The BMP2/4 ortholog Dpp functions as an inter-organ signal that regulates developmental timing in Drosophila

For many organisms, developmental transitions are triggered by a neuroendocrine axis and are contingent upon multiple organs achieving sufficient growth and maturation. How the status of peripheral organs is communicated to the neuroendocrine axis is not known. In Drosophila larvae, metamorphosis is triggered by the steroid hormone ecdysone, secreted by the prothoracic gland (PG). Here we show that the BMP2/4 ortholog Dpp, which regulates growth and patterning of larval imaginal discs, also functions as a systemic signal to regulate developmental timing. Dpp from peripheral tissues, mostly imaginal discs, can reach the PG and inhibit ecdysone biosynthesis. As the discs grow, Dpp signaling decreases in the PG, thus alleviating the inhibition of ecdysone biosynthesis, and permitting entry into metamorphosis. We suggest that if a tissue can trap more morphogen locally as it grows and matures, then circulating levels of morphogen can provide a systemic readout of organ size and maturation.