Epigenetics in ecology and evolution: what we know and what we need to know

Protein-based inheritance

Epigenetic mechanisms of inheritance have come to occupy a prominent place in our understanding of living systems, primarily eukaryotes. There has been considerable and lively discussion of the possible evolutionary significance of transgenerational epigenetic inheritance. One particular type of epigenetic inheritance that has not figured much in general discussions is that based on conformational changes in proteins, where proteins with altered conformations can act as templates to propagate their own structure. An increasing number of such proteins - prions and prion-like - are being discovered. Phenotypes due to the structurally altered proteins are transmitted along with their structures. This review discusses the properties and implications of "classical" amyloid-forming prions, as well as the broader class of proteins with intrinsically disordered domains, which are proving to have fascinating properties that appear to play important roles in cell organisation and function, especially during stress responses.

Local parasite pressures and host genotype modulate epigenetic diversity in a mixed-mating fish

Parasite-mediated selection is one of the main drivers of genetic variation in natural populations. The persistence of long-term self-fertilization, however, challenges the notion that low genetic variation and inbreeding compromise the host's ability to respond to pathogens. DNA methylation represents a potential mechanism for generating additional adaptive variation under low genetic diversity. We compared genetic diversity (microsatellites and AFLPs), variation in DNA methylation (MS-AFLPs), and parasite loads in three populations of Kryptolebias hermaphroditus, a predomintanly self-fertilizing fish, to analyze the potential adaptive value of DNA methylation in relation to genetic diversity and parasite loads. We found strong genetic population structuring, as well as differences in parasite loads and methylation levels among sampling sites and selfing lineages. Globally, the interaction between parasites and inbreeding with selfing lineages influenced DNA methylation, but parasites seemed more important in determining methylation levels at the local scale.

Current evidence for a role of epigenetic mechanisms in response to ionizing radiation in an ecotoxicological context

The issue of potential long-term or hereditary effects for both humans and wildlife exposed to low doses (or dose rates) of ionising radiation is a major concern. Chronic exposure to ionising radiation, defined as an exposure over a large fraction of the organism's lifespan or even over several generations, can possibly have consequences in the progeny. Recent work has begun to show that epigenetics plays an important role in adaptation of organisms challenged to environmental stimulae. Changes to so-called epigenetic marks such as histone modifications, DNA methylation and non-coding RNAs result in altered transcriptomes and proteomes, without directly changing the DNA sequence. Moreover, some of these environmentally-induced epigenetic changes tend to persist over generations, and thus, epigenetic modifications are regarded as the conduits for environmental influence on the genome. Here, we review the current knowledge of possible involvement of epigenetics in the cascade of responses resulting from environmental exposure to ionising radiation. In addition, from a comparison of lab and field obtained data, we investigate evidence on radiation-induced changes in the epigenome and in particular the total or locus specific levels of DNA methylation. The challenges for future research and possible use of changes as an early warning (biomarker) of radiosensitivity and individual exposure is discussed. Such a biomarker could be used to detect and better understand the mechanisms of toxic action and inter/intra-species susceptibility to radiation within an environmental risk assessment and management context.

The Importance of Validating the Demethylating Effect of 5-aza-2'-deoxycytidine in Model Species (A Comment on Cook et al., "DNA Methylation and Sex Allocation in the Parasitoid Wasp Nasonia vitripennis")

The use of DNA demethylating agents has been popular in epigenetic studies. Recently, Cook and colleagues, in a 2015 American Naturalist article, claimed an effect of 5-aza-2'-deoxycytidine (5-aza-dC) on the sex ratio of a parasitoid wasp without verifying its effect on DNA methylation. We repeated the 5-aza-dC feeding treatment to test its effectiveness. We used bisulfite amplicon sequencing of 10 genes that either were heavily methylated, previously showed a response to 5-aza-dC, or were suggested to regulate fatty acid synthesis epigenetically, and we demonstrate that wasps fed 5-aza-dC did not show reduced DNA methylation at these loci. Therefore, the conclusion that demethylation shifts sex ratios upward needs reconsideration.

Cloudy with a Chance of Insights: Context Dependent Gene Regulation and Implications for Evolutionary Studies

Research in various fields of evolutionary biology has shown that divergence in gene expression is a key driver for phenotypic evolution. An exceptional contribution of cis-regulatory divergence has been found to contribute to morphological diversification. In the light of these findings, the analysis of genome-wide expression data has become one of the central tools to link genotype and phenotype information on a more mechanistic level. However, in many studies, especially if general conclusions are drawn from such data, a key feature of gene regulation is often neglected. With our article, we want to raise awareness that gene regulation and thus gene expression is highly context dependent. Genes show tissue- and stage-specific expression. We argue that the regulatory context must be considered in comparative expression studies.

Contrasting DNA methylation responses of inbred fish lines to different rearing environments

Epigenetic mechanisms generate plastic phenotypes that can become locally adapted across environments. Disentangling genomic from epigenomic variation is challenging in sexual species due to genetic variation among individuals, but it is easier in self-fertilizing species. We analysed DNA methylation patterns of two highly inbred strains of a naturally self-fertilizing fish reared in two contrasting environments to investigate the obligatory (genotype-dependent), facilitated (partially depend on the genotype) or pure (genotype-independent) nature of the epigenetic variation. We found higher methylation differentiation between genotypes than between environments. Most methylation differences between environments common to both strains followed a pattern where the two genotypes (inbred lines) responded to the same environmental context with contrasting DNA methylation levels (facilitated epialleles). Our findings suggest that, at least in part, DNA methylation could depend on the dynamic interaction between the genotype and the environment, which could explain the plasticity of epigenetically mediated phenotypes.

