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Adaptive Significance of ERα Splice Variants in Killifish (Fundulus heteroclitus) Resident in an Estrogenic Environment
Citation:
Cotter, K., D. Nacci, D. Champlin, A. Yeo, T. Gilmore, AND G. Callard. Adaptive Significance of ERα Splice Variants in Killifish (Fundulus heteroclitus) Resident in an Estrogenic Environment. ENDOCRINOLOGY. Endocrine Society, 157(6):2294-2308, (2016).
Impact/Purpose:
This manuscript describes experimental studies that contribute to our understanding of the ecological risks associated with chronic contaminant exposures to wildlife populations. Here, we assessed reproductive patterns associated with long-term responses from exposures to an important class of highly toxic environmental pollutants. Specifically, chemical-specific tolerance has rapidly and repeatedly evolved in an estuarine fish species resident to estuaries of the Atlantic U.S. coast. We used laboratory studies to characterize variation in genes related to reproduction among laboratory-reared progeny of fish from populations known to vary in their sensitivity to these pollutants to infer mechanisms of toxicity and tolerance. Results of these studies demonstrate the value of molecular tools to diagnose and predict effects of chemical stressors and characterize the mechanisms and costs of toxic and compensatory responses to chemical stressors by wild populations. General impacts from this contribution include improved understanding by managers and scientists of links between human activities, natural dynamics, ecological stressors and ecosystem condition.
Description:
The possibility that chronic, multigenerational exposure to environmental estrogens selects for adaptive hormone response phenotypes is a critical unanswered question. Embryos/larvae of killifish from an estrogenic polluted environment (New Bedford Harbor, NBH), as compared to those from a reference site, overexpress estrogen receptor a (ERa) mRNA but are hypo-responsive to estradiol (E2). Analysis of ERa mRNAs in the two populations revealed differences in splicing of the gene encoding ERa (esr1). Here we tested the transactivation functions of four differentially expressed ERa mRNAs and tracked their association with the hypo-responsive phenotype for three generations after transfer of NBH parents to a clean environment. Deletion variants ERaΔ6 and ERaΔ6 – 8 were specific to NBH killifish; had dominant negative functions in an in vitro reporter assay; and were heritable. Morpholino-mediated induction of ERaΔ6 mRNA in zebrafish embryos verified its role as a dominant negative ER on natural estrogen-responsive promoters. Alternate long (ERaL) and short (ERaS) 5'-variants were similar transcriptionally but differed in estrogen responsiveness (ERaS >> ERaL). ERaS accounted for high total ERa expression in F1 NBH embryos/ larvae but this trait was abolished by transfer to clean water. By contrast, the hypo-responsive phenotype of F1 NBH embryos/larvae persisted after long term lab holding but reverted to a normal or hyper-responsive phenotype after two or three generations, suggesting the acquisition of physiological or biochemical traits that compensate for ongoing expression of negative-acting ERaΔ6 and ERaΔ6 – 8 isoforms. We conclude that a heritable change in the pattern of alternative splicing of ERa pre-mRNA is part of a genetic adaptive response to estrogens in a polluted environment.
