Fig. 2. Vg expression and wing development in vg^b clones; evidence that QE activity requires `priming' by Vg. (A-C,E,E') vg<sup>b</sup> (A,B,E,E') and vg<sup>0</sup> (C) clones (black by the absence of GFP, green) induced before (A,E,E') or after (B,C) D-V segregation. Both vg<sup>b</sup> and vg<sup>0</sup> clones contribute normally to the proximal hinge and notum primordia. However, vg<sup>b</sup> clones induced before the D-V segregation often fail to contribute to the wing pouch (A,F) or to express 1XQE-lacZ (E', red), whereas clones induced afterwards succeed (B, inset; see F). vg<sup>0</sup> clones invariably fail to contribute to the wing pouch (C). For A-D, the sibling `twin' clones are marked by doubled GFP expression, bright green. For E, the Minute technique was used to give the vg^b clone a growth advantage. Numbers correspond to time of clone induction in hours after egg laying (h AEL); the D-V segregation occurs at 60 h AEL. (D) Early-induced vg^b clones generated in homozygous rp49-vg discs contribute to the wing pouch and express 1XQE-lacZ (red). (F) Bar charts showing the survival of vg^b or control (vg⁺) clones in the wing pouch, relative to that of their wild-type twin clones, depending on the time of induction (h AEL) and the absence or presence of rp49-vg. Bars represent the percentage of wild-type twin clones that contribute to the pouch (1) with an associated vg^b clone that contributes to the pouch (green), (2) without an associated vg^b clone (red), or (3) with an associated vg^b clone that contributes only to the hinge primordium (yellow). n, total number of clones scored for each experimental condition. In the absence of the rp49-vg transgene, early-induced vg^b clones contribute only rarely to the pouch, and appear instead to sort into the hinge primordium or to be lost. The ratio of vg^b clones that contribute to the pouch (type 1, green) increases at the expense of the other two types as a function of time of induction, reaching the wild-type distribution after the D-V segregation. In rp49-vg discs, both early- and late-induced vg^b clones contribute almost normally to the pouch.