Abstract
Complete vitellogenesis (synthesis and incorporation of vitellogenin, Vg) in the female catfish, Clarias batrachus, during different reproductive phases (preparatory, prespawning, and postspawning) of its annual ovarian cycle is induced by administering heterologous Vg (semipurified; ULT-I and purified; HA-I and HA-II) isolated from the Indian major carp, mrigal (Cirrhinus mrigala). During the prespawning period, ULT-I, which contains both HA-I and HA-II, showed a dose-dependent response in relation to the induction of complete vitellogenesis in female catfish. In the preparatory period, daily treatment with 1 mg of ULT-I for 12 days resulted in an increase in plasma Vg level, ovarian weight, and number of stage-II (vitellogenic) oocytes along with the appearance of stage III (fully formed) yolky oocytes. In the prespawning period, 12-day administration of HA-I at the dose level of 0.5 mg/fish/day enhanced complete vitellogenesis in intact female catfish and induced Vg synthesis in hypophysectomized female catfish, whereas HA-II, at the same dose level, could only stimulate Vg synthesis, indicating their different roles during vitellogenesis. The results so far obtained may be due to the estradiol-17β contamination in the Vg fractions. Estimation of E2 in ULT-I, HA-I, and HA-II by E2 ELISA indicates that HA-II contains only a small amount (106 pg/mg protein). Therefore, daily treatment with 1 mg each of delipidated ULT-I and HA-I for 12 days could synthesize very little Vg, whereas intact ULT-I at the same dose level induced not only a high plasma Vg level but also increased GSI, indicating the importance of lipid in the native molecule for the induction of vitellogenesis. During postspawning period, administration of HA-I (0.5 mg/fish/day for 21 days) to reproductively regressed female catfish pretreated with HA-II (10 μg/fish/day for 7 days) and maintained at a long photoperiod (LD 14:10) and high temperature (30°), induced complete vitellogenesis.
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