Estrogen responses in killifish (Fundulus heteroclitus) from polluted and unpolluted environments are site- and gene-specific

Independently evolved pollution resistance in four killifish populations is largely explained by few variants of large effect

The genetic architecture of phenotypic traits can affect the mode and tempo of trait evolution. Human-altered environments can impose strong natural selection, where successful evolutionary adaptation requires swift and large phenotypic shifts. In these scenarios, theory predicts that adaptation is due to a few adaptive variants of large effect, but empirical studies that have revealed the genetic architecture of rapidly evolved phenotypes are rare, especially for populations inhabiting polluted environments. Fundulus killifish have repeatedly evolved adaptive resistance to extreme pollution in urban estuaries. Prior studies, including genome scans for signatures of natural selection, have revealed some of the genes and pathways important for evolved pollution resistance, and provide context for the genotype-phenotype association studies reported here. We created multiple quantitative trait locus (QTL) mapping families using progenitors from four different resistant populations, and using RAD-seq genetically mapped variation in sensitivity (developmental perturbations) following embryonic exposure to a model toxicant PCB-126. We found that one to two large-effect QTL loci accounted for resistance to PCB-mediated developmental toxicity. QTLs harbored candidate genes that govern the regulation of aryl hydrocarbon receptor (AHR) signaling. One QTL locus was shared across all populations and another was shared across three populations. One QTL locus showed strong signatures of recent natural selection in the corresponding wild population but another QTL locus did not. Some candidate genes for PCB resistance inferred from genome scans in wild populations were identified as QTL, but some key candidate genes were not. We conclude that rapidly evolved resistance to the developmental defects normally caused by PCB-126 is governed by few genes of large effect. However, other aspects of resistance beyond developmental phenotypes may be governed by additional loci, such that comprehensive resistance to PCB-126, and to the mixtures of chemicals that distinguish urban estuaries more broadly, may be more genetically complex.

Independently evolved pollution resistance in four killifish populations is largely explained by few variants of large effect

The genetic architecture of phenotypic traits can affect the mode and tempo of trait evolution. Human-altered environments can impose strong natural selection, where successful evolutionary adaptation requires swift and large phenotypic shifts. In these scenarios, theory predicts the influence of few adaptive variants of large effect, but empirical studies that have revealed the genetic architecture of rapidly evolved phenotypes are rare, especially for populations inhabiting polluted environments. Fundulus killifish have repeatedly evolved adaptive resistance to extreme pollution in urban estuaries. Prior studies, including genome scans for signatures of natural selection, have revealed some of the genes and pathways important for evolved pollution resistance, and provide context for the genotype-phenotype association studies reported here. We created multiple quantitative trait locus (QTL) mapping families using progenitors from four different resistant populations, and genetically mapped variation in sensitivity (developmental perturbations) following embryonic exposure to a model toxicant PCB-126. We found that a few large-effect QTL loci accounted for resistance to PCB-mediated developmental toxicity. QTLs harbored candidate genes that govern the regulation of aryl hydrocarbon receptor (AHR) signaling, where some (but not all) of these QTL loci were shared across all populations, and some (but not all) of these loci showed signatures of recent natural selection in the corresponding wild population. Some strong candidate genes for PCB resistance inferred from genome scans in wild populations were identified as QTL, but some key candidate genes were not. We conclude that rapidly evolved resistance to the developmental defects normally caused by PCB-126 is governed by few genes of large effect. However, other aspects of resistance beyond developmental phenotypes may be governed by additional loci, such that comprehensive resistance to PCB-126, and to the mixtures of chemicals that distinguish urban estuaries more broadly, may be more genetically complex.

Assessing integrated biomarkers of triplefin fish Forsterygion capito inhabiting contaminated marine water - A multivariate approach

The presence of multiple chemicals in aquatic ecosystems makes evaluation of their real impact on the biota difficult. Integrated biomarkers are therefore needed to evaluate how these chemicals contribute to environmental degradation. The aims of the present study were to evaluate responses to and effects of marine pollution using a series of biomarkers through multivariate analyses. Transcriptional responses of cyp1a (cytochrome P450), mt (metallothionein), vtg (vitellogenin) and cyp19b (cytochrome P450 aromatase); branchial and hepatic histological alterations; and Fulton condition factors (CF) were evaluated, as well as the metals and polycyclic aromatic hydrocarbons present in Forsterygion capito in Auckland, New Zealand. Sites were selected along a contamination gradient: four highly contaminated sites and four less contaminated. Molecular responses with a higher relative expression of the mt and cyp1a genes were detected at a highly contaminated site (Panmure). Several histological lesion types were found in the livers of fish inhabiting both types of sites, but gill lesions were present primarily at highly contaminated sites. In terms of general health status, the lowest CF values were overwhelmingly found in fish from the same site (Panmure). The multivariate approach revealed that telangiectasia and hyperplasia were associated with the presence of chemicals, and these showed negative associations with the CF values, with fish from three highly contaminated sites being most affected. In conclusion, the multivariate approach helped to integrate these biological markers in this blennioid fish, thus providing a more holistic view of the complex chemical mixtures involved. Future studies should implement these analyses.

