Fig. 1. Patterned expression of the bHLH transcription factor Atonal is essential for R8 formation in wild-type discs. (A) Atonal expression is patterned in the morphogenetic furrow of wild-type eye imaginal discs (red); terminally differentiated R8 cells express the protein Boss (green). (B,C) A close-up of Atonal immunostaining shows initial expression in proneural groups; the nuclei of two to three cells at the posterior edge of these groups rise apically, forming the R8 equivalence group. One cell from this group retains Atonal expression and differentiates as the R8 photoreceptor. (D) Scanning electron micrograph of a wild-type adult eye shows normal pattern formation. (E) Egfr^Ellipse is an activated allele of Egfr; Egfr^Ellipse/+ flies have slightly smaller, mispatterned eyes. (F) Loss of one copy of echinoid in trans to Egfr^Ellipse enhances the rough eye phenotype. Anterior is towards the left.