Amphioxus Evx genes: implications for the evolution of the Midbrain-Hindbrain Boundary and the chordate tailbud.

Evx genes are widely used in animal development. In vertebrates they are crucial in gastrulation, neurogenesis, appendage development and tailbud formation, whilst in protostomes they are involved in gastrulation and neurogenesis, as well as segmentation at least in Drosophila. We have cloned the Evx genes of amphioxus (Branchiostoma floridae), and analysed their expression to understand how the functions of Evx have evolved between invertebrates and vertebrates, and in particular at the origin of chordates and during their subsequent evolution. Amphioxus has two Evx genes (AmphiEvxA and AmphiEvxB) which are genomically linked. AmphiEvxA is prototypical to the vertebrate Evx1 and Evx2 genes with respect to its sequence and expression, whilst AmphiEvxB is very divergent. Mapping the expression of AmphiEvxA onto a phylogeny shows that a role in gastrulation, dorsal-ventral patterning and neurogenesis is probably retained throughout bilaterian animals. AmphiEvxA expression during tailbud development implies a role for Evx throughout the chordates in this process, whilst lack of expression at the homologous region to the vertebrate Midbrain-Hindbrain Boundary (MHB) is consistent with the elaboration of the full organiser properties of this region being a vertebrate innovation.