Imaginal Discs: The Genetic and Cellular Logic of Pattern Formation by Lewis I. Held, Jr.
Imaginal Discs
by Lewis I. Held, Jr.
Chapter 5: The Leg Disc

Figure 5.1 | Figure 5.2 | Figure 5.3 | Figure 5.4 | Figure 5.5 | Figure 5.6 | Figure 5.7 | Figure 5.8 | Figure 5.9 | Figure 5.10 | Figure 5.11 | Figure 5.12
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Figure 5.9

Figure 5.9
Logic of the Hh-Dpp-Wg circuitry that governs leg patterning (cf. Fig. 5.4c).
a. Regions (cf. Figs. 5.1c, 5.8, 5.9) where key genes are transcribed (en, engrailed; dpp, decapentaplegic; wg, wingless; al, aristaless) or where protein is detected (Hh, Hedgehog) or where discrete types of competence prevail (dAC vs. vAC are dorsal and ventral parts of the anterior compartment). Shading or hatching symbol is shown under each name.

b. Core circuit (cf. Fig. 2.7 for symbol key), whose logic is as follows. Like en, hh is transcribed in the posterior compartment (PC), but unlike En, Hh protein diffuses some distance into the AC. PC cells are 'deaf' to Hh signal because they express En (cf. Fig. 6.4) [4229], but AC cells within Hh's range can respond. Hh's diffusion range in the AC is depicted as two opposing sectors (after the first 'AND'). Cells in the dAC and vAC areas respond differently to Hh: dAC cells transcribe dpp, while vAC cells transcribe wg. Dpp and Wg (like Hh) are diffusible signals, but their ranges (graded shading) and activity levels are uncertain. Cells at the center (where dpp and wg sectors touch) make Al. From this diagram it would seem that Dpp and Wg need Hh continually, but in fact, turning Hh OFF during the last day of larval life causes no notable defects aside from missing claws [1472].

c. Evidence for the Dpp + Wg = Al circuit. Ubiquitous expression of Hh expands dpp and wg transcription into the entire dAC or vAC [231] (assessed by lacZ reporters). As a result, cells bordering the dAC/vAC boundary are exposed to the same high-Dpp, high-Wg conditions as cells at the center. Hence, the Al spot extends into a stripe along this line. When hh is turned ON (transiently) during 1st instar, discs become grossly misshapen, and Dpp antibody (above) or a wg reporter-gene (below) reveals that Dpp and Wg continue to be expressed -- hence suggesting that each acts as a mitogen within its own (dAC or vAC) territory. The PC remains normal in size (despite a ~3-fold larger AC), while the Al stripe elongates to span the enlarged AC (not shown). Similar results are seen with ubiquitous Hh expression during 3rd instar [2466].

d. Further evidence for the Dpp + Wg = Al circuit. Ubiquitous expression of Wg causes the Al spot to lengthen into a stripe along the Dpp sector.

e. Mutual antagonism between Dpp and Wg. The act of synthesizing Dpp apparently precludes synthesis of Wg within the same cell, and vice versa. AC cells appear to adopt different states of competence (cf. Fig. 3.2) based upon whether they receive a Dpp or a Wg signal: cells that receive Dpp (dAC plus ? sector) remain competent to make Dpp in response to Hh, while cells that receive Wg (vAC plus ? sector) stay able to make Wg in response to Hh. This see-saw can be forced the wrong way by (1) blocking Dpp [1811, 1812] or its transduction [753, 3329, 3932, 4277] in D cells, (2) blocking Wg [177, 880, 1673, 1811, 2110] or its transduction [519, 2262, 3317] in V cells, (3) forcing D cells to make Wg [487, 2082, 4277], or (4) forcing V cells to make Dpp [487, 2954, 4277].

Data for panel a comes from refs. in Figs. 5.8-5.9. Sources for other panels are: b [231, 620], c [231, 617], d [620], e [1811].

N.B.: When Wg is ubiquitously expressed (d) during 1st instar, discs also deform (not shown) but do not grow as much as with excess Hh (c) [617]. Also, Distal-less behaves like Al but in a broader area. Ventral sector of dpp transcription is omitted (see text), as are coxal and tibial regions of Al expression [620].

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