DRAWING A STRIPE IN DROSOPHILA IMAGINAL DISKS - NEGATIVE REGULATION OF DECAPENTAPLEGIC AND PATCHED EXPRESSION BY ENGRAILED

During development of the Drosophila adult appendage precursors, the l arval imaginal disks, the decapentaplegic (dpp) gene is expressed in a stripe just anterior to the anterior/posterior (A/P) compartment boun dary. Here, we investigate the genetic controls that lead to productio n of this stripe. We extend previous observations on leaky engrailed ( en) mutations by showing that mutant clones completely lacking both en and invected (inv) activity ectopically express dpp-lacZ reporter gen es in the posterior compartment, where dpp activity ordinarily is repr essed. Similarly, patched (ptc) is also ectopically expressed in such posterior compartment en(-)inv(-) null clones. In contrast, these en(- )inv(-) clones exhibit loss of hedgehog (hh) expression. We suggest th at the absence of dpp expression in the posterior compartment is due t o direct repression by en. Ubiquitious expression of en in imaginal di sks, produced by a hs-en construct, eliminates the expression of dpp-l acZ in its normal A/P boundary stripe. We identify three in vitro Engr ailed binding sites in one of our dpp-lacZ reporter gene. Mutagenesis of these Engrailed binding sites results in ectopic expression of this reporter gene, but does not alter the normal stripe of expression at the A/P boundary. We propose that the en-hh-ptc regulatory loop that i s responsible for segmental expression of wingless in the embryo is re utilized in imaginal disks to create a stripe of dpp expression along the A/P compartment boundary.