Characterization of Neuronal Circuits for coordinated limb movements in avian
Department of Medical Neurobiology, IMRIC, Hadassah Medical School, Hebrew Univeristy, Jerusalem.
Two main features that support birds’ flying are morphological changes that patterned the wings from limbs, and the transition from alternate gait to synchronous flapping. Birds evolved from quadrupedal reptiles. Comparative genomics analysis revealed that dozens of syntenic gene clusters were deleted in birds. We hypothesis that the evolvement of wing flapping is a direct consequence of the genome size reduction, either loss of genes or enhancer elements. We found that gene encoding the axon guidance molecule Ephrin-B3, that serves as a midline barrier for midline axonal-crossing of excitatory interneurons in the mouse spinal cord, is missing in the genome of chick. Notably, mice null for Ephrin-B3 hop by synchronous gait. Analysis of neuronal circuits at the lumbar (legs) and brachial (wings) chick spinal cord levels, reveals axonal decussation of excitatory and pre-motor interneurons at the brachial, but not lumbar, spinal cord. Midline crossing at the lumbar spinal cord is impeded by level-specific expression of Ephrin-B1. Concomitantly, knock-in of the mouse Ephrin-B3, at the chick brachial spinal cord, is sufficient to prevent axonal decussation. Hence, supporting a role for gene-loss in gain of a trait, synchronous flapping of the wings.