One might reasonably expect the edge of the Drosophila wing to be analogous to the edge of a piece of paper, a simple cessation of structure. The edge of the wing serves as a boundary between dorsal and ventral structures, and is shaped by complex boundary phenomenon.
Fringe plays a critical role in boundary formation in the developing wing.
This critical role bespeaks the complexity of the edge, and exemplifies the importance of boundaries in developmental processes. To begin to understand fringe , is to begin to understand many important principles of developmental biology. Clones of cells with mutated fringe , on a background of unmutated cells, form ectopic ectopic wing margins along the edge. The pattern of apterous expression in the wing disc is established by wingless as an early event in disc development. As the basis of the early apterous - wingless interaction is unknown, it is not known whether the wingless effects on apterous are direct.