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.
Department of Physiology and Neuroscience, NYU Langone Medical Center, New York, New York 10016, USA.
Leung-Hang
LH
Ma
Edwin
E
Gilland
Andrew H
AH
Bass
Robert
R
Baker
D058500:Animal Fins / Q000294:innervation*; D000818:Animals; D005399:Fishes; D005552:Forelimb / Q000294:innervation*; D009046:Motor Neurons / Q000166:cytology*