A switch to non-proliferative growth sustains Drosophila wing development during the early pupal stage

Understanding how final organ size is established during development remains a fundamental yet unresolved question in biology. Size determination depends on two key processes: how organs increase their mass and how they stop growing upon reaching an appropriate size. Over the past few decades, organ transplantation and regeneration experiments have provided broad insights into size determination.^1,2 In particular, research on Drosophila imaginal discs has started disentangling the complex integration of growth control with developmental processes.^3,4,5 The wing disc is a highly proliferating monolayer epithelium during larval stages, with cell proliferation slowing and eventually halting at the larva-to-pupa (L/P) transition. Since growth has traditionally been associated with proliferation, the current model postulates that proliferation arrest determines final tissue size. Consequently, the L/P transition is believed to mark a switch between growth and morphogenesis, and understanding proliferation arrest at that stage has been the focus of both experimental and theoretical studies.^{6,7,8,9,10,11} Here, through 3D reconstruction and volume measurements, we show that wing disc growth continues throughout the L/P transition, only arresting later during the pupal period. This reveals a previously uncharacterized phase of growth in the early pupal stage that takes place during wing eversion, expansion, and elongation. Furthermore, we demonstrate that pupal wing growth is driven by an increase in cell volume, with an important contribution from insulin/insulin growth factor (IGF) signaling activated by fat body-derived Dilp6. These findings challenge the prevailing model of imaginal wing development and open new avenues for the study of growth arrest and organ size determination.