Autonomous termination of proliferation in the Drosophila wing disc is TORC1 dependent

Cells in a developing organ stop proliferating when the organ reaches a correct, final size. The underlying mechanisms are not understood. Although many signaling pathways and cell cycle components are required to sustain cell proliferation, which one of these turns off to terminate proliferation is not known. Here we study proliferation termination using Drosophila wing discs. We extend larval development to provide wing discs a constant growth-sustaining environment, allowing them to terminate proliferation autonomously. We find that the wing pouch, which forms the adult wing blade, terminates proliferation in the absence of brinker or warts, indicating that neither Dpp signaling nor Hippo/Yorkie signaling control final wing size. Instead, termination of proliferation coincides with reduced TORC1 activity and is bypassed by reactivating TORC1. Hence proliferation ceases due to reduced cell growth. Experimental manipulation of Dpp or Yki signaling can bypass proliferation termination in hinge and notum regions, suggesting that the mechanisms regulating proliferation termination may be distinct in different regions of the disc. One Sentence SummaryUsing Drosophila, Strassburger et al. investigate the termination of proliferation of an organ when it reaches its final size, and show this occurs due to a drop in TORC1 signaling.