| Naturaly, one of biological constraints to the development of commercial farming of tilapia is its early sexual maturity that result in spawing before fish reach market size Therefore, commercial production of tilapia often relies on monosex culture of males. All-male tilapia populations are desirable because males achieve a larger final size than females.The most common techniques for producing male fingerlings are steroid-induced sex inversion and interspecific hybridization. To study the masculinization and 17α- Methyltestosterone(MT) residues, the Nile tilapia fry were treated by MT. However, the molecular mechanisms involved in these changes and the sex-linked locus remain unclear. SSR, RT-PCR and liquid chromatography method were used in this study. The main achievements of investigation were as follows:1. At 10 days post-fertilization (DPF), Nile tilapia (Oreochromis niloticus) fry were fed by17α-Methyltestosterone (MT)-treated diet (0, 5, 10, 30, 50 and 60 mg?kg-1) for 30 days, then the MT-treated diet were replaced by MT-free commercial feed. After four months experiment, the absolute growth rate of length (AGRL), absolute growth rate of weight (AGRw), specific growth rate of length(SGRL) and specific growth rate of weight (SGRW) had a significant difference among MT treated groups and control group respectively. In 50 mg?kg-1 and 60 mg?kg-1 MT-treated groups, the male population was more than 99%, while in 5 mg?kg-1 and 10 mg?kg-1MT-treated groups, the male population was less than 80%. The residue of MT in muscle tissue at 30 days in each groupwas ranked as 0μg?kg-1 (control group) < 23.19μg?kg-1 (5mg?kg-1) < 30.82μg?kg-1 (10mg?kg-1) < 37.76μg?kg-1 (30mg?kg-1) < 44.75μg?kg-1 (50mg?kg-1)< 68.41μg?kg-1 (60mg?kg-1). However, the residue of MT was not detected in any treated group at 60 days. The results indicated that the MT acted as sex-inversion agents by functionally masculinizing Nile tilapia and improved the fish growth rate. After administering MT-free commercial feed for 60 days, the residue of MT could not be detected in muscle tissue.2.To study the expression levels of P450arom,11β-HSD2 and P450Scc, 10mg?kg-1 of MT and Letrozole were injected in the Nile Tilapia and the expression levels were investigated after injction-treated for 12h, 24h, 48h, 7d, 14d, 21d. The results showed that expressions of P450arom, 11β-HSD2 and P450Scc were inhibited firstly compared to the untreated group, then they recovered their initial levels. In the MT- treated group, P450Scc was firstly inhibited, P450arom,11β-HSD2 were later; While In the Letrozole-treated group, P450arom and P450Scc were firstly inhibited, 11β-HSD2 were later. The results indicated that MT may indirect inhibition of the expression of P450arom; while Letrozole direct inhibition of the expression of P450arom, then have a influence on the sex-inversion of fish.3.Ten microsatellite markers were selected from GenBank(UNH104,GM041,GM663,UNH168,UNH846,UNH931,UNH148,UNH985,UNH995,GM258). After PCR amplifying by 10 pairs of microsatellite DNA primers,electrophoresingin PAGE gel and Sliver Staining,the results showed that the polymorphism information contents ranged from 0.3416~0.6741 for the female O.niloticus population, the male O.niloticus population was 0.2641~0.6769. The ten locus were all the Moderate polymorphism loci or Highly polymorphism loci. The average observation heterozygosity and the average expectation heterozygosity of the female O.niloticus population were 0.4442 and 0.5913; the male O.niloticus population were 0.5500 and 0.5660. All of these indices indicated that genetic diversity of two populations was a high level. Otherwise, the ten microsatellite markers were detected in the Nile Tilipia, the results showed that it was no difference between the female O.niloticus population and the male O.niloticus population. |
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