ENGRAILED AND POLYHOMEOTIC INTERACTIONS ARE REQUIRED TO MAINTAIN THE A P BOUNDARY OF THE DROSOPHILA DEVELOPING WING/

Engrailed is a nuclear regulatory protein with essential roles in embr yonic segmentation and wing morphogenesis. One of its regulatory targe ts in embryos was shown to be the Polycomb group gene, polyhomeotic, W e show here that transheterozygous adult flies, mutant for both engrai led and polyhomeotic, show a gap in the fourth vein. In the correspond ing larval imaginal discs, a polyhomeotic-lacZ enhancer trap is not no rmally activated in anterior cells adjacent to the anterior-posterior boundary. This intermediary region corresponds to the domain of low en grailed expression that appears in the anterior compartment, during L3 , Several arguments show that engrailed is responsible for the inducti on of polyhomeotic in these cells. The role of polyhomeotic in this in termediary region is apparently to maintain the repression of hedgehog in the anterior cells abutting the anterior-posterior boundary, since these cells ectopically express hedgehog when polyhomeotic is not act ivated. This leads to ectopic expressions first of patched, then of cu bitus interruptus and decapentaplegic in the posterior compartment, ex cept for the dorsoventral border cells that are not affected, Thus pos terior cells express a new set of genes that are normally characterist ic of anterior cells, suggesting a change in the cell identity, Altoge ther, our data indicate that engrailed and polyhomeotic interactions a re required to maintain the anterior-posterior boundary and the poster ior cell fate, just prior to the evagination of the wing.