Dl overexpression in R8 cells A. Overexpressing Dl in differentiating R8 cells leads to a rough eye of similar size to wild type (699 ± 21 ommatidia were recorded in male eyes and 790 ± 49 in females). Genotype is male 109-68>DESI (line A10). Overexpressing Dl in spl heterozygotes was similar (not shown; 693 ± 47 facets per eye were counted in spl/+; 109-68>A10 females, compared with 790 ± 49 in +/+;109-68>A10 females). 109-68 drives expression in R8 cells from column 1 onwards, too late to affect initial R8 specification but possibly in time to affect abnormal ommatidium development that occurs in spl mutants. B. 109-68>DESI (line A10) eye discs labelled for the R8-specific Boss protein (green) and photoreceptor protein elav (magenta). No ommatidia or R8 cells were obviously missing as a result of Dl overexpression, nor were ectopic R8 cells seen. C. spl/Y; UAS DESI (line A10) males showed the small, rough eye typical of spl (386 ± 54 ommatidia). The Dl transgene is expected to remain unexpressed in this background. D. spl/Y; UAS DESI (line A10) eye discs show the typical spl phenotype. Some R8 cells and ommatidia are missing, other ommatidia have abnormal R8 cells and lack other photoreceptor cells. E. spl/Y; 109-68>DESI (line A10) eyes. Dl overexpression in R8 cells did not rescue the spl mutant. In fact there was a statistically insignificant tendency for eyes tended to appear smaller than inspl. (312 ± 101 ommatidia were counted in spl/Y; 109-68> A10 male eyes). F. spl/Y; 109-68>DESI (line A10) eye discs also resembled spl. No rescue of the R8 or outer photoreceptor defects was apparent.