Fig. 4. Hh moves from cell to cell. All the wing discs were derived from third instar larvae. Anterior is towards left and dorsal is upwards in all the wing disc images (A-H). (A-C') dpp-lacZ staining (red) in a wild-type wing disc (A), and discs carrying sfl^9B4 mutant clones (B,B',C,C'). In the wild-type wing disc, dpp-lacZ is expressed in eight to ten cells anterior to the AP boundary in response to Hh signal (A). dpp-lacZ expression was only observed in a maximum of three cells at the posterior edge of sfl^9B4 clones (marked by broken lines in B and C, and by the absence of GFP in B' and C'), and high levels of lacZ expression were only detected in one or two cells adjacent to posterior wild-type cells. (D-H) GFP-Dpp expression under the control of dpp^Gal4 in a wild-type wing disc (D) and discs carrying mutant clones of sfl^9B4 (E,F), ttv⁶³ (G), and dally⁸⁰-dly^A187 (H). The mutant clones are marked by the absence of Myc (E,F,H) or ß-gal (G) staining (red) and are outlined with broken lines. In all cases, GFP-Dpp expression is restricted to the posterior-most row of cells in the mutant clones. Hh movement towards anterior cells is blocked by even one cell diameter of mutant clones (arrows in B,C,E,H) as indicated by dpp-lacZ or GFP-Dpp expression.