Integrative and Comparative Biology
Integrative and Comparative Biology, volume 53, number 2, pp. 359–372
doi:10.1093/icb/ict019
Society for Integrative and Comparative Biology
SYMPOSIUM
The Role of Methylation of DNA in Environmental Adaptation
Kevin B. Flores,1,* Florian Wolschin† and Gro V. Amdam*,†
From the symposium ‘‘Ecological Epigenetics’’ presented at the annual meeting of the Society for Integrative and
Comparative Biology, January 3–7, 2013 at San Francisco, California.
1
E-mail: kbflores@asu.edu
Synopsis Methylation of DNA is an epigenetic mechanism that influences patterns of gene expression. DNA methylation
marks contribute to adaptive phenotypic variation but are erased during development. The role of DNA methylation in
adaptive evolution is therefore unclear. We propose that environmentally-induced DNA methylation causes phenotypic
heterogeneity that provides a substrate for selection via forces that act on the epigenetic machinery. For example,
selection can alter environmentally-induced methylation of DNA by acting on the molecular mechanisms used for the
genomic targeting of DNA methylation. Another possibility is that specific methylation marks that are environmentallyinduced, yet non-heritable, could influence preferential survival and lead to consistent methylation of the same genomic
regions over time. As methylation of DNA is known to increase the likelihood of cytosine-to-thymine transitions, nonheritable adaptive methylation marks can drive an increased likelihood of mutations targeted to regions that are consistently marked across several generations. Some of these mutations could capture, genetically, the phenotypic advantage
of the epigenetic mark. Thereby, selectively favored transitory alterations in the genome invoked by DNA methylation
could ultimately become selectable genetic variation through mutation. We provide evidence for these concepts using
examples from different taxa, but focus on experimental data on large-scale DNA sequencing that expose between-group
genetic variation after bidirectional selection on honeybees, Apis mellifera.
Introduction
The methylation of DNA in eukaryotes is a chemical
modification that involves the addition of a methyl
group onto the position 5 of a pyrimidine ring on
cytosines (5mC), primarily within cytosine–phosphate–guanine (CpG) dinucleotides. DNA methylation can affect structural changes to chromatin by
attracting protein complexes that modify the histone
scaffolds holding the DNA coil. DNA methylation in
promoter regions can induce a tightly packed form
of DNA with attached proteins, called heterochromatin, that restrict the access of the transcriptional machinery. The outcome is the silencing of proximal
gene expression (Klose and Bird 2006). In species
that contain DNA methylation, the mechanism has
been functionally linked to development, behavior,
and phenotypic plasticity (Day and Sweatt 2010;
Feng et al. 2010; Law and Jacobsen 2010; Boyko
and Kovalchuk 2011; Lyko and Maleszka 2011).
DNA methylation is present in genomes across
taxa and likely pre-dates the divergence of plants
and animals. However, the amount and distribution
of DNA methylation in the genome varies widely
among species. For instance, 470% of CpGs are
methylated in humans, whereas this number is
18% in Arabidopsis thaliana and 51% in Apis
mellifera (honeybee) (Flores and Amdam 2011).
Moreover, some species have lost the enzymes necessary for DNA methylation despite possession of
complex development, behavior, and expression of
phenotypic plasticity. For example, the fruit fly
Drosophila melanogaster has no CpG DNA methylation, but its molecular biology is similar enough to
that of mammals to model development, behavior,
human disease, and nutrition (Beckingham et al.
2005). In addition, the fly shares complex programs
such as metamorphosis with the honeybee, in which
methylation of CpGs contributes to developmental
Advanced Access publication April 25, 2013
ß The Author 2013. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved.
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*School of Life Sciences, Arizona State University, PO Box 874501, Tempe, AZ 85287, USA; †Department of
Biotechnology, Chemistry and Food Science, Norwegian University of Life Sciences, PO Box 5003, Aas N-1432, Norway
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DNA methylation: targeting, gene
regulatory functions, and programming
The de novo methylation of DNA in eukaryotic genomes is performed by the DNA methyltransferase
DNMT3. The maintenance DNA methyltransferase
DNMT1 carries out the methylation of the cytosine
on the complementary strand subsequent to de novo
methylation and during replication of DNA (Law
and Jacobsen 2010). Although DNA methylation is
predominantly found on cytosines within CpG dinucleotides, it also occurs to a much lesser extent in
the context of CHG and CHH sequences (H ¼ A, C,
or T) (Chan et al. 2005; Lister et al. 2008, 2009).
