Hedgehog activity, independent of Decapentaplegic, participates in wing disc patterning

In the Drosophila wing imaginal disc, the Hedgehog (Hh) signal molecule induces the expression of decapentaplegic (dpp) in a band of cells abutting the anteroposterior (A/P) compartment border. It has been proposed that Dpp organizes the patterning of the entire wing disc. We have tested this proposal by studying the response to distinct levels of ectopic expression of Hh and Dpp, using the sensory organ precursors (SOPs) of the wing and notum and the presumptive wing veins as positional markers. Here, we show that Dpp specifies the position of most SOPs in the notum and of some of them in the wing. Close to the A/P compartment border, however, SOPs are specified by Hh rather than by Dpp alone. We also show that late signaling by Hh, after setting up dpp expression, is responsible for the formation of vein 3 and the scutellar region, and also for the determination of the distance between veins 3 and 4. One of the genes that mediates the Hh signal is the zinc-finger protein Cubitus interruptus (Ci). These results indicate that Hh has a Dpp-independent morphogenetic effect in the region of the wing disc near the A/P border.