Insight from Lamprey Genomics: Brain and Pituitary Reproductive Hormones of Lampreys
Stacia A. Sower, Mihael Freamat, and Scott I. Kavanaugh
Abstract.—This chapter summarizes reproduction and the latest findings on reproductive endocrinology in one of the only two living representatives of the most ancient lineage of vertebrates, agnathans. Modern vertebrates are classified into two major groups, the gnathostomes (jawed vertebrates) and the agnathans (jawless vertebrates). The agnathans are classified into two groups, myxinoids (hagfishes) and petromyzonids (lampreys), while the gnathostomes constitute all the other living vertebrates, including the bony and cartilaginous fishes and the tetrapods. During the past two decades, there have been rapid advances in our knowledge of the structure and function of reproductive hormones in lamprey. Lampreys are the earliest evolved vertebrates for which there are demonstrated functional roles for two (possibly three) gonadotropin-releasing hormones (GnRHs) that act via the hypothalamic-pituitary-gonadal axis controlling reproductive processes. From our structural and functional studies, we have determined the primary amino acid and cDNA sequences of two forms of GnRH, lamprey GnRH-I and -III, one GnRH receptor, and one gonadotropin-beta (GTH-b) hormone. Since 2006, with the availability of the lamprey genome, we have identified an additional GnRH isoform (lamprey GnRH-II) and two glycoprotein hormone receptors (one gonadotropic-like and one thyrotropic-like). The high conservation of these hormones and their receptors throughout vertebrate species makes the lamprey model highly appropriate for examining the neuroendocrine system. Here, we present a summary on our current knowledge of reproductive endocrinology in these basal vertebrates.