Fig. 8. The feed-forward circuit of Wg-dependent vg autoregulation in Drosophila. DSL-Notch signaling (purple) induces wg (red) and BE-dependent Vg (VgB, green) expression in border cells, causing them to send Wg (red arrows), as well as the short-range feed-forward signal, `X' (black arrows). X entrains neighboring cells to activate QE-dependent Vg expression (VgQ, blue) in response to Wg, and these cells become a new source for X so that the process reiterates, propagating recruitment of surrounding cells into the growing wing primordium (as illustrated on the right). Continuous exposure to X (short black arrows on right), together with Wg, may also be required to sustain QE-dependent vg expression in prospective wing cells once they are recruited. This circuitry is integrated with other signaling processes, including the stimulation of growth of the surrounding cell population (white), from which vg-expressing cells are recruited, to control the expansion in wing size as discussed in the text and the accompanying paper (Zecca and Struhl, 2007).