The nonmammalian-mammalian transition through neurohypophysial peptides

Neurohypophysial hormones are particularly proper evolutionary tracers. Whereas Eutherian mammals have oxytocin and virtually always arginine vasopressin, nonmammalian tetrapods possess instead mesotocin and vasotocin. The transitions of mesotocin-oxytocin and vasotocin-vasopressin involved in the passage of reptiles-mammals seem to have occurred independently. Australian marsupials are endowed with mesotocin but American marsupials have either oxytocin (South-American opossums) or both oxytocin and mesotocin (North-American opossum). These results suggest that Australian Metatherians have preserved reptilian mesotocin and used it for milk-ejecting function whereas the change mesotocin-oxytocin appeared only in the American line. All marsupials have either arginine vasopressin or lysipressin and phenypressin (Australians) or lysipressin and arginine vasopressin (Americans). It is assumed that the change of vasotocin into arginine vasopressin occurred very early, perhaps in mammal-like reptiles, and duplication of the gene with subsequent mutations has led to the presence of two vasopressin-like peptides in most Metatherians.