Development Of Gonad At Early Stages In Zebrafish(danio Rerio) And Effects Of Mt On Its Sex Differentiation

The process of early gonadal development and the effect of methyltestosterone(MT) on sex differentiation were studied in zebrafish (Danio rerio). The laws of early gonadal development and sex reversal effected by exogenous hormones were explored in zebrafish. It will provide a theoretical basis for the application of sex hormones induced monosexual variety to cultivate. The results were shown as follows:(1) Zebrafish belongs to the differentiated species of gonochoristic fish, its ovarian differentiation started from 19 days when the oogonia in ovary increased quickly. The cytologic symbol of ovarian differentiation was the emergence of oocytes, which appeared after hatching for 30 days. TheⅡstage oocytes emerged until 60 days.There were two types of testis differentiation in zebrafish, some testis differentiated at the same time with ovaries, and the others differentiated later than ovarian, at first the gonads in these fish were developed into ovaries, the oogonia began to degenerate at the same time, and then the gonads developed into testis. On 25 days post hatching, the appearances of spermatogonia and spermatocytes in the slices of these gonads were observed in the first kind of juvenile gonads, which was the cytologic symbol of testis differentiation. The other kind of juvenile gonads developed to ovaries first, then the ovaries began to degenerate at about 25 days post hatching, the oogonium degenerated to somatic cell, at the same time spermatogonia emerged, and then the gonads developed into testis. This process of degradation lasted until 50 days post hatching, and the primary spermatocytes appeared in the testis on 60 days post hatching.(2) In the study of sex reversal, five treatment groups (20, 30, 100μg/g, the negative control group and blank control group) were set up. After MT treatment, there were no appearance of female individuals in all the test groups. Compared with the control group, the sex ratio of zebrafish in test groups has a significant difference. Low dose of MT played a significant role in promoting the development of testis. In the high-dose group, the development of juvenile testis has been inhibited obviously. There were some facultative gonads in the process of treatment, but by the end of hormone treatment there were no facultative gonads observed. After treatment and resumed normal feeding conditions for 30 days, some facultative phenomena were found in the middle-dose group, suggesting that hormone-induced sex reversal may still be resumed. MT had a certain role in promoting the growth of zebrafish, but there was no significant difference in body length between test groups and control group, and at the same time, the mortality rates of juvenile zebrafish in middle-dose and high-dose group were significantly higher.