Fig. 1. Hedgehog response zones and wing patterning. The dorsal surface of a wing is on the left and a cartoon of wing imaginal disc is on the right. Five longitudinal veins (L1-L5) punctuate the anteroposterior axis of the wing. The center of the disc, the wing pouch, is the primordium for the wing and is colored. The horizontal line across the center of the wing pouch indicates where dorsal and ventral compartments meet at the future wing margin. Hh is secreted by cells of the posterior compartment (towards the right in this and all other figures), shaded gray. The anterior compartment responds to the gradient of Hh that forms near its border because it produces the Hh receptor Patched (Ptc) (Chen and Struhl, 1996; Marigo et al., 1996; Stone et al., 1996). The first two or three cells have high levels of Hh (blue), which allows nuclear access of Ci155, depletes Ci155 to make CiA and blocks production of CiR. This activates transcription of en, ptc, col and dpp (Blair, 1992; Guillen et al., 1995; Ohlmeyer and Kalderon, 1998). The next four to six cells see intermediate or low levels of Hh (green and yellow, respectively), which allow nuclear access of Ci155, and make little or no CiA and little or no CiR so that Ci155 accumulates. ptc, col, dpp and iro are made in the intermediate zone, whereas only dpp and iro are made in the low zone (Mullor et al., 1997; Strigini and Cohen, 1997; Vervoort et al., 1999; Mullor and Guerrero, 2000). Far from Hh sources (red), Ci155 is depleted to produce CiR, and Hh target genes are repressed. En expression defines the high response zone where it prevents Iro expression and where Ptc sequesters Hh to limit the range of signaling (Hidalgo, 1994; Chen and Struhl, 1996; de Celis and Barrio, 2000; Crozatier et al., 2002). The intermediate zone is defined by the overlap of iro and col. Col in the high and intermediate zones downregulates Dpp responses; the result is the L3/4 intervein (Vervoort et al., 1999; de Celis and Barrio, 2000; Mohler et al., 2000; Crozatier et al., 2002). Col also activates transcription of the secreted EGFR ligand Vein, which signals to posterior adjacent cells to make vein L4 (Mohler et al., 2000; Crozatier et al., 2002). In the low zone, only dpp and iro are induced. They cooperate to specify vein L3 (Mullor and Guerrero, 2000). Dpp also promotes growth along the anteroposterior axis and acts as a morphogen to pattern deeper in the anterior and posterior compartments (Lawrence and Struhl, 1996). Finally, G1 cyclins are activated by Hh to promote growth and proliferation (Duman-Scheel et al., 2002).

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