Noncanonical role of Hox14 revealed by its expression patterns in lamprey and shark

Hox genes are arranged in uninterrupted clusters in vertebrate genomes, and the nested patterns of their expression define spatial identities in multiple embryonic tissues. The ancestral Hox cluster of vertebrates has long been thought to consist of, maximally, 13 Hox genes. However, recently, Hox14 genes were discovered in three chordate lineages, the coelacanth, cartilaginous fishes, and amphioxus, but their expression patterns have not yet been analyzed. We isolated Hox14 cDNAs from the Japanese lamprey and cloudy catshark. These genes were not expressed in the central nervous systems, somites, or fin buds/folds but were expressed in a restricted cell population surrounding the hindgut. The lack of Hox14 expression in most of the embryonic axial elements, where nested Hox expressions define spatial identities, suggests a decoupling of Hox14 genes' regulation from the ancestral regulatory mechanism. The relaxation of preexisting constraint for collinear expression may have permitted the secondary losses of this Hox member in the tetrapod and teleost lineages.