Fig. 1. Dynamic Atonal expression within the morphogenetic furrow. (A) The morphogenetic furrow (MF, blue vertical stripe) traverses the eye imaginal disc from posterior to anterior. Photoreceptors are recruited progressively – ommatidia close to the MF are immature and consist of fewer photoreceptors than do ommatidia located more posteriorly. The first photoreceptor to differentiate with passage of the MF is R8, which then coordinates recruitment of all subsequent photoreceptors. Anterior is towards the left in all panels. (B) Region of box in A. Atonal (Ato) is expressed in a dynamic pattern. Anterior Ato expression is ubiquitous, while posteriorly Ato is resolved to single cells that will become R8s. (C) Schematic of Ato expression. Ato expression occurs in four distinct stages. In this and subsequent figures, gray shading corresponds to the stage of Ato expression. Stage 1 (dark gray): Ato is expressed in a broad band of virtually all nuclei within and just anterior to the morphogenetic furrow. Stage 2 (medium gray): Ato is detected in alternating clusters (intermediate groups) of approximately 10 nuclei each. These intermediate groups are separated by a ‘bridge’ of three or four Ato-expressing cells. Stage 3 (medium gray): Two or three posterior nuclei of the intermediate group migrate apically to form the R8 equivalence group (blue outlines), a group of cells believed to be equipotent to differentiate as the R8 photoreceptor. Stage 4 (light gray): three columns of Ato-expressing nuclei are positioned exactly out of phase with one another to mark the future R8 cell and prefigure the adult hexagonal array. An enhancer located 3' to the ato-coding region controls ato transcription during stage 1 and is not dependent on endogenous ato function, whereas a 5' enhancer is autoregulatory and directs ato transcription during stages 2-4. (D) Lateral view of Ato expression. Ato-expressing nuclei are positioned basally during stages 1 and 2, but migrate toward the apical surface of the imaginal disc during stage 3 and remain apical throughout stage 4.