Developmental system drift in motor ganglion patterning between distantly related tunicates

Area of Science:

• Developmental Biology
• Neuroscience
• Evolutionary Biology

Background:

• The tunicate Ciona serves as a model for chordate neurodevelopment.
• The motor ganglion (MG) development in Ciona is well-studied, but less is known in other tunicates.

Purpose of the Study:

• To investigate the conservation and divergence of motor ganglion development in the tunicate Molgula occidentalis compared to Ciona.
• To explore the regulatory mechanisms of Dmbx gene expression in cross-species comparisons.

Main Methods:

• Comparative gene expression analysis in developing Molgula MG.
• Generation of transgenic Molgula embryos with Dmbx reporter plasmids.
• Cross-species transgenic assays using cis-regulatory DNAs from Ciona and Molgula.

Main Results:

• Molgula MG patterning is highly conserved with Ciona, with similar neuronal subtype positioning.
• The timing of subtype-specific gene expression onset was earlier in Molgula.
• Morphology of Molgula descending decussating neurons (ddNs) is similar to Ciona ddNs and vertebrate Mauthner cells.
• Cross-species assays revealed significant changes in the regulatory logic of Dmbx transcription, with Ciona cis-regulatory DNAs active in Molgula but not vice versa.

Conclusions:

• Functional conservation of cis-regulatory DNAs masks divergence in molecular mechanisms.
• Tunicate genetic plasticity may stem from rigid spatial organization of embryonic cell lineages.