Movement of endogenous Dpp through receptor mutant clones refutes the receptor-mediated transcytosis model.

Third instar wing imaginal discs with LOF clones for tkv brk (A–C) and tkv brk in a sax mutant background (D–F). (A–F) The Dpp production region is indicated by staining for the Hh target Patched (green), and receptor mutant clones are shown by the loss of lac-Z expression (A,B) or nGFP (C–F). The Dpp gradient is visualized indirectly, by staining for the pathway activity (pMad) (B′,C′,E′,F′), or by staining for the Dpp target gene spalt (A′,D′). (A″–F″) Sketches of the analyzed wing discs. The green line indicates the Dpp production region, considered clones are shown in gray, and red arrows indicate the regions behind clones in which high Dpp signaling activity can only occur if Dpp moves through receptor mutant tissue, which is indeed the case, as high Spalt and pMad signals can be observed in these regions.