The amphioxus genome in Evo-Devo: archetype or "cul de sac'?

The new discipline of "Evo-Devo" is facing the fascinating paradox of expla ining morphological evolution using similar pieces or genes to build highly divergent animals. The cephalochordate amphioxus has the privilege situati on of being the closest living relative to vertebrates, retaining a vertebr ate-like simple body plan, and a preduplicative genome. We report two examp les showing that the amphioxus genome may well be archetypal, but has been evolving since the divergence from the vertebrate lineage. Firstly, the amp hioxus 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 amph ioxus 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 serv e as key landmark to understand morphological evolution.