Fig. 4. 5XQE transgene expression requires the vg-dependent feed-forward signal. (A) vg⁰ disc carrying two types of clones: Tub1>vg clones, black by the absence of GFP (red) and 5XQE>vg clones, black within the prospective Drosophila wing pouch (outlined by dotted line) by the absence of CD2 expression (green). Here (and in B,C), expression of the 5XQE>vg transgene is monitored indirectly in Tub1>GFP>vg tissue by robust expression of 1XQE-lacZ (appears lavender). As illustrated at the bottom, the Tub1>vg clone (green) has induced adjacent cells in the abutting 5XQE>vg clone (lavender) to express the 5XQE>vg transgene, and induction of the 5XQE>vg transgene has propagated through the clone and induced 5XQE>CD2>vg expression (yellow) in neighboring cells on the other side. Expression of both the 1XQE-lacZ and 5XQE>CD2>vg transgenes are rescued within the Tub1>vg clone (as in Fig. 1F,G). (B,C) vg⁰ discs that carry abutting Tub1>vg and 5XQE>vg clones, as in A, except that the 5XQE>vg and Tub1>vg clones are black by, respectively, absence of GFP (green, owing to excision of a >Tub1-GFP> Flp-out cassette) and absence of DsRed (red). As in A, the Tub1>vg clones in both discs (green in the diagram) have induced 5XQE>vg expression (lavender) that propagates into the abutting 5XQE>vg clones, rescuing wing development.