Uni-directional misexpression driven by the combination of UY333 and GMR-GAL4 caused R-cell defects identical to the GSd427 phenotype. (A) Relative positions of GSd427 and UY333 P-element insertion sites in the genome. Both P elements are inserted into the cytological region 50C23. GSd427 is inserted in a region ~5.5 kb upstream of RN-Tre and ~5 kb downstream of the mam transcriptional initiation site, while UY333 is inserted into a site ~588 base pairs upstream of RN-tre. GSd427 carries bi-directional UAS elements (red arrows) that can potentially drive gene transcription in both plus- and minus directions, while UY333 carries one UAS element that can drive gene expression only at the direction of RN-tre. (B-E) Apical (B and C) and basal (D and E) view of third-instar larval eye discs stained with anti-Elav antibody. (B and D) Apical (B) and basal (D) optic sections of wild-type eye discs. The distance between the sections shown in B and D is ~13 mm. (C and E) Apical (C) and basal (E) optic sections of GMR-GAL4-UY333 eye discs. Many R-cell nuclei were mis-localized to the basal region (E) (~34%, n = 1026 ommatidia in 6 eye discs). The distance between the sections shown in C and E is ~13 μm. Scale bar: B-E, 50 μm.