Discontinuities in Rap1 activity determine epithelial cell morphology within the developing wing of Drosophila

► In the Drosophila wing epithelium, vein cell precursors are not hexagonal. ► Adherens junctions are asymmetrically distributed within presumptive vein cells. ► Two-cell patches of ectopic vein tissue phenocopy Rap1 loss-of-function. ►Rap1 over-expression disrupts cell-shape refinement of vein precursors. ► The Egfr/Ras signaling pathway down regulates the Rap1 effector Cno in vein cells.