Heat-stress induced flowering can be a potential adaptive response to ocean warming for the iconic seagrass Posidonia oceanica

The Mediterranean Sea is particularly vulnerable to warming and the abrupt declines experienced by the endemic Posidonia oceanica populations after recent heatwaves have forecasted severe consequences for the ecological functions and socio-economical services this habitat forming species provides. Nevertheless, this highly clonal and long-lived species could be more resilient to warming than commonly thought since heat-sensitive plants massively bloomed after a simulated heatwave, which provides the species with an opportunity to adapt to climate change. Taking advantage of this unexpected plant response, we investigated for the first time the molecular and physiological mechanisms involved in seagrass flowering through the transcriptomic analysis of bloomed plants. We also aimed to identify if flowering is a stress-induced response as suggested from the fact that heat-sensitive but not heat-tolerant plants flowered. The transcriptomic profiles of flowered plants showed a strong metabolic activation of sugars and hormones and indications of an active transport of these solutes within the plant, most likely to induce flower initiation in the apical meristem. Preflowered plants also activated numerous epigenetic-related genes commonly used by plants to regulate the expression of key floral genes and stress-tolerance genes, which could be interpreted as a mechanism to survive and optimize reproductive success under stress conditions. Furthermore, these plants provided numerous molecular clues suggesting that the factor responsible for the massive flowering of plants from cold environments (heat-sensitive) can be considered as a stress. Heat-stress induced flowering may thus be regarded as an ultimate response to survive extreme warming events with potential adaptive consequences for the species. Fitness implications of this unexpected stress-response and the potential consequences on the phenotypic plasticity (acclimation) and evolutionary (adaptation) opportunity of the species to ocean warming are finally discussed.

UNVEILing connections between genotype, phenotype, and fitness in natural populations

Understanding the links between genetic variation and fitness in natural populations is a central goal of evolutionary genetics. This monumental task spans the fields of classical and molecular genetics, population genetics, biochemistry, physiology, developmental biology, and ecology. Advances to our molecular and developmental toolkits are facilitating integrative approaches across these traditionally separate fields, providing a more complete picture of the genotype-phenotype map in natural and non-model systems. Here, we summarize research presented at the first annual symposium of the UNVEIL Network, an NSF-funded collaboration between the University of Montana and the University of Nebraska, Lincoln, which took place from the 1st to the 3rd of June, 2018. We discuss how this body of work advances basic evolutionary science, what it implies for our ability to predict evolutionary change, and how it might inform novel conservation strategies.

Evolution of DNA Methylation in Papio Baboons

Changes in gene regulation have long been thought to play an important role in primate evolution. However, although a number of studies have compared genome-wide gene expression patterns across primate species, fewer have investigated the gene regulatory mechanisms that underlie such patterns, or the relative contribution of drift versus selection. Here, we profiled genome-scale DNA methylation levels in blood samples from five of the six extant species of the baboon genus Papio (4-14 individuals per species). This radiation presents the opportunity to investigate DNA methylation divergence at both shallow and deeper timescales (0.380-1.4 My). In contrast to studies in human populations, but similar to studies in great apes, DNA methylation profiles clearly mirror genetic and geographic structure. Divergence in DNA methylation proceeds fastest in unannotated regions of the genome and slowest in regions of the genome that are likely more constrained at the sequence level (e.g., gene exons). Both heuristic approaches and Ornstein-Uhlenbeck models suggest that DNA methylation levels at a small set of sites have been affected by positive selection, and that this class is enriched in functionally relevant contexts, including promoters, enhancers, and CpG islands. Our results thus indicate that the rate and distribution of DNA methylation changes across the genome largely mirror genetic structure. However, at some CpG sites, DNA methylation levels themselves may have been a target of positive selection, pointing to loci that could be important in connecting sequence variation to fitness-related traits.

Common garden experiments disentangle plant genetic and environmental contributions to ectomycorrhizal fungal community structure

The interactions among climate change, plant genetic variation and fungal mutualists are poorly understood, but probably important to plant survival under drought. We examined these interactions by studying the ectomycorrhizal fungal (EMF) communities of pinyon pine seedlings (Pinus edulis) planted in a wildland ecosystem experiencing two decades of climate change-related drought. We established a common garden containing P. edulis seedlings of known maternal lineages (drought tolerant, DT; drought intolerant, DI), manipulated soil moisture and measured EMF community structure and seedling growth. Three findings emerged: EMF community composition differed at the phylum level between DT and DI seedlings, and diversity was two-fold greater in DT than in DI seedlings. EMF communities of DT seedlings did not shift with water treatment and were dominated by an ascomycete, Geopora sp. By contrast, DI seedlings shifted to basidiomycete dominance with increased moisture, demonstrating a lineage by environment interaction. DT seedlings grew larger than DI seedlings in high (28%) and low (50%) watering treatments. These results show that inherited plant traits strongly influence microbial communities, interacting with drought to affect seedling performance. These interactions and their potential feedback effects may influence the success of trees, such as P. edulis, in future climates.

Opportunities and limitations of reduced representation bisulfite sequencing in plant ecological epigenomics

Contents Summary 738 I. Introduction 738 II. RRBS loci as genome-wide epigenetic markers 739 III. Exploiting functional annotation of RRBS loci 739 IV. Limitations of RRBS methods for nonmodel species 740 V. Maximising the impact of RRBS in plants 741 VI. Conclusions 741 Acknowledgements 741 SUMMARY: Investigating the features and implications of epigenetic mechanisms across the breadth of organisms and ecosystems is important for understanding the ecological relevance of epigenetics. Several cost-effective reduced representation bisulfite sequencing approaches (RRBS) have been recently developed and applied to different organisms that lack a well annotated reference genome. These new approaches improve the assessment of epigenetic diversity in ecological settings and may provide functional insights. We assess here the opportunities and limitations of RRBS in nonmodel plant species. Well thought out experimental designs that include complementary gene expression studies, and the improvement of genomics resources for the target group, promise to maximize the effect of future RRBS studies.