Mechanisms targeting DNA methylation
The targeting of DNA methylation both in plants
and mammals is controlled by an RNA-directed
mechanism that allows different genomic sites to be
independently methylated in response to growth and
developmental, and environmental cues (Mette et al.
2000; Aravin and Hannon 2008; KuramochiMiyagawa et al. 2008; Morris 2009; Mahfouz 2010).
This process involves members of the family of PAZ
Piwi domain proteins that are capable of binding to
24- to 26-nt-long RNAs transcribed from non-coding
regions, called Piwi-interacting RNAs (piRNAs). The
PIWI/piRNA complex is guided to specific sequences
in the genome by RNA–DNA or RNA–RNA pairing
recognition (Wassenegger et al. 1994; Pélissier and
Wassenegger 2000). This PIWI complex then attracts
DNMT3 to perform de novo methylation. It is possible that the PIWI/piRNA mechanism of directed
DNA methylation may affect the placement of
other epigenetic modifications such as H3K4 demethylation, which then attract de novo DNA
methyltransferases, but it has been shown that the
PIWI/piRNA pathway is at least upstream of
de novo DNA methylation (Aravin et al. 2008).
Recently, it has also been shown that other small
RNAs similarly mediate de novo DNA methylation
by associating with PIWI proteins in plants. In
these instances, siRNAs or miRNAs that arose from
miRNA-coding regions guided DNA methylation at
some of their generation sites and in trans at their
target sites (Chellappan et al. 2010; Wu et al. 2010).
The genomic functional roles of DNA methylation
DNA methylation is known to affect transcriptional
silencing when it occurs in gene-promoter regions,
transposons, and repeats. In contrast, intragenic
DNA methylation (inside gene bodies) is frequently
associated with actively transcribed genes, suggesting
that the precise role of DNA methylation in transcriptional regulation may vary between promoter
and intragenic regions and between genes (Zhang
et al. 2006; Hellman and Chess 2007; Zilberman
et al. 2007; Ball et al. 2009; Rauch et al. 2009).
Although the conserved regulatory function(s) of intragenic DNA methylation remains elusive, an
emerging theory congruent with these findings is
that one conserved function of exon methylation is
the regulation alternative splicing (Laurent et al.
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outcomes (Kucharski et al. 2008). Thus, it appears
that quite dramatic changes in interspecific amounts
and distributions of DNA methylation are permitted—and, as we will argue, perhaps accommodated—by evolutionary processes.
Methylation of DNA is unique from other epigenetic mechanisms that affect the structure of chromatin because its usage by genomes carries an
evolutionary ramification in the form of increased
mutability. For example, the rate of C-to-T mutations is 10-fold to 50-fold higher in humans’ methylated cytosines (Duncan and Miller 1980; Bulmer
1986; Britten et al. 1988; Sved and Bird 1990).
Genomes with DNA methylation, overall, show a depletion of CpG dinucleotides that reflects the occurrence of mutations induced by DNA methylation in
the germline (Flores and Amdam 2011). It is unclear
whether such patterns of depletion include adaptive
mutations or reflect neutral and tolerated genomic
changes.
Here, we propose a four-stage mechanism that
may explain how methylation of DNA can play a
role in adaptive evolution: (1) environmental exposures contribute to variability in targeting of DNA
methylation, (2) targeting that benefits reproduction
and survival are perpetuated over generations when
environmental exposures remain unchanged, (3) targeted genomic regions experience increased mutability, and (4) mutations can accommodate the
phenotype achieved by methylation targeting and
make it available to natural selection.
To arrive at this explanatory framework, we begin
by discussing the functional roles of DNA methylation at the cellular and organismic levels. We then
discuss studies that exemplify how changes in patterns in genomic DNA methylation can occur in response to environmental variability and the degree to
which those changes are transferred to offspring.
