Figure 2.7
Circuitry that assigns shaft vs. socket cell fates in the SOP lineage.
Logic symbols (key at right of image cf. [2284, 4735]) denote activation, inhibition, combinatorial criteria (AND gate), and redundancy (OR gate). The NOT symbol is a blend of genetic and electronic icons [1751, 2770], and the reciprocal NOT condition (mutual antagonism) means that if one partner is ON (1), then the other must be OFF (0). These relations are summarized in the truth tables, where 1 means 'true' and 0 means 'false' [399, 1433]. The 'driver' is the element that chiefly dictates the outcome, and 'intercellular' connotes a ligand (arrowhead) and receptor (V).

a. Core pathway of proteins that decide shaft vs. socket cell identities (cf. Fig. 2.2 and text for details). Abbreviations: Dl (Delta), H (Hairless), Ser (Serrate), Su(H) (Suppressor of Hairless). In plain English, what this circuit means is: "Activate the Notch receptor if it binds either Dl or Ser on an adjacent cell (but go no farther if Numb is present!), then let it bind Su(H) and (as long as there's not too much Hairless around!) let the Notch-Su(H) complex turn ON genes for socket identity; otherwise turn ON genes for shaft identity."

b. The circuit as it functions in daughters of the IIa cell (Fig. 2.1) in wild-type flies. Numb is normally the decisive factor in setting cell fate (see text). The daughter that lacks Numb (above) has an active Notch pathway and hence forms a socket, while the daughter that inherits Numb (below) has an inactive Notch pathway and hence forms a shaft.

c. The circuit as it malfunctions when neurons are forced (via neuron-specific Gal4-UAS driver "31-1") to make excess Delta [2008]. In this situation, the prospective shaft cell makes a socket instead, presumably because hyperstimulation of Notch receptors supersaturates the limited amount of Numb. The ability of such excesses to short-circuit the system (thick line) implies that care must be exercised in interpreting GOF phenotypes (in general) whenever proteins flood a network.