Genome-wide DNA methylation changes in two Brassicaceae species sampled alongside a radiation gradient in Chernobyl and Fukushima

The long-term radiological impact to the environment of the nuclear accidents in Chernobyl and Fukushima is still under discussion. In the course of spring of 2016 we sampled two Brassicacea plants, Arabidopsis thaliana and Capsella bursa-pastoris native to Ukraine and Japan, respectively, alongside a gradient of radiation within the exclusion and difficult to return zones of Chernobyl (CEZ) and Fukushima (FEZ). Ambient dose rates were similar for both sampling gradients ranging from 0.5 to 80 μGy/h at plant height. The hypothesis was tested whether a history of several generations of plants growing in enhanced radiation exposure conditions would have led to changes in genome-wide DNA methylation. However, no differences were found in the global percentage of 5-methylated cytosines in Capsella bursa pastoris plants sampled in FEZ. On the other hand a significant decrease in whole genome methylation percentage in Arabidopsis thaliana plants was found in CEZ mainly governed by the highest exposed plants. These data support a link between exposure to changed environmental conditions and changes genome methylation. In addition to methylation the activity concentration of different radionuclides, ¹³⁷Cs, ⁹⁰Sr, ²⁴¹Am and Pu-238,239,240 for CEZ and ^{137, 134}Cs for FEZ, was analysed in both soil and plant samples. The ratio of 5.6 between ¹³⁷Cs compared to ¹³⁴Cs was as expected five years after the FEZ accident. For CEZ ¹³⁷Cs is the most abundant polluting radionuclide in soil followed by ⁹⁰Sr. Whereas ²⁴¹Am and Pu-isotopes are only marginally present. In the plant tissue, however, higher levels of Sr than Cs were retrieved due to a high uptake of ⁹⁰Sr in the plants. The ⁹⁰Sr transfer factors ranged in CEZ from 5 to 20 (kg/kg) depending on the locality. Based on the activity concentrations of the different radionuclides the ERICA tool was used to estimate the total dose rates to the plants. It was found that for FEZ the doses was mainly contributable to the external Cs-isotopes and as such estimated total dose rates (0.13-38 μGy/h) were in the same range as the ambient measured dose rates. In strong contrast this was not true for CEZ where the total dose rate was mainly due to high uptake of the ⁹⁰Sr leading to dose rates ranging from 1 to 370 μGy/h. Hence our data clearly indicate that not taking into account the internal contamination in CEZ will lead to considerable underestimation of the doses to the plants. Additionally they show that it is hard to compare the two nuclear accidental sites and one of the main reasons is the difference in contamination profile.

Egg incubation temperature affects the timing of the Atlantic salmon Salmo salar homing migration

Here, we show that adult Atlantic salmon Salmo salar returned about 2 weeks later from the feeding areas in the North Atlantic Ocean to the Norwegian coast, through a phenotypically plastic mechanism, when they developed as embryos in c. 3°C warmer water than the regular incubation temperature. This finding has relevance to changes in migration timing caused by climate change and for cultivation and release of S. salar.

Contribution of epigenetic variation to adaptation in Arabidopsis

In plants, transgenerational inheritance of some epialleles has been demonstrated but it remains controversial whether epigenetic variation is subject to selection and contributes to adaptation. Simulating selection in a rapidly changing environment, we compare phenotypic traits and epigenetic variation between Arabidopsis thaliana populations grown for five generations under selection and their genetically nearly identical ancestors. Selected populations of two distinct genotypes show significant differences in flowering time and plant architecture, which are maintained for at least 2–3 generations in the absence of selection. While we cannot detect consistent genetic changes, we observe a reduction of epigenetic diversity and changes in the methylation state of about 50,000 cytosines, some of which are associated with phenotypic changes. Thus, we propose that epigenetic variation is subject to selection and can contribute to rapid adaptive responses, although the extent to which epigenetics plays a role in adaptation is still unclear.

Whether plant epigenetic variation is subject to selection and contributes to adaptation is under debate. Here, the authors compare DNA methylation and phenotypes of Arabidopsis lines subject to simulated selection and their nearly isogenic ancestors and provide evidence that epigenetic variation contributes to adaptive responses.

Crop domestication: anthropogenic effects on insect-plant interactions in agroecosystems

Although crop domestication is considered a model system for understanding evolution, the eco-evolutionary effects of domesticated crops on higher trophic levels have rarely been discussed. Changes in size, shape, quality, or timing of plant traits during domestication can influence entire arthropod communities. The plant traits specific to crop plants can be rare in nature. In the face of such novelty, it is important to understand how species and trophic levels vary in their responses. Although the evidence is still limited, crop domestication can influence the ecology, genetics, and evolution of plants, insect herbivores, natural enemies, and pollinators. We call for more study on how eco-evolutionary processes operate under domestication to provide new insight on the sustainability of species interactions within agroecosystems.

Methylation divergence of invasive Ciona ascidians: Significant population structure and local environmental influence

The geographical expansion of invasive species usually leads to temporary and/or permanent changes at multiple levels (genetics, epigenetics, gene expression, etc.) to acclimatize to abiotic and/or biotic stresses in novel environments. Epigenetic variation such as DNA methylation is often involved in response to diverse local environments, thus representing one crucial mechanism to promote invasion success. However, evidence is scant on the potential role of DNA methylation variation in rapid environmental response and invasion success during biological invasions. In particular, DNA methylation patterns and possible contributions of varied environmental factors to methylation differentiation have been largely unknown in many invaders, especially for invasive species in marine systems where extremely complex interactions exist between species and surrounding environments. Using the methylation-sensitive amplification polymorphism (MSAP) technique, here we investigated population methylation structure at the genome level in two highly invasive model ascidians, Ciona robusta and C. intestinalis, collected from habitats with varied environmental factors such as temperature and salinity. We found high intrapopulation methylation diversity and significant population methylation differentiation in both species. Multiple analyses, such as variation partitioning analysis, showed that both genetic variation and environmental factors contributed to the observed DNA methylation variation. Further analyses found that 24 and 20 subepiloci were associated with temperature and/or salinity in C. robusta and C. intestinalis, respectively. All these results clearly showed significant methylation divergence among populations of both invasive ascidians, and varied local environmental factors, as well as genetic variation, were responsible for the observed DNA methylation patterns. The consistent findings in both species here suggest that DNA methylation, coupled with genetic variation, may facilitate local environmental adaptation during biological invasions, and DNA methylation variation molded by local environments may contribute to invasion success.

