Fig. 4. Changes in rho and iro expression in larval discs prefigure the abnormal vein pattern of col¹ wings. (A,B) rho expression in wild-type and col¹ third instar wing discs. In col¹ discs (B), rho expression in the presumptive L3 vein is wider than in wild type (A), while it is strongly downregulated dorsally or absent ventrally (white arrow) in the L4 vein primordium. The L3 and residual L4 primordia are closer to each other than in wild type. (C,D) Double in situ hybridisation for rho (blue) and col (pink, red bar) transcripts. In wild-type discs, there is no overlap between rho and col expression (C). The overlap observed in col¹ discs (D) indicates a posterior shift of L3 vein. (E,F) Double in situ hybridisation for ara (blue, grey bar) and col (pink, red bar) transcripts. In col¹ discs (F), the anterior border of ara expression is shifted closer to the AP boundary. (G,H) Dpp signalling in wing discs, revealed by the distribution of phosphorylated P-Mad. In wild-type discs (G), Dpp signalling is strongly downregulated in the AP organiser (white bar) and peaks on either side (black bars). In col¹ discs (H), a uniform labelling is observed in the centre of the disc (black bar). (I) Schematic diagram of the expression domains of En (orange box), Col (red box), iro (grey box), rho (heavy grey hatching) and high Dpp signalling (Mad*, blue) in wild-type (top) and col¹ (bottom) wing discs, based on data presented in A-H (see Blair, 1992; Gomez-Skarmeta et al., 1996; de Celis and Barrio, 2000). The L3 provein is indicated in light grey hatching and the AP border as a vertical black bar. This scheme postulates that the anterior shift of iro expression in col¹ mutants is due to changes in sal/salr activity in response to changes in Dpp signalling, while increased rho expression reflects the loss of repression by Col in posterior-most iro-expressing cells.

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