Fish full life-cycle testing for androgen methyltestosterone on medaka (Oryzias latipes)

Abstract-We studied the chronic effects of methyltestosterone (MT) on reproductive status of medaka (Oryzias latipes) over two generations under continuous exposure to verify the applicability of the fish full life-cycle test (FFLC) for this androgen with this species. The exposure of parental (F0) medaka to MT was begun on embryos within 12 h postfertilization and continued for up to 101 d; assessment endpoints included embryological development, hatching, posthatch survival, growth, sexual differentiation, reproduction, and hepatic vitellogenin (VTG) levels under flow-through exposure to MT at each mean measured concentration of 0.35, 1.09, 3.29, 9.98, and 27.75 ng/L. Eggs (F1) spawned from the F0 fish at 98, 99, and 100 d posthatch were examined for hatchability, survival after hatching, growth, sexual differentiation, and hepatic VTG level until 60 d posthatch. In the FFLC with medaka, MT induced masculinization of both secondary sex characteristics and gonads. We observed that all F0 fish in the 27.75-ng/L treatment group showed male secondary sex characteristics in which no fish with ovary could be discerned. Several fish with ovaries in F0 and F1 generations treated with 9.98 ng/L showed male secondary sex characteristics. We also observed swollen abdomens in the F0 and F1 female fish in the 9.98-ng/L treatment group. These swollen abdomens were induced by enlarged ovaries and were accompanied with declined fecundity and fertility in the F0 generation. These results indicate that MT reduces the reproductive potential of medaka and that the FFLC with this species is applicable to the evaluation of androgens.