The Amphioxus Hox Cluster: Characterization, Comparative Genomics, and Evolution

The amphioxus Hox cluster is often viewed as “archetypal” for the chordate lineage. Here we present a descriptive account of the 448kb region spanning the Hox cluster of the amphioxus Branchiostoma floridae from Hox14 to Hox1.We provide complete coding sequences of all 14 previously described amphioxus sequences and describe a detailed analysis of the conserved non-coding regulatory sequence elements. We find that the posterior part of the Hox cluster is so highly derived that even the complete genomic sequence is insufficient to decide whether the posterior Hox genes arose by independent duplications or whether they are true orthologs of the corresponding gnathostome paralog groups. In contrast, the anterior region is much better con- served. The amphioxus Hox cluster strongly excludes repetitive elements with the exception of two repeat islands in the posterior region. Repeat exclusion is also ob- served in gnathostomes, but not protostome Hox clusters. We thus hypothesize that the much shorter vertebrate Hox clusters are the result of extensive resolution of the redundancy of regulatory DNA following the genome duplications rather than the consequence of a selection pressure to remove non-functional sequence from the cluster.