Fig. 9. Model for Faf, Lqf and Neur function. (A) Early events in ommatidial assembly (see Wolff and Ready, 1993). The morphogenetic furrow (mf) moves in the direction of the arrow. A is anterior and P is posterior. The first several columns (0-4) of developing ommatidia are shown. Atonal-expressing cells are black. R1-R8 are indicated. Three processes (I, II, III) that require Notch/Delta signaling are shown. (I) Proneural enhancement: Atonal expression is upregulated. (II) Lateral inhibition: Atonal expression is limited to groups of 10 cells and ultimately to R8s in column 0. (III) R-cell restriction: R2/3/4/5 precursors signal their neighbors to prevent excessive neural determination. As the ectopic cells in faf mutants appear to arise between R3/4, they may be the orange cells. As depicted by the black bars, Faf is essential only for event III, Lqf is essential for events I, II and III, and Neur is essential for event III but is required to a lesser extent for the events I and II. (B) A model showing how Faf/Lqf may function with Neur in the Delta signaling cells is shown. The blue circles are ubiquitin. Lqf is deubiquitinated by Faf, which increases Lqf levels. Ubiquitination of Delta by Neur may stimulate interactions between Delta and Lqf and thereby facilitate Delta internalization.