During the CW window, starvation or loss of TOR signaling disrupted the formation of Snail peaks and endocycle synchronization, whereas later starvation had no effect on snail expression. Indeed, PG-specific loss of snail function prior to the CW checkpoint causes larval arrest due to a cessation of endoreplication in PG cells, whereas impairing snail after the CW checkpoint no longer affected endoreplication and further development. This suggests that the synchronization of PG cells into S phase via Snail represents the mechanistic link used to terminate the CW checkpoint. Snail levels then decline continuously, and endoreplication becomes nonsynchronized in the PG after the CW checkpoint. Interestingly, the second Snail peak occurs at the exit of the CW checkpoint. Remarkably, these Snail peaks coincide with two peaks of PG cells entering S phase and a slowing of DNA synthesis between the peaks. We observed two Snail peaks in the PG, one before and one after the molt from the second to the third instar. Before and during the CW checkpoint, snail is highly expressed in the larval prothoracic gland (PG), an endocrine tissue undergoing endoreplication and primarily dedicated to the production of the steroid hormone ecdysone. We show here that the Drosophila transcription factor Snail plays a key role in this process. How insects assess body size via the CW checkpoint is still poorly understood on the molecular level. This irreversible decision, termed the critical weight (CW) checkpoint, ensures that larvae have acquired sufficient nutrients to complete and survive development to adulthood. In holometabolous insects, the growth period is terminated through a cascade of peptide and steroid hormones that end larval feeding behavior and trigger metamorphosis, a nonfeeding stage during which the larval body plan is remodeled to produce an adult. Both mammals and insects use target of rapamycin (TOR) and insulin signaling pathways to coordinate growth with nutrition. The final body size of any given individual underlies both genetic and environmental constraints.
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