a, GFP expression driven by wg[KO;Gal4] at different stages. b, QUAS-Tomato expressing clones in larvae of the genotype hs-Flp QUAS-Tomato; wg[KO; FRT.NRT-Wg.FRT QF]/Cyo. Larvae were heat shocked 18 hours before fixation at the times indicated. White arrows indicate the presence of Tomato-expressing cells in the middle of the prospective wing blade, far from the prospective margin. c, Fluorescent RNA in situ hybridization (FISH) with a wingless probe in wing imaginal discs obtained 84±3 hours after egg laying. Red arrow indicates incipient wingless expression in prospective thorax. White marks indicate the plane of transverse section. White arrow shows FISH signal in the apical domain. d, HRP activity (detected by DAB staining) in a late 3rd instar wg[KO;Gal4]/+, UAS-HRP-CD8-GFP wing imaginal disc. Activity is detectable throughout the pouch (black arrow) but not in the prospective thorax (red arrow).