STRUCTURAL AND FUNCTIONAL EVOLUTION OF THE VASOPRESSIN OXYTOCIN SUPERFAMILY - VASOPRESSIN-RELATED CONOPRESSIN IS THE ONLY MEMBER PRESENT INLYMNAEA, AND IS INVOLVED IN THE CONTROL OF SEXUAL-BEHAVIOR

It has been suggested that the gene duplication that led to the format ion of the vasopressin/oxytocin two-gene family occurred early during vertebrate evolution. However, the existence of both vasopressin- and oxytocin-related peptides in invertebrates suggests that this duplicat ion may have occurred much earlier, although there is no evidence for the co-occurrence of vasopressin- and oxytocin-related peptides in the same invertebrate species, We report here that in Lymnaea only the va sopressin-related peptide Lys-conopressin, but not an oxytocin-related peptide, is present. Moreover, it is very likely that an oxytocin-lik e cDNA or gene is absent. The conopressin gene is expressed in neurons that control male sexual behavior, and its gene products are present in the penis nerve and the vas deferens, Conopressin induces muscular contractions of the vas deferens and inhibits central neurons that con trol female reproductive behavior. Thus, although structurally related to vasopressin, conopressin has functional and behavioral characteris tics typical for oxytocin. Physiological and receptor binding data sug gest that conopressin and [Ile(8)]-conopressin, a synthetic oxytocin-l ike analog of conopressin, are functionally equivalent in Lymnaea, and that the chemical nature of the amino acid residue at position 8 does not result in a functional difference. Therefore, we suggest that inv ertebrates contain only a single member of the vasopressin/oxytocin ge ne family and that the amino acid change that distinguishes vasopressi n from oxytocin is functionally neutral in invertebrates.