Figure 3.8
Origin of microchaete SOPs on the notum.
a. Heminotum (minus humerus), showing proneural stripes where Achaete (Ac) protein (c) foreshadows the 5 medial rows of microchaetes (e). Stripes 1 and 5 converge and fuse at their posterior tips (not shown), and other stripes develop laterally. Stripe 5 overlaps the ADC and PDC macrochaete sites, whose SOPs arise >12 hrs before pupariation (Fig. 3.7). These SOPs may still exude inhibition since no microchaetes form between them.

b. Enlarged swath, showing cells (hexagons). Delta (Dl) protein appears at ~0 hrs after pupariation (AP), is expressed in all 5 stripes by 12 hrs AP (cf. table), and is detectable at low levels ('(Dl)') elsewhere. Notch (N) protein is also ubiquitous but more intense between Dl stripes. Whether these stripes arise via AS-C cis-enhancers like those that govern macrochaetes is unknown [1538]. Also unknown is whether the early stripes constrain later ones [3966].

c. By ~8 hrs AP, Ac is congruent with Dl but is confined to incipient SOPs (black cells outlined in white) by 12 hrs AP. SOPs (sensory organ precursors) appear in stripe 5, then 1 and 3, then 2 and 4 (cf. table). Inside each stripe, SOPs arise asynchronously.

d. Most SOPs are evident by 12 hrs AP, but some emerge as late as 20 hrs AP. By that time the earlier SOPs have not formed bristles as shown here, but have divided and appear as 4-cell tetrads. Late SOPs arise in the largest spaces between extant SOPs, as Wigglesworth found in hemipterans. He imagined that SOPs stifle would-be SOPs nearby by emiting an inhibitor [4660] or absorbing an inducer [4657]. Only when a competent (Ac-expressing) cell finds itself outside inhibitory fields can it become a SOP. These rules produce a roughly even spacing of bristles in the available area. Proneural stripes will yield zigzag bristle rows if the fields are as wide (3-5 cells) as the stripes. Depending on how the signal is transmitted, fields need not be circular.

e. Heminotum, showing all bristles.

f. Effect of lowering Dl function on Ac expression. When t.s. Dl^LOF mutants are heat-pulsed from 5-10 hrs AP, Ac is expressed ubiquitously except along the midline (the same band that remains bare in N^LOF and shi^LOF mutants [1742, 3863]). The implication is that Dl normally represses ac between the 'Dl stripes'. Conceivably, this area defaults to an ac-ON state in the absence of N signaling.

This diagram is based on data from [3270, 4428]. N.B.: it makes sense that bristles would be positioned late in development (after proliferation has ceased) since any pattern that is created earlier would be 'messed up' by scattered mitoses [695]. This system thus obeys the general rule that proliferation and differentiation are mutually exclusive in eukaryotic cells [4882].