Reconstructing the ancestral vertebrate brain

Area of Science:

• Neuroscience
• Evolutionary Biology
• Developmental Biology

Background:

• The origin and early evolution of vertebrate brains are poorly understood due to limited data on jawless vertebrates (agnathans).
• Comparative studies of cyclostomes (lampreys and hagfish) and jawed vertebrates (gnathostomes) are crucial for reconstructing ancestral brain architecture.

Purpose of the Study:

• To review recent findings on lamprey and hagfish brain development.
• To reconstruct the brain architecture of the last common ancestor of vertebrates.
• To discuss modifications in vertebrate brain development across lineages.

Main Methods:

• Comparative analysis of brain development in lampreys and hagfish.
• Review of existing developmental studies on cyclostomes and gnathostomes.

Main Results:

• Recent studies reveal the presence of brain compartments (medial ganglionic eminence, rhombic lip) in hagfish embryos, previously thought to be absent in cyclostomes.
• These findings suggest that these brain regions evolved before the divergence of cyclostomes and gnathostomes.
• A more complex ancestral vertebrate brain state is proposed, accounting for derived traits in separate lineages.

Conclusions:

• The ancestral vertebrate brain was likely more complex than previously assumed based on lamprey data alone.
• Brain developmental plans have been independently modified in various vertebrate lineages.
• Understanding hagfish and lamprey brain development is key to deciphering vertebrate brain evolution.