Thereafter, we build support for the evolutionary
role of DNA methylation from genome-wide resequencing data of honeybees that were subject to
37 generations of bidirectional selection (Page and
Fondrk 1995).
K. B. Flores et al.
Role of methylation of DNA
2010; Lyko et al. 2010; Park et al. 2011). This is
especially relevant for the honeybee because over
80% of the DNA methylation in its genome was
found to be located within exons (Lyko et al.
2010), and DNA methylation distinctly ends at intron–exon boundaries (Flores and Amdam 2011).
Programming of DNA methylation
or behavioral phenotypes similar to genetic mutations. Changes in the developmental program of
DNA methylation could lead to differences in cellular
differentiation and be causal to differences in postdevelopmental physiology. Changes in the program
of neuronal DNA methylation could elicit novel behaviors or behavioral responses to the environment.
In the next two sections, we discuss how the environment affects variability in DNA methylation and
the epigenetic mechanisms that could transmit such
variable DNA methylation to offspring.
The environment as a modifier of DNA
methylation
There is increasing evidence that environmental variability can cause variation in the program of DNA
methylation in developing offspring. Because DNA
methylation also plays a functional role in transcriptional regulation, it is possible that the altered
patterns of DNA methylation signaled by the environment may, in turn, signal changes in gene expression. Thereby, variations in DNA-methylation
induced by environmental changes may be functional
and allow a population to display phenotypic variability despite being genetically homogeneous.
Recent studies in several plant species show that
alternative phenotypes can occur in populations with
little or no genetic variation, but instead correlate
with increased variation in DNA methylation
(Lukens and Zhan 2007; Gao et al. 2010; LiraMedeiros et al. 2010). In the dandelion, Taraxcum
officinale, such variability may be induced by environmental stress (Verhoeven et al. 2010). Other data
support the view that DNA methylation is required
for phenotypic responses to environmental exposures. For example, mutations in the targeting pathway of DNA methylation jin A. thaliana can reduce
global genomic DNA methylation along with changes
in the plant’s adaptive responses to heat, cold, salt,
drought, and flood (Boyko et al. 2010). In animals,
moreover, environmental factors such as the maternal diet (Lillycrop et al. 2005, 2007), neonatal diet
(Plagemann et al. 2009), rearing behavior (Weaver
et al. 2004) and folic acid supplementation (Wolff
et al. 1998) can alter de novo programming of
DNA methylation during development of the offspring. For example, feeding a protein-restricted
diet to pregnant rats results in gene-specific hypomethylation in the offspring. These differences in
DNA methylation in the offspring correlate with
changes in their adult phenotype, such as alterations
to glucose production in response to stress (Lillycrop
et al. 2007) and an increase in systolic blood pressure
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The targeting of DNA methylation in the genome is
both internally regulated, and as discussed in the
next section, sensitive to extrinsic signaling. The internal regulation constitutes a program of DNA
methylation which interacts with other dynamic molecular processes, such as transcription. Programmed
changes in DNA methylation are thought to help
regulate cellular differentiation during development
by inducing stable alterations in gene expression
(Monk et al. 1987; Kafri et al. 1992; Reik 2007;
Sasaki and Matsui 2008; Cedar and Bergman 2009).
Recent studies of genome-wide DNA methylation
support the tenet that programmed locus-specific
changes in DNA methylation correlate with changes
in cell phenotype (Lister et al. 2009; Laurent et al.
2010; Li et al. 2010). Experimental perturbation of
the intrinsic developmental program of DNA methylation can cause drastic changes in phenotype or be
lethal to organisms across eukaryotic taxa, including
plants (Lindroth et al. 2001; Cao and Jacobsen 2002a,
2002b; Kankel et al. 2003; Xiao et al. 2006), vertebrates (Li et al. 1992; Okano et al. 1999; Stancheva
et al. 2001; Li 2002), and invertebrates (Kucharski
et al. 2008; Shi et al. 2011).
