Figure 6.3
Morphogen gradients that dictate wing cell fates (cf. Table 6.1).
The wing pouch from a mature right wing disc (above left) is idealized as an oval (cf. Fig. 6.1) with solid lines (A/P or D/V) between compartments. Dashed lines denote veins 2-5 (vein 1 = part of the margin). Scale bar applies to the enlarged oval. Black triangles around the oval are gradients of Hh, Wg, or Dpp, though Hh is also present in the P region (dotted rectangle). Outer bars indicate levels of target gene transcription, though en is also ON in P cells (dotted bar) and ptc is ON at a basal level in remaining A cells (not shown). For gene abbreviations see Fig. 6.2 , App. 6, or below. For additional details and perspective see App. 7.

Hh diffuses into the A region [4228] where it turns ON (1) dpp [4136, 4479], (2) patched (ptc) [2728, 4136, 4479, 4539], (3) knot (a.k.a. collier) [1840, 2894, 3077, 4479], and (4) en [4136] (late 3rd instar only [350, 4228]) at successively higher thresholds (though thresholds for ptc and knot are comparable). Because en —|dpp [1647, 2980, 3747, 4229], dpp only remains ON in the front 2/3 of the Hh gradient [4136]. Light shading denotes suppression. Ptc's level should also drop where it overlaps en-ON [277, 350, 2728, 3372] since en —|ci and Ci is needed for ptc transcription [2832, 3747, 4229] (cf. Figs. 5.6-5.7). Why it does not is unclear [4539]. At the same threshold as dpp, Hh also turns ON master of thickveins (not shown) [1327]. At a lower threshold than any of those listed above, Hh turns ON the Iro-C genes ara and caup (not shown; cf. Fig. 6.11) [320, 1537, 2993].

Cells in the dpp-ON stripe make Dpp, which diffuses to turn ON (1) vg [2217, 2219], (2) omb [2455, 3074], and (3) spalt [2455, 3074] at successively higher thresholds.

Cells at the D/V boundary make Wg, which diffuses to turn ON (1) vg [678, 2217, 3091, 4849], (2) Dll [166, 1040, 3089, 3091, 4849], and (3) achaete (ac; also scute, not shown) [166, 912, 3689, 3982] at successively higher thresholds, though Dll and vg respond in a graded manner. So do Dfz3 (activated) [3977] and vvl (repressed) [703, 990, 995], and the Iro-C genes ara and caup (repressed) [1537] (not shown). The ac stripe splits into two bands (each ~4 cells wide; cf. Fig. 6.8) [2079, 3689, 3690] apparently due to Cut —|ac [988] -- possibly via Emc [913] or Mβ [871] or both [205]. Cut was once thought to be a Wg target [352, 353], but the link is indirect [2839, 3089] via Vg [3936] and Scalloped [2940]. Oddly, Wg also turns ON omb (not shown) [1626] via the same cis-enhancer that is under Brinker-mediated Dpp control [3978]. The overall diagram is adapted from [3087, 4137] but amended for vg regulation [703, 2217], with specific pathway data from: Hh [2992, 2993, 4136, 4478, 4479, 4539], Dpp [2455, 3074, 3406, 3972], Wg [4849]. Cell dimensions (scale bar) reflect the mean of 10 counts along 25 mm transects in Fig. 8b of [350].