The amphioxus genome in Evo-Devo: archetype or “cul de sac”?

The new discipline of "Evo-Devo" is facing the fascinating paradox of explaining morphological evolution using similar pieces or genes to build highly divergent animals. The cephalochordate amphioxus has the privilege situation of being the closest living relative to vertebrates, retaining a vertebrate-like simple body plan, and a preduplicative genome. We report two examples showing that the amphioxus genome may well be archetypal, but has been evolving since the divergence from the vertebrate lineage. Firstly, the amphioxus Hox cluster has at least 14 genes, and illustrates the phenomenon of "posterior flexibility", or a lesser constraint of the Hox posterior genes to evolve. Secondly, an ancestral Evx gene was tandemly duplicated in the amphioxus genome: one of the copies (amphiEvx-A) has retained the chordate-specific tasks of Chordate Evx, while a fast evolving copy (amphiEvx-B) is not longer involved in archetypal tasks. Our results indicate that the amphioxus genome has particularities and oddities that remind: amphioxus is not the ancestor of the vertebrates, but its fortunate position as the closest living relative to the ancestor give amphioxus genes the privilege to serve as key landmark to understand morphological evolution.