Figure 8.2
Expression of Ubx in thoracic disc development. Abbreviations: PS (parasegments), T2 (2nd thoracic segment), T3 (3rd thoracic segment). Axes are indicated by the compass: A-P, anterior-posterior; D-V, dorsal-ventral. See also App. 7.

a. Fate map of the left half of an egg (cf. Fig. 4.1), showing the ectodermal region (PS 5-13) where Ubx is expressed (shaded) at Stage 11 (cf. Fig. 4.2). Black ovals mark where discs arise in T2 and T3.

b. Expression of Ubx in T2 (wing and 2nd leg) and T3 (haltere and 3rd leg) discs from the left side a mature larva. Degrees of shading signify amounts. Each thick vertical line bisects the discs into A and P compartments (cf. Fig. 4.4). Note that Ubx expression has been altered since the embryonic stage. These changes are likely due to the facts that (1) embryonic and imaginal cis-enhancers are separate at the Ubx locus and (2) the hegemony of the Pc-G or Trx-G complex can be overturned by sufficiently powerful trans-acting factors (cf. Fig. 8.1) [3397]. In T2, Ubx persists in the leg disc's P compartment, but it has vanished from the wing disc except for the peripodial membrane (not shown). A new spot (arrow) arises in the anterior tibial area of the 2nd-leg disc. The 3rd-leg disc has a similar spot, along with an intense P patch that will form transverse rows of the tarsus (d) and tibia. In both T3 discs Ubx expression is stronger than in the T2 discs, and it is stronger in the P than the A compartment. In the haltere disc, the intensity is also high in the A region (arrow) near the center (future capitellum) and in the surrounding fold (future pedicel).

c. Derivatives of the haltere disc (pedicel and capitellum). Basal hinge (scabellum) and flank sclerite (hemi-metanotum) are not labeled.

d. Bristle pattern of the 3rd-leg basitarsus, showing transverse rows that are unique to T3. Unlike the 1st leg, whose transverse rows are anterior, these rows are on the P side, while the 2nd leg lacks transverse rows altogether (cf. Fig. 3.10).

Fate map in a is based on [1739]. Data on Ubx expression in the embryo is from [53, 324, 675, 684, 1990], which should be consulted for spatio-temporal subtleties. Ubx expression in the discs (b) is from [496, 4564, 4626]. The latter references are not entirely consistent with one another, and the sum total of the published domains is depicted here. For details of anatomy (c and d) and fate maps see [17, 526, 1358, 1714, 1883].