Muscle-Derived TGF-β Factors Control Animal Body Size via an Epigenetic Integration Clock

How animals determine their final body size remains a central unresolved question in developmental biology. In insects, developmental transitions such as metamorphosis are traditionally thought to be controlled by size-threshold mechanisms. However, the molecular basis linking tissue growth to developmental timing remains unclear. Here we analyze genetic perturbation phenotypes of the muscle-derived TGF-β factor myoglianin (Myo) in the cricket Gryllus bimaculatus . Loss of Myo function produces a striking paradoxical phenotype: Myo knockout animals exhibit muscle hypertrophy yet fail to complete normal developmental progression, whereas partial reduction of Myo signaling leads to prolonged growth and gigantism. These observations cannot be explained by conventional size-threshold models. To interpret these data, we analyze the phenotypes using a temporal-integration framework in which systemic growth signals are accumulated as physiological time. In this model, developmental transitions occur when the integrated internal state reaches an epigenetic readiness threshold. The framework quantitatively links growth rate (R), developmental duration (D), and final body size (S), yielding an approximate area-constancy relationship (S·D ≈ constant) under defined conditions. The model further provides a conceptual explanation for several puzzling developmental regimes, including delayed growth under nutritional limitation, gigantism in Myo RNAi animals, developmental arrest in Myo knockout mutants, and timer-dominant progression observed after removal of juvenile hormone signaling. These findings suggest that muscle-derived TGF-β signals may function as systemic integrators of physiological time, coupling tissue growth to organismal developmental progression. This perspective provides a unified framework for understanding body size determination and suggests that final body size may emerge as a consequence of temporal integration of growth signals rather than instantaneous size thresholds. We propose to call this overarching timing mechanism the Epigenetic Integration Clock.