Genetic and epigenetic changes during the invasion of a cosmopolitan species (Phragmites australis)

While many introduced invasive species can increase genetic diversity through multiple introductions and/or hybridization to colonize successfully in new environments, others with low genetic diversity have to persist by alternative mechanisms such as epigenetic variation. Given that Phragmites australis is a cosmopolitan reed growing in a wide range of habitats and its invasion history, especially in North America, has been relatively well studied, it provides an ideal system for studying the role and relationship of genetic and epigenetic variation in biological invasions. We used amplified fragment length polymorphism (AFLP) and methylation-sensitive (MS) AFLP methods to evaluate genetic and epigenetic diversity and structure in groups of the common reed across its range in the world. Evidence from analysis of molecular variance (AMOVA) based on AFLP and MS-AFLP data supported the previous conclusion that the invasive introduced populations of P. australis in North America were from European and Mediterranean regions. In the Gulf Coast region, the introduced group harbored a high level of genetic variation relative to originating group from its native location, and it showed epigenetic diversity equal to that of the native group, if not higher, while the introduced group held lower genetic diversity than the native. In the Great Lakes region, the native group displayed very low genetic and epigenetic variation, and the introduced one showed slightly lower genetic and epigenetic diversity than the original one. Unexpectedly, AMOVA and principal component analysis did not demonstrate any epigenetic convergence between native and introduced groups before genetic convergence. Our results suggested that intertwined changes in genetic and epigenetic variation were involved in the invasion success in North America. Although our study did not provide strong evidence proving the importance of epigenetic variation prior to genetic, it implied the similar role of stable epigenetic diversity to genetic diversity in the adaptation of P. australis to local environment.

Local adaptation through genetic differentiation in highly fragmented Tilia cordata populations

We assessed the level of geographic differentiation of Tilia cordata in Denmark based on tests of 91 trees selected from 12 isolated populations. We used quantitative analysis of spring phenology and population genetic analysis based on SSR markers to infer the likely historical genetic processes within and among populations. High genetic variation within and among populations was observed in spring phenology, which correlated with spring temperatures at the origin of the tested T. cordata trees. The population genetic analysis revealed significant differentiation among the populations, but with no clear sign of isolation by distance. We infer the findings as indications of ongoing fine scale selection in favor of local growth conditions made possible by limited gene flow among the small and fragmented populations. This hypothesis fits well with reports of limited fruiting in the investigated Danish T. cordata populations, while the species is known for its ability to propagate vegetatively by root suckers. Our results suggest that both divergent selection and genetic drift may have played important roles in forming the genetic patterns of T. cordata at its northern distribution limit. However, we also speculate that epigenetic mechanism arising from the original population environment could have created similar patterns in regulating the spring phenology.

Maternal Effects in a Wild Songbird Are Environmentally Plastic but Only Marginally Alter the Rate of Adaptation

Despite ample evidence for the presence of maternal effects (MEs) in a variety of traits and strong theoretical indications for their evolutionary consequences, empirical evidence to what extent MEs can influence evolutionary responses to selection remains ambiguous. We tested the degree to which MEs can alter the rate of adaptation of a key life-history trait, clutch size, using an individual-based model approach parameterized with experimental data from a long-term study of great tits (Parus major). We modeled two types of MEs: (i) an environmentally plastic ME, in which the relationship between maternal and offspring clutch size depended on the maternal environment via offspring condition, and (ii) a fixed ME, in which this relationship was constant. Although both types of ME affected the rate of adaptation following an abrupt environmental shift, the overall effects were small. We conclude that evolutionary consequences of MEs are modest at best in our study system, at least for the trait and the particular type of ME we considered here. A closer link between theoretical and empirical work on MEs would hence be useful to obtain accurate predictions about the evolutionary consequences of MEs more generally.

Genetic and epigenetic variations associated with adaptation to heterogeneous habitat conditions in a deciduous shrub

Environmentally induced phenotypic plasticity is thought to play an important role in the adaption of plant populations to heterogeneous habitat conditions, and yet the importance of epigenetic variation as a mechanism of adaptive plasticity in natural plant populations still merits further research. In this study, we investigated populations of Vitex negundo var. heterophylla (Chinese chastetree) from adjacent habitat types at seven sampling sites. Using several functional traits, we detected a significant differentiation between habitat types. With amplified fragment length polymorphisms (AFLP) and methylation-sensitive AFLP (MSAP), we found relatively high levels of genetic and epigenetic diversity but very low genetic and epigenetic differences between habitats within sites. Bayesian clustering showed a remarkable habitat-related differentiation and more genetic loci associated with the habitat type than epigenetic, suggesting that the adaptation to the habitat is genetically based. However, we did not find any significant correlation between genetic or epigenetic variation and habitat using simple and partial Mantel tests. Moreover, we found no correlation between genetic and ecologically relevant phenotypic variation and a significant correlation between epigenetic and phenotypic variation. Although we did not find any direct relationship between epigenetic variation and habitat environment, our findings suggest that epigenetic variation may complement genetic variation as a source of functional phenotypic diversity associated with adaptation to the heterogeneous habitat in natural plant populations.

Early experiences mediate distinct adult gene expression and reproductive programs in Caenorhabditis elegans

Environmental stress during early development in animals can have profound effects on adult phenotypes via programmed changes in gene expression. Using the nematode C. elegans, we demonstrated previously that adults retain a cellular memory of their developmental experience that is manifested by differences in gene expression and life history traits; however, the sophistication of this system in response to different environmental stresses, and how it dictates phenotypic plasticity in adults that contribute to increased fitness in response to distinct environmental challenges, was unknown. Using transcriptional profiling, we show here that C. elegans adults indeed retain distinct cellular memories of different environmental conditions. We identified approximately 500 genes in adults that entered dauer due to starvation that exhibit significant opposite (“seesaw”) transcriptional phenotypes compared to adults that entered dauer due to crowding, and are distinct from animals that bypassed dauer. Moreover, we show that two-thirds of the genes in the genome experience a 2-fold or greater seesaw trend in gene expression, and based upon the direction of change, are enriched in large, tightly linked regions on different chromosomes. Importantly, these transcriptional programs correspond to significant changes in brood size depending on the experienced stress. In addition, we demonstrate that while the observed seesaw gene expression changes occur in both somatic and germline tissue, only starvation-induced changes require a functional GLP-4 protein necessary for germline development, and both programs require the Argonaute CSR-1. Thus, our results suggest that signaling between the soma and the germ line can generate phenotypic plasticity as a result of early environmental experience, and likely contribute to increased fitness in adverse conditions and the evolution of the C. elegans genome.