Medicines as an emergent contaminant: the review of microbial biodegration potential

Emerging environmental contaminants, such as medicine waste, are of great concern to the scientific community and to the local environmental and health departments because of their potential long-term effects and ecotoxicological risk. Besides the prolonged use of medicines for the development of modern society, the elucidation of their effect on the ecosystem is relatively recent. Medicine waste and its metabolites can, for instance, cause alterations in microbial dynamics and disturb fish behavior. Bioremediation is an efficient and eco-friendly technology that appears as a suitable alternative to conventional methods of water waste and sludge treatment and has the capacity to remove or reduce the presence of emerging contaminants. Thus, this review has the objective of compiling information on environmental contamination by common medicines and their microbial biodegradation, focusing on five therapeutic classes: analgesics, antibiotics, antidepressants, non-steroidal anti-inflammatory drugs (NSAIDs), and contraceptives. Their effects in the environment will also be analyzed, as well as the possible routes of degradation by microorganisms.

Sex-Specific Differences of Steroid Receptors Following Exposure to Environmentally Relevant Concentrations of Phenothiazine in Fundulus heteroclitus

Phenothiazine (PTZ) is a heterocyclic thiazine compound used for industrial and medical purposes. Through environmental surveillance studies, PTZ was found being discharged into a local river in Connecticut. Phenothiazine has been shown to act similarly to endocrine disrupting chemicals. This study sought to identify sex specific hormone receptor changes in Fundulus heteroclitus in response to PTZ exposure. Fundulus heteroclitus, also known as mummichog, are small fish native to the Atlantic coast of the United States and Canada. They reside in brackish waters and can survive harsh toxic environments. This model organism is native to the polluted waters found in Connecticut. In this study, fish were exposed to PTZ concentrations of 0.5 ppm, 1.0 ppm, and 2.0 ppm for 1 week. Following exposure, brain, liver, and gonad tissues were harvested; cDNA was synthesized; and mRNA expression was assessed for 6 different hormone receptors. Compared with vehicle control (ethanol) differences in mRNA expression, levels of hormone receptors were observed in various tissues from male and female fish. Many of the tissues assessed showed changes in expression level, while only female liver and testis showed no change. These results implicate PTZ as a potential endocrine disrupting compound to mummichog at environmentally relevant concentrations.

Evolutionary Toxicogenomics of the Striped Killifish (Fundulus majalis) in the New Bedford Harbor (Massachusetts, USA)

In this paper, we used a Genotyping-by-Sequencing (GBS) approach to find and genotype more than 4000 genome-wide SNPs (Single Nucleotide Polymorphisms) from striped killifish exposed to a variety of polychlorinated biphenyls (PCBs) and other aromatic pollutants in New Bedford Harbor (NBH, Massachusetts, USA). The aims of this study were to identify the genetic consequences of exposure to aquatic pollutants and detect genes that may be under selection. Low genetic diversity (H_E and π) was found in the site exposed to the highest pollution level, but the pattern of genetic diversity did not match the pollution levels. Extensive connectivity was detected among sampling sites, which suggests that balanced gene flow may explain the lack of genetic variation in response to pollution levels. Tests for selection identified 539 candidate outliers, but many of the candidate outliers were not shared among tests. Differences among test results likely reflect different test assumptions and the complex pollutant mixture. Potentially, selectively important loci are associated with 151 SNPs, and enrichment analysis suggests a likely involvement of these genes with pollutants that occur in NBH. This result suggests that selective processes at genes targeted by pollutants may be occurring, even at a small geographical scale, and may allow the local striped killifish to resist the high pollution levels.

When evolution is the solution to pollution: Key principles, and lessons from rapid repeated adaptation of killifish (Fundulus heteroclitus) populations

For most species, evolutionary adaptation is not expected to be sufficiently rapid to buffer the effects of human‐mediated environmental changes, including environmental pollution. Here we review how key features of populations, the characteristics of environmental pollution, and the genetic architecture underlying adaptive traits, may interact to shape the likelihood of evolutionary rescue from pollution. Large populations of Atlantic killifish (Fundulus heteroclitus) persist in some of the most contaminated estuaries of the United States, and killifish studies have provided some of the first insights into the types of genomic changes that enable rapid evolutionary rescue from complexly degraded environments. We describe how selection by industrial pollutants and other stressors has acted on multiple populations of killifish and posit that extreme nucleotide diversity uniquely positions this species for successful evolutionary adaptation. Mechanistic studies have identified some of the genetic underpinnings of adaptation to a well‐studied class of toxic pollutants; however, multiple genetic regions under selection in wild populations seem to reflect more complex responses to diverse native stressors and/or compensatory responses to primary adaptation. The discovery of these pollution‐adapted killifish populations suggests that the evolutionary influence of anthropogenic stressors as selective agents occurs widely. Yet adaptation to chemical pollution in terrestrial and aquatic vertebrate wildlife may rarely be a successful “solution to pollution” because potentially adaptive phenotypes may be complex and incur fitness costs, and therefore be unlikely to evolve quickly enough, especially in species with small population sizes.

Hepatic transcriptional responses to copper in the three-spined stickleback are affected by their pollution exposure history

Some fish populations inhabiting contaminated environments show evidence of increased chemical tolerance, however the mechanisms contributing to this tolerance, and whether this is heritable, are poorly understood. We investigated the responses of two populations of wild three-spined stickleback (Gasterosteus aculeatus) with different histories of contaminant exposure to an oestrogen and copper, two widespread aquatic pollutants. Male stickleback originating from two sites, the River Aire, with a history of complex pollution discharges, and Siblyback Lake, with a history of metal contamination, were depurated and then exposed to copper (46μg/L) and the synthetic oestrogen ethinyloestradiol (22ng/L). The hepatic transcriptomic response was compared between the two populations and to a reference population with no known history of exposure (Houghton Springs, Dorset). Gene responses included those typical for both copper and oestrogen, with no discernable difference in response to oestrogen between populations. There was, however, some difference in the magnitude of response to copper between populations. Siblyback fish showed an elevated baseline transcription of genes encoding metallothioneins and a lower level of metallothionein induction following copper exposure, compared to those from the River Aire. Similarly, a further experiment with an F1 generation of Siblyback fish bred in the laboratory found evidence for elevated transcription of genes encoding metallothioneins in unexposed fish, together with an altered transcriptional response to 125μg/L copper, compared with F1 fish originating from the clean reference population exposed to the same copper concentration. These data suggest that the stickleback from Siblyback Lake have a differential response to copper, which is inherited by the F1 generation in laboratory conditions, and for which the underlying mechanism may include an elevation of baseline transcription of genes encoding metallothioneins. The genetic and/or epigenetic mechanisms contributing to this inherited alteration of metallothionein transcription have yet to be established.

