DecapentaplegicOverexpression AffectsDrosophilaWing and Leg Imaginal Disc Development andwinglessExpression

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We have used the GAL4-UAS expression system to increase the level of expression of theDrosophilagenedecapentaplegic(dpp) in a pattern approximating its normal pattern in leg and wing imaginal discs. Intermediate increases ofdppexpression have little effect in wing discs but high levels ofdppoverexpression lead to reduction of the scutellum and duplication of posterior wing structures. In leg discs intermediate increases cause supernumerary outgrowths of ventral leg structures in the anterior–ventral region. Greater increases ofdppexpression cause the loss of ventral leg structures with the concomitant fusion of left and right dorsal forelegs. The defects observed in both legs and wings appear to arise through dose-dependent effects ofdpponwingless(wg) expression. A high level ofdppoverexpression in the wing disc causes reduction ofwgexpression in the presumptive scutellar region, consistent with the subsequent reduction of the scutellum. An intermediate increase ofdppexpression in leg discs induces the expansion ofwgexpression into the ventral outgrowths. At higherdppexpression levels, ventralwgexpression in leg discs is eliminated, consistent with the loss of ventral leg cuticle. In the leg disc end knob and in the wing margin primordium, wherewganddppcooperate in producing distal outgrowth,dppoverexpression has no detectable effect either on patterning or onwgexpression. We propose that a critical role fordppin other regions of the leg and wing discs is to reduce or block the expression ofwg.This role ofdppis supported by the observation that ectopicwgexpression is detected in imaginal discs wheredppsignaling is compromised by lowering the activity of one of its receptors,tkv.This antagonism betweendppandwgexpression may be critical to assigning only one disc region as the distal organizer.