Environmental stress during early development in animals can have profound effects on adult behavior and physiology due to programmed changes in gene expression. However, whether different stresses result in distinct changes in traits that allow stressed animals to better survive and reproduce in future adverse conditions is largely unknown. Using the animal model system, C. elegans, we show that adults that experienced starvation exhibit opposite (“seesaw”) genome-wide gene expression changes compared to adults that experienced crowding, and are distinct from animals that experienced favorable conditions. Genes that are similarly up- or downregulated due to either starvation or crowding are located in clusters on the same chromosomes. Importantly, these gene expression changes of differently-stressed animals result in corresponding changes in progeny number, a life history trait of evolutionary significance. These distinct gene expression programs require different signaling pathways that communicate across somatic and germline tissue types. Thus, different environmental stresses experienced early in development induce distinct signaling mechanisms to result in changes in gene expression and reproduction in adults, and likely contribute to increased survival in future adverse conditions.

Advancing behavioural genomics by considering timescale

Animal behavioural traits often covary with gene expression, pointing towards a genomic constraint on organismal responses to environmental cues. This pattern highlights a gap in our understanding of the time course of environmentally responsive gene expression, and moreover, how these dynamics are regulated. Advances in behavioural genomics explore how gene expression dynamics are correlated with behavioural traits that range from stable to highly labile. We consider the idea that certain genomic regulatory mechanisms may predict the timescale of an environmental effect on behaviour. This temporally minded approach could inform both organismal and evolutionary questions ranging from the remediation of early life social trauma to understanding the evolution of trait plasticity.

Developmental plasticity: Bridging research in evolution and human health

Early life experiences can have profound and persistent effects on traits expressed throughout the life course, with consequences for later life behavior, disease risk, and mortality rates. The shaping of later life traits by early life environments, known as 'developmental plasticity', has been well-documented in humans and non-human animals, and has consequently captured the attention of both evolutionary biologists and researchers studying human health. Importantly, the parallel significance of developmental plasticity across multiple fields presents a timely opportunity to build a comprehensive understanding of this phenomenon. We aim to facilitate this goal by highlighting key outstanding questions shared by both evolutionary and health researchers, and by identifying theory and empirical work from both research traditions that is designed to address these questions. Specifically, we focus on: (i) evolutionary explanations for developmental plasticity, (ii) the genetics of developmental plasticity and (iii) the molecular mechanisms that mediate developmental plasticity. In each section, we emphasize the conceptual gains in human health and evolutionary biology that would follow from filling current knowledge gaps using interdisciplinary approaches. We encourage researchers interested in developmental plasticity to evaluate their own work in light of research from diverse fields, with the ultimate goal of establishing a cross-disciplinary understanding of developmental plasticity.

A naturally occurring epiallele associates with leaf senescence and local climate adaptation in Arabidopsis accessions

Epigenetic variation has been proposed to facilitate adaptation to changing environments, but evidence that natural epialleles contribute to adaptive evolution has been lacking. Here we identify a retrotransposon, named “NMR19” (naturally occurring DNA methylation variation region 19), whose methylation and genomic location vary among Arabidopsis thaliana accessions. We classify NMR19 as NMR19-4 and NMR19-16 based on its location, and uncover NMR19-4 as an epiallele that controls leaf senescence by regulating the expression of PHEOPHYTIN PHEOPHORBIDE HYDROLASE (PPH). We find that the DNA methylation status of NMR19-4 is stably inherited and independent of genetic variation. In addition, further analysis indicates that DNA methylation of NMR19-4 correlates with local climates, implying that NMR19-4 is an environmentally associated epiallele. In summary, we discover a novel epiallele, and provide mechanistic insights into its origin and potential function in local climate adaptation.

Epigenetic variation underlies various aspects of phenotypic diversity of plants. Here, He et al show a naturally occurring epiallele controls Arabidopsis leaf senescence by regulating the expression of PHEOPHYTIN PHEOPHORBIDE HYDROLASE (PPH), and is associated with local climate adaptation.

Context-Dependent Developmental Effects of Parental Shade Versus Sun Are Mediated by DNA Methylation

Parental environment influences progeny development in numerous plant and animal systems. Such inherited environmental effects may alter offspring phenotypes in a consistent way, for instance when resource-deprived parents produce low quality offspring due to reduced maternal provisioning. However, because development of individual organisms is guided by both inherited and immediate environmental cues, parental conditions may have different effects depending on progeny environment. Such context-dependent transgenerational plasticity suggests a mechanism of environmental inheritance that can precisely interact with immediate response pathways, such as epigenetic modification. We show that parental light environment (shade versus sun) resulted in context-dependent effects on seedling development in a common annual plant, and that these effects were mediated by DNA methylation. We grew replicate parents of five highly inbred Polygonum persicaria genotypes in glasshouse shade versus sun and, in a fully factorial design, measured ecologically important traits of their isogenic seedling offspring in both environments. Compared to the offspring of sun-grown parents, the offspring of shade-grown parents produced leaves with greater mean and specific leaf area, and had higher total leaf area and biomass. These shade-adaptive effects of parental shade were pronounced and highly significant for seedlings growing in shade, but slight and generally non-significant for seedlings growing in sun. Based on both regression and covariate analysis, inherited effects of parental shade were not mediated by changes to seed provisioning. To test for a role of DNA methylation, we exposed replicate offspring of isogenic shaded and fully insolated parents to either the demethylating agent zebularine or to control conditions during germination, then raised them in simulated growth chamber shade. Partial demethylation of progeny DNA had no phenotypic effect on offspring of shaded parents, but caused offspring of sun-grown parents to develop as if their parents had been shaded, with larger leaves and greater total canopy area and biomass. These results contribute to the increasing body of evidence that DNA methylation can mediate transgenerational environmental effects, and show that such effects may contribute to nuanced developmental interactions between parental and immediate environments.