The genomic landscape of rapid repeated evolutionary adaptation to toxic pollution in wild fish

Atlantic killifish populations have rapidly adapted to normally lethal levels of pollution in four urban estuaries. Through analysis of 384 whole killifish genome sequences and comparative transcriptomics in four pairs of sensitive and tolerant populations, we identify the aryl hydrocarbon receptor-based signaling pathway as a shared target of selection. This suggests evolutionary constraint on adaptive solutions to complex toxicant mixtures at each site. However, distinct molecular variants apparently contribute to adaptive pathway modification among tolerant populations. Selection also targets other toxicity-mediating genes and genes of connected signaling pathways; this indicates complex tolerance phenotypes and potentially compensatory adaptations. Molecular changes are consistent with selection on standing genetic variation. In killifish, high nucleotide diversity has likely been a crucial substrate for selective sweeps to propel rapid adaptation.

Evolutionary toxicology: Meta-analysis of evolutionary events in response to chemical stressors

The regulatory decision-making process regarding chemical safety is most often informed by evidence based on ecotoxicity tests that consider growth, reproduction and survival as end-points, which can be quantitatively linked to short-term population outcomes. Changes in these end-points resulting from chemical exposure can cause alterations in micro-evolutionary forces (mutation, drift, selection and gene flow) that control the genetic composition of populations. With multi-generation exposures, anthropogenic contamination can lead to a population with an altered genetic composition, which may respond differently to future stressors. These evolutionary changes are rarely discussed in regulatory or risk assessment frameworks, but the growing body of literature that documents their existence suggests that these important population-level impacts should be considered. In this meta-analysis we have compared existing contamination levels of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) that have been documented to be associated with evolutionary changes in resident aquatic organisms to regulatory benchmarks for these contaminants. The original intent of this project was to perform a meta-analysis on evolutionary events associated with PCB and PAH contamination. However, this effort was hindered by a lack of consistency in congener selection for "total" PCB or PAH measurements. We expanded this manuscript to include a discussion of methods used to determine PCB and PAH total contamination in addition to comparing regulatory guidelines and contamination that has caused evolutionary effects. Micro-evolutionary responses often lead populations onto unique and unpredictable trajectories. Therefore, to better understand the risk of population-wide alterations occurring, we need to improve comparisons of chemical contamination between affected locations. In this manuscript we offer several possibilities to unify chemical comparisons for PCBs and PAHs that would improve comparability among evolutionary toxicology investigations, and with regulatory guidelines. In addition, we identify studies documenting evolutionary change in the presence of PCB and PAH contamination levels below applicable regulatory benchmarks.

Cross-resistance in Gulf killifish (Fundulus grandis) populations resistant to dioxin-like compounds

The Houston Ship Channel (HSC) in Houston, Texas is an aquatic environment with a long history of contamination, including polychlorinated dibenzodioxins (PCDD), polychlorinated dibenzofurans (PCDF), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and heavy metals. Populations of Gulf killifish (Fundulus grandis) from the HSC have adapted to resist developmental cardiac deformities caused by dioxin-like compounds (DLCs). Contaminants in the HSC have acted as a strong selective pressure on resident Gulf killifish populations. Rapid adaptation can lead to fitness costs, some as a direct result of the mechanisms involved in the adaptive process, whereas other adaptations may be more general. To explore potential fitness costs, we evaluated two Gulf killifish populations with documented resistance to DLC-induced cardiac teratogenesis (Patrick Bayou and Vince Bayou), and one previously characterized reference population (Gangs Bayou). We also characterized a previously unstudied population from Galveston Bay as an additional reference population (Smith Point). We tested the sensitivity of F1 larvae from these four populations to two classes of pesticides (pyrethroid (permethrin) and carbamate (carbaryl)) and two model pro-oxidants (tert-butyl hydroquinone (tBHQ) and tert-butyl hydroperoxide (tBOOH)). In addition, we explored their responses to hypoxia and measured resting metabolic rates (M.O2). Both adapted populations were cross-resistant to the toxicity of carbaryl and both pro-oxidants tested. There were no population differences in sensitivity to permethrin. On the other hand, one reference population (Gangs Bayou) was less sensitive to hypoxia, and maintained a lower M.O2 . However, there were no differences in hypoxia tolerance or resting metabolic rate between the second reference and the two adapted populations. This investigation emphasizes the importance of including multiple reference populations to clearly link fitness costs or cross-resistance to pollution adaptation, rather than to unrelated environmental or ecological differences. When compared to previous literature on adapted populations of Fundulus heteroclitus, we see a mixture of similarities and differences, suggesting that F. grandis adapted phenotypes likely involve multiple mechanisms, which may not be completely consistent among adapted populations.

