Ancestry of motor innervation to pectoral fin and forelimb.
Motor innervation to the tetrapod forelimb and fish pectoral fin is assumed to share a conserved spinal cord origin, despite major structural and functional innovations of the appendage during the vertebrate water-to-land transition. In this paper, we present anatomical and embryological evidence showing that pectoral motoneurons also originate in the hindbrain among ray-finned fish. New and previous data for lobe-finned fish, a group that includes tetrapods, and more basal cartilaginous fish showed pectoral innervation that was consistent with a hindbrain-spinal origin of motoneurons. Together, these findings support a hindbrain-spinal phenotype as the ancestral vertebrate condition that originated as a postural adaptation for pectoral control of head orientation. A phylogenetic analysis indicated that Hox gene modules were shared in fish and tetrapod pectoral systems. We propose that evolutionary shifts in Hox gene expression along the body axis provided a transcriptional mechanism allowing eventual decoupling of pectoral motoneurons from the hindbrain much like their target appendage gained independence from the head.
Ma
Leung-Hang
LH
Department of Physiology and Neuroscience, NYU Langone Medical Center, New York, New York 10016, USA.
Gilland
Edwin
E
Bass
Andrew H
AH
Baker
Robert
R
D058500:Animal Fins / Q000294:innervation*; D000818:Animals; D005399:Fishes; D005552:Forelimb / Q000294:innervation*; D009046:Motor Neurons / Q000166:cytology*