Polychaete trunk neuroectoderm converges and extends by mediolateral cell intercalation

Area of Science:

• Developmental Biology
• Evolutionary Biology
• Cell Biology

Background:

• Convergent extension transforms spherical embryos into elongated forms, a key feature in bilaterian development.
• This process is modified or absent in model organisms like Caenorhabditis elegans and Drosophila melanogaster.
• The mechanisms of body elongation in polychaete annelids, such as Platynereis dumerilii, remain largely unknown.

Purpose of the Study:

• To investigate the cellular and molecular mechanisms underlying axis elongation in Platynereis dumerilii.
• To compare convergent extension in polychaetes with that in vertebrates (frogs) to understand its evolutionary history.

Main Methods:

• In vivo time-lapse imaging of Platynereis embryos.
• Analysis of cellular movements, specifically mediolateral cell intercalation.
• Investigation of molecular pathways, including actin, microtubules, JNK, and the noncanonical Wnt pathway.

Main Results:

• Platynereis neuroectoderm elongates via mediolateral cell intercalation on both sides of the neural midline.
• This process separates the mouth and anus, both derived from the blastopore.
• Tissue elongation is dependent on actin and JNK activity but independent of microtubules, involving the noncanonical Wnt pathway.

Conclusions:

• Convergent extension in Platynereis shares similarities and differences with frog development.
• The findings suggest that trunk neuroectoderm convergent extension is an ancestral bilaterian trait.
• This ancestral mechanism likely played a role in separating the mouth and anus during early bilaterian evolution.