Besides organismal development, programmed
changes in DNA methylation are also essential to
regulation of synaptic plasticity in memory and of
stress-induced behavior (Miller and Sweatt 2007;
LaPlant et al. 2010; Miller et al. 2010). For example,
various locations in the mouse brain undergo dynamic changes in DNA methylation in connection
with neuronal activity (Guo et al. 2011), while inhibition of DNMT enzymes after associative learning
in honeybees can interfere with the consolidation of
memory (Lockett et al. 2010). Programmed changes
in locus-specific DNA methylation also occur in
the bee brain during behavioral transitions that
are essential for colony fitness (Herb et al. 2012)
(Fig. 1). These data suggest that the functional role
of the programming of de novo DNA methylation in
the brain is conserved between vertebrates and
invertebrates.
Changes to the epigenetic code, such as differences
in the programming of DNA methylation, can
impact fitness by inducing alternative developmental
361
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K. B. Flores et al.
that may ultimately lead to hypertension (Bogdarina
et al. 2007).
Honeybees are an invertebrate species for which
the sensitivity of DNA methylation to the environment is conserved. This sensitivity has been socially
co-opted to regulate caste fate in female larvae. These
larvae can develop into either reproductive queens or
sterile workers depending on the diet they receive.
The diet is tightly controlled by the larvae’s adult
sisters that are nurse bees within the hive. If the
rearing of larvae is perturbed, the process of caste
differentiation, including the developmental program
of DNA methylation is altered, and this process involves changes in the expression of DNMT3 and the
locus-specific placement of DNA methylation (Shi
et al. 2011). The role of DNA methylation in caste
fate was further cemented by results showing that
queens can develop from larvae that are artificially
reared on a combination of a worker’s diet and
silencing DNMT3 with double-stranded RNA
(Kucharski et al. 2008).
These studies suggest that DNA methylation is a
conserved molecular mechanism in plants, vertebrates, and invertebrates that can be used to convert
environmental heterogeneity into phenotypic differences. Thus, we may gain a better understanding of
how phenotypic variability arises in populations by
studying how the cellular pathways that regulate the
genomic targeting of DNA methylation are signaled
by environmental change (Fig. 2). Studying these
pathways may also allow us to determine how patterns of DNA methylation evolve because genetic
changes to these pathways could lead to differences
in targeting of DNA methylation and in developmental programming.
The evolutionary role of environmentally-induced
phenotypic heterogeneity mediated by DNA
methylation
The capacity for the pathway of de novo DNA methylation to transduce spatial or temporal environmental variation into phenotypic variation implies a
potential role for DNA methylation in adaptive evolution. In honeybees, phenotypic variation that is
signaled by the environment and mediated by DNA
methylation may affect fitness of the colony.
Phenotypic heterogeneity among the honeybee workers underlies division of labor within the hive, and
an increase in division of labor can increase colony
fitness (Waibel et al. 2006; Oldroyd and Fewell
2007). It has been proposed that DNA methylation
could be involved in processing the internalization of
variations in micro-environments of larvae during
development, of workers, thereby helping generate
phenotypic heterogeneity in the worker population
despite genetic homogeneity (Flores and Amdam
2011). DNA methylation is known to have a functional role in transducing differences in the composition of the diet of larvae between queens and
workers (Kucharski et al. 2008). It is possible that
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Fig. 1 DNA methylation in the honeybee brain is dynamic and associated with an individual’s life history. Honeybee workers usually
progress with increasing age from tasks in the nest, such as nursing the brood, to foraging outside the nest. This behavioral switch is
essential for colony fitness because it regulates the allocation of workers dedicated to resource-harvesting in response to environmental
conditions. When forcing foragers to assume nursing tasks, some, but not all of the patterns of DNA methylation characteristic of
foragers will revert to patterns characteristic of nest bees. The patterns of DNA methylation in the brains of nurse bees and foragers
differ (Herb et al. 2012).
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Role of methylation of DNA
DNA methylation may have a similar function
among the worker caste, in which altering the
larval diet has the capacity to modulate physiological
traits that regulate the worker’s heterogeneity, such
as the number of ovarioles (Kaftanoglu et al. 2011).
Similarly, variability in the environment of queen
larvae could signal changes in the queen’s developmental program of DNA methylation. This could
lead to an increase in the phenotypic heterogeneity
of the worker population that the queen produces if
those changes in DNA methylation affect her germline. Alternatively, environmentally signaled DNA
methylation could mediate biological effects that
are manifest over the lifetime of individual bees,
also leading to phenotypic heterogeneity.
