Gonadal Sex Differentiation in Tilapia mossambica, with Special Regard to the Time of Estrogen Treatment Effective in Inducing Complete Feminization of Genetic Males

Abstract

In Tilapia mossambica, paired primordial gonads are formed 8-10 days after hatching, when germ cells begin their gradual multiplication and development into gonial ones. Sex differentiation of gonads either into ovaries or into testes becomes histologically discernible about 20 days of age: the development of some germ cells into meiotic prophase and the concomitant start of ovarian cavity formation oharacterize ovarian differentiation, while the ooourrence of efferent duot anlages in gonadal stroma oonfirms testioular differentiation. Henoeforth, germ oells in the ovary advanoe their oogenetio processes smoothly, but those in the testis remain quiescent until about 50 days of age. By oral administration of ethinylestradiol at a constant dose of 50 p,g per g diet, a complete feminization of genetio males was obtained in an experiment in which the treatment was done during a period from 6 to 25 days of age covering the sexually indifferent stage through the stage of sex differentiation. The treatments lasting for 10 days in either of the two stages were without effeot in causing any sexual modification, and an experiment carried out for 10 days extending through the two stages resulted in the occurrenoe of intersexual gonads in genetio males. It is presumed from these results that, in T. mossambica, the period sensitive to exogenous sex hormones for attaining complete sex reversal may lie between 10 and 25 days after hatohing during whioh gonadal sex differentiation begins and proceeds. Since Yamamoto1) achieved the functional feminization of Oryzias latipes by treatment with estrone, it has become generally thought that a complete and functional sex reversal can be induced rather easily in teleost fishes by the administration of heterologous sex hormones. From a viewpoint of fish propagation, artificial control of sex of fishes may be an important and interesting subject which awaits future development. There still remain, however, many problems to be solved in practicing the artificial direction of sex by exogenous sex hormones. According to Yamamot02), the conditions to be fulfilled to attain a complete and functional sex reversal of fishes are that a heterologous sex hormone should be administered at adequate dosage levels to fish fry starting at the sexually indifferent stage and lasting through the stage of gonadal sex differentiation. The cases of functional sex reversal experimented so far in Oryzias latipes1)3), Lebistes reticula* Laboratory of Fresh-Water Fi8h-CUlture, Faculty of Fisheries, Hokkaido University (;ft~m*~7.kgi~$~7.k~~~~1!l!)