Fig. 1. Development of the peripodial side of the wing disc. (A) PE structure in a third instar wing disc. Characteristic low-density nuclear distribution and squamous cell morphology are noticed in the PE. Nuclei are stained with the DNA dye TO-PRO-3 (red) and Arm localization in the zonula adherens (white) reveals cell shape. A white shadow delimits the surrounding cubic cells. The different cell types are described in a transverse section of a disc. (B-D) Ubx expression in the developing PE. In mid second instar (B), major differences in cell shape throughout the disc are not observed. In late second instar (C), squamous cells are seen in a proximal territory of the peripodial side of the disc (arrowheads). At this time, the expression of Ubx is still in poor correlation with squamous morphology. GFP expression in the wing-notum side of second instar discs, driven by ap-GAL4, allows comparison of cell shape between the two sides of the disc in the transversal sections. In third instar discs (D), Ubx is expressed in squamous and some cubic cells. Notice the decreasing gradient of Ubx expression in cubic cells (brackets and inset at higher magnification). (E-G) zfh-2 (green) and iro-C (red) expression domains in developing wing discs. In early mid second instar (E), zfh-2^MS209 GAL4 (see Materials and methods) and iro-C are expressed in proximal and distal cubic cells, respectively. Note that in early third instar discs (F) zfh-2 and iro-C domains now overlap in cubic posterior cells (arrowhead). During late larval development (G), iro-C-lacZ expression appears in some distal cubic cells (arrowhead). Discs are oriented anterior towards the left and distal upwards in all panels and figures.