Investigating estrogenic activity of the dispersant Corexit 9500 in the American alligator and diamondback terrapin
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McNabb, Nicole Alexandra
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During the Deepwater Horizon oil spill, millions of gallons of Corexit 9500 were applied to the surface water and at the wellhead to disperse the oil. This massive release mandates evaluation of the long-term effects of Corexit. Estrogen signals play critical roles in temperature-dependent sex determination (TSD) and gonadal differentiation in some turtles and all crocodilians, in which egg incubation temperatures during a thermosensitive period (TSP) determine sex of the embryos. A single exposure to exogenous estrogen during TSP leads to skewed sex ratios by inducing ovarian development, even at a male-producing temperature. Due to this sensitivity to estrogen, reptiles that exhibit TSD, including the American alligator (<i>Alligator mississippiensis</i>) and diamondback terrapin (<i>Malaclemys terrapin</i>), can be considered ideal sentinel species to investigate chronic exposure to estrogenic environmental contaminants. Both estrogen receptor isoforms, ESR1 and ESR2, were cloned from the diamondback terrapin and found to be highly homologous to painted turtle, alligator, and chicken ESRs. The cloned ESRs were characterized using nuclear hormone receptor transactivation assays <i>in vitro</i> with 17β-estradiol, Corexit, and dispersed oil. Additionally, alligator eggs were exposed to Corexit at 0.25, 2.5, and 25 μg/g egg weight prior to TSP to investigate the potential endocrine disruption and estrogenic effects on gonadal development <i>in ovo</i>. Gonadal tissues dissected from alligators at one week of age were analyzed using histological methods and quantitative PCR to determine resulting sex ratios and understand the outcome of developmental exposure. Corexit exposure <i>in ovo</i> did not alter sex ratios or testicular mRNA abundance at one week of age, suggesting Corexit alone does not have strong estrogenic activity <i>in ovo</i>, whereas Corexit and dispersed oil showed estrogenic activities <i>in vitro</i> using transactivation assays. Further investigations are required to assist in understanding the effects of Corexit exposure on the reproductive health of coastal aquatic reptiles.