Fig. 1. EGFR/Ras signaling is required for notum development. (A,B) Organization of the mature wing imaginal disc. (A) Subdivision of a wild-type wing imaginal disc into AP compartments (as marked by en-Gal4 driven GFP expression in posterior cells; green), DV compartments (as marked by the expression of Ap in dorsal cells; blue) and notum-wing primordium (as marked by elevated expression of mirr-lacZ in prospective notum cells; red). The notum-wing boundary is marked by an arrowhead; mirr-lacZ is also expressed in prospective pleural cells in the mature wing disc (to the right and below the arrowhead). (B) mirr-lacZ expression (red) relative to Wg expression (blue) in a wild-type wing imaginal disc. Overlapping expression appears in pink. Note that mirr-lacZ expression is generally restricted to the prospective lateral notum (region III), which is demarcated dorsally by a stripe of Wg expression (arrows in B-E) and ventrally by a characteristic deep fold (arrowheads in A-E). However, as noted in the text, the Iro-C genes are also expressed in additional domains in mature, third instar wing discs, including in a stripe of cells extending ventrally along the edge of the disc. The prospective medial notum (region IV) is located dorsal to this stripe of Wg expression, and the prospective wing blade (region I) is encircled by two closely associated rings of Wg expression (only one ring is visible here, asterisk), distinguishing it from the surrounding prospective wing hinge (region II). Wg is also expressed along the DV compartment boundary, bisecting the wing blade primordium into D and V halves. (C-F) Egfr– (C) and ras– (D-F) clones marked by the absence of either GFP expression (C-E, green) or CD2 expression (F, green). Wg expression is shown in blue (C-E) and faint green (F); tsh-lacZ expression, a marker for prospective wing hinge, is shown in blue (F). (C) Distribution of Egfr– clones induced during late second/early third larval instar using the Minute technique. Note that clones are present throughout the disc, except in an area corresponding to the prospective lateral notum (between the arrow and arrowhead). (D) Distribution of ras– clones generated in a non-Minute background during first larval instar. ras+ twin spots are marked by bright GFP expression. ras– clones generally failed to survive in the prospective lateral notum (see also Fig. 2B). Note the presence of a large ras– clone in the prospective medial notum (asterisk), which abuts the prospective lateral notum (defined by the stripe of Wg expression), which straddles the boundary between the two domains. (E) Disc with at least two large ras– clones generated during first larval instar using the Minute technique. Note that the mutant clones populate most of the prospective wing blade and wing hinge and show a normal pattern of Wg expression; however, the clones do not contribute to the notum, which also appears to develop normally and shows normal Wg expression. (F) Disc with large ras– clones generated during first larval instar using the Minute technique. The notum is ablated and the disc is composed largely of prospective wing hinge tissue (marked by high levels of tsh-lacZ expression; blue) and wing blade tissue (encircled by a thin stripe of Wg expression and bisected by an additional stripe of Wg expression along the DV compartment boundary (faint green).
