Evolution of vasopressin and oxytocin family of nonapeptides
From Gwee et al. (here) © 2008 Gwee et al; licensee BioMed Central Ltd.
Delivery of the fetus and placenta depends in part on the action of oxytocin (a pituitary hormone) on oxytocin receptors in the myometrium. Duplication of the vasotocin (VT) gene of lampreys was the basis for the evolution of oxytocin (OT) and arginine vasopressin (VP).
A fresh study (here) confirms this scenario but takes a closer look at the evolution of the receptors. There are three receptors for vasopressin and one for oxytocin. In essence the hypothesis is that these arose during the whole genome duplications that are thought to have occurred early in the evolution of vertebrates. It is noted that these two rounds of duplication would in theory have produced four vasotocin genes. If so, three were lost and the fourth only later gave rise to the oxytocin gene by a separate tandem duplication event. The gist of the argument is that the receptor evolved first.
Whilst oxytocin acts on the mammary glands and uterus of mammals, vasopressin acts on the kidney. In addition, both peptides act as neurotransmitters in the brain. Yamashita and Kitano suggest the neurotransmitter role came first. Only later were receptors expressed in peripheral organs such as the uterus and the peptides secreted as hormones from the posterior pituitary.