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Growth regulation of the Drosophila wing imaginal disc critically depends on the Decapentaplegic (Dpp) morphogen gradient. How a graded Dpp signal is interpreted by cells to control homogeneous tissue growth remains unclear. Wartlick et al. (p. 1154; see the Perspective by Le Goff and Lecuit) address this question by measuring the spatial and temporal changes of Dpp concentration and signaling activity during the disc growth phase and by quantifying cell proliferation parameters in the discs. Both modeling and experimental findings suggest that both Dpp concentration and signaling gradients scale with tissue size so that, on average, the onset of mitosis occurs when Dpp signaling levels have increased by 50% since the beginning of the cell cycle.
Morphogens, such as Decapentaplegic (Dpp) in the fly imaginal discs, form graded concentration profiles that control patterning and growth of developing organs. In the imaginal discs, proliferative growth is homogeneous in space, posing the conundrum of how morphogen concentration gradients could control position-independent growth. To understand the mechanism of proliferation control by the Dpp gradient, we quantified Dpp concentration and signaling levels during wing disc growth. Both Dpp concentration and signaling gradients scale with tissue size during development. On average, cells divide when Dpp signaling levels have increased by 50%. Our observations are consistent with a growth control mechanism based on temporal changes of cellular morphogen signaling levels. For a scaling gradient, this mechanism generates position-independent growth rates.