Fig. 2. Loss-of-function phenotypes affecting R8 spacing. (A) Loss-of-function N clones result in low levels of Ato expression in stage 1. This represents a failure of ‘proneural enhancement.’ No Ato expression is detected from stage 2-4. (B) Removal of Notch (N) function with a temperature-sensitive mutation results in failure of lateral inhibition. Stage 2 intermediate groups do not resolve and large clusters of well-spaced R8 precursors develop. Stage 1 Ato expression is also reduced at the restrictive temperature. Return to the permissive temperature restores lateral inhibition and single R8 precursors develop. (C) Loss of scabrous (sca) function prevents formation of stage 2 intermediate groups and nuclei at this stage are unpatterned. Notch-mediated inhibition of R8 occurs in stage 2, resulting in single R8 precursors in stage 4, but these cells are too closely spaced and phase relationships are lost. (D) Absence of both N and sca function results in massive overinduction of R8 precursors and nearly all nuclei express Ato during stage 2, owing to a failure of both intermediate group establishment and lateral inhibition. Stage 1 Ato expression is reduced at the restrictive temperature. Upon return to the permissive temperature, single R8 precursors develop, but they are unpatterned. (E) Loss-of-function Epidermal Growth Factor Receptor (EGFR) clones fail to form discrete intermediate groups and have additional Ato-expressing cells in stages 2 and 3. R8 precursors are too closely spaced and lack phase relationships. Note resemblance to sca^– phenotype. (F) Removal of both EGFR and sca function leads to a more severe phenotype than either alone. Intermediate groups do not form in stage 2 and many unpatterned Ato-expressing R8 precursors develop in stage 4.