In other species: fowls , chicken , turkey , dog , Japanese quail

Categories: Nervous system phene

Single-gene trait/disorder: unknown

Disease-related: no

Cross-species summary: Most vertebrates have left/right independent unilateral motor control. The left brain hemisphere controls the right side of the body and the right brain hemisphere controls the left side of the body. This is achieved by motor neurons in the corticospinal tract crossing to the contralateral side at the level of the brainstem. In the spinal cord, a midline barrier exists that prevents axons from re-crossing. The transmembrane protein ephrin-B3 (EFNB3) is a key molecule required for establishing this spinal midline barrier and correct axon guidance (Kullander et al. 2001; Yokoyama et al. 2001).
Flight in birds evolved through patterning of the wings from forelimbs and transition from alternating gait to synchronous wing flapping. During bird evolution, EFNB3 function and the spinal midline barrier was lost, which resulted in synchronous wing flapping (Haimson et al. 2021).

Species summary: Haimson et al. (2021): "In mammals, the spinal midline guidance molecule ephrin-B3 instructs the wiring that enables limb alternation, and its deletion leads to synchronous hopping gait.  Here, we show that the ephrin-B3 protein in birds lacks several motifs present in other vertebrates, diminishing its affinity for the EphA4 receptor. The avian ephrin-B3 gene [EFNB3] lacks an enhancer that drives midline expression and is missing in galliforms."