Figure 1.

Ectopic expression of hh in the absence of mtv activity. (A) Wing disc labelled to visualize expression of the mtv-lacZ reporter gene (antibody to β-Gal, green) in an hh-GAL4;UAS-GFP background (GFP, blue). (F) Wing disc labelled to visualize expression of a hh-lacZ reporter gene (antibody to β-gal, blue) and Mtv protein expression (green or white). (B–E,G,H) Clones of cells mutant for mtv (mtv⁶; B–E) or for both engrailed/invected and mtv (Df(2R)en[E] mtv⁶; G,H) and labelled by the absence of the GFP marker (green). The contour of the clone is marked by a red or white line. hh-lacZ (blue or white; B,C,G), Engrailed (En, red; C), Hedgehog (Hh, purple; D) or Patched (Ptc, purple or white; E,H) is ectopically expressed in a clone abutting the anterior–posterior (ap) and dorsal–ventral (dv; dashed lines) boundaries. Note in (D) that the marked clone with ectopic Hh protein expression was born in the anterior (a) compartment (twin clone is marked by an arrowhead) but crosses into the posterior (p) compartment. A neighbouring p clone (white asterisk) and its twin (white arrow) are shown. Violation of the ap compartment was very frequently observed in these clones (data not shown). (I,J) Two different mechanisms to repress hh expression in a cells. (I) En expression in p cells induces hh expression and represses cubitus interruptus (ci) expression. Hh activity in a cells stabilizes the transcriptional activator form of Ci (Ci^act). In the absence of Hh signal, Ci is transformed to a transcriptional repressor (Ci^rep). Ci^rep represses Hh expression. (J) Proposed model illustrating the role of Mtv and Gro in the repression of hh in a cells. GFP, green fluorescent protein; Gro, Groucho; Mtv, master of thickveins.