Figure 8. Ectopic misexpression of reph up-regulates Eph expression in the developing visual system.

To determine whether reph⁺ was sufficient for Eph expression, the UAS, GAL4 system was used to drive expression of a UAS-reph⁺ transgene in cell-specific patterns or within somatic clones. GAL4-expressing cells and clones were positively marked by membrane-bound GFP expressed from a UAS-CD8::GFP transgene (green color in B and C; shown alone in B′. The axonal architecture was visualized by staining with anti-HRP (green in A, red in C). A,A′,A″) A wild type specimen showing the medulla and its characteristic high midline-low dorsoventral gradient of Eph (anti-Eph, blue in A, shown alone in A″). B,B′,B″) Expression of UAS-reph⁺ in cortical neurons using an elav-GAL4 driver flattens the Eph gradient, notably at the dorsoventral margins (anti-Eph, blue in B, shown alone in B″). Defects in medulla development seen as gaps in the neuropil (yellow arrowheads in B′,B″) result from this up-regulation of Eph expression. C,C′,C″) Several UAS-reph⁺ cortical clones (red arrowheads in C′,C″) generated using a flip-out tubGAL4 driver can be seen in this specimen. Within these clones, Eph expression (anti-Eph, shown alone in C″) was up-regulated. The enhanced Eph expression was associated with defects manifest as large HRP⁺ cortical inclusions. Disruption of the normal Eph expression pattern also affected medulla neuropil development (yellow arrowhead in C′, C″). Abbreviations: lobula (lob), medulla neuropil (med n).