Figure 7.5.

Gradient versus cascade models. (A) The gradient model, highlighting that, depending on a cell’s position in the disc, Dll and dac CRMs must interpret very different ratios of Dpp:Wg signaling. (B) The cascade model, in which Wg+ Dpp are only required to initiate PD axis formation by activating Dll and ligands for the EGFR pathway. Dll in turn activates dac, and both Dll and dac maintain their expression in a Wg+Dpp-independent manner. EGFR activity maintains dac repression, while Wg+Dpp repress dac in the center of the leg disc early in leg development.