Evolution of bilaterian central nervous systems: a single origin?

Area of Science:

• Evolutionary developmental biology
• Neuroscience
• Comparative genomics

Background:

• The origin of the central nervous system (CNS) in bilaterians is debated, with theories proposing either a single origin or independent evolution in protostomes and deuterostomes.
• The role of ancestral organizing centers in vertebrate brain development and their potential homology in other bilaterians remains unclear.

Purpose of the Study:

• To analyze evidence for and against the single-origin theory of the CNS.
• To investigate the role of gene networks in CNS patterning and homology across bilaterian groups.
• To clarify the phylogenetic position of hemichordates in relation to CNS evolution.

Main Methods:

• Comparative analysis of gene expression patterns in protostomes and deuterostomes.
• Review of morphological and genetic data related to CNS development.
• Phylogenetic analysis of conserved gene networks.

Main Results:

• The bulk of evidence supports a single origin of the CNS in the ancestral bilaterian.
• Mediolateral patterning gene expression provides stronger evidence for CNS homology than anteroposterior patterning genes.
• Hemichordates present an ambiguous case, with potential loss or independent evolution of CNS centralization.

Conclusions:

• The CNS likely evolved once in the ancestral bilaterian, with subsequent internalization and specialization of neurogenic ectoderm.
• Conserved gene expression patterns in CNS patterning suggest homology across protostomes and deuterostomes.
• Ancestral organizing centers were likely present in early chordates and elaborated upon in vertebrates.