Adaptive Significance of ERα Splice Variants in Killifish (Fundulus heteroclitus) Resident in an Estrogenic Environment

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, MA [NBH]) compared with those from a reference site overexpress estrogen receptor alpha (ERα) mRNA but are hyporesponsive to estradiol. Analysis of ERα mRNAs in the two populations revealed differences in splicing of the gene encoding ERα (esr1). Here we tested the transactivation functions of four differentially expressed ERα mRNAs and tracked their association with the hyporesponsive phenotype for three generations after transfer of NBH parents to a clean environment. Deletion variants ERαΔ6 and ERαΔ6-8 were specific to NBH killifish, had dominant negative functions in an in vitro reporter assay, and were heritable. Morpholino-mediated induction of ERαΔ6 mRNA in zebrafish embryos verified its role as a dominant negative ER on natural estrogen-responsive promoters. Alternate long (ERαL) and short (ERαS) 5'-variants were similar transcriptionally but differed in estrogen responsiveness (ERαS ≫ ERαL). ERαS accounted for high total ERα expression in first generation (F1) NBH embryos/larvae but this trait was abolished by transfer to clean water. By contrast, the hyporesponsive phenotype of F1 NBH embryos/larvae persisted after long-term laboratory 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 ERαΔ6 and ERαΔ6-8 isoforms. We conclude that a heritable change in the pattern of alternative splicing of ERα pre-mRNA is part of a genetic adaptive response to estrogens in a polluted environment.

Environmental fate and ecological risks of nonylphenols and bisphenol A in the Cape D'Aguilar Marine Reserve, Hong Kong

Nonylphenols (NPs) and bisphenol A (BPA) are the most common endocrine disruptors detected in the coastal waters of Hong Kong. The Cape D'Aguilar Marine Reserve (CAMR), the only marine reserve in Hong Kong is close to urbanized areas, thus the resident marine organisms are inevitably influenced by partially treated wastewater from adjacent sewage treatment plants (STPs). Elevated levels of NPs and BPA were detected in all seawater, sediment and biota samples collected from the CAMR. Estrogenic activities of seawater from the CAMR, and sludge and sewage from a nearby STP were assessed using yeast estrogen screen assay. We found aromatase, estrogen receptor and vitellogenin genes in the marine medaka fish Oryzias melastigma were significantly up-regulated after exposure to the reserve's seawater. According to a tissue-residue-based probabilistic risk assessment, the marine species living in the CAMR are having 35% and 21% of chance to be at risk due to exposure to NPs and BPA, respectively.

Cloning of multiple ERα mRNA variants in killifish (Fundulus heteroclitus), and differential expression by tissue type, stage of reproduction, and estrogen exposure in fish from polluted and unpolluted environments

To test the hypothesis that alternative splicing could be an adaptive mechanism for populations subject to multi-generational estrogenic exposures, we compared estrogen receptor alpha (ERα) splicing variants in two populations of killifish (Fundulus heteroclitus): one resident in an estrogenic polluted environment (New Bedford Harbor, NBH, MA, USA) and one from a relatively uncontaminated reference site (Scorton Creek, SC, MA, USA). In total we identified 19 ERα variants, each with deletions of one or more coding exons. Four of the variants with potential functional relevance were analyzed by qPCR to test for population differences in expression by tissue type, site, sex, seasonal reproductive status and estrogen treatment. Significantly, a 5'-truncated short form variant (ERαS) was highly expressed in liver and ovary, and was associated with seasonal reproductive activity in SC but not NBH fish. Both ERαS and the full-length long variant (ERαL) were estrogen-inducible (ERαS>ERαL) but the induction response was lower in NBH than in SC fish. In contrast, NBH killifish were hyper-responsive to estrogen as measured by expression of two other estrogen responsive genes: vitellogenin (Vtg) and aromatase B (AroB). Most strikingly, two ERα deletion variants (Δ6 and Δ6-8), lacking ligand binding and activation function domains, were identified in a subset of NBH fish, where they were associated with reduced responsiveness to estrogen treatment. Together, these results support the hypothesis that alternative splicing of the esr1 gene of killifish could be an autoregulatory mechanism by which estrogen modulates the differential expression of ERα, and suggests a novel and adaptive mechanistic response to xenoestrogenic exposure.

Effects of chronic, parental pharmaceutical exposure on zebrafish (Danio rerio) offspring

In this study we explored how parental exposure to pharmaceuticals influences reproduction in offspring. Adult zebrafish (Danio rerio) were exposed for 6 weeks to 10 μgL(-1) of carbamazepine (CBZ) and gemfibrozil (GEM), two commonly prescribed drugs. Embryos were collected, reared in clean water until sexual maturity and then assessed for reproductive output, courtship, sperm function and organ histology. While 34% of the control pairs produced clutches, only 11% of the fish with CBZ exposed parents or 17% of the fish with GEM exposed parents produced clutches. Reciprocal crosses indicated that exposure in males had more profound reproductive effects. When a control F1 male was crossed with either a F1 female whose parents were CBZ or GEM exposed; no differences were observed in embryo production compared to controls. However, when a control F1 female was crossed with either a CBZ or GEM F1 male, 50% less embryos were produced. Male courtship was reduced in both CBZ and GEM F1 fish but the deficits in courtship displays were drug specific. Compared to control males, the sperm from GEM F1 males had shorter head lengths and midpieces whereas sperm from CBZ F1 males had longer midpieces. Although it remains unclear how specifically these morphological differences influenced sperm velocity, the sperm from GEM F1 males and from CBZ F1 males swam faster than the sperm of control F1 at 20s post activation. No significant differences were observed in the histology of the liver, kidney and gonads across treatment groups. These data are important as they show that chronic, low dose pharmaceutical exposure of parental fish is sufficient to cause significant reproductive effects in offspring.

