Dioxin-like compounds are toxic to most vertebrates, but significant differences in sensitivity exist among species. A recent study suggests that the amino acid residues corresponding to Ile324 and Ser380 in the chicken aryl hydrocarbon receptor 1 (AHR1) are important determinants of differential biochemical responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in chickens and common terns. Here, we investigate whether the identity of these amino acid residues can predict embryonic sensitivity to dioxin-like compounds in a wide range of birds. AHR1 sequences were determined in species for which sensitivity data were available. Of all the species surveyed, chickens were unique in having the Ile/Ser genotype and were also the most sensitive to dioxin-like compounds. Turkeys, ring-necked pheasants, and Eastern bluebirds (intermediate Ile/Ala genotype) were less sensitive than chickens but more sensitive than American kestrels, common terns, double-crested cormorants, Japanese quail, herring gulls, or ducks (Val/ Ala genotype). Our work suggests that key amino acids in the AHR1 ligand binding domain are predictive of broad categories of dioxin sensitivity in avian species. Given the large degree of variation in species sensitivity and the paucity of species-specific toxicity data, a genetic screen based on these findings could substantially improve risk assessment for dioxin-like compounds in wild birds.

We performed two controlled experiments to determine the amount of mass-dependent and mass-independent fractionation (MDF and MIF) of methylmercury (MeHg) during trophic transfer into fish. In Experiment 1, juvenile yellow perch (Perca flavescens) were raised in captivity on commercial food pellets and then their diet was either maintained on un-amended food pellets (0.1 µg/g MeHg), or was switched to food pellets with 1.0 µg/g or 4.0 µg/g of added MeHg, for a period of 2 months. The difference in δ202Hg (MDF) and Δ199Hg (MIF) between fish tissues and food pellets with added MeHg were within the analytical uncertainty (δ202Hg; 0.07 ‰, Δ199Hg; 0.06 ‰) indicating no isotope fractionation. In Experiment 2, lake trout (Salvelinus namaycush) were raised in captivity on food pellets, and then shifted to a diet of bloater (Coregonus hoyi) for 6 months. The δ202Hg and Δ199Hg of the lake trout equaled the isotopic composition of the bloater after 6 months, reflecting re-equilibration of the Hg isotopic composition of the fish to new food sources and a lack of isotope fractionation during trophic transfer. We suggest that the stable Hg isotope ratios in fish can be used to trace environmental sources of Hg in aquatic ecosystems.

DNA methylation refers to the addition of a methyl group to nucleotides within DNA. As with other epigenetic endpoints, patterns of DNA methylation are susceptible to alterations due to exposure to environmental stressors, including contaminants. These alterations can persist in the absence of the initial stressor as cells divide, and can even be inherited between generations if they occur in the germ line. Although our knowledge concerning patterns of DNA methylation in animals is increasing, there remains a gap in the literature when it comes to species outside of those typically used for biomedical research. Here, I review the literature relating to DNA methylation in an array of taxa (mammals, fish, birds, amphibians, reptiles, and invertebrates) and discuss these data from an ecotoxicological perspective. The pattern and extent of DNA methylation is well conserved across species of vertebrates; methylation appears mainly on cytosine residues within a CpG context, and much of the genome is methylated, with the notable exception of cytosines within CpG islands in the promoters of genes. Highly methylated genes in vertebrates tend to be transcriptionally repressed. However, large differences occur between classes of vertebrates in terms of the timing and nature of reprogramming and genomic imprinting: epigenetic processes that establish patterns of DNA methylation in the early embryo and which are sensitive to environmental stress. In invertebrates, patterns of DNA methylation are extremely variable and differ significantly from the condition observed in vertebrates. Some invertebrate genomes exhibit no DNA methylation while others are methylated to a level that is comparable to vertebrates. Additionally, DNA methylation may have different functions in invertebrates, e.g., alternative splicing. This variability in basic patterns of DNA methylation among species during sensitive periods of development suggests that responses to epigenetically active environmental contaminants may be similarly variable. For example, the timing of exposure to a contaminant may be a critical factor when considered in the light of variable reprogramming schedules among species. With this in mind, I review data relating to the effects of contaminants on DNA methylation in animals, focusing on non-model organisms and on exposures in natural environments, when possible. An ecotoxicological perspective on patterns of DNA methylation in animals may improve our understanding of the range and diversity of epigenetic phenomena in the natural world.

Early-life stage (ELS) toxicity tests are recognized as an advancement over current testing methodologies in terms of cost, animal use, and biological relevance. However, standardized ELS tests are not presently available for some vertebrate taxa, including birds. The present study describes a Japanese quail (Coturnix japonica) ELS test that is a promising candidate for standardization and applies it to test 8 environmental chemicals (ethinylestradiol, benzo[a]pyrene, chlorpyrifos, fluoxetine, lead(II)nitrate, trenbolone, seleno-L-methionine, hexabromocyclododecane). Individual chemicals were injected into the air cell of unincubated Japanese quail eggs at 3 concentrations, all predicted to cause ≤20% mortality. Survival to embryonic day 16 was consistently high (>90%) among the vehicle-injected controls. All chemicals, except ethinylestradiol, were detected in liver tissue, most at concentrations suggestive of embryonic clearance. Adverse effects were observed for 5 of the 8 chemicals; chlorpyrifos (41.1 µg/g) significantly increased developmental abnormalities and decreased embryo and gallbladder mass. Ethinylestradiol (54.2 µg/g) and hexabromocyclododecane (0.02 µg/g) decreased embryo mass and tarsus length, respectively. Benzo[a]pyrene (0.83 µg/g) and fluoxetine hydrochloride (32.7 µg/g) exceeded the 20% mortality cutoff. No effects were observed following lead(II)nitrate, seleno-L-methionine, or trenbolone exposure up to 10.7, 0.07, and 4.4 µg/g, respectively. Overall, our ELS approach was time-and cost-effective, caused minimal mortality in controls, effectively delivered diverse chemicals to the embryo, and permitted identification of apical outcomes, all of which provide support toward standardization.