Figure 7.3
Patterns of gene expression in the eye-antenna disc during 3rd instar. Smaller outlines indicate earlier stages as noted. Black areas chart mRNA (transcribed from the endogenous gene or a reporter). Shades of gray denote degrees of expression. Directions are shown in the compass at the lower right: A-P (anterior-posterior) and D-V (dorsal-ventral) refer to prospective axes of the adult, while 'medial' and 'lateral' denote directions within the larval body.

Genes (DO = details omitted; 'ambiguous' means that conflicting reports have here been summed into a maximal-domain composite): ci (cubitus interruptus) [1135], dac (dachshund; early 3rd instar) [2353]^Δ, Dll (Distal-less) [1085, 3242]^Δ, dpp (decapentaplegic; above is early 3rd instar [930]^Δ) [2004]^Δ, en (engrailed) [4168]^Δ, ey (eyeless; early 3rd instar) [930]^Δ, eya (eyes absent; early 3rd instar) [2353]^Δ, fj (four jointed; antennal expression is approximate) [4851]^Δ, fng (fringe; ambiguous) [696]^Δ, hairy [1616]^Δ, hh (hedgehog; above is 2nd instar [1082]) [406]^Δ, hth (homothorax) [1085, 3380]^Δ, N (Notch; 2nd instar) [2353], mβ [E(spl)mβ] [871]^Δ (but see [991]), mirr (mirror; ambiguous) [4797]^Δ, omb (optomotor-blind) [4849], pnr (pannier; mid-3rd instar) [2752], so (sine oculis; early 3rd instar) [2353]^Δ, spalt [1085]^Δ, ss (spineless) [1119], toy (twin of eyeless; 2nd instar) [2353]^Δ, wg (wingless; above is early 2nd instar [696]) [3762]^Δ, and the enhancer trap WR122-lacZ [1781]. Genes omitted (expression patterns is in parentheses): araucan and caupolican (≈ mirr) [3380]^Δ, extradenticle (Exd is nuclear wherever Hth is present) [677], orthodenticle (ocellar region and ommatidia) [3663]^Δ, vein (just posterior to the Hh ocellar spot) [80]. Annular expression domains of BarH1 and dachshund in the antenna (not shown) appear identical to corresponding tarsal domains (Fig. 5.11) [3762]. Links (—> activation; —| inhibition) indicate genetic interactions; 'AND' means that both inputs are needed to activate the target gene; dashed lines denote weaker effects. The flow of control begins at upper left in 2nd instar. Whether Egfr inhibits toy directly or via N is unknown [2353]. Egfr is probably activated in the antenna (cf. Fig. 6.9) [2353, 2362, 3382]. The antenna resembles the leg (cf. Fig. 5.8) insofar as 'En —> Hh —> {dpp AND wg}' [1037]. For the eye's D-V axis, polarity is established by mirr (upper right; cf. Fig. 7.4). Wg may not actually inhibit dpp at the level of transcription [4390]. A dac —> eya link detected elsewhere does not work in the eye [554]. The 'AND' logic for the early eye genes has been omitted for clarity. For an exegesis of this nexus see [1787]. For enhancer-trap patterns see [315, 3388, 4210] and FlyView [2027], and for expression of hh, wg, and dpp in the peripodial membrane see [773].

N.B.: In comparing these expression patterns with those in the leg and wing discs (Figs. 5.8 and 6.2) it is important to recall that the eye disc rotates ~180° after emerging from the embryonic ectoderm [4146] -- thereby inverting its D-V axis relative to the D-V axes of the thoracic discs (cf. Fig. 4.3a). Circuitry of the 'early eye' genes (lower left) is based on [744, 930, 1780]. For ideas about how this network evolved, see [1419, 2034, 3895].