Further studies are needed to test whether variability in the environments of larvae or adults (e.g.,
diet or temperature) could imprint differences in
patterns of DNA methylation in workers’ brains,
thereby altering behavioral regulation that is critical
for division of labor, such as the transition from
nurse to forager.
Inheritance of DNA methylation
The capacity to transmit environmentally-induced
DNA methylation marks from parent to offspring
can be evolutionarily advantageous because it may
prepare the offsprings’ phenotype for the environmental stress that the parent(s) may have endured
(Mousseau and Fox 1998; Jablonka and Lamb 2005;
Youngson and Whitelaw 2008; Jablonka and Raz
2009). A genome-wide erasure of DNA methylation
during development would prevent the transfer of
DNA methylation marks, and any phenotypic traits
caused by them, from parent(s) to offspring. Patterns
of DNA methylation are reprogrammed genomewide during plant and mammalian development,
thereby limiting the capacity for transgenerational
inheritance of DNA methylation, at least on a
genome-wide scale.
DNA methylation reprogramming
during plant and mammalian
development
The degree to which DNA methylation is erased and
then re-established during development differs
between plants and mammals; evidence for DNA
demethylation is lacking in insects. The erasure of
DNA methylation in plants is carried out by a demethylation pathway, which includes the DNA glycosylases and AP lyases ROS1 (repressor of silencing
1), DME (demeter), DML2 (Demeter-like 2), and
DML3 (demeter-like 3), to excise cytosine’s that are
methylated. The nucleotide gap in DNA is then
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Fig. 2 Environment and RNA-dependent mechanisms act to influence the methylation of DNA. Patterns of DNA methylation depend
on the environment and are directed by piRNAs and the RNA-binding protein PIWI. For simplification, only one gene is shown to be
affected under each environment. Under environmental condition A, genomic regions that do not encode a gene (non-coding red part
of the DNA) produce piRNAs that bind to PIWI and then attract PIWI to genes that bear near 100% homology on the piRNA
sequence. Subsequently, DNMT3 is recruited and the respective gene is methylated. Under environmental condition B, piRNAs derived
from another genomic region are produced and a different gene is methylated, leading to an alternate molecular phenotype.
364
methylation (Cedar and Bergman 2009). Molecules
involved in the targeting of de novo DNA methylation, such as piRNAs, could be passed through the
germ line and lead to de novo DNA methylation at
specific loci during development in offspring. It has
also been demonstrated that DNA methylation imprinted during the development of offspring can be
transgenerationally inherited through recapitulation
of maternal traits. For example, an increase in the
licking and grooming (LG) of pups and arched-back
nursing (ABN) by rat mothers alters the pattern of
DNA methylation (compared with low-LG–ABN
mothers) in the promoter region of the glucocorticoid receptor (GR) in the offspring’s hippocampus.
The GR gene regulates the hypothalamic–pituitary–
adrenal (HPA) axis and stress response in the
hippocampal neurons, and the offspring of highLG–ABN mothers have a reduced reactivity and anxiety. These more relaxed offspring are then more
likely to adopt the same approach to the rearing of
young as did their mothers, thereby perpetuating
their GR DNA methylation patterns in the next generation (Weaver et al. 2004; Diorio and Meaney
2007).
Evidence for transgenerational
inheritance of DNA methylation
Transgenerational inheritance of DNA methylation is
more plausible in plants due to the possibility for
asexual vegetative reproduction and because in
sexual reproduction gametes are derived from
almost fully matured vegetative tissue. Transmission
of DNA methylation from parent(s) to offspring may
also be more adaptive in plants than in animals,
since gametes and vegetative offspring are derived
from tissue that has been subject to the environmental stress that occurred during almost the entire life
history of the parent generation. The exposure to
environmental stress has been shown to induce phenotypic changes that can persist to the next generation in plants and animals (Pembrey et al. 2005;
Grant-Downton and Dickinson 2006; Koturbash
et al. 2006; Molinier et al. 2006) and DNA methylation may play a critical role in the transgenerational
perpetuation of such environmentally-induced phenotypes (Mirouze and Paszkowski 2011). For example, in A. thaliana, transgenerational inheritance of
stress-induced responses is dependent on de novo
DNA methylation (Boyko et al. 2010). A similar
study in the dandelion T. officinale found that environmental stress, specifically chemical induction of
defensed against herbivores and pathogens, induces
differences in DNA methylation and that most of
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presumably filled by DNA repair polymerase and
ligase enzymes (Zhu 2009). This demethylation pathway affects genome-wide hypo-methylation in
Arabidopsis endosperm, especially within transposable elements (Gehring et al. 2009), and mutation
of DME partially restores endosperm DNA methylation to the amount found in other tissues (Hsieh
et al. 2009). The expression of DME in maternalspecific cells of the endosperm results in the demethylation, and consequently changes in expression, at
specific genes, i.e., maternal allele-specific imprinting
(Huh et al. 2008).