Transgenerational stress-adaption: an opportunity for ecological epigenetics

In the recent years, there has been considerable interest to investigate the adaptive transgenerational plasticity of plants and how a "stress memory" can be transmitted to the following generation. Although, increasing evidence suggests that transgenerational adaptive responses have widespread ecological relevance, the underlying epigenetic processes have rarely been elucidated. On the other hand, model plant species have been deeply investigated in their genome-wide methylation landscape without connecting this to the ecological reality of the plant. What we need is the combination of an ecological understanding which plant species would benefit from transgenerational epigenetic stress-adaption in their natural habitat, combined with a deeper molecular analysis of non-model organisms. Only such interdisciplinary linkage in an ecological epigenetic study could unravel the full potential that epigenetics could play for the transgenerational stress-adaption of plants.

Variability in DNA Methylation and Generational Plasticity in the Lombardy Poplar, a Single Genotype Worldwide Distributed Since the Eighteenth Century

In the absence of genetic diversity, plants rely on the capacity of phenotypic plasticity to cope with shifts in environmental conditions. Understanding the mechanisms behind phenotypic plasticity and how local phenotypic adjustments are transferred to clonal offspring, will provide insight into its ecological and evolutionary significance. Epigenetic changes have recently been proposed to play a crucial role in rapid environmental adaptation. While the contribution of epigenetic changes to phenotypic plasticity has been extensively studied in sexual reproducing model organisms, little work has been done on vegetative generations of asexual reproducing plant species. We studied the variability of DNA methylation and bud set phenology of the Lombardy poplar (Populus nigra cv. Italica Duroi), a cultivated tree representing a single genotype worldwide distributed since the eighteenth century. Bud set observations and CpG methyl polymorphisms were studied on vegetative offspring resulting from cuttings grown for one season in a common glasshouse environment. The cuttings were collected from 60 adult Lombardy poplars growing in different environments. The physiological condition of the cuttings was determined by measuring weight and nutrient condition. Methylation sensitive amplified polymorphisms were used to obtain global patterns of DNA methylation. Using logistic regression models, we investigated correlations among epigenotype, bud phenology, and the climate at the home site of the donor trees, while accounting for physiological effects. We found significant epigenetic variation as well as significant variation in bud phenology, in the absence of genetic variation. Remarkably, phenology of bud set observed at the end of the growing season in the common environment was significantly correlated with climate variables at the home site of the mother trees, specifically the average temperature of January and monthly potential evapotranspiration. Although we could not directly detect significant effects of epigenetic variation on phenology, our results suggest that, in the Lombardy poplar, epigenetic marks contribute to the variation of phenotypic response that can be transferred onto asexually reproduced offspring resulting in locally adapted ecotypes. This contributes to the growing evidence that epigenetic-based transgenerational inheritance might be relevant for adaptation and evolution in contrasting or rapidly changing environments.

An ecotoxicological view on neurotoxicity assessment

The numbers of potential neurotoxicants in the environment are raising and pose a great risk for humans and the environment. Currently neurotoxicity assessment is mostly performed to predict and prevent harm to human populations. Despite all the efforts invested in the last years in developing novel in vitro or in silico test systems, in vivo tests with rodents are still the only accepted test for neurotoxicity risk assessment in Europe. Despite an increasing number of reports of species showing altered behaviour, neurotoxicity assessment for species in the environment is not required and therefore mostly not performed. Considering the increasing numbers of environmental contaminants with potential neurotoxic potential, eco-neurotoxicity should be also considered in risk assessment. In order to do so novel test systems are needed that can cope with species differences within ecosystems. In the field, online-biomonitoring systems using behavioural information could be used to detect neurotoxic effects and effect-directed analyses could be applied to identify the neurotoxicants causing the effect. Additionally, toxic pressure calculations in combination with mixture modelling could use environmental chemical monitoring data to predict adverse effects and prioritize pollutants for laboratory testing. Cheminformatics based on computational toxicological data from in vitro and in vivo studies could help to identify potential neurotoxicants. An array of in vitro assays covering different modes of action could be applied to screen compounds for neurotoxicity. The selection of in vitro assays could be guided by AOPs relevant for eco-neurotoxicity. In order to be able to perform risk assessment for eco-neurotoxicity, methods need to focus on the most sensitive species in an ecosystem. A test battery using species from different trophic levels might be the best approach. To implement eco-neurotoxicity assessment into European risk assessment, cheminformatics and in vitro screening tests could be used as first approach to identify eco-neurotoxic pollutants. In a second step, a small species test battery could be applied to assess the risks of ecosystems.

Epigenetics: A way to bridge the gap between biological fields

The concept of epigenetics has evolved since Waddington defined it from the late 1930s as the study of the causal mechanisms at work in development. It has become a multi-faceted notion with different meanings, depending on the disciplinary context it is used. In this article, we first analyse the transformations of the concept of epigenetics, from Waddington to contemporary accounts, in order to identify its different meanings and traditions, and to come up with a typology of epigenetics throughout its history. Second, we show on this basis that epigenetics has progressively turned its main focus from biological problems regarding development, toward issues concerning evolution. Yet, both these different epistemological aspects of epigenetics still coexist. Third, we claim that the classical opposition between epigenesis and preformationism as ways of thinking about the developmental process is part of the history of epigenetics and has contributed to its current various meanings. With these objectives in mind, we first show how Waddington introduced the term "epigenetics" in a biological context in order to solve a developmental problem, and we then build on this by presenting Nanney's, Riggs' and Holliday's definitions, which form the basis for the current conception of "molecular epigenetics". Then, we show that the evo-devo research field is where some particular uses of epigenetics have started shifting from developmental issues to evolutionary problems. We also show that epigenetics has progressively focused on the issue of epigenetic inheritance within the Extended Evolutionary Synthesis' framework. Finally, we conclude by presenting a typology of the different conceptions of epigenetics throughout time, and analyse the connections between them. We argue that, since Waddington, epigenetics, as an integrative research area, has been used to bridge the gap between different biological fields.

