| Research Abstract |
The present research was planned to elucidate physiological mechanisms supporting phenomena of viviparity in some rockfish species of the genera Sebastes and Sebastiscus.
Seasonal reproductive cycles were examined histologically in male and female Sebastes taczanowskii. On the basis of a comparison of reproductive cycles among four Sebastes species, it was suggested that the northern species tended to prolong gametogenesis in both sexes, and that sperm storage in the ovary also tended to be longer in the northern than in the southern species. In vitro incubation experiments revealed that 17alpha, 20beta- dihydroxy-4-pregnen-3-one (17alpha,20beta-DP) had the most significant effect in inducing oocyte maturation. Furthermore, changes in serum steroid hormone levels were studied during the reproductive cycles. It was suggested that estradiol-17beta was the most important steroid involved in vitellogenesis and 17alpha,20beta-DP possibly played an important role in gestation in S.taczanowski
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i and Sebastiscus marmoratus. In two oviparous teleosts, Oryzias latipes and Gasterosteus aculeatus, on the other hand, fertilization and early embryonic development could be experimentally induced in their ovaries after artificial insemination. These results indicate that physiological innovation on the maternal side, which is concerned with the maintenance of embryonic development during late pregnancy and the induction of parturition, may be essential for successful attainment of viviparity.
Female-specific serum proteins (FSSPs) in S. faczanowskii were identified and partially characterized by immunochemical procedures. It was suggested that, among multiple FSSPs, vitellogenin was used only for yolk formation in ovarian oocytes and was not a direct nutritional source for developing embryos during gestation.
In S. taczanowskii and S. schlegeli, it was confirmed morphologically and histochemically that developing embryos ingested yolk-like materials through their hindgut during late pregnancy. An increase in levels of Na,K-ATPase activity during the same period was observed in embryos of these two species. Furthermore, some ultrastructural features observed in ovarian tissues suggested possible underlying physiological mechanisms correlated with the maintenance of pregnancy in Sebastes. Less
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