The amphioxus Hox cluster: deuterostome posterior flexibility and Hox14

The amphioxus (Branchiostoma floridae) Hox cluster is a model for the ances tral vertebrate cluster, prior to the hypothesized genome-wide duplications that may have facilitated the evolution of the vertebrate body plan. Here we describe the posterior (5') genes of the amphioxus cluster, and report t he isolation of four new homeobox genes. Vertebrates possess 13 types of Ho x gene (paralogy groups), but we show that amphioxus possesses more than 13 Hox genes. Amphioxus is now the first animal in which a Hox14 gene has bee n found. Our mapping and phylogenetic analysis of amphioxus "Posterior Clas s" Hox genes reveals that these genes are evolving at a faster rate in deut erostomes than in protostomes, a phenomenon we term Posterior Flexibility.