Fig. 3. A PD axis does not require Dpp signaling in the absence of Brk. (A) dpp^discs mutants have very small discs and the normal distal and medial Dll and dac domains are absent. The Dll and dac expression that remains corresponds to the Dpp- and Wg-independent trochanter region (arrow), which also expresses Homothorax (Hth, blue). (B) brk^XA discs are overgrown, and the Dll and dac expression domains are expanded in the ventral and lateral parts of the disc. (C) brk^XA; dpp^discs discs are overgrown and Dll, Dll+Dac and Dac expression domains are all present. Dll expression is expanded dorsally and the dorsal Dac-only domain is not present (arrow). Dac expression is expanded laterally compared with wild type (asterisks; compare with Fig. 1A'). (D) In brk^XA; dpp^discs discs, wg (red) is expressed along the entire DV axis. Dll is also expanded dorsally, following the dorsal Wg expression (arrow). (E) In brk^XA; dpp^discs discs, the distal domain of al is not present (arrow), whereas other, Dpp- and Wg-independent domains of al are still present (compare with wild type, F). (F) Expression of al in a wild-type disc. The distal al domain is indicated by the arrow.