Genetic variation at aryl hydrocarbon receptor (AHR) loci in populations of Atlantic killifish (Fundulus heteroclitus) inhabiting polluted and reference habitats

Background

The non-migratory killifish Fundulus heteroclitus inhabits clean and polluted environments interspersed throughout its range along the Atlantic coast of North America. Several populations of this species have successfully adapted to environments contaminated with toxic aromatic hydrocarbon pollutants such as polychlorinated biphenyls (PCBs). Previous studies suggest that the mechanism of resistance to these and other “dioxin-like compounds” (DLCs) may involve reduced signaling through the aryl hydrocarbon receptor (AHR) pathway. Here we investigated gene diversity and evidence for positive selection at three AHR-related loci (AHR1, AHR2, AHRR) in F. heteroclitus by comparing alleles from seven locations ranging over 600 km along the northeastern US, including extremely polluted and reference estuaries, with a focus on New Bedford Harbor (MA, USA), a PCB Superfund site, and nearby reference sites.

Results

We identified 98 single nucleotide polymorphisms within three AHR-related loci among all populations, including synonymous and nonsynonymous substitutions. Haplotype distributions were spatially segregated and F-statistics suggested strong population genetic structure at these loci, consistent with previous studies showing strong population genetic structure at other F. heteroclitus loci. Genetic diversity at these three loci was not significantly different in contaminated sites as compared to reference sites. However, for AHR2 the New Bedford Harbor population had significant F_ST values in comparison to the nearest reference populations. Tests for positive selection revealed ten nonsynonymous polymorphisms in AHR1 and four in AHR2. Four nonsynonymous SNPs in AHR1 and three in AHR2 showed large differences in base frequency between New Bedford Harbor and its reference site. Tests for isolation-by-distance revealed evidence for non-neutral change at the AHR2 locus.

Conclusion

Together, these data suggest that F. heteroclitus populations in reference and polluted sites have similar genetic diversity, providing no evidence for strong genetic bottlenecks for populations in polluted locations. However, the data provide evidence for genetic differentiation among sites, selection at specific nucleotides in AHR1 and AHR2, and specific AHR2 SNPs and haplotypes that are associated with the PCB-resistant phenotype in the New Bedford Harbor population. The results suggest that AHRs, and especially AHR2, may be important, recurring targets for selection in local adaptation to dioxin-like aromatic hydrocarbon contaminants.

Multiple structurally distinct ERα mRNA variants in zebrafish are differentially expressed by tissue type, stage of development and estrogen exposure

It is well established that estrogen-like environmental chemicals interact with the ligand-binding site of estrogen receptors (ERs) to disrupt transcriptional control of estrogen responsive targets. Here we investigate the possibility that estrogens also impact splicing decisions on estrogen responsive genes, such as that encoding ERα itself. Targeted PCR cloning was applied to identify six ERα mRNA variants in zebrafish. Sequencing revealed alternate use of transcription and translation start sites, multiple exon deletions, intron retention and alternate polyadenylation. As determined by quantitative (q)PCR, N-terminal mRNA variants predicting long (ERαA(L)) and short (ERα(S)) isoforms were differentially expressed by tissue-type, sex, stage of development and estrogen exposure. Whereas ERα(L) mRNA was diffusely distributed in liver, brain, heart, eye, and gonads, ERα(S) mRNA was preferentially expressed in liver (female>male) and ovary. Neither ERα(L) nor ERα(S) transcripts varied significantly during development, but 17β-estradiol selectively increased accumulation of ERα(S) mRNA (∼170-fold by 120 hpf), an effect mimicked by bisphenol-A and diethylstilbestrol. Significantly, a C-truncated variant (ERα(S)-Cx) lacking most of the ligand binding and AF-2 domains was transcribed exclusively from the short isoform promoter and was similar to ERα(S) in its tissue-, stage- and estrogen inducible expression. These results support the idea that promoter choice and alternative splicing of the esr1 gene of zebrafish are part of the autoregulatory mechanism by which estrogen modulates subsequent ERα expression, and further suggest that environmental estrogens could exert some of their toxic effects by altering the relative abundance of structurally and functionally distinct ERα isoforms.

Determination of estrogenic steroids and microbial and photochemical degradation of 17α-ethinylestradiol (EE2) in lake surface water, a case study

In this study, a GC-MS technique was applied to determine 17α-ethinylestradiol (EE2), an active ingredient of oral contraceptives, and its fate in Lake Quinsigamond, Massachusetts, USA. To the knowledge of the authors, this is the first study of EE2 and its microbial and photochemical degradation in a lake ecosystem. EE2 was detected at a concentration up to 11.1 ng L(-1). At this concentration EE2 may affect the reproduction of fish and other aquatic organisms in the lake due to its high estrogenic activity. EE2 was persistent to the biodegradation by the microorganisms in the lake. Under aerobic conditions a long lag phase (42 days) was observed before the biodegradation of EE2 and a half-life of 108 days was estimated. Under anaerobic conditions, EE2 experienced even a longer acclimation stage (63 days) and a slower microbial degradation in the lake water. The photodegradation of EE2 was rapid in the lake surface water under natural sunlight, with a half-life of less than 2 days in summer sunny days. Compared to biodegradation, photodegradation may represent a predominant removal mechanism for EE2 in natural surface waters.