Community evolution increases plant productivity at low diversity

Species extinctions from local communities negatively affect ecosystem functioning. Ecological mechanisms underlying these impacts are well studied, but the role of evolutionary processes is rarely assessed. Using a long-term field experiment, we tested whether natural selection in plant communities increased biodiversity effects on productivity. We re-assembled communities with 8-year co-selection history adjacent to communities with identical species composition but no history of co-selection ('naïve communities'). Monocultures, and in particular mixtures of two to four co-selected species, were more productive than their corresponding naïve communities over 4 years in soils with or without co-selected microbial communities. At the highest diversity level of eight plant species, no such differences were observed. Our findings suggest that plant community evolution can lead to rapid increases in ecosystem functioning at low diversity but may take longer at high diversity. This effect was not modified by treatments simulating co-evolutionary processes between plants and soil organisms.

Rapid response to changing environments during biological invasions: DNA methylation perspectives

Dissecting complex interactions between species and their environments has long been a research hot spot in the fields of ecology and evolutionary biology. The well-recognized Darwinian evolution has well-explained long-term adaptation scenarios; however, "rapid" processes of biological responses to environmental changes remain largely unexplored, particularly molecular mechanisms such as DNA methylation that have recently been proposed to play crucial roles in rapid environmental adaptation. Invasive species, which have capacities to successfully survive rapidly changing environments during biological invasions, provide great opportunities to study molecular mechanisms of rapid environmental adaptation. Here, we used the methylation-sensitive amplified polymorphism (MSAP) technique in an invasive model ascidian, Ciona savignyi, to investigate how species interact with rapidly changing environments at the whole-genome level. We detected quite rapid DNA methylation response: significant changes of DNA methylation frequency and epigenetic differentiation between treatment and control groups occurred only after 1 hr of high-temperature exposure or after 3 hr of low-salinity challenge. In addition, we detected time-dependent hemimethylation changes and increased intragroup epigenetic divergence induced by environmental stresses. Interestingly, we found evidence of DNA methylation resilience, as most stress-induced DNA methylation variation maintained shortly (~48 hr) and quickly returned back to the control levels. Our findings clearly showed that invasive species could rapidly respond to acute environmental changes through DNA methylation modifications, and rapid environmental changes left significant epigenetic signatures at the whole-genome level. All these results provide fundamental background to deeply investigate the contribution of DNA methylation mechanisms to rapid contemporary environmental adaptation.

Epigenetics and adaptive phenotypic variation between habitats in an asexual snail

In neo-Darwinian theory, adaptation results from a response to selection on relatively slowly accumulating genetic variation. However, more rapid adaptive responses are possible if selectable or plastic phenotypic variation is produced by epigenetic differences in gene expression. This rapid path to adaptation may prove particularly important when genetic variation is lacking, such as in small, bottlenecked, or asexual populations. To examine the potential for an epigenetic contribution to adaptive variation, we examined morphological divergence and epigenetic variation in genetically impoverished asexual populations of a freshwater snail, Potamopyrgus antipodarum, from distinct habitats (two lakes versus two rivers). These populations exhibit habitat specific differences in shell shape, and these differences are consistent with adaptation to water current speed. Between these same habitats, we also found significant genome wide DNA methylation differences. The differences between habitats were an order of magnitude greater than the differences between replicate sites of the same habitat. These observations suggest one possible mechanism for the expression of adaptive shell shape differences between habitats involves environmentally induced epigenetic differences. This provides a potential explanation for the capacity of this asexual snail to spread by adaptive evolution or plasticity to different environments.

Evolutionary biology today and the call for an extended synthesis

Evolutionary theory has been extended almost continually since the evolutionary synthesis (ES), but except for the much greater importance afforded genetic drift, the principal tenets of the ES have been strongly supported. Adaptations are attributable to the sorting of genetic variation by natural selection, which remains the only known cause of increase in fitness. Mutations are not adaptively directed, but as principal authors of the ES recognized, the material (structural) bases of biochemistry and development affect the variety of phenotypic variations that arise by mutation and recombination. Against this historical background, I analyse major propositions in the movement for an 'extended evolutionary synthesis'. 'Niche construction' is a new label for a wide variety of well-known phenomena, many of which have been extensively studied, but (as with every topic in evolutionary biology) some aspects may have been understudied. There is no reason to consider it a neglected 'process' of evolution. The proposition that phenotypic plasticity may engender new adaptive phenotypes that are later genetically assimilated or accommodated is theoretically plausible; it may be most likely when the new phenotype is not truly novel, but is instead a slight extension of a reaction norm already shaped by natural selection in similar environments. However, evolution in new environments often compensates for maladaptive plastic phenotypic responses. The union of population genetic theory with mechanistic understanding of developmental processes enables more complete understanding by joining ultimate and proximate causation; but the latter does not replace or invalidate the former. Newly discovered molecular phenomena have been easily accommodated in the past by elaborating orthodox evolutionary theory, and it appears that the same holds today for phenomena such as epigenetic inheritance. In several of these areas, empirical evidence is needed to evaluate enthusiastic speculation. Evolutionary theory will continue to be extended, but there is no sign that it requires emendation.

Ancestry-Specific Methylation Patterns in Admixed Offspring from an Experimental Coyote and Gray Wolf Cross

Reduced fitness of admixed individuals is typically attributed to genetic incompatibilities. Although mismatched genomes can lead to fitness changes, in some cases the reduction in hybrid fitness is subtle. The potential role of transcriptional regulation in admixed genomes could provide a mechanistic explanation for these discrepancies, but evidence is lacking for nonmodel organisms. Here, we explored the intersection of genetics and gene regulation in admixed genomes derived from an experimental cross between a western gray wolf and western coyote. We found a significant positive association between methylation and wolf ancestry, and identified outlier genes that have been previously implicated in inbreeding-related, or otherwise deleterious, phenotypes. We describe a pattern of site-specific, rather than genome-wide, methylation driven by inter-specific hybridization. Epigenetic variation is thus suggested to play a nontrivial role in both maintaining and combating mismatched genotypes through putative transcriptional mechanisms. We conclude that the regulation of gene expression is an underappreciated key component of hybrid genome functioning, but could also act as a potential source of novel and beneficial adaptive variation in hybrid offspring.

