Figure 3.7
Origin of macrochaete SOPs on the notum.
a, b. Heminota (minus humeri) composed of diploid (2n, a) vs. haploid (1n, b) cells should look grossly alike because ploidy does not alter body size in animal species [1190, 1191]. The chief effect of ploidy is a proportional change in cell volume [1801]. Ovals denote macrochaete PNCs (proneural clusters), most of which arise (asynchronously) before the notal region acquires the shape shown (cf. Fig. 2.6 for acronyms and Fig. 3.3 for a realistic depiction of a PNC). Dashed lines mark when each SOP (sensory organ precursor) appears [1925, 3374]. This temporal sequence is essentially irrelevant to the pattern of axonal fasciculation within the disc [4431, 4432] and axonal projection within the CNS [1455] (not shown).

c, d. Area where the PDC macrochaete arises (enlarged). Ploidy uncouples mechanisms that depend on cell size (inhibitory-field diameter?) from those that do not (PNC diameter?). Assuming that PNCs (dark shading) come from a disc's global prepattern, a 1n PNC will have the same area as a 2n PNC, but should contain more cells (hexagons) because 1n cells are ~21% smaller in diameter (0.79³ = 0.5; 1n wing cells have 40% less area [3750]). Central cell (black with white outline) is the SOP.

e, f. SOPs may use a diffusible inhibitor (circle = range) to prevent excess SOPs. If smaller cells secrete less inhibitor, then 1n SOPs may not be able to inhibit outlying cells, some of which will become SOPs and set up their own fields. Indeed, 1n-2n mosaics do have extra bristles at MC sites (and higher bristle densities overall) when occupied by 1n tissue [3750, 3754]. However, there is usually only one extra bristle per site, and it tends to arise anteriorly or posteriorly, but not laterally. This result favors the Lateral Inhibition Model over the Mutual Inhibition Model (Fig. 3.6; see text).