Fig. 5. N activity and expression of its ligands in second and third-instar eye discs. (A-A'') In late-second-instar eye discs, Ser (arrow, green) is expressed in a broad swath about the D-V midline, whereas Dl (red) expression is more general. (B) Mid-third-instar N activation evidenced by N-GV/UAS-lacZ staining. Ahead of the wave front, strong N activity is in a line running from the D-V midline and curving up into the anterior dorsal region (arrow), and weak N activity is detected in the ventral part (arrowhead). (C) Ser (arrows and arrowheads, green) is expressed in a domain corresponding to the lines of both strong and weak N activation but occupying broader areas. (C') Dl (arrows, red) is upregulated in a line corresponding to the profile of strong N activation, and DI expression can also be seen weakly in a position corresponding to the weaker N activation (arrowhead). (D) Late-third-instar N activity (blue) is detected in the region of the D-V midline and head capsule on both the dorsal (arrow) and ventral (arrowhead) side. (D') Ser (green) is expressed in a broader region corresponding to N activity. (D'') Dl (red) is mainly expressed behind the MF, but is also expressed at the anterior extreme in both dorsal (arrow) and ventral (arrowhead) regions.