Heterochronic activation of TGF-β signaling drives the diversity of the avian sterna.

Adaptive evolution has shaped vertebrate skeletal morphology, supporting diverse locomotor modes and lifestyles. In volant birds (carinates), the sternum develops a ventral keel for the attachment of massive pectoral muscles essential for powered flight. In contrast, flightless ratites lack both flight and the keel, creating a marked contrast in sternal shape. Here, we interrogated the cellular and molecular basis underlying this divergence using chicken and emu embryos as models. Through a series of analyses including spatiotemporal transcriptomics and a spheroid culture system, we found that TGF-β signaling, which promotes proliferation of sternal chondroprogenitors, is activated in both species until the left and right progenitors meet. In chicken, this activation persists, driving ventral extension of the keel primordium, whereas in emu it shuts off early, preventing keel formation. Our findings suggest that skeletal morphological changes associated with behavioral transitions can emerge from heterochronic shifts in developmental signaling, thereby deepening our understanding of the evolutionary logic shaping skeletal diversification.