scRNA-seq analysis of cells comprising the amphioxus notochord

Area of Science:

• Evolutionary developmental biology
• Comparative genomics
• Chordate evolution

Background:

• Cephalochordates are crucial for understanding chordate origins, predating urochordates and vertebrates.
• The notochord, a defining chordate feature, has coin-shaped myofibrillar cells in amphioxus.
• Notochord-derived hedgehog signaling patterns the dorsal nerve cord in vertebrates, but single-cell properties remain unclear.

Purpose of the Study:

• To investigate the single-cell properties and gene expression profiles of amphioxus notochordal cells.
• To identify distinct cell types within the cephalochordate notochord and their potential functions.
• To compare cephalochordate notochord characteristics with those of vertebrates to infer evolutionary relationships.

Main Methods:

• Iso-seq analysis for full-length transcript sequencing.
• Single-cell RNA-seq analysis to determine individual cell gene expression.
• In situ hybridization (ISH) to visualize gene expression patterns within the notochord.

Main Results:

• Gene expression broadly categorizes notochordal cells into myofibrillar and non-myofibrillar (Müller) cells.
• Myofibrillar cells are central, with some extending notochordal horns; some myofibrillar genes are notochord-specific.
• Müller cells (dorsal, lateral, ventral) express hedgehog and Brachyury; dorsal Müller cells interact with the nerve cord, lateral with myotomes, and ventral with endoderm via signaling pathways.

Conclusions:

• Cephalochordate notochord cells display specialized functions analogous to vertebrate notochord roles in patterning adjacent tissues.
• The presence of conserved signaling pathways (hedgehog, Brachyury) and cell types suggests an ancestral form of the vertebrate notochord in cephalochordates.
• This study provides molecular evidence for the evolutionary origin of the vertebrate notochord within cephalochordates.