Epigenetic variation between urban and rural populations of Darwin's finches

Background

The molecular basis of evolutionary change is assumed to be genetic variation. However, growing evidence suggests that epigenetic mechanisms, such as DNA methylation, may also be involved in rapid adaptation to new environments. An important first step in evaluating this hypothesis is to test for the presence of epigenetic variation between natural populations living under different environmental conditions.

Results

In the current study we explored variation between populations of Darwin’s finches, which comprise one of the best-studied examples of adaptive radiation. We tested for morphological, genetic, and epigenetic differences between adjacent “urban” and “rural” populations of each of two species of ground finches, Geospiza fortis and G. fuliginosa, on Santa Cruz Island in the Galápagos. Using data collected from more than 1000 birds, we found significant morphological differences between populations of G. fortis, but not G. fuliginosa. We did not find large size copy number variation (CNV) genetic differences between populations of either species. However, other genetic variants were not investigated. In contrast, we did find dramatic epigenetic differences between the urban and rural populations of both species, based on DNA methylation analysis. We explored genomic features and gene associations of the differentially DNA methylated regions (DMR), as well as their possible functional significance.

Conclusions

In summary, our study documents local population epigenetic variation within each of two species of Darwin’s finches.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-1025-9) contains supplementary material, which is available to authorized users.

Genotypic diversity in root-endophytic fungi reflects efficient dispersal and environmental adaptation

Studying community structure and dynamics of plant-associated fungi is the basis for unravelling their interactions with hosts and ecosystem functions. A recent sampling revealed that only a few fungal groups, as defined by internal transcribed spacer region (ITS) sequence similarity, dominate culturable root endophytic communities of nonmycorrhizal Microthlaspi spp. plants across Europe. Strains of these fungi display a broad phenotypic and functional diversity, which suggests a genetic variability masked by ITS clustering into operational taxonomic units (OTUs). The aims of this study were to identify how genetic similarity patterns of these fungi change across environments and to evaluate their ability to disperse and adapt to ecological conditions. A first ITS-based haplotype analysis of ten widespread OTUs mostly showed a low to moderate genotypic differentiation, with the exception of a group identified as Cadophora sp. that was highly diverse. A multilocus phylogeny based on additional genetic loci (partial translation elongation factor 1α, beta-tubulin and actin) and amplified fragment length polymorphism profiling of 185 strains representative of the five dominant OTUs revealed a weak association of genetic differences with geography and environmental conditions, including bioclimatic and soil factors. Our findings suggest that dominant culturable root endophytic fungi have efficient dispersal capabilities, and that their distribution is little affected by environmental filtering. Other processes, such as inter- and intraspecific biotic interactions, may be more important for the local assembly of their communities.

Maximizing ecological and evolutionary insight in bisulfite sequencing data sets

Genome-scale bisulfite sequencing approaches have opened the door to ecological and evolutionary studies of DNA methylation in many organisms. These approaches can be powerful. However, they introduce new methodological and statistical considerations, some of which are particularly relevant to non-model systems. Here, we highlight how these considerations influence a study's power to link methylation variation with a predictor variable of interest. Relative to current practice, we argue that sample sizes will need to increase to provide robust insights. We also provide recommendations for overcoming common challenges and an R Shiny app to aid in study design.

Epigenetics in natural animal populations

Phenotypic plasticity is an important mechanism for populations to buffer themselves from environmental change. While it has long been appreciated that natural populations possess genetic variation in the extent of plasticity, a surge of recent evidence suggests that epigenetic variation could also play an important role in shaping phenotypic responses. Compared with genetic variation, epigenetic variation is more likely to have higher spontaneous rates of mutation and a more sensitive reaction to environmental inputs. In our review, we first provide an overview of recent studies on epigenetically encoded thermal plasticity in animals to illustrate environmentally-mediated epigenetic effects within and across generations. Second, we discuss the role of epigenetic effects during adaptation by exploring population epigenetics in natural animal populations. Finally, we evaluate the evolutionary potential of epigenetic variation depending on its autonomy from genetic variation and its transgenerational stability. Although many of the causal links between epigenetic variation and phenotypic plasticity remain elusive, new data has explored the role of epigenetic variation in facilitating evolution in natural populations. This recent progress in ecological epigenetics will be helpful for generating predictive models of the capacity of organisms to adapt to changing climates.

Genetic and epigenetic variation in Spartina alterniflora following the Deepwater Horizon oil spill

Catastrophic events offer unique opportunities to study rapid population response to stress in natural settings. In concert with genetic variation, epigenetic mechanisms may allow populations to persist through severe environmental challenges. In 2010, the Deepwater Horizon oil spill devastated large portions of the coastline along the Gulf of Mexico. However, the foundational salt marsh grass, Spartina alterniflora, showed high resilience to this strong environmental disturbance. Following the spill, we simultaneously examined the genetic and epigenetic structure of recovering populations of S. alterniflora to oil exposure. We quantified genetic and DNA methylation variation using amplified fragment length polymorphism and methylation sensitive fragment length polymorphism (MS‐AFLP) to test the hypothesis that response to oil exposure in S. alterniflora resulted in genetically and epigenetically based population differentiation. We found high genetic and epigenetic variation within and among sites and found significant genetic differentiation between contaminated and uncontaminated sites, which may reflect nonrandom mortality in response to oil exposure. Additionally, despite a lack of genomewide patterns in DNA methylation between contaminated and uncontaminated sites, we found five MS‐AFLP loci (12% of polymorphic MS‐AFLP loci) that were correlated with oil exposure. Overall, our findings support genetically based differentiation correlated with exposure to the oil spill in this system, but also suggest a potential role for epigenetic mechanisms in population differentiation.