line therefore regulates the development of the follicle c ells through two complementary signaling pathways: Gurken signals twice to control spatial patterning, whereas Delta signals twice to exert temporal c ontrol.