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Circuitry (cf. Fig. 2.7 for key) controlling the identity of mechanosensory (MS) vs. chemosensory (CS) bristles vs. chordotonal (CT) organs. Those organs are cartooned with their neurons (N) but without their support cells. They are created from single-cell SOPs (sense organ precursors, black rectangles) via differentiative mitoses (not shown; cf. Fig. 2.1). In upper left of each panel are transcriptional states (1= ON; 0 = OFF) of cut and poxn depicted as a binary code.
The circuit (middle area) is simple. 'Prepattern' genes (cf. Ch. 3) activate atonal (ato) at certain spots in the skin, and Ato suppresses Cut. If the SOP also lacks Paired box-neuro (Poxn), then it makes a CT organ. If Ato is absent, but other proneural proteins (not shown) are present (cf. Ch. 3), then Cut will be expressed, and the cell will make a bristle. The bristle type will be CS if poxn is ON and MS if poxn is OFF. The state of poxn (like ato and cut) must be set by region-specific genes upstream (not shown). Thus, LOF or GOF mutations in poxn interconvert MS and CS bristles, and cutLOF or atoGOF transform MS bristles into CT organs. Transformations of bristles into olfactory sensilla (not shown) are elicited by amosGOF .
CT organs lack protrusions [378, 1454, 2787, 2831] and arise from the SOP via a different lineage tree . Stretching of their dendrites (springs attached at two anchor points -- 'A') causes the neurons to fire an action potential to the CNS [789, 2018]. CT sensilla on the fly femur have 2 neurons , whose cell bodies are embedded along the stretching axis (unlike depicted here).
MS bristles transduce touch stimuli by lever action : deflection of the rigid shaft (triangle) toward the body surface (horizontal line) causes it to pivot. Neural depolarization appears to be due to stretching of the dendritic membrane (vs. compression of the tubular body) [789, 874, 2787] (but see ).
CS bristles should technically be termed 'chemo-mechanosensory' since they use one neuron to sense touch like MS bristles, but they have four extra neurons whose dendrites extend up the shaft to a pore at the tip [2502, 4125], where they 'taste' salts or sugars [111, 2955, 3061, 3529, 4841].
This schematic is adapted from  (MS),  (CS), and [378, 2128] (CT), with logic based on . Many authors have conjectured that cells differentiate by making a series of binary choices. A favorite metaphor for decision trees has been a rail yard of bifurcating railroad tracks . N.B.: Expression of cut at the wing margin goes through two phases; only the later phase controls SOP identity .