Genome-wide DNA methylation alterations of Alternanthera philoxeroides in natural and manipulated habitats: implications for epigenetic regulation of rapid responses to environmental fluctuation and phenotypic variation

Patterns of DNA methylation throughout a range expansion of an introduced songbird

The spread of invasive species presents a genetic paradox: how do individuals overcome the genetic barriers associated with introductions (e.g., bottlenecks and founder effects) to become adapted to the new environment? In addition to genetic diversity, epigenetic variation also contributes to phenotypic variation and could influence the spread of an introduced species in novel environments. This may occur through two different (non-mutually exclusive) mechanisms. Individuals may benefit from existing (and heritable) epigenetic diversity or de novo epigenetic marks may increase in response to the new environment; both mechanisms might increase flexibility in new environments. Although epigenetic changes in invasive plants have been described, no data yet exist on the epigenetic changes throughout a range expansion of a vertebrate. Here, we used methylation sensitive-amplified fragment length polymorphism to explore genome-wide patterns of methylation in an expanding population of house sparrows (Passer domesticus). House sparrows were introduced to Kenya in the 1950s and have significant phenotypic variation dependent on the time since colonization. We found that Kenyan house sparrows had high levels of variation in methylation across the genome. Interestingly, there was a significant, potentially compensatory relationship between epigenetic and genetic diversity: epigenetic diversity was negatively correlated with genetic diversity and positively correlated with inbreeding across the range expansion. Thus, methylation may increase phenotypic variation and/or plasticity in response to new environments and therefore be an important source of inter-individual variation for adaptation in these environments, particularly over the short timescales over which invasions occur.

Epigenetic variation creates potential for evolution of plant phenotypic plasticity

Heritable variation in plant phenotypes, and thus potential for evolutionary change, can in principle not only be caused by variation in DNA sequence, but also by underlying epigenetic variation. However, the potential scope of such phenotypic effects and their evolutionary significance are largely unexplored. Here, we conducted a glasshouse experiment in which we tested the response of a large number of epigenetic recombinant inbred lines (epiRILs) of Arabidopsis thaliana--lines that are nearly isogenic but highly variable at the level of DNA methylation--to drought and increased nutrient conditions. We found significant heritable variation among epiRILs both in the means of several ecologically important plant traits and in their plasticities to drought and nutrients. Significant selection gradients, that is, fitness correlations, of several mean traits and plasticities suggest that selection could act on this epigenetically based phenotypic variation. Our study provides evidence that variation in DNA methylation can cause substantial heritable variation of ecologically important plant traits, including root allocation, drought tolerance and nutrient plasticity, and that rapid evolution based on epigenetic variation alone should thus be possible.

DNA methylation as a mechanism of nutritional plasticity: limited support from horned beetles

Epigenetic changes to DNA, potentially heritable alterations above the sequence level, such as DNA methylation, are thought to underlie many instances of adaptive phenotypic plasticity. Our understanding of the links between epigenetic variation and adaptive phenotypic plasticity in natural populations is limited. If DNA methylation underlies adaptive responses to different nutritional environments, methylation patterns should be correlated with differences in performance across nutritional environments, and respond to changes in the environment. Additionally, genotypes that can cope with a broader range of nutritional environments are expected to have greater flexibility in methylation patterns. We tested these predictions using horned beetles (genus Onthophagus), which can cope with a wide range of variation in larval nutrition. We surveyed levels of methylation across several methylated loci in lab-reared beetles originating from natural populations using a methylation-specific amplified fragment length polymorphism (AFLP) analysis. For less than half the of the loci investigated, methylation level was correlated with performance, measured as adult body size attained on a given diet, in different nutritional environments, with an overall greater effect in males (the more nutritionally plastic sex) than females. Methylation levels at most sites were influenced more by genotype (iso-female line) than by environment (dung type). Only 1 site (of 12) showed a significant genotype-by-environment interaction. Taken together, our results provide modest support for the hypothesis that DNA methylation underlies nutritional plasticity, as only 8-16% of methylated sites conformed to all of our predictions.

A cascade of recently discovered molecular mechanisms involved in abiotic stress tolerance of plants

Today, agriculture is facing a tremendous threat from the climate change menace. As human survival is dependent on a constant supply of food from plants as the primary producers, we must aware of the underlying molecular mechanisms that plants have acquired as a result of molecular evolution to cope this rapidly changing environment. This understanding will help us in designing programs aimed at developing crop plant cultivars best suited to our needs of a sustainable agriculture. The field of systems biology is rapidly progressing, and new insight is coming out about the molecular mechanisms involved in abiotic stress tolerance. There is a cascade of changes in transcriptome, proteome, and metabolome of plants during these stress responses. We have tried to cover most pronounced recent developments in the field of "omics" related to abiotic stress tolerance of plants. These changes are very coordinated, and often there is crosstalk between different components of stress tolerance. The functions of various molecular entities are becoming more clear and being associated with more precise biological phenomenon.

Genetic and epigenetic regulation of stress responses in natural plant populations

Plants have developed intricate mechanisms involving gene regulatory systems to adjust to stresses. Phenotypic variation in plants under stress is classically attributed to DNA sequence variants. More recently, it was found that epigenetic modifications - DNA methylation-, chromatin- and small RNA-based mechanisms - can contribute separately or together to phenotypes by regulating gene expression in response to the stress effect. These epigenetic modifications constitute an additional layer of complexity to heritable phenotypic variation and the evolutionary potential of natural plant populations because they can affect fitness. Natural populations can show differences in performance when they are exposed to changes in environmental conditions, partly because of their genetic variation but also because of their epigenetic variation. The line between these two components is blurred because little is known about the contribution of genotypes and epigenotypes to stress tolerance in natural populations. Recent insights in this field have just begun to shed light on the behavior of genetic and epigenetic variation in natural plant populations under biotic and abiotic stresses. This article is part of a Special Issue entitled: Plant gene regulation in response to abiotic stress.

A truly ecological epigenetics study

Until a few years ago, epigenetics was a field of research that had nothing to do with ecology and that virtually no ecologist had ever heard of. This is now changing, as more and more ecologists learn about epigenetic processes and their potential ecological and evolutionary relevance, and a new research field of ecological epigenetics is beginning to take shape. One question that is particularly intriguing ecologists is to what extent epigenetic variation is an additional, and hitherto overlooked, source of natural variation in ecologically important traits. In this issue of Molecular Ecology, Herrera & Bazaga (2011) provide one of the first attempts to truly address this question in an ecological setting. They study variation of DNA methylation in a wild population of the rare, long-lived violet Viola cazorlensis, and they use these data to explore interrelations between environmental, genetic and epigenetic variation, and in particular the extent to which these factors are related to long-term differences in herbivore damage among plants. They find substantial epigenetic variation among plant individuals. Interestingly, this epigenetic variation is significantly correlated with long-term differences in herbivory, but only weakly with herbivory-related DNA sequence variation, which suggests that besides habitat, substrate and genetic variation, epigenetic variation may be an additional, and at least partly independent, factor influencing plant–herbivore interactions in the field. Although the study by Herrera & Bazaga (2011) raises at least as many new questions as it answers, it is a pioneering example of how epigenetics can be incorporated into ecological field studies, and it illustrates the value and potential novel insights to be gained from such efforts.