Competition among growing organs and developmental control of morphological asymmetry

Fluctuating asymmetry is often used as a measure of developmental instability, although its developmental basis is poorly understood. Theoretical models and experimental studies have suggested that feedback interactions between structures on the left and right body sides play a pivotal role in the control of asymmetry. Here we provide experimental evidence that competition for a limiting resource can generate such interactions between growing organs. In our experiments in the butterfly Precis coenia (Lepidoptera: Nymphalidae), hindwing imaginal discs were removed from one or both body sides of caterpillars. Emerging butterflies were thus missing one or both hindwings, but had heavier forewings, mid– and hindlegs than untreated controls. When only one hindwing was removed, the forewing and hindleg on the treated side were heavier than on the untreated side. The asymmetry and overall weight increase in response to wing disc removal diminished with increasing physical distance of the responding tissue from the imaginal disc removed. Our findings are consistent with the hypothesis that growing imaginal discs compete for a haemolymph–borne resource, such as a nutrient or growth factor. Such competition is a possible mechanism for feedback interactions and may thus participate in the developmental control of asymmetry.