Figure 3.9
Origin of bristle SOPs on the leg.
a. Distal half of a left second leg (mid-tibia to claws), showing stripes of Hairy protein that appear at pupariation [665, 3193]. Only 3 of the 4 stripes are visible from this posterior view. Transverse stripes are omitted. Dorsal (D) and ventral (V) midlines are marked. At this stage, leg segments are shorter and wider than drawn here.

b. Entire 360° surface of an enlarged swath (filleted at D midline), showing cells (hexagons) and all 4 Hairy stripes (hatched). Black bar (below) shows the extent of Engrailed (En) expression, which characterizes the posterior compartment.

c. Same swath, showing 8 stripes of Achaete (Ac) protein that all appear at 5-6 hrs AP (after pupariation) [3193]. These proneural stripes (each 3-4 cells wide) foreshadow the tarsal bristle rows (e). The number of bristles per row (SOPs per stripe) decreases with distance from the V midline [1714]. Positions of SOPs (sensory organ precursors; black cells outlined in white) are hypothetical. Their emergence has not been examined histologically. Lineage studies indicate that SOPs in stripe 1 (at least) originate from >2 columns of cells since row 1 bristles can come from both compartments [1800, 2449]. SOPs in stripe 1 are depicted posterior (6 at left) or anterior (4 at right) to the boundary (white zigzag line). Inhibitory fields around SOPs are omitted.

d. Inferred movements (white arrows). Lineage studies suggest that (1) bracts and bristles in each row come from separate columns of cells and (2) bristle cells move ventrally (V arrows; stripes 2-7) or dorsally (D arrows; stripes 1 and 8) to one edge of their stripe. Adjustments of this kind (previously seen with scale precursor cells on moth wings [3051]) would explain why leg bristles are more aligned than notal bristles. Other 'fine-tuning' shifts may occur vertically, based on abnormal spacing in mutants with misoriented bristles [1810]. The horizontal and vertical shifts have been combined into one vector per SOP, though they probably happen at different times [1803] (Fig. 3.11).

e. Adult left leg (posterior view), showing bristle rows 1-4.

f. Effect of eliminating h function on Ac: Ac now appears in 4 broad stripes -- implying that the 8 Ac stripes arise by repression of ac within Hairy stripes. This 'subtractive' logic is similar to the strategy used on the notum (cf. Fig. 3.8). Dl and N protein distributions have not yet been ascertained for the legs.