Fig. 6. Dpp signalling sequentially establishes the notum-hinge boundary in second instar wing disc (L2) and the medial-lateral subdivision of the notum in third instar wing disc (L3). A and P, anterior and posterior compartment. Dpp source is hatched in blue. During the second instar, Iro-C is activated by Vn/EFGR in the most proximal part of the wing disc (Wang et al., 2000; Zecca and Struhl, 2002a; Zecca and Struhl, 2002b). The distal border of the Iro-C domain, which defines the notum-hinge boundary (Diez del Corral et al., 1999), is established by repression by Dpp signalling, which at this stage only functions in the distal parts of the disc. In the third instar, dpp is expressed in the notum territory and again negatively regulates Iro-C in the medial notum. In the third instar disc, additional, uncharacterized factors (question marks) help maintain the notum-hinge border of Iro-C expression, since this border is generated, at least in part, under conditions of strong depletion of Dpp signalling (Fig. 4D, Fig. 5G).

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