In mammals, direct transmission of DNA methylation marks from parent(s) to offspring is limited to
specific loci because of the waves of genome-wide
demethylation, followed by the re-establishment of
DNA methylation that occurs during gametogenesis
in primordial germ cells and in the embryo immediately following fertilization (Reik 2007; Surani et al.
2007; Sasaki and Matsui 2008; Hemberger et al.
2009). Demethylation in the embryonic stage has
the additional complexity that the paternal genome
from the sperm is demethylated, but the maternal
genome is not and may be protected from demethylation (Mayer et al. 2000; Oswald et al. 2000; Santos
et al. 2002; Nakamura et al. 2007). Similarly, differences in the timing of remethylation of DNA occur
between the male and female germs cells, in which
maternal-specific methylation is established after the
male germ cells are initially methylated (Bartolomei
and Ferguson-Smith 2011). A combination of active
and passive mechanisms of demethylation may contribute to genome-wide demethylation in mammals.
Passive demethylation involves the loss of DNA
methylation through the lack of maintenance
through cell division, resulting in hemi-methylated
substrates during the G2 phase of the cell cycle.
Several molecular mechanisms have been proposed
for active demethylation in mammals, including
5mC modification enzymes, DNA deaminases, and
the base excision repair pathway (Hajkova et al.
2010; Popp et al. 2010). It still remains unclear
how extensively the active or passive demethylation
pathways contribute to the genome-wide erasure of
DNA methylation; however, it has been demonstrated that some parental DNA methylation marks
can be transmitted to offspring (Richards 2006;
Hitchins et al. 2007).
In any case, DNA methylation patterns of the parents could be re-established after demethylation has
occurred. This could be facilitated by other epigenetic mechanisms, such as the differential inheritance
of DNA-binding proteins, including modified histones, which have the capacity to mediate DNA
K. B. Flores et al.
Role of methylation of DNA
DNA methylation increases mutability
Genomic regions that are methylated, either due to
programmed or environmental signaling, are subject
to an increased mutation rate because methylated
cytosines spontaneously deaminate to thymine,
which is then followed by replacement of guanine
by adenine on the opposite DNA strand due to the
mismatch repair of DNA (Duncan and Miller 1980).
Evidence for these DNA-methylation-induced mutations (DMIMs) in the form of CpG depletion is
found at genomic loci that have presumably been
methylated over evolutionary time, i.e., across multiple generations, in germ line cells. For example,
approximately half of all honeybee genes are methylated, leaving a pattern in which half of all genes
have less CpGs than expected (Elango et al. 2009)
(Fig. 3 [top]). This pattern of depletion of CpG in
honeybees’ genome is also apparent at the exon level,
in concordance with the observation that honeybees’
DNA methylation is largely targeted to exons
(Figs. 3 [bottom] and 4). Similar patterns of CpG
depletion are found in the genomes of Acyrthosiphon
pisum, Ciona intestinalis, and humans, but are absent
from species that do not show any significant levels
of CpG methylation, such as D. melanogaster (Flores
and Amdam 2011).
