Figure 3. Refractoriness to Wg is defined by Notch.

(A) Schematic representation of the results pointed out by (B) in regard of the expression pattern in DV boundary and neighboring cells. Several cell populations can be distinguished at and around the DV boundary in terms of the expression of Notch protein and the activity of Notch and Wg pathways. Orange, green, and violet squares highlight the D and V compartments, and the DV boundary, respectively. (B) High magnification of the DV boundary of mature wild-type wing discs showing, in red, Wg and Cut protein expression in boundary cells as well as higher levels of Notch protein expression in boundary and neighboring cells. Non-boundary cells show, in blue, high levels of Delta and Senseless (Sens) protein and naked mRNA. As revealed by Cut and Sens expression, the width of the boundary and flanking cell populations comprise a small number (two-three) of cells. (C,D) Mature wing discs expressing the intracellular domain of Notch, N^intra, (C) or Wg (D) under dpp^Gal4 control. In the first case, Gal4-expressing cells do not respond to Wg whereas in the second case they do. (E) Small clones of cells in the boundary lacking Notch activity marked by the absence of GFP (red) and pointed out by means of a white arrowhead. Sens protein (in blue) starts to be expressed in boundary mutant cells. (F) Mature wing disc that expresses a dominant negative form of Delta (Delta^DN) under dpp^Gal4 control. Note that boundary cells lacking Cut expression (in red) start to express Sens (blue). (C'–F') In silico counterparts of the results shown in (C–F). Ectopic expression of activation of Notch (C') or Wg (D'). (E'–F') As occurs in vivo, the absence of Notch activity in the boundary causes those cells to lose Cut expression and to start responding to Wg.