Control of cuticle formation by wing imaginal discs in vitro

Wing discs from late final-instar Ephestia larvae form only pupal cuticle when immediately implanted into pupae which subsequently undergo metamorphosis. However, either pupal or adult structures are made in vitro depending on (1) the ecdysterone dose and/or (2) disc cell proliferation. Continuous culture in ecdysterone (0.5-5.0 μg/ml) results in the appearance of transparent cuticle. On the basis of several criteria, this untanned cuticle is postulated to be scaleless adult cuticle. Discs pulsed with 0.5 μg/ml ecdysterone for 48-120 hr, or with 5.0 μg/ml for 24 hr, formed tanned cuticle. Lower doses of ecdysterone (i.e., 0.5 μg/ml for 24 hr or continuous exposure to 0.05 μg/ml) trigger adult scale formation. Enhancement of [³H]thymidine incorporation by these latter doses suggests the occurrence of disc cell divisions and polyploidization. The choice between pupal and adult pathways by wing discs of this age can be controlled exclusively by ecdysterone; juvenile hormone need not be involved in vitro.