Monday 13 February 2006
Hemichordates, the phylum of bilateral animals most closely related to chordates, could reveal the evolutionary origins of chordate traits such as the nerve cord, notochord, gill slits and tail.
The anteroposterior maps of gene expression domains for 38 genes of chordate neural patterning are highly similar for hemichordates and chordates, even though hemichordates have a diffuse nerve-net.
The map of dorsoventral expression domains, centered on a Bmp-Chordin axis, differs between the two groups; hemichordates resemble protostomes more than they do chordates.
The dorsoventral axis might have undergone extensive modification in the chordate line, including centralization of the nervous system, segregation of epidermis, derivation of the notochord, and an inversion of organization.
Gerhart J, Lowe C, Kirschner M. Hemichordates and the origin of chordates. Curr Opin Genet Dev. 2005 Aug;15(4):461-7. PMID: 15964754