MP13 Aquatic Ecotoxicology
Monday, 14 November 2005: 8:00 AM - 6:30 PM in Exhibit Hall
(HIR-1116-322145) 'Point of no return' on formation of testis-ova in Japanese medaka exposed to 17
-Estradiol.
Hirai, N1, Nanba, A1, Abe, R1, Koshio, M1, Kondo, T1, Tatarazako, N1, 1 National Institute for Environmental Studies, Tsukuba, Ibaraki, Japan
ABSTRACT- Several fish assays have being conducted to evaluate the dose-dependent influence of estrogen activity, but there are few study related to evaluate the period-dependent effects of estrogens. Point of no return (PNR), which is an indicator to evaluate the lethal starvation period (Blaxter and Hempel 1961) in study on early life mortality of fishes, is expected as a suitable endpoint to evaluate the time influence when fish are exposed to estrogens. We examined that PNR on formation of testis-ova was examined in Japanese medaka exposed to 17-estradiol (E2: 171 ng/L) during egg and larval period. The 10 series of eggs within six hours after fertilizations were transported to 2-liter glass tanks by 20 eggs, and each series was exposed to E2 for 3, 6, 9, 12, 15, 18, 21, 24, 27, 30 days after fertilization, respectively. Testis-ova in genetic-male gonad were observed in larvae exposed to E2 for over 18 days after fertilization, which is 6 days post-hatch. Males only exposed during egg period do not have testis-ova in their testis. Our previous study indicates testis-ova are first found at 12 days post hatch larvae when medaka is transferred to E2 during egg and larval period. Taken together with this study, the value PNR for 171 ng/L E2 was assessed as 18 days after fertilization, and suggesting that exposure in larval period may be related to the formation of testis-ova in genetic-males.
Key words: Point of no return, Medaka, Testis-ova, Estrogen