Figure 3.4
Spatiotemporal control of macrochaete development on the head and thorax.
a. The genotype-phenotype 'switchboard' for MCs. Half head and thorax of a wild-type fly (dorsal view; mCs omitted). Black circles below symbolize MCs seriated in posterior-to-anterior order, except that some are grouped (cf. Fig. 2.6 for key; P=posterior, A=anterior, D=dorsal, V=ventral, M=middle). The double helix spans the ~100 kb achaete-scute Complex near the tip of the X chromosome. Arrows denote transcription units (kink = intron), with distal to the left and zero as an arbitrary EcoR1 site. Four 'proneural' genes (achaete, ac; scute, sc; lethal-at-scute, l'sc; asense, ase) encode bHLH proteins, though only ac and sc normally affect MCs. Enhancers (rectangles) were mapped genetically (light shading; double arrows mark unknown limits) [636, 3688] or by their ability to drive reporter genes in proneural clusters (dark shading) [1538, 2891]. The minimal effective sequence per rectangle is unknown except for the DC enhancer [1380]. Note that the MCs and enhancers are not colinear (dotted lines). Loci for ANP and APA enhancers (5-7 kb and -1 to -7 kb respectively) differ trivially (a few kb) from those depicted (as assessed by reporters). The ac¹ allele affects DCs (Fig. 3.3) due to an 18-kb (64-82) deletion that overlaps the DC enhancer. The functional core of this 5.7 kb enhancer is really < 1.4 kb long [1380]. The DC enhancer may be looped into contact with the sc promoter (30 kb away) when Pannier (a transcription factor) binds it and forms a complex with bHLH dimers that bind the promoter [3504]. The yellow gene (y) has its own enhancers (not shown) [1442, 1534]. In ref. [1538], ASA and PPA are (mistakenly?) ascribed to the same enhancer. See also App. 7.

b. Times (hours after pupariation) when sites become insensitive to γ-irradiation. Since the high dose that was used (10⁴ Rad) kills dividing cells but spares postmitotic ones, onset of insensitivity probably signifies completion of SOP mitoses [3420]. Indeed, onset times agree roughly with histological detection of four SOP progeny at these various sites [1925]. More precisely, times may reflect the start of endoreplication since shaft cells become insensitive and endoreplicate ~1-3 hrs. before socket cells [1741, 3420, 3421]. No colinearity exists between onset times and the overall posterior-anterior seriation (dotted lines), though posterior bristles precede anterior ones within most pairs -- a correlation that may have less to do with location than with shaft length (see c). Times for mCs are 19-24 hrs. AP (not plotted).

c. Inverse correlation between bristle length and onset of radio-insensitivity. This trend may stem from a need for SOPs to begin mitoses or endoreplication early enough to finish differentiating before cuticle deposition. Strangely, the earlier a MC SOP arises (cf. Fig. 3.7), the longer it waits before starting mitosis [1925]. Lengths are for wild-type flies raised at 17°C [2837], times are from [3420], and the line was plotted by best-fit regression. N.B.: For calibration purposes, the actual lengths for pDC and aDC bristles are 336 µm and 250 µm, respectively [2544].