Fig. 1.

Specification of R8 cells. In all figures, anterior is to the left. (A) In the eye imaginal disc epithelium, precisely constructed ommatidia each differentiate around a single R8 cell. R8 specification is accompanied by a transient contraction of disc cells in the “morphogenetic furrow” (MF) that sweeps anteriorly across the disc (arrow gives MF direction). Posterior to the furrow are vertical columns of R8s. Ato protein (green) appears just ahead of the furrow and is accompanied, then replaced, by Sens (magenta), expressed stably in R8 cells. (B) Ato expression evolves from a low uniform stripe into periodic intermediate groups (IGs) that then resolve into single R8 cells. Sens tracks Ato from the IGs. [Because the eye disc epithelium is pseudostratified, there are more unstained cells between R8s than it might appear from the size of the stained R8 nuclei (2, 4).] (C) Ato expression in intermediate groups and R8 cells. (Ato is lost in R8s posterior to the MF through a regulatory mechanism not discussed in this paper; ref. 33). (D) Sens expression. (E) Schematic of Ato (green) and Sens (magenta rings) expression in the furrow. Black dotted lines outline an IG, which has higher Ato expression than the stripe to its anterior. Yellow scale bars indicate the extent of the MF. (Scale bars: A and B, 18 μm.)