| Yellow drum(Nibea albiflora)is an important commercial marine fish species in China.The fish exhibits significant sexual size dimorphism,with the female growing faster than the males.Therefore,the efficiency and profitability of its aquaculture could be maximized by all-female production.The present study consists of two experiments.1.Sexually undifferentiated gynogenetic yellow drum(Nibea albiflora)juveniles at 30 days-post-hatching(dph)were treated by immersing method at different concentrations(0,0.2,2,10,50 ppm).The juveniles were immersed in 17?-methyltestosterone(17?-MT)for 2 hours per day for 60 days.The effects of 17?-MT on growth,sex ratios and gonadal differentiation were investigated at 60,70,90,120 dph by body size measurement and gonadal histology.The results showed that treatments with 17?-MT played negative roles on the growth of yellow drum.The growth of low concentration treatments(0.2ppm)did not significantly differ with that in the controls,but the growth was significantly lower in medium(2,10 ppm)and high concentration treatments(50 ppm)than that in control.Histological examinations showed that all the treatment groups yielded the 100% male.However,the treatments with 17?-MT are likely to exert a negative impact on the development of gonad,characterizing by small gonads,deformity gonad and facultative gonads.The ratios of abnormal gonads in different treatments increased with the concentration of 17?-MT.In contrast,abnormal gonads were not observed in the low concentration group(0.2 ppm),implying treatments with low concentration of 17-MT did not have significant influence to the normal gonadal development.Taken together,the optimal concentration for the induction of sex reversal yellow drum was 0.2 ppm.The male specific gene dmrt1 and female specific gene cyp19a1 a were used to detect the sex-specific gene expression.RT-PCR revealed that dmrt1 was expressed only in the fish from treatment groups induced by 17?-MT,while cyp19a1 a was only expressed in the gynogenetic control group.The results were consistent with the histological examinations,further confirmed that 17?-MT could successfully induce pseudo-male of yellow drum from molecule level.2.Effect of letrozole on the sex differentiation and ratios of Nibea mitsukurii were investigated in this experiment.The letrozole at different doses(0,1,10,100 mg/kg)mixed with commercial pellets were fed to sexually undifferentiated juvenile of Nibea mitsukurii at 20 days-post-hatching,and the growth,sex ration and gonadal differentiation were investigated.The result showed that no significant differences in growth among the groups,which revealed that the oral administration of letrozole did not affect the growth of Nibea mitsukurii.The sex ratios in control group was 1:1,and 100% males were observed in letrozole-treated groups.However,intersex gonad at high ratios were observed in these groups,and the ratio was approximately 50%.We speculated that genotype of these intersex fish were females because of 1;1 sex ratios in control groups.In addition,the development of testis in letrozole-treated groups was faster than that in control groups.However,gonadosomatic index(GSI)was significantly different among the control group and letrozole-treated groups.The present results showed that letrozole administration are likely to accelerate the testis development,but it inhibits the gonad development to some extent. |
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