Fig. 8. Expression patterns and function of dally contribute to shaping the Dpp morphogen gradient in wing disc. (A) Model for Dally function in Dpp signaling. Dally (red) forms a signaling complex with Dpp (green) and receptor molecules (yellow) on the cell surface (left). Purple arrows represent signaling activity. Increased levels of Dally can enhance Dpp signaling by stabilizing the signaling complex (middle). However, excess levels of Dally sequester Dpp protein and show an inhibitory effect on signaling (right). (B) Levels of tkv expression (yellow), dally expression (red) and Dpp signaling (purple). Expression of both Tkv (a Dpp receptor) and Dally (a Dpp co-receptor) is regulated by several common molecular pathways in the wing. (1) Hh signaling suppresses tkv and activates dally. (2) En induces tkv and represses dally. (3) Dpp signaling downregulates both genes. As the Tkv receptor and Dally co-receptor mediate Dpp signaling, this regulatory pathway forms a negative-feedback loop. At anterior and posterior edges of the wing pouch, lower levels of Dpp signaling result in high levels of Tkv and Dally, which sensitize cells to Dpp.