That Vein acts to engender cell proliferation is made clear from studies of mutant vein clones. Effects on cell proliferation are not only locally restricted. The wings carrying mutant clones for either vein or Rhomboid are smaller than those of control sibling flies, while the wings of double mutants are smaller still. In addition to their small size, the wings of double mutants lack all longitudinal and transverse veins. The adult Drosophila wing consists of two wing surfaces, apposed by their basal membranes, which first come into contact at metamorphosis, following wing disc eversion.
Contact is crucial.
Dorsal-ventral contact apposition was prevented during wing imaginal disc morphogenesis by implanting fragments of discs into metamorphosing hosts. In these implants, longitudinal veins differentiate in both surfaces, but exhibit wider than normal corrugation. When apposed to a lack of vein differentiation in the dorsal surface, vein differentiation fails in distal vein territories, preferentially in the ventral surface. Conversely, extra differentiation as in plexus , extramachrochaetae, HS-rhomboid 27B, Notch and Delta mutants, causes extra veins in clones, not only in the wing surface of the clone, but also in the opposite wing surface. Non-autonomous effects are observed in the same wing surface, a phenomenon called 'connectivity'.