SEQUENTIAL ACTIVATION OF THE EGF RECEPTOR PATHWAY DURING DROSOPHILA OOGENESIS ESTABLISHES THE DORSOVENTRAL AXIS

Previous work has demonstrated a role for the Drosophila EGF receptor (Torpedo/DER) and its ligand, Gurken, in the determination of anteriop osterior and dorsoventral axes of the follicle cells and oocyte, The r oles of DER in establishing the polarity of the follicle cells were ex amined further, by following the expression of DER-target genes. One c lass of genes (e.g. kekon) is induced by the DER pathway at all stages , Broad expression of kekon at the stage in which the follicle cells m igrate posteriorly over the oocyte, demonstrates the capacity of the p athway to pattern all follicle cells except the ventral-most rows, Thi s may provide the spatial coordinates for the ventral-most follicle ce ll fates, A second group of target genes (e.g. rhomboid (rho)) is indu ced only at later stages of oogenesis, and may require additional inpu ts by signals emanating from the anterior, stretch follicle cells. The function of Rho was analyzed by ectopic expression in the stretch fol licle cells, and shown to induce a non-autonomous dorsalizing activity that is independent of Gurken, Rho thus appears to be involved in pro cessing a DER ligand in the follicle cells, to pattern the egg chamber and allow persistent activation of the DER pathway during formation o f the dorsal appendages.