Intra-organ growth coordination in Drosophila is mediated by systemic ecdysone signaling

Regulation of final organ size is a complex developmental process that involves the integration of systemic and organ-specific processes. Previously, we have shown that in developing Drosophila, perturbing the growth of one imaginal disc - the parts of a holometabolous larva that become the external adult organs - retards growth of other discs and delays development, resulting in tight inter-organ growth coordination and the generation of a correctly proportioned adult. Whether different parts of the same imaginal disc similarly coordinate their growth to generate a functioning adult organ is, however, unclear. In this study, we use the wing imaginal disc in Drosophila to study and identify mechanisms of intra-organ growth coordination. We generate larvae in which the two compartments of the wing imaginal disc have ostensibly different growth rates (wild-type or growth-perturbed). We find that there is tightly coordinated growth between the wild-type and growth-perturbed compartments, where growth of the wild-type compartment is retarded to match that of the growth-perturbed compartment. Crucially, this coordination is disrupted by application of exogenous 20-hydroxyecdysone (20E), which accelerates growth of the wild-type compartment. We further elucidate the role of 20E signaling in growth coordination by showing that in wild-type discs, compartment-autonomous up-regulation of 20E signaling accelerates compartment growth and disrupts coordination. Interestingly, growth acceleration through exogenous application of 20E is inhibited with suppression of the Insulin/Insulin-like Growth Factor Signaling (IIS) pathway. This suggests that an active IIS pathway is necessary for ecdysone to accelerate compartment growth. Collectively, our data indicate that discs utilize systemic mechanisms, specifically ecdysone signaling, to coordinate intra-organ growth.