Fig. 8. Model of action of Ubx on the Dpp pathway. (A) Scheme of the relationship between Ubx and different elements of this pathway in the central domains of wing (W) and haltere (H) discs. In the haltere disc, Ubx reduces dpp transcription, eliminates dally and mtv expression in most of the pouch and elevates Tkv levels, thus reducing the extent of Dpp spread and activity (but increasing it in AB cells). (B,C) Cartoons representing the distribution of Dpp (blue balls) in wing (B) and haltere (C) discs, showing less Dpp and less Dpp spread in the latter. The Tkv expression is indicated by the size of the Y symbol and the colours in the nuclei represent Dpp activity. (D,E) Effect of Ubx^- (D) and Ubx^- tkv⁺ (E) mutant clones in Dpp signaling. In Ubx^- clones the levels of Tkv are reduced so that Dpp can travel through the clone and reach the wild-type cells in more peripheral positions (D). If tkv is expressed in these clones, it retains Dpp and reduces the extent of Dpp activity (E).