Acute exposure to 17α-ethinylestradiol disrupts audience effects on male-male interactions in Siamese fighting fish, Betta splendens

Endocrine disrupting chemicals can have profound effects on the behavior of aquatic organisms residing in polluted waters. Males are especially sensitive to the effects of estrogen mimics and both courtship and aggression may be dramatically reduced by chemical exposure. Population-level impacts may occur if these chemicals decrease the ability of males to obtain mates or defend territories. Exposure might also have far-reaching impacts by interfering with information transfer within a network of individuals. For example, males exposed to an endocrine disruptor may be less sensitive to the presence of an audience. Male Siamese fighting fish were used to examine how short-term exposure to 17α-ethinylestradiol (EE2) alters audience effects on male-male interactions. Males either received a nominal dose of EE2 or remained unexposed and then interacted with an opponent in one of three treatments (female, male, or no audience). EE2 altered audience effects in this study. Opponent-directed gill flaring was lower when a female audience was present compared to when there was a male or no audience in both EE2 and control males. The number of opponent-directed tail beats did not differ as a function of audience type in EE2 males. In contrast, unexposed males increased opponent-directed tail beats when a female audience is present. Therefore, EE2 reduces the ability of males to communicate with multiple individuals simultaneously. If this is the case, endocrine disruptor exposure may alter population structure as selection should favor individuals that are able to readily adjust their signaling behavior as a function of social context.

The effects of short-term exposure to an endocrine disrupter on behavioral consistency in male juvenile and adult Siamese fighting fish

17α-Ethinyloestradiol (EE2) is known to impact courtship and aggression but how exposure affects the consistency with which individuals express these behaviors over time is not commonly addressed. In addition, how juvenile and adult male fish differ in levels of behavioral consistency, both before and after EE2 exposure, is unknown. To examine these questions, juvenile and adult male Siamese fighting fish were presented with a dummy male and dummy female simultaneously both before and after acute exposure to a nominal (15 ng/L) dose of EE2. Multiple trials were conducted to measure how consistent individual differences are affected by age and EE2 exposure. Both female- and male-directed behaviors decreased after short-term exposure to EE2. Juvenile and adult fish differed in the number of female-directed tail beats and bites they performed, with adult fish performing more tail beats and juvenile fish performing more bites. EE2 exposure had a greater effect on consistent individual differences in female-directed than in male-directed behaviors, which may be a byproduct of intense artificial selection for aggression in this species. Repeatability values were lower both before and after exposure in juvenile than in adult fish, suggesting that individuals become more consistent with age. This study suggests that male Siamese fighting fish vary in their sensitivity to short-term EE2 exposure and stresses the importance of examining behavior over multiple time points both within and across age classes to gain a more thorough understanding of the effects of endocrine disruptors on behavior.

Inhibition of vitellogenin gene induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin is mediated by aryl hydrocarbon receptor 2 (AHR2) in zebrafish (Danio rerio)

Vitellogenins are hepatically derived yolk-protein precursors required for oogenesis in all oviparous teleosts. Altered gene-regulation of vitellogenesis by environmental contaminants can have profound effects on reproductive success, and ultimately population sustainability. To better understand chemical effects on vitellogenin gene regulation, we tested the hypothesis that activation of the aryl hydrocarbon receptor 2 (AHR2) by dioxin inhibits the estrogen receptor pathway regulation of 3 vitellogenin genes (vtg1-3) in vivo, using zebrafish (Danio rerio) as a model teleost. Using an embryo-larval bioassay, embryos were either treated with 1000 pptr (parts-per-trillion, pg/mL) 17α-ethynylestradiol (EE2) alone from 6h post fertilization (hpf) to 4 days post fertilization (dpf), or pre-treated with dioxin (4-5 hpf) prior to EE2. Pre-treatment with 400 pptr 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) or 1,2,3,7,8-pentachlorodibenzo-p-dioxin inhibited the EE2 induction of vtg1, vtg2 and vtg3 by >95% (p≤0.05). In comparison, a splice-blocking AHR2 morpholino used to down-regulate ahr2 expression significantly reduced the inhibition of vtg1, vtg2 and vtg3 by 400 pptr 2,3,7,8-TCDD (20.7-27.4% rescue). These studies demonstrate that 2,3,7,8-TCDD directly inhibits the vitellogenin pathway in vivo through activation of the AHR2. This work provides evidence for AHR2 dependent cross-talk inhibition of vitellogenin genes and offers insight into anti-estrogenic reproductive effects observed in oviparous species exposed to AHR agonist contaminants.

Atorvastatin up-regulate toxicologically relevant genes in rainbow trout gills

There are large and increasing discharges of statins into the aquatic environment. Statins are cholesterol-lowering pharmaceuticals, inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A reductase, an enzyme in the cholesterol synthesis pathway. Earlier studies have shown that statins will affect the expression of a range of genes in mammalian tissues and this group of pharmaceuticals has also been shown to affect membrane transporters. Changes in gene expression and ion transport in aquatic organisms may have dramatic consequences for the individual. The aim of the present study was to clarify whether waterborne exposure to a selected statin, atorvastatin, would affect gene expression in rainbow trout (Oncorhynchus mykiss) gill or liver or ion regulation in gills. Juvenile rainbow trout were exposed to two atorvastatin acid and atorvastatin lactone concentrations for 7 days (nominal concentrations 200 ng L(-1) and 10 μg L(-1)). The exposures caused up-regulated gene expression in gill, not liver, and only at the lowest concentration. Genes involved in membrane transport (pgp, mrp1), oxidative stress response (sod, mt), apoptosis (bax) and biotransformation (sult2b) were differentially expressed whereas the expression of genes involved in cholesterol biosynthesis (hmgr, fdps) or peroxisomal proliferation (ppar) were not affected. There were no significant changes in gill Na(+)/K(+) ATPase activity following exposure to atorvastatin. The pattern of differentially expressed genes in rainbow trout gills differ from responses previously observed in mammalian tissues following statin exposure.

