BACKGROUND: The mammalian neurohypophysial hormones, vasopressin and oxytocin 
are involved in osmoregulation and uterine smooth muscle contraction 
respectively. All jawed vertebrates contain at least one homolog each of 
vasopressin and oxytocin whereas jawless vertebrates contain a single 
neurohypophysial hormone called vasotocin. The vasopressin homolog in 
non-mammalian vertebrates is vasotocin; and the oxytocin homolog is mesotocin in 
non-eutherian tetrapods, mesotocin and [Phe2]mesotocin in lungfishes, and 
isotocin in ray-finned fishes. The genes encoding vasopressin and oxytocin genes 
are closely linked in the human and rodent genomes in a tail-to-tail 
orientation. In contrast, their pufferfish homologs (vasotocin and isotocin) are 
located on the same strand of DNA with isotocin gene located upstream of 
vasotocin gene separated by five genes, suggesting that this locus has 
experienced rearrangements in either mammalian or ray-finned fish lineage, or in 
both lineages. The coelacanths occupy a unique phylogenetic position close to 
the divergence of the mammalian and ray-finned fish lineages.
RESULTS: We have sequenced a coelacanth (Latimeria menadoensis) BAC clone 
encompassing the neurohypophysial hormone genes and investigated the 
evolutionary history of the vertebrate neurohypophysial hormone gene locus 
within a comparative genomics framework. The coelacanth contains vasotocin and 
mesotocin genes like non-mammalian tetrapods. The coelacanth genes are present 
on the same strand of DNA with no intervening genes, with the vasotocin gene 
located upstream of the mesotocin gene. Nucleotide sequences of the second exons 
of the two genes are under purifying selection implying a regulatory function. 
We have also analyzed the neurohypophysial hormone gene locus in the genomes of 
opossum, chicken and Xenopus tropicalis. The opossum contains two tandem copies 
of vasopressin and mesotocin genes. The vasotocin and mesotocin genes in chicken 
and Xenopus, and the vasopressin and mesotocin genes in opossum are linked 
tail-to-head similar to their orthologs in coelacanth and unlike their homologs 
in human and rodents.
CONCLUSION: Our results indicate that the neurohypophysial hormone gene locus 
has experienced independent rearrangements in both placental mammals and teleost 
fishes. The coelacanth genome appears to be more stable than mammalian and 
teleost fish genomes. As such, it serves as a valuable outgroup for studying the 
evolution of mammalian and teleost fish genomes.