Fig. 1. brk is not required for DV axis specification. (A) Schematic representation of a third instar leg imaginal disc, summarizing the functions of Dpp and Wg in DV and PD axis development. The Wg (orange), Dpp (blue), Dll (green), Dll+Dac (yellow) and Dac-only (red) domains are indicated for a third instar leg disc. The Dac-only domain is larger in the dorsal disc. While Dpp promotes dorsal fates and Wg promotes ventral fates, Dpp and Wg act combinatorially to establish the PD axis. (A') Expression of Dll (green) and dac (red) in a third instar leg disc. All images of leg discs are oriented anterior leftwards and dorsal upwards. (B) A wild-type third instar leg disc stained for brk-lacZ (red), Wg (green) and P-Mad (blue). brk-lacZ expression is reciprocal to P-Mad, which is highest dorsally but is also weakly observed ventrally. The inset shows brk expression, which is weakly downregulated in ventral cells at this stage. (C-F'') brk^- clones (absence of GFP, arrows) have no effect on dpp-lacZ (C-C''), wg-lacZ (D-D''), omb-lacZ (E-E'') or H15-lacZ (F-F''). Engrailed (En, blue) marks the posterior compartment. The ' and '' panels show enlargements of the clones as indicated.