Fig. 8. The genetic hierarchy that controls pattern formation along the PD axis. For the sake of simplicity, the contribution of the AP patterning system is not included in the cartoon. (A) In the first step, the activity of Wg (red) and Dpp (not shown) define the position and extent of the wing field. Within this field, the Notch pathway induces the expression of Vg along the DV boundary. Wg and possibly Vg are required to induce the expression of Nub (blue) in a disc-like domain slightly larger than that of the early Wg domain (Ng et al., 1996; del Alamo Rodriguez et al., 2002). (B) Vg associates with Sd to form a bipartite transcription factor (Vg/Sd) that controls the expression of an unknown diffusible factor. This factor activates the expression of dve (light green) and rn (yellow) independently from each other and in domains of different sizes. The combined activity of Nub and Rn activate the inner ring-like domain of wg expression (dashed red circle). Dve, together with Nub and other factors, establishes the distal region of the PW, which is defined along the PD axis by the length of the medial and distal costal regions. Wg organizes the formation of the medial part of the PW. At the DV boundary, Vg/Sd activates the expression of Wg together with the Notch pathway. Wg suppresses the expression of Dve in cells near the DV boundary. (C) The cells at the DV boundary divide and the daughter cells that are displaced from the boundary form the wing pouch. Expression of Vg in the pouch cells is maintained by Wg, secreted from the DV boundary, and Dpp, secreted from the AP boundary (not shown). As a consequence, Vg is expressed in a gradient with the peak at the DV boundary. In the nascent pouch, Vg activates the expression of several patterning genes, such as Dll, and together with Wg, the genes of the achaete-scute complex, adjacent to the cells at the DV boundary (Neumann and Cohen, 1997; Zecca et al., 1996; Klein and Martinez-Arias, 1999), and fj. The activation of these targets is indicated by the three red arrows in the lower drawing of Step C. Fj is required for the establishment of a region within the pouch, and for the generation of planar polarity of the wing. The cartoon highlights the fact that all of the genes controlled by Vg/Sd are expressed in disc-like domains of different sizes. Their expression leads to concentric areas with different combinations of gene activities. These combinations are likely to establish different parts of the PW (see Step C).