Flight in birds has evolved through the structuring of the wings of the forelimbs and the transition from an alternating gait to a synchronous beat. In mammals, the spinal midline guide molecule, ephrin-B3, instructs the wiring that allows limb alternation, and its removal leads to a synchronous jumping gait. Here, we show that the ephrin-B3 protein in birds lacks several motifs present in other vertebrates, decreasing its affinity for the EphA4 receptor. Avian ephrine-B3 gene lacks an activator that drives midline expression and is absent in galliformes. The morphology and wiring at the brachial level of the chicken embryonic spinal cord resembles that of ephrine-B3 lousy mice. The dorsal midline decussation, evident in the mutant mouse, is apparent at the brachial level of the chick and is prevented by the expression of exogenous substances. ephrine-B3 to the roof plate. Our results support a role of loss of ephrin-B3 function in the formation of avian brachial spinal cord circuits and the facilitation of synchronous wing flapping.
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