OOGENESIS IN ATLANTIC SALMON (SALMO-SALAR L.) OCCURS BY ZONAGENESIS PRECEDING VITELLOGENESIS IN-VIVO AND IN-VITRO

Fish oogenesis represents pleiotropic cytodifferentiative programs inc luding hepatic synthesis of the molecular components for both the eggs hell and the oocytic energy deposits. Both hepatic processes are direc tly controlled by plasma levels of estradiol (E-2), and injected E-2 i nduces both biogenetic processes in prepubertal fish of both sexes. Th is work compares the temporal pattern of E-2-induced biosynthesis of z ona radiata proteins (zr-proteins) and vitellogenin in Atlantic salmon (Salmo salar L.) in vivo and in vitro. We monitored the presence of p lasma zr-proteins and vitellogenin, using homologous polyclonal antise rum to zr-proteins and a monoclonal antibody to vitellogenin. Zr-prote ins were induced by all E-2 concentrations (0.001-1.1 mg/kg body weigh t (bw)) within one week of exposure while vitellogenin was not induced until two weeks post-injection and then only in plasma from fish inje cted with high E-2 concentrations (0.4 mg or 1.1 mg/kg bw). After E-2 treatment, hepatocytes isolated from male fish synthesized zr-proteins and vitellogenin in vitro. However, zr-proteins were secreted into th e medium two days before vitellogenin, as measured by ELISA. The data indicate a preferential induction of zr-proteins compared with vitello genin, both with regard to E-2 sensitivity and response time to E-2 tr eatment. These findings suggest an obligate sequence in salmon oogenes is. During sexual maturation low E-2 levels at first induce only zonag enesis, while increasing levels of E-2 subsequently induce both zonage nesis and vitellogenesis. In nature, the interval between zonagenesis and vitellogenesis may, therefore, be considerable. The data suggest n ew control mechanisms in fish oogenesis.