Brain and Pituitary Hormones of Lampreys, Recent Findings and their Evolutionary Significance

SYNOPSIS. Lampreys of the Class Agnathan are of particular importance in understanding endocrinological relationships since they represent one of the oldest lineages of extant vertebrates, which evolved over 550 million years ago. In this review, agnathans are considered to be paraphyletic in origin, with the modern agnathans classified into two groups, myxinoids (hagfish) and petromyzonids (lamprey). Recent paleontological analysis of modern groups suggests that the jawed vertebrates are more closely related to lampreys than either is to the hagfishes. It has been proposed that there have been two periods of gene duplication during the course of vertebrate evolution, one of which occurred close to the origin of the vertebrates. This paper will summarize the most recent information on the structure and function of newly identified hormones and genes of the brain (lamprey GnRH-I and -III, somatostatin-14; neuropeptide Y-related peptide and a lamprey tachykinin) and pituitary (ACTH, MSH-A, MSH-B, NHF-nasohypophysial factor, POM, and POC) in lampreys. In addition, these data provide further information for the prediction of gene duplication during the early development of vertebrates and increases our understanding of the molecular evolution and functional diversity of these hormones.