ABSTRACT
Migratory, pluripotential neural crest cells are confined to vertebrates; their origin paved the way for the origin of the vertebrate body plan. Here we examine whether protochordates such as amphioxus and ascidia have cell types or cell behaviours that are homologous to forerunners of neural rest. Expression patterns of gene classes such as HNF-3, hh, Pax-3/7 and BMP-2/4 reveal that amphioxus and ascidia pattern their neural plate along the mediolateral (=dorsoventral) axis: this fulfils one prerequisite of neural crest evolution. A second prerequisite is cell migration from the neural plate. This has not been directly demonstrated, although the origin of non-neural Msx positive cells in amphioxus embryos deserves further investigation. Finally, we consider anteroposterior patterning of neural crest derivatives. We find that amphioxus Hox genes have spatial expression patterns only in the developing neural tube; we suggest that the regulatory sequences responsible for this expression were elaborated during vertebrate evolution to facilitate neural crest patterning.
