Dpp of posterior origin patterns the proximal region of the wing

The decapentaplegic (dpp) gene encodes a long-range morphogen that plays a key role in the patterning of the wing imaginal disc of Drosophila (Nellen, D., Burke, R., Struhl, G. and Basler, K. 1996. Direct and long-range action of a DPP morphogen gradient. Cell 85, 357-368.). The current view is that dpp is transcriptionally active in a narrow band of anterior compartment cells close to the anterio-posterior (A/P) compartment border. Once the Dpp protein is synthesised, it travels across the A/P border and diffuses forming concentration gradients in the two compartments (reviewed in Lawrence, P.A., Struhl, G. 1996. Morphogens, compartments, and pattern: lessons from drosophila? Cell 85, 951-961.). We have found a new site of dpp expression in the posterior wing compartment which appears during the third larval period. This source of Dpp signal generates a local gradient of Dpp pathway activity, which is independent of that originating in the anterior compartment. We show that this posterior tier of Dpp activity is functionally required for normal wing development: the elimination of dpp expression in the posterior compartment results in defective adult wings in which pattern elements such as the alula and much of the axillary cord are not formed. Moreover, these structures develop normally in the absence of anterior dpp expression. Thus the normal wing pattern requires distinct Dpp organizer activities in the anterior and posterior compartments. We further show that, unlike the anterior dpp expression domain, the posterior one is not dependent on Hedgehog activity but is dependant on the activity of the IRO complex gene mirror. Since there is a similar expression in the haltere disc, we suggest that this late appearing posterior Dpp activity may be an attribute of dorsal thoracic discs.