Axial patterning in cephalochordates and the evolution of the organizer.

The organizer of the vertebrate gastrula is an important signalling centre that induces and patterns dorsal axial structures. Although a topic of long-standing interest, the evolutionary origin of the organizer remains unclear. Here we show that the gastrula of the cephalochordate amphioxus expresses dorsal/ventral (D/V) patterning genes (for example, bone morphogenetic proteins (BMPs), Nodal and their antagonists) in patterns reminiscent of those of their vertebrate orthlogues, and that amphioxus embryos, like those of vertebrates, are ventralized by exogenous BMP protein. In addition, Wnt-antagonists (for example, Dkks and sFRP2-like) are expressed anteriorly, whereas Wnt genes themselves are expressed posteriorly, consistent with a role for Wnt signalling in anterior/posterior (A/P) patterning. These results suggest evolutionary conservation of the mechanisms for both D/V and A/P patterning of the early gastrula. In light of recent phylogenetic analyses placing cephalochordates basally in the chordate lineage, we propose that separate signalling centres for patterning the D/V and A/P axes may be an ancestral chordate character.