Combined activities of Gurken and Decapentaplegic specify dorsal chorion structures of the Drosophila egg

During Drosophila oogenesis Gurken, associated with the oocyte nucleus, act ivates the Drosophila EGF receptor in the follicular epithelium. Gurken fir st specifies posterior follicle cells, which in turn signal back to the ooc yte to induce the migration of the oocyte nucleus from a posterior to an an terior-dorsal position. Here, Gurken signals again to specify dorsal follic le cells, which give rise to dorsal chorion structures including the dorsal appendages. If Gurken signaling is delayed and starts after stage 6 of oog enesis the nucleus remains at the posterior pole of the oocyte, Eggs develo p with a posterior ring of dorsal appendage material that is produced by ma in-body follicle cells expressing the gene Broad-Complex. They encircle ter minal follicle cells expressing variable amounts of the TGF beta homologue, decapentaplegic. By ectopically expressing decapentaplegic and clonal anal ysis with Mothers against dpp we show that Decapentaplegic signaling is req uired for Broad-Complex expression. Thus, the specification and positioning of dorsal appendages along the anterior-posterior axis depends on the inte rsection of both Gurken and Decapentaplegic signaling, This intersection al so induces rhomboid expression and thereby initiates the positive feedback loop of EGF receptor activation, which positions the dorsal appendages alon g the dorsal-ventral egg axis.