Induction of fertilizable eggs by conspecific vitellogenin implantation in captive female walking catfish, Clarias batrachus (Linn.)

The synthesis of vitellogenin (Vg) is induced by conspecific Vg (Vg1 and Vg2) and estradiol-17 (E-2) as demonstrated by the pattern of H-3-serine incorporation in the liver and plasma proteins. The incorporation studies indicated that the label was first incorporated into the liver after which it appeared in the blood in both E-2- and Vg-treated male catfish. Since Vg was capable of inducing its own synthesis, experiments were conducted in females during preparatory-prespawning period (March-May) to make them gravid by implanting Vg pellets. Two implantations of 4mg Vg1 pellets into female catfish with an interval of 15days, followed by laboratory maintenance for 45days of initial implantation showed a significant increment in ovarian weight with concomitant formation of yolky oocytes through synthesis and incorporation of Vg, whereas Vg2 implantation was not effective in this regard. Histological observation of yolky oocytes in Vg1-treated group showed the peripheral migration of germinal vesicle (eccentric germinal vesicle), which indicates the onset of maturation. On 45th day, third implantation with 2mg Vg pellets was performed and after 15days, fish were hormonally induced with a single injection of hCG (2,000IU/kg fish). Six groups were considered such as initial control, BSA-implanted control, Vg1-implanted, Vg2-implanted, catfish collected from the field on the last day of the experiment and catfish collected during spawning period in this experiment with 3-7 fish in each group. Each of the experimental fish was sexually mature and the body weight was between 100 and 125g. The percentage of ovulation and fertilization in the eggs of Vg1-implanted group was 91% and 78%, respectively, which was almost similar to that of gravid female catfish collected during breeding period (July). The breeding performance in BSA- and Vg2-treated females was very poor. The fertilized eggs were hatched in the laboratory conditions. Thus, in the female catfish, Vg1 not only induces vitellogenesis but also makes the oocytes viable for fertilization.