Fig. 6. Proposed model for the interplay between the Hh, Dpp and EGF signalling pathways in the AP organiser and patterning of the medial region of the wing. Anterior cells that receive high doses of Hh (the AP wing organiser) activate Col, which, in turn, upregulates expression of BS, vn, emc and mtv, and represses expression of EGFR (^aMohler et al., 2000). BS (D-SRF) is required for commitment of Col-expressing cells to an intervein fate (L3-L4 intervein) and Emc for their proliferation. Repression of tkv by Mtv leads to low levels of Dpp signalling in the AP organiser cells and high levels in both A and P flanking cells (^bFunakoshi et al., 2001). Vn diffuses and activates EGF signalling in posterior cells (visualised by rho expression; heavy grey hatching). Col-mediated upregulation of Vn expression and modulation of Dpp signalling are both required for activation of EGF signalling in posterior cells and formation of L4 vein. rho repression in posterior-most iro-expressing cells positions the L3 vein competent domain anterior to the AP organiser (see Fig. 4I for the role of sal/salr in positioning iro expression). The signals triggering EGF activation in L3 vein and positioning rho expression at the centre of L3 and L4 provein domains (Biehs et al., 1998) remain unknown.