Primary Studies On Several Factors Possibly Involved In The Gonadal Sex Determination And Differentiation Of Southern Catfish, Silurus Meridionalis

The southern catfish (Silurus meridionalis), obtained by artificial propogation, was all-female, but the sex ratio in nature was approximately 1:1. Previous experiments from our laboratory have showed that illumination, feed and water temperature after hatching had no effects on sex differentiation of southern catfish. On the other hand, estrogen played important roles in the sexual differentiation, whereas androgen had no effect on sex differentiation of southern catfish. Up to now, the mechanisms of feminization and sex differentiation of southern catfish by artificial propogation remain elusive.To obtain male fish, fry of southern catfish, obtained by artificial propogation were treated by oral administration of aromatase inhibitor (Fadrozole) and estrogen receptor antagonist (Tamoxifen). The histological observations on gonad of 5~130days after hatching (dah.) fish from treated group and control group indicated that the morphological sexual differentiation of southern catfish took place at around 7dah. The gonadal morphology between male and female was distinguishable after 7dah. The female gonadal transverse section was tubby. Its top was blunt and forms two outgrowths at about Sdah which fused and formed ovarian cavity at about 12dah. The male gonadal transverse section was spindly and its top was cuspate, with no outgrowths. The male gonad keeps this shape during the whole male gonadal development. Several obvious differences between female and male gonads were observed during the gonadal sex differentiation. In female, the ovarian cavity was formed around 12 dan. Rapid proliferation of germ cells started at about 35 dah. Meiosis of germ cells initiated about 55 dah; whereas in male fish germ cells proliferated rapidly only after 130 dah and meiosis of the germ cell initiated at the sametime. The gonadal differentiation in females occured much earlier than that in males.The histological observations of southern catfish gonad(5~130dah)showed that, there was no evidence that female reversed to male, and previous experiments have showed that the sex ratio of southern catfish of 4~7months after hatching in the nature was about 1:1, which confirmed the possibility that southern catfish in the nature had sex reversal was little. It also suggested the sex ratio (approximately 1:1) in nature was not caused by sex reversal after sex differentiation, but determined at sex differentiation or before sex differentiation. So the all-female of southern catfish by artificial fertilization was probably determined during or before sex differentiation, which was influenced by ecological factors largely. Previous experiments have showed that illumination, feed and water temperature after hatching had no effects on sex differentiation of southern catfish. In order to further ascertain if the all-female southern catfish was caused by gynogenesis during artificial fertilization or-the sperms containing Y chromosome were destroyed by the substance in ovary fluid, especially the acidic substance during artificial fertilization by dry method, we performed the observation of fertilization cytological process of southern catfish after artificial fertilization and the experiment of artificial fertilization using wet method.In this study, we performed artificial fertilization using wet method after 2-, 5-, and 100-fold dilution of the matured eggs and ovary fluid mixture of southern catfish with natural water from Jialingjiang River. However, we didn't find any male individuals in any treated group at 130dah. The results indicated that all female population of southern catfish after artificial propogation was not caused by dry method.We observed the artificial fertilization processes of southern catfish cytologically under the water temperature of 17 and 24 . Under the water temperature of 17, the sperm entered into the egg at about 20s and transformed into male pronucleus at around 25min after fertilization, the second polar body formed and was extruded at about 20min, while the female pronucleus was formed at around 25min after fertilization. The male pronucleus and female pronucleus fused partly in 40~45min and fused completely in 50min after insemination, In the water temperature of 24, the cytological behaviors were almost the same as the behaviors observed under the 17 , except that the time sequences of these behaviors appeared earlier. The results showed that the southern catfish eggs fertilized...