Neuroectodermal and endodermal expression of the ascidian Cdx gene is separated by metamorphosis

Area of Science:

• Developmental biology
• Evolutionary developmental biology
• Marine invertebrate biology

Background:

• Ascidians, or sea squirts, are invertebrate chordates with a complex biphasic life history.
• Chordate-specific structures develop during ascidian embryogenesis and metamorphosis.
• Vertebrate Cdx genes play crucial roles in posterior development and gastrulation.

Purpose of the Study:

• To characterize the expression pattern of the caudal/Cdx gene homologue, Hec-Cdx, in the ascidian Herdmania curvata.
• To understand the role of Hec-Cdx in the development of the ascidian body plan.
• To compare Hec-Cdx expression with vertebrate Cdx gene expression.

Main Methods:

• Gene expression analysis of Hec-Cdx in Herdmania curvata.
• RNA abundance quantification during different life stages (gastrulation, neurulation, tailbud extension, juvenile stages).
• Localization of Hec-Cdx expression during embryogenesis and metamorphosis.

Main Results:

• Hec-Cdx expression begins at gastrulation and peaks during neurulation and tailbud extension.
• Expression is localized to specific posterior neural plate precursor cells during tailbud formation.
• During metamorphosis, Hec-Cdx expression is found in the juvenile intestine.
• Hec-Cdx expression patterns are temporally separated in different tissues, similar to Otx gene expression.

Conclusions:

• The evolution of the ascidian biphasic body plan did not involve the deployment of Hec-Cdx into new developmental pathways.
• Instead, the novel body plan likely arose from a temporal separation of tissue-specific gene expression.
• This suggests conserved roles for Cdx genes in chordate posterior development, adapted to different life histories.