Figure 2.

Hypothetical model for neural crest-like cell migration in amphioxus. The expression of characteristic neural genes in scattered individual cells in the epidermis indicates that neuronal precursors are generated from the normal epidermal cells following a particular genetic program switched on during neurulation. These unknown genetic programs should endow these particular cells with additional distinctive properties, which might include the acquisition of individual migratory behaviour. The migration and terminal differentiation of those neuronal precursors, in particular locations, could depend on the pre-patterning of the neurula epidermis. This model summarises and gives an oversimplified view of the epidermal expression of different genes that seem to participate in the epidermal patterning of the amphioxus neurula. As in the neural tube, Hox genes are expressed in a nested manner and, together with the expression of AmphiPax6, AmphiEn and AmphiCdx, divide the entire epidermis into meridians of differential expression. All these genes are expressed in a scattered manner, but are delimited along the anteroposterior axis as indicated by colour code. In the case of the Hox genes, the meridians were established by the combinatorial expression of AmphiHox 1, 3, 4 and 6. Under this hypothetical model, those individual cells (red dots) mentioned above, would migrate vertically (arrows) through defined routes, as delimited by each meridian of combinatorial expression.