Imaginal Discs: The Genetic and Cellular Logic of Pattern Formation by Lewis I. Held, Jr.
Imaginal Discs
by Lewis I. Held, Jr.
Chapter 2: The Bristle

Figure 2.1 | Figure 2.2 | Figure 2.3 | Figure 2.4 | Figure 2.5 | Figure 2.6 | Figure 2.7 | Figure 2.8
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Figure 2.6

Figure 2.6
Effects of Hairless mutations on bristle cell fates (a, b), and interactions between Hairless and Suppressor of Hairless (c-e).
a. Half head and thorax of a wild-type fly (above), with all bristles omitted except macrochaetes. Beneath, macrochaetes (black circles) are seriated in posterior-to-anterior order, except that some are grouped [2560] as scutellars (SC), post-alars (PA), dorsocentrals (DC), supra-alars (SA), notopleurals (NP), humerals (HU), verticals (VT), or orbitals (OR). Relative positions of intragroup members are posterior (P), anterior (A), dorsal (D), ventral (V), or middle (M). Presutural (PS), postvertical (PV), and ocellar bristles (OC; oval is an ocellus) are unpaired. The histograms show how each site is affected by Hairless genotypes. 'Hs' and 'Hw' are strong and weak mutant alleles of Hairless respectively (+ = wild type). Black bars are frequencies (percent) of 'normal' bristles (shaft and socket); shaded bars are frequencies of 'double sockets', which likely arise from a shaft-to-socket cell transformation (key at right). H function is evidently more limiting in shaft cells than in incipient SOPs because mild reductions in H levels cause more double sockets than absent bristles [198]. The variation in sensitivity among sites makes no sense in terms of their seriation.

b. Intermediate phenotypes between a normal bristle (type 1) and a 'double socket' (type 5). In type 2 the shaft has abnormal fluting and pigmentation, and the socket fails to form a complete circle around the base. In types 3 and 4, the shaft is a vestige that ultimately (in type 5) resembles a stunted socket.

c. Number of 'normal' (shaft and socket both present) macrochaetes on the head and thorax of HLOF/+ flies (H1 allele) carrying varying doses of the wild-type allele of Su(H). 'Def.' and 'Dup.' are a deficiency and duplication for the Su(H) locus.

d. Domains in the H and Su(H) proteins. Dashed ovals connect regions needed for H-Su(H) binding [492]. 'Basic' here means a segment of the protein where the average pI is 11 (sic!) [2657] . The DNA-binding domain of Su(H) is probably bipartite (i.e., same limits but centrally inert), given the properties of its mouse homolog [788].

e. Threshold Model of Bang et al. [197]. In this model, H and Su(H) titrate one another, and whichever protein remains undimerized determines cell fate. That is, excess Su(H) causes a cell to become a socket, and excess H causes it to become a shaft. N.B.: In other contexts, Hairless may act independently of Su(H) [725].

Data in a are from Table 2 of [198], b is a schematic of that article's Fig. 5, c is replotted from [117], d incorporates data culled from [492, 1331, 2657, 3826], and e is based on [197].

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