Sustained high temperature increases the vitellogenin response to 17α-ethynylestradiol in mummichog (Fundulus heteroclitus)

Mummichog (Fundulus heteroclitus), an estuarine fish of the western Atlantic, were acclimated to three salinities (0, 16 or 32 ppt) or three temperatures (10, 20 or 26 °C) and exposed to nominal 50 or 250 ng/L 17α-ethynylestradiol (EE2) for 14 days. In a separate experiment, fish were exposed to the same levels of EE2 and were subjected to a 1h heat shock (20-30 °C) on the 14th day and allowed to recover for 20 h. We were interested in whether or not susceptibility to EE2 exposure, as indicated by increases in vitellogenin (vtg) gene expression would change with high and low salinity, warm or cold temperature acclimation or acute heat shock. We also investigated the potential role of heat shock proteins (HSPs) under these conditions. Liver vtg1 mRNA was significantly induced in male mummichog exposed to 50 and 250 ng/L EE2, but salinity acclimation or acute heat shock did not further affect this induction. Males acclimated to 26 °C and exposed to 250 ng/L EE2 induced 3.5-fold more vtg1 mRNA than EE2 exposed males acclimated to 10 °C. HSP90 and HSP70 protein were largely unaffected by EE2 exposure. Our findings suggest that mummichog are more susceptible to EE2 under sustained temperature increases that may occur seasonally or with warming of coastal waters.

Toxicity assessment and vitellogenin expression in zebrafish (Danio rerio) embryos and larvae acutely exposed to bisphenol A, endosulfan, heptachlor, methoxychlor and tetrabromobisphenol A

Organochlorine pesticides and brominated flame retardants, such as tetrabromobisphenol A and polybrominated diphenyl ethers, pose an environmental hazard owing to their persistence, low solubility and estrogenic effects, and concerns have been raised regarding their effects on aquatic biota. In the present study, zebrafish embryos and larvae were used as a model to investigate the sublethal and lethal effects of three different organochlorine pesticides, namely methoxychlor, endosulfan and heptachlor, as well as the flame retardant tetrabromobisphenol A, and its precursor compound bisphenol A. Preliminary data for chemical exposure tests were obtained by determining the 96 h median effective concentration EC50 (hatching rate) and 96 h median lethal concentration LC50 . Quantitative polymerase chain reaction was used to investigate the gene expression levels of the biomarker vitellogenin (vtg1) after 96 h exposures to 10, 25, 50 and 75% of the 96 h EC50 value for embryos and 96 h LC50 value for larvae. The use of vtg1 mRNA induction in zebrafish embryos and larvae was found to be a sensitive biomarker of exposure to these organic compounds, and was helpful in elucidating their adverse effects and setting water quality guidelines.

Evolutionary origins of the estrogen signaling system: insights from amphioxus

Classically, the estrogen signaling system has two core components: cytochrome P450 aromatase (CYP19), the enzyme complex that catalyzes the rate limiting step in estrogen biosynthesis; and estrogen receptors (ERs), ligand activated transcription factors that interact with the regulatory region of target genes to mediate the biological effects of estrogen. While the importance of estrogens for regulation of reproduction, development and physiology has been well-documented in gnathostome vertebrates, the evolutionary origins of estrogen as a hormone are still unclear. As invertebrates within the phylum Chordata, cephalochordates (e.g., the amphioxus of the genus Branchiostoma) are among the closest invertebrate relatives of the vertebrates and can provide critical insight into the evolution of vertebrate-specific molecules and pathways. To address this question, this paper briefly reviews relevant earlier studies that help to illuminate the history of the aromatase and ER genes, with a particular emphasis on insights from amphioxus and other invertebrates. We then present new analyses of amphioxus aromatase and ER sequence and function, including an in silico model of the amphioxus aromatase protein, and CYP19 gene analysis. CYP19 shares a conserved gene structure with vertebrates (9 coding exons) and moderate sequence conservation (40% amino acid identity with human CYP19). Modeling of the amphioxus aromatase substrate binding site and simulated docking of androstenedione in comparison to the human aromatase shows that the substrate binding site is conserved and predicts that androstenedione could be a substrate for amphioxus CYP19. The amphioxus ER is structurally similar to vertebrate ERs, but differs in sequence and key residues of the ligand binding domain. Consistent with results from other laboratories, amphioxus ER did not bind radiolabeled estradiol, nor did it modulate gene expression on an estrogen-responsive element (ERE) in the presence of estradiol, 4-hydroxytamoxifen, diethylstilbestrol, bisphenol A or genistein. Interestingly, it has been shown that a related gene, the amphioxus "steroid receptor" (SR), can be activated by estrogens and that amphioxus ER can repress this activation. CYP19, ER and SR are all primarily expressed in gonadal tissue, suggesting an ancient paracrine/autocrine signaling role, but it is not yet known how their expression is regulated and, if estrogen is actually synthesized in amphioxus, whether it has a role in mediating any biological effects. Functional studies are clearly needed to link emerging bioinformatics and in vitro molecular biology results with organismal physiology to develop an understanding of the evolution of estrogen signaling. This article is part of a Special Issue entitled 'Marine organisms'.

