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(PH161) Comparison of genetic, behavioural and developmental effects of acetochlor on Rana catesbeiana tadpoles.
J, L1, Ovaska, K2, Helbing, C1, 1 Department of Biochemistry & Microbiology, Victoria, BC, Canada2
ABSTRACT- The thyroid hormone, 3,5,3'-triiodothyronine (T3), is important in regulating growth, metabolism, and development of vertebrates and is essential for the metamorphosis of frog tadpoles. Endogenous T3 levels are extremely low in premetamorphic tadpoles and increase during prometamorphosis resulting in the overt remodeling of the tadpole into a frog. Exogenous administration of T3 to premetamorphic tadpoles induces a precocious metamorphosis. Previous studies have shown that acetochlor, a widely-used preemergent herbicide, accelerates T3-induced metamorphosis in premetamorphic tadpoles and alters gene expression prior to overt morphological change. However, previous work was not able to detect any alterations in natural metamorphosis when premetamorphic Rana pipiens tadpoles were exposed to acetochlor likely due to lack of endpoint sensitivity (Cheek et al, 1999 Arch Env Contam Toxicol 37: 70). Here we show that prometamorphic tadpoles exposed to 1 and 10 nM acetochlor show an elevation in the expression of thyroid hormone receptors, TR and , in the tail fin biopsies. Although comparison of acetochlor treated and control animals did not reveal a significant difference in developmental parameters such as timing to metamorphosis, TR expression levels, emergence of first leg and end stage attained were significantly correlated within individuals. The brain is particularly sensitive to T3 and we detected a significant increase for TR expression in animals exposed to acetochlor. A more marked increase is observed in the brains of premetamorphic animals exposed to 10 nM acetochlor ± 100 nM T3 for 4 days. These animals showed an alteration in escape behaviour which could lead to decreased survival in the wild. Therefore, TR gene expression levels may serve as a useful marker for exposure for disruption of TH action.
Key words: Rana pipiens
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