Reproductive effects of life-cycle exposure to difenoconazole on female marine medaka (Oryzias melastigma)

Difenoconazole (DFZ) is a widely used triazole fungicide which has been detected in some estuaries and embayments. This study was conducted to investigate the effects of DFZ on ovarian development in female marine medaka (Oryzias melastigma). After 180 days exposure of the embryo to DFZ (0, 1, 10, 100 and 1000 ng/L), the gonadosomatic index and percentage of mature oocytes produced were significantly reduced in the 1, 10 and 100 ng/L treatments but not the 1000 ng/L treatment compared to the control, thus exhibiting a U-shaped dose response curve. The relative mRNA levels of brain follicle-stimulating hormone, ovarian cytochrome P450 aromatase (CYP19s), hepatic estrogen receptors and vitellogenin, and the ratio of 17 beta-estradiol to testosterone in the muscle, also showed a U-shaped dose response, which was consistent with the development of oocytes. In addition, glutathione S-transferase activity in the ovary showed a U-shaped dose-response. These results gave an explanation for this U-shaped dose-response. The egg number produced, the hatch ability and the swim-up success in the F1 generation all showed a U-shaped dose response, indicating that exposure to DFZ at low concentrations can cause a decrease of fecundity and viability of the next generation. Thus, a more extensive evaluation of the impact of DFZ on marine fish reproduction at realistic environmental concentrations is needed.