Fundulus heteroclitus: ovarian reproductive physiology and the impact of environmental contaminants

Fundulus heteroclitus, the mummichog or Atlantic killifish, is the dominant small-bodied fish species of the east coast estuaries and salt marshes of Canada and the USA, where it is present as two subspecies, the northern F. h. macrolepidotus and the southern F. h. heteroclitus. Recently identified as the premier teleost model in environmental biology, the species has long been of value in understanding evolved tolerance to toxicants and more lately in adding to our knowledge about reproductive effects of environmental endocrine disruptors. The body of literature on F. heteroclitus ovarian physiology and reproduction, from both field and laboratory studies, provides the foundation for present work focused on understanding the reproductive effects and modes of action of environmental toxicants. In this paper, we review the environmental and endocrine factors controlling ovarian and reproductive cycling in F. heteroclitus, noting specifics related to field and laboratory studies on the two subspecies as well as key research gaps compared to other fish species. We also summarize recent development of methodologies to study the effects of environmental contaminants on endocrine signalling and egg production in F. heteroclitus. Continued efforts to progress both our fundamental understanding of reproductive physiology in mummichog, coupled with studies focused on the modes of action of environmental contaminants, have high potential to further develop this teleost model. While the model may presently lag behind those based on other species of fish, the unique biochemical and physiological adaptations which allow F. heteroclitus to adapt to changing environmental and toxic conditions provide a valuable experimental system for comparative physiologists, ecotoxicologists and evolutionary biologists.

Short-term exposure to an endocrine disruptor affects behavioural consistency in male threespine stickleback

It has been established that exposure to xenooestrogens can have profound effects on reproductive and aggressive behaviours in aquatic organisms. Studies on the effects of 17α-ethinyloestradiol (EE2) on behaviour, however, rarely examine these effects over multiple trials, across time, or across contexts. In addition, it is possible that individuals vary in their sensitivity to EE2 and individuals may differ in the degree to which their behaviour is affected. The study of consistent individual differences in behaviour has grown in popularity in recent years perhaps because of the evolutionary questions this behavioural variation raises. Adult male threespine stickleback (Gasterosteus aculeatus) from an anadromous population repeatedly received the simultaneous presentation of a dummy male and dummy female conspecific both before and after exposure to a short-term, nominal (15 ng L(-1)) concentration of 17α-ethinyloestradiol (EE2). These before and after measures were then compared against one another as well as against responses of males in a control group that was not exposed to EE2. Courtship, aggression, and nesting behaviours were all affected by EE2 exposure. In addition, behavioural consistency over trials was markedly reduced after EE2 exposure, as shown by lower repeatability values in the after exposure measures and in weaker correlations between the before and after exposure measures in the EE2 group compared to the control group. This suggests that male threespine stickleback vary in their sensitivity to EE2. This study is one of the first to examine the effects of short-term EE2 exposure on behaviour across trials and has important implications for population-level fitness consequences of xenooestrogens.

Decreased vitellogenin inducibility and 17β-estradiol levels correlated with reduced egg production in killifish (Fundulus heteroclitus) from Newark Bay, NJ

Aquatic species inhabiting polluted estuaries are exposed to complex mixtures of xenobiotics which can alter normal reproduction. We previously reported that female Atlantic killifish (Fundulus heteroclitus) from the highly contaminated Newark Bay, NJ (USA) exhibited an inhibition of oocyte development due to reduced vitellogenin (egg-yolk precursor) levels. Our hypothesis was that the inhibition of oocyte development in Newark Bay killifish is due to (1) deficient levels of circulating 17β-estradiol, and (2) a decreased sensitivity of the vitellogenin pathway to physiological doses of 17β-estradiol. In the first study, adult naïve killifish from Tuckerton, NJ (reference) were caged at Tuckerton and Newark Bay. After 1 month, males caged at Newark Bay exhibited inductions of hepatic vitellogenin and estrogen receptor α, which were transient and returned to basal levels after 2 months (p≤0.05). In the second study, fecundity and 17β-estradiol levels were measured in reproductively active adult females from Tuckerton and Newark Bay. Tuckerton females produced 140 eggs per female and Newark Bay females produced 11 eggs per female. Embryos from Newark Bay had 34% greater mortality and 28% less hatch, relative to Tuckerton. In addition, embryo mass and yolk-volume of Newark Bay embryos compared to Tuckerton embryos was 16% and 25% lower, respectively. Circulating 17β-estradiol levels in Newark Bay females (0.26 ng/mL) were measured to be 8-fold lower than Tuckerton females (2.25 ng/mL). In the third study, adult killifish from both sites were dosed with 17β-estradiol to assess the sensitivity of the vitellogenin pathway. At doses of 0.01, 0.1, 1 and 10 ng/g body weight, induction levels of circulating vitellogenin in Newark Bay males were significantly inhibited by 97, 99, 98 and 44%, respectively, compared to Tuckerton males. At doses of 0.01, 0.1, 1, 10 and 100 ng/g body weight, induction levels of circulating vitellogenin in Newark Bay females were inhibited by 89, 79, 61, 40 and 30%, respectively, compared to Tuckerton females. These differences in inducibility could not be explained by altered hepatic expression of estrogen receptors α, βa or βb. Based on the caged and dose-response studies, contaminants that down-regulate vitellogenin would interfere with its ability to be used as a biomarker for xeno-estrogen exposures. These studies demonstrate that contaminants within Newark Bay exert both estrogenic and anti-estrogenic responses which results in an overtly anti-estrogenic phenotype (reduced egg production due to inhibition of vitellogenesis).