Fig. 4. Control of wing disc development by EGFR signaling. (I) Early during larval development, high levels of EGFR signaling activate ap (hatched) and the Iro-C (yellow) genes in overlapping domains within the dorsoproximal region of the wing disc. (II) During subsequent development, Iro-C expression depends on continuous EGFR input. Hence, Iro-C expression remains confined to the area of high EGFR signaling (yellow), segregating the disc into distinct notum (yellow) and wing (white) primordia. By contrast, ap expression, once activated, becomes heritable, segregating the disc into D (ap^ON, hatched) and V (ap^OFF, unhatched) compartments. Hence, as the disc grows, ventrally situated cells within the D compartment continue to express ap, even after they move out of the range of high EGFR signaling. The shift of the DV boundary into a region of low EGFR signaling appears essential to allow D and V compartment cells to interact to initiate a positive feedback loop of reciprocal Notch signaling that drives the expression of Wg, Vg and other ‘boundary’ genes (green). (III) The products of these genes then organize the subsequent growth and differentiation of the prospective wing blade (including the long-range induction of Vg, which defines the wing blade primordium, blue), and the prospective wing hinge (defined by the absence of Vg and Iro-C expression, white).