Fig. 1. Relationship between Notch activation and DV compartmentalization. (A) Signaling interactions at the DV boundary. Delta (Dl) is expressed by both dorsal and ventral cells in response to Notch activation (blue arrow), but signals most effectively (thick black arrow) to dorsal cells owing to the presence of Fng, and only signals poorly (thin arrow) to ventral cells. Activated Notch in dorsal cells acts together with the dorsal-specific gene ap (not shown) to promote expression of Ser. Ser is blocked (black T) from signaling (gray arrow) to other dorsal cells by Fng, and thus is limited to signaling back across the compartment boundary (thick black arrow) to ventral cells, where it activates Notch. As shown here, Notch activation also results in the elevation of F-actin (green lines) along the cell interface where peak signaling occurs; the resolution of confocal microscopy is such that this usually appears as a single line (see also Fig. 4). (B-D) Third instar wing imaginal discs, stained for ap-lacZ expression (red) to mark dorsal cells, and Wg (blue), to mark Notch activation. Expression of ap-lacZ is a reliable marker of dorsal provenance, as it is not affected by Notch signaling or changes in cell location (Blair et al., 1994; Micchelli and Blair, 1999; Rauskolb et al., 1999). In this and subsequent figures, discs are oriented with ventral down and anterior to the left. (B'-D''') Individual stains of the discs shown in B-D. (B) AyGal4 UAS-fng, with a clone of Fng-expressing cells (arrow) marked by co-expression of GFP (green). Ectopic expression of Fng can effectively reposition the compartment boundary away from the normal DV interface by simultaneously creating an ectopic stripe of Notch activation within ventral cells, and eliminating normal Notch activation at the DV interface (Rauskolb et al., 1999). The normal DV interface (arrowhead) is relatively straight and smooth, and is disturbed by the Fng-expressing clone. (C,D) AyGal4 UAS-Dl, with clones of Dl-expressing cells (arrows) marked by co-expression of GFP (green). Ectopic expression of Dl can effectively reposition the compartment boundary away from the normal DV interface by simultaneously creating an ectopic stripe of Notch activation within dorsal cells, and eliminating normal Notch activation at the DV interface. In C, the clone edge is smoother apically, as evidenced by Wg expression, than basally, as evidenced by GFP expression. In D, the ectopic Notch activation stripes are smooth, but do not completely register with the wild-type DV boundary.