THE REGULATION OF HEDGEHOG AND DECAPENTAPLEGIC DURING DROSOPHILA EYE IMAGINAL DISC DEVELOPMENT

The hedgehog signalling pathway is a conserved mechanism which acts in inductive processes in both vertebrate and invertebrate development t o direct growth and patterning. In Drosophila, the secreted Hedgehog p rotein acts as a signal to induce non-autonomous activation in adjacen t cells of either the decapentaplegic or wingless genes (both of which encode growth factor-like molecules), via inactivation of patched act ivity. In the eye disc, this pathway drives progression of the morphog enetic furrow, while in the wing (and leg and antennal) discs it is re quired to set up an organising centre along the anteroposterior compar tment boundary. We have compared the regulation and function of hedgeh og pathway activity in the eye and wing discs, and find that there are significant differences. Whereas in the wing disc, engrailed function is required for hedgehog expression, in the eye disc activation and m aintenance of hedgehog expression is achieved independently of engrail ed. Regulation of decapentaplegic expression also differs: in the wing disc it is repressed in the anterior compartment by patched and in th e posterior compartment by engrailed. In the eye disc, however, it is repressed posterior to the morphogenetic furrow in the absence of eith er parched or engrailed activity. We conclude that in the eye disc the re are novel aspects to hedgehog pathway function. Moreover, engrailed does not: play an essential conserved role.