Fig. 1. Inner photoreceptors exhibit differential gene expression during Drosophila eye development. (A-C) Adult photoreceptor (PR) subtypes and relevant cell-specific factors. (A) Inner photoreceptor cells (IPRs) versus outer photoreceptor cells (OPRs): R7 and R8 IPRs express Salm and are surrounded by R1-R6 OPRs. (B) Polarization versus color-sensitive IPRs: all R7s express Pros (pink nucleus), whereas only pale/yellow R8 PRs express Sens (green nucleus). (C) Ommatidial subtypes: DRA ommatidia (two dorsal rows) express Rh3 in both R7 and R8, 30% ommatidia (termed pale) express Rh3 and Rh5 in R7 and R8, respectively, and 70% ommatidia (termed yellow) express Rh4 and Rh6 in R7 and R8, respectively. (D) Transcription factor expression timeline during PR development. From R8 specification at the morphogenetic furrow (MF) onward, Sens expression is R8-specific. Early sens expression is ato-dependent, whereas later expression is Sal gene-dependent. R7 recruitment occurs more posteriorly than R8 specification and coincides with Pros and Salm expression (Domingos et al., 2004; Kauffmann et al., 1996). Otd expression begins at the end of PR recruitment and is expressed in all PRs throughout development (Vandendries et al., 1996). (E-G) Larval (E), 50% pupal (F) and adult (G) eye co-staining with Otd (purple) and Sens (green). E is oriented anterior to posterior, F is oriented dorsal right and includes the DRA (not indicated), and G is oriented dorsal up, distal right, with the DRA boxed. From R8 specification at the MF onward, Sens expression is R8-specific. Sens is expressed in all R8 cells during larval and 50% pupation (E,F), but is absent from adult DRA (G). It should be noted that Sens and Hth are coexpressed in DRA R8 cells from 15-50% pupation (data not shown; and M. Wernet, personal communication) (Wernet et al., 2003).