Evolution of motor innervation to vertebrate fins and limbs

The evolution and diversification of vertebrate behaviors associated with locomotion depend highly on the functional transformation of paired appendages. Although the evolution of fins into limbs has long been a focus of interest to scientists, the evolution of neural control during this transition has not received much attention. Recent studies have provided significant progress in the understanding of the genetic and developmental bases of the evolution of fin/limb motor circuitry in vertebrates. Here we compare the organization of the motor neurons in the spinal cord of various vertebrates. We also discuss recent advances in our understanding of these events and how they can provide a mechanistic explanation for the evolution of fin/limb motor circuitry in vertebrates.

Research highlights
► Motor neurons have undergone sequential changes in their organization in the spinal cord in accordance with the emergence of their peripheral targets.
► The appearance of the Hox-dependent LMCs seems to be linked to the emergence of paired limbs.
► In the lineage of actinopterygian fishes, a Hox-dependent LMC has not been acquired.
► Primitive pectoral innervation has shown to be originated from both the hindbrain and the spinal cord.
► The Hox-dependent mechanism of appendicular innervation has not been established with regard to pelvic fin innervation.