Fig. 5. The yolk and midgut are hotspots for ligand sensor activation. GAL4-LBD activation patterns are depicted for the yolk and midgut during embryogenesis (A-J) and in the midgut at the onset of metamorphosis (K-T). Representative embryos are shown for control (A,B), DHR3 (C,D), DHR38 (E,F), HNF4 (G,H), and FTZ-F1 (I,J) ligand sensors. The yolk is a major site of activation for GAL4-DHR3 (C), GAL4-DHR38 (E), GAL4-HNF4 (G) and GAL4-FTZ-F1 (I) embryos at stages 14-15 (arrowheads). Yolk activation remains prominent for GAL4-FTZ-F1 during stages 16-17 (J), but switches to the gut epithelium (arrows) for GAL4-DHR3 (D), GAL4-DHR38 (F) and GAL4-HNF4 (H). At later stages, GAL4-DHR3 displays strong and widespread activation in the proventriculus and midgut of late third instar larvae (M), and selectively reduced activation in the proventriculus after pupariation (arrow in N), while activation in the rest of the midgut is maintained. GAL4-DHR38 and GAL4-HNF4 display spatially restricted activation at the junction of the midgut, proventriculus (small arrow in Q) and gastric caeca (arrowheads in Q) (O-R). The FTZ-F1 ligand sensor is activated in the anterior midgut in a spatially and temporally specific fashion at puparium formation (S,T).