SERRATE AND WINGLESS COOPERATE TO INDUCE VESTIGIAL GENE-EXPRESSION AND WING FORMATION IN DROSOPHILA

Background: The appendages of insects, like the limbs of vertebrates, grow out of the body wall after the establishment of a proximo-distal axis among a group of primordial cells. In Drosophila, the wing develo ps in the limbless larva from one of the imaginal discs of the thorax, which give rise to the adult epidermis. The earliest identified requi rement in wing development is for the induction of vestigial (vg) gene expression at the interface between ventral cells and dorsal cells of the wing disc. It has been proposed that this event requires two reci procal signals - one from the dorsal to the ventral cells and the othe r from the ventral to the dorsal cells - which trigger vg expression a t the presumptive wing margin and hence initiate the development of th e wing tissue. Results: We have identified four genes - Serrate (Ser), wingless (tug), Notch and Suppressor of Hairless (Str(H)) - whose act ivity is required during the second and early third larval instars for the expression of vg. Analysis of the functions and patterns of expre ssion of these genes at the time of the inductive event indicates that the Ser protein acts as a dorsal signal, and the Wg protein as a vent ral signal for the induction of vg expression. Furthermore, the expres sion of both Ser and Wg is sufficient to trigger ectopic wing developm ent in the wing disc and leg discs. The product of the Notch gene, whi ch encodes a receptor, is also required for this event and we suggest that its role is to integrate the inputs of Ser and Wg. Conclusions: W e show that the induction of vg, which initiates wing development in D rosophila, requires the combined activities of Ser, tug and Notch. Bas ed on the patterns of expression and requirements for Ser and wg in th is process, we propose that Ser is a dorsal signal and that Wg is a ve ntral signal, and that their combination at the dorso-ventral interfac e activates the Notch receptor and leads to vg expression.