| In this paper, the migration of primordial germ cells (PGCs), formation of genital ridges and gonad sex differentiation of teleost, Pelteobagrus vachelli was investigated using histological methods, and the distribution of P450 aromatase in some tissues of the fish was examined by immunohistochemistry, too. Also in the paper, the effect of temperature upon expression of P450 aromatase gene was studied by RT-PCR techniques.The results were as follows:On day 1, the PGCs were at the splanchnic mesoderm above the gut. On day 5, PGCs migrated to the peritoneal epithelium. The genital ridge was formed on day 8, and there was no PGCs in it. The primordial gonads were formed on day 14. On day 23, the primordial gonads differentiated into ovaries or testes. From day 10 on, the larvae were cultivated in different water temperatures (20±0.5℃, 24±1.0℃, 30±0.5℃, 34±0.5℃) for 25 days. The control group was cultivated at natural water temperature (24±1.0℃). The sex ratio and percentage of male larvae were calculated after the experiment. At natural water temperature (24±1.0℃) and lower water temperature (20±0.5℃), the sex ratio was approximately 1:1 (1:1.09 and 1.22:1 respectively). However in the group cultivated at 30±0.5℃, the sex ratio was 1:4.89 and the percentage of male was 83.3±0.7%. The sex ratio was 2.85:1 and the percentage of male rate was only 26.4±0.4% in the group cultivated at 34±0.5℃. These results suggested that water at 30±0.5℃influenced the larval gonad towards differentiating into a testis, and water at 34±0.5℃gave the opposite result, i.e. the gonad differentiated into an ovary. This experiment showed that the sex determination of P. vachelli was temperature-dependent sex determination.The P450 aromatase was mainly orientated in the brain, oogonia and follicular cells in the ovaries, and interstitial tissue in the testes. Before gonad sex differentiation, there was weak P450 aromatase activity in larvae of both control and experimental groups. With the process of gonad sex differentiation, the activity of P450 aromatase increased. In the group cultivated at 30±0.5℃, P450 aromatase activity was lower than that in the group cultivated at 34±0.5℃. In the larvae with completion of gonad differentiation, P450 aromatase activity in the ovaries was higher than that in the testes. P450aromA mRNA was detected only in ovaries. Whereas P450aromB mRNA was identified in brain, gonads, stomach, intestines and kidney, with more amount in brain. In the ovaries, the amount of P450aromA mRNA was about 2.2 times as much as P450aromB mRNA.
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