Fig. 7. Ser is sequentially regulated by the Ap, N and Wg pathways during larval development. (A) Alignment of the Ser minimal wing enhancer regions in D. melanogaster (mel; 794 bp) and D. pseudoobscura (pse; 810 bp). Both enhancers are located within 1 kb downstream of the Ser 3'UTR. Locations of Ap (green), Su(H) (bold type) and dTCF (red) binding sites are indicated, which are defined based on the in vitro DNA-protein binding data in this study. Non-canonical sites are italicized. Conserved nucleotides of the two species are denoted in blue. (B) Sequential regulation of Ser by the Ap, N and Wg pathways represents an integration point for proper wing development. The selector protein Ap directly activates Ser expression in the dorsal compartment during early third instar, which induces N activation at the DV border in mid third instar. The N pathway then regulates Ser expression by a positive-feedback loop along the DV boundary. Finally, as a result of Wg signaling, at the end of the third instar, Ser is expressed in two stripes flanking the DV boundary.