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Synopsis Epigenetic mechanisms are increasingly understood to have major impacts across ecology. However, one molecular epigenetic mechanism, DNA methylation, currently dominates the literature. A second mechanism, histone modification, is likely important to ecologically relevant phenotypes and thus warrants investigation, especially because molecular interplay between methylation and histone acetylation can strongly affect gene expression. There are a limited number of histone acetylation studies on non-model organisms, yet those that exist show that it can impact gene expression and phenotypic plasticity. Wild birds provide an excellent system to investigate histone acetylation, as free-living individuals must rapidly adjust to environmental change. Here, we screen histone acetylation in the house sparrow (Passer domesticus); we studied this species because DNA methylation was important in the spread of this bird globally. This species has one of the broadest geographic distributions in the world, and part of this success is related to the way that it uses methylation to regulate its gene expression. Here, we verify that a commercially available assay that was developed for mammals can be used in house sparrows. We detected high variance in histone acetylation among individuals in both liver and spleen tissue. Further, house sparrows with higher epigenetic potential in the Toll Like Receptor-4 (TLR-4) promoter (i.e., CpG content) had higher histone acetylation in liver. Also, there was a negative correlation between histone acetylation in spleen and TLR-4 expression. In addition to validating a method for measuring histone acetylation in wild songbirds, this study also shows that histone acetylation is related to epigenetic potential and gene expression, adding a new study option for ecological epigenetics. 

Abstract As a highly successful introduced species, house sparrows (Passer domesticus) respond rapidly to their new habitats, generating phenotypic patterns across their introduced range that resemble variation in native regions. Epigenetic mechanisms likely facilitate the success of introduced house sparrows by aiding particular individuals to adjust their phenotypes plastically to novel conditions. Our objective here was to investigate patterns of DNA methylation among populations of house sparrows at a broad geographic scale that included different introduction histories: invading, established, and native. We defined the invading category as the locations with introductions less than 70 years ago and the established category as the locations with greater than 70 years since introduction. We screened DNA methylation among individuals (n = 45) by epiRADseq, expecting that variation in DNA methylation among individuals from invading populations would be higher when compared with individuals from established and native populations. Invading house sparrows had the highest variance in DNA methylation of all three groups, but established house sparrows also had higher variance than native ones. The highest number of differently methylated regions were detected between invading and native populations of house sparrow. Additionally, DNA methylation was negatively correlated to time-since introduction, which further suggests that DNA methylation had a role in the successful colonization’s of house sparrows. 

Abstract Organisms are experiencing higher average temperatures and greater temperature variability because of anthropogenic climate change. Some populations respond to changes in temperature by shifting their ranges or adjusting their phenotypes via plasticity and/or evolution, while others go extinct. Predicting how populations will respond to temperature changes is challenging because extreme and unpredictable climate changes will exert novel selective pressures. For this reason, there is a need to understand the physiological mechanisms that regulate organismal responses to temperature changes. In vertebrates, glucocorticoid hormones mediate physiological and behavioral responses to environmental stressors and thus are likely to play an important role in how vertebrates respond to global temperature changes. Glucocorticoids have cascading effects that influence the phenotype and fitness of individuals, and some of these effects can be transmitted to offspring via trans- or intergenerational effects. Consequently, glucocorticoid-mediated responses could affect populations and could even be a powerful driver of rapid evolutionary change. Here, we present a conceptual framework that outlines how temperature changes due to global climate change could affect population persistence via glucocorticoid responses within and across generations (via epigenetic modifications). We briefly review glucocorticoid physiology, the interactions between environmental temperatures and glucocorticoid responses, and the phenotypic consequences of glucocorticoid responses within and across generations. We then discuss possible hypotheses for how glucocorticoid-mediated phenotypic effects might impact fitness and population persistence via evolutionary change. Finally, we pose pressing questions to guide future research. Understanding the physiological mechanisms that underpin the responses of vertebrates to elevated temperatures will help predict population-level responses to the changing climates we are experiencing. 

Abstract Variation in DNA methylation is associated with many ecological and life history traits, including niche breadth and lifespan. In vertebrates, DNA methylation occurs almost exclusively at “CpG” dinucleotides. Yet, how variation in the CpG content of the genome impacts organismal ecology has been largely overlooked. Here, we explore associations between promoter CpG content, lifespan and niche breadth among 60, amniote vertebrate species. The CpG content of 16 functionally relevant gene promoters was strongly, positively associated with lifespan in mammals and reptiles, but was not related to niche breadth. Possibly, by providing more substrate for CpG methylation to occur, high promoter CpG content extends the time taken for deleterious, age-related errors in CpG methylation patterns to accumulate, thereby extending lifespan. The association between CpG content and lifespan was driven by gene promoters with intermediate CpG enrichment—those known to be predisposed to regulation by methylation. Our findings provide novel support for the idea that high CpG content has been selected for in long-lived species to preserve the capacity for gene expression regulation by CpG methylation. Intriguingly, promoter CpG content was also dependent on gene function in our study; immune genes had on average 20% less CpG sites than metabolic- and stress-related genes. 

Hypothalamic–pituitary–adrenal axis (HPA) flexibility is an emerging concept recognizing that individuals that will cope best with stressors will probably be those using their hormones in the most adaptive way. The HPA flexibility concept considers glucocorticoids as molecules that convey information about the environment from the brain to the body so that the organismal phenotype comes to complement prevailing conditions. In this context, FKBP5 protein appears to set the extent to which circulating glucocorticoid concentrations can vary within and across stressors. Thus,FKBP5expression, and the HPA flexibility it causes, seem to represent an individual's ability to regulate its hormones to orchestrate organismal responses to stressors. AsFKBP5expression can also be easily measured in blood, it could be a worthy target of conservation-oriented research attention. We first review the known and likely roles of HPA flexibility and FKBP5 in wildlife. We then describe putative genetic, environmental and epigenetic causes of variation in HPA flexibility andFKBP5expression among and within individuals. Finally, we hypothesize how HPA flexibility andFKBP5expression should affect organismal fitness and hence population viability in response to human-induced rapid environmental changes, particularly urbanization. This article is part of the theme issue ‘Endocrine responses to environmental variation: conceptual approaches and recent developments’. 

Animals encounter many novel and unpredictable challenges when moving into new areas including pathogen exposure. Because effective immune defenses against such threats can be costly, plastic immune responses could be particularly advantageous, as such defenses can be engaged only when context warrants activation. DNA methylation is a key regulator of plasticity via its effects on gene expression. In vertebrates, DNA methylation occurs exclusively at CpG dinucleotides, and typically, high DNA methylation decreases gene expression, particularly when it occurs in promoters. The CpG content of gene regulatory regions may therefore represent one form of epigenetic potential (EP), a genomic means to capacitate gene expression and hence adaptive phenotypic plasticity. Non-native populations of house sparrows (Passer domesticus) - one of the world's most cosmopolitan species – have high EP in the promoter of a key microbial surveillance gene, Toll-like receptor 4 (TLR4), compared to native populations. We previously hypothesized that high EP may enable sparrows to balance the costs and benefits of inflammatory immune responses well, a trait critical to success in novel environments. In the present study, we found support for this hypothesis: house sparrows with high EP in TLR4 promoter were better able to resist a pathogenic Salmonella enterica infection than sparrows with low EP. These results support the idea that high EP contributes to invasion and perhaps adaptation in novel environments, but the mechanistic details whereby these organismal effects arise remain obscure.