The origin of developmental mechanisms underlying vertebral elements: implications from hagfish evo-devo

Area of Science:

• Vertebrate Paleontology
• Developmental Biology
• Comparative Anatomy

Background:

• The axial skeletal morphology of hagfish, a jawless vertebrate clade, is poorly understood.
• Hagfish are generally thought to possess only a notochord and fin rays, with vertebral elements presumed absent.
• It remains unclear if hagfish axial skeletal features are ancestral or derived.

Purpose of the Study:

• To re-examine the axial skeletal morphology and development in the Japanese inshore hagfish (Eptatretus burgeri).
• To investigate the presence and origin of vertebral elements in hagfish.
• To infer the ancestral state of vertebral elements in early vertebrates.

Main Methods:

• Anatomical analyses of hagfish axial skeletal systems.
• Histological examination of hagfish skeletal development.
• Molecular phylogenetic analyses of cyclostomes and gnathostomes.

Main Results:

• Cartilaginous vertebral elements, derived from sclerotome, were identified at the ventral aspect of the hagfish notochord.
• Phylogenetic and morphological evidence supports the presence of sclerotome-derived vertebral elements in the common ancestor of all vertebrates.
• Developmental mechanisms for vertebral element formation appear conserved across vertebrate evolution.

Conclusions:

• Hagfish possess distinct vertebral elements, contrary to previous assumptions.
• Vertebral elements were likely present in the common ancestor of all vertebrates, with sclerotomal cells forming these structures.
• The developmental pathways for vertebral elements have been conserved throughout vertebrate evolution.