Dorsoventral patterning in hemichordates: insights into early chordate evolution.

Area of Science:

• Developmental Biology
• Evolutionary Biology
• Genetics

Background:

• The dorsoventral (D/V) axis organization in hemichordates and chordates offers insights into their common ancestor.
• Understanding this axis is crucial for identifying evolutionary changes in the chordate body plan post-lineage divergence.

Purpose of the Study:

• To compare D/V development in hemichordates and chordates.
• To infer the organization of their common ancestor.
• To identify evolutionary modifications of the chordate body axis.

Main Methods:

• Comparative analysis of gene expression patterns (Bmp, Chordin, Admp, transcription factors) in hemichordate embryos.
• Experimental manipulation of Bmp signaling (excess Bmp protein, small interfering RNA knockdown).
• Examination of D/V patterning independence from anterior/posterior patterning.

Main Results:

• A conserved Bmp-Chordin developmental axis precedes the anatomical D/V axis in hemichordates, suggesting ancient origins.
• Specific gene expression domains in hemichordate germ layers are regulated by Bmp signaling.
• D/V patterning is independent of A/P patterning in hemichordates, unlike chordates.
• Neural gene expression in hemichordates is not repressed by high Bmp levels, correlating with a diffuse nervous system.

Conclusions:

• The Bmp-Chordin axis in the common ancestor patterned germ layers and neural cell fates within a diffuse nervous system.
• Chordates later evolved neural centralization by segregating neural and epidermal ectoderm using this pre-existing axis.
• The relative orientation of the mouth and the Bmp-Chordin axis may have shifted in the chordate lineage.