Because the developmental program of DNA
methylation is sensitive to the environment, the
level of methylation in some genomic regions in offspring may be a probabilistic function of the environment. In genomic regions at which methylation
Fig. 3 Methylation correlates with depletion of CpG in honeybees’ genes and exons. The CpG[O/E] ratio is used as a measure of the
depletion of CpG in a genomic region; it is calculated as the number of observed CpGs divided by the number of expected CpGs based
on GC content (Elango et al. 2009; Flores and Amdam 2011). (Top) Honeybees’ genes are separated into methylated and unmethylated
categories and the distributions of CpG[O/E] ratios is shown for each category. Methylated genes are more depleted of CpGs than are
unmethylated genes, likely because of the increased rate of C/T transitions due to deamination of nucleotides. (Bottom) Honeybees’
exons are separated into methylated and unmethylated categories and the distributions of CpG[O/E] ratios is shown for each category.
Methylated exons are more depleted of CpGs than are unmethylated exons. Data on DNA methylation were obtained from bisulfitesequencing of queens and workers [31].
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these differences are inherited by the offspring
(Verhoeven et al. 2010).
A key feature of the study of dandelions was that
it involved a species with asexual reproduction,
allowing the variability in DNA methylation to be
associated with variability in environments of the
parents instead of with genetic variation. DNA methylation may play a role in evolutionary adaptation by
providing an epigenetic layer of inheritance on top of
genetic inheritance. However, substantiating this
concept will require further studies that control for,
or accurately measure, genetic variation on a
genome-wide scale in order to negate the possibility
that observed heritable changes in DNA methylation
are caused by genetic mutations.
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K. B. Flores et al.
only occurs during an environmental change, mechanisms of inheritance of transgenerational DNA
methylation may help to perpetuate methylation in
these genomic regions for several generations after
the environmental change has occurred. If this methylation is causal to a phenotype that has a selective
advantage during a period of environmental change,
then DMIMs could allow that phenotype to become
fixed (i.e., genetically programmed). The fixation of
phenotypes may be caused by DMIMs that emulate
the function of the DNA methylation that was induced by the environmental change. One way this
might occur is if DMIMs alter the propensity for a
genomic region to be methylated, causing a locusspecific change in DNA methylation (Fig. 5).
Alternatively, DMIMs could fix an adaptive phenotype by changing the sequence of a gene-coding
region, such as an exon if a DMIM is caused by
exon-targeted DNA methylation, or a gene regulatory
sequence, such as a transcription factor-binding site
if a DMIM is caused by promoter DNA methylation.
Circumstantial evidence for the
evolutionary role of DNA methylation
DMIMs could ultimately circumvent the demethylation waves in the animal germ line and lead to a
stable transmission of differences. Experimental evidence supporting the idea of DMIMs comes from
studies of the honeybee. Honeybees can be
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Fig. 4 Exon-specific depletion of CpG reflects exon-specific DNA methylation in the honeybee gene GB16921 (homolog of Ptip, part
of the H3K4 methyltransferase complex). (Top) Base-pair resolution of the intensity of methylation is shown for the length of the gene
GB16921. Solid (dotted) lines above (below) the x-axis indicate the intensity of methylation in the queen (worker). The nine exons (1–
9 from left to right) for this gene are shown on the x-axis as open boxes. The x-axis is labeled as base-pairs (bp) from translation start
site (TSS). Plus signs just above (below) the x-axis indicate CpG coverage from bisulfite-sequencing data in queens (workers). Exons 2
and 3 are heavily methylated in both queens and workers. (Bottom) The CpG[O/E] ratio is calculated with a 200-bp sliding window
along the length of the gene (see Fig. 3 for discussion of CpG[O/E]). These data show that depletion of CpG in GB16921 is confined to
exons that are heavily methylated (compare with top panel). Data on DNA methylation were obtained at base-pair resolution from
genome-wide bisulfite-sequencing of honeybee queens and workers [31].
Role of methylation of DNA
bidirectionally selected for the amount of pollen
(source of protein) versus nectar (source of carbohydrate) that is stored by colonies. This colony-level
selection for high and low pollen-hoarding was first
described by Hellmich et al. (1985) and was subsequently perfected by Page and Fondrk [14]. Page and
Amdam (2007) described differences between the resulting genotypes in several traits such as workers’
lifespan, sucrose sensitivity, and ovariole number.
Interestingly, in wild-type (unselected) honeybees,
similar suites of differences in traits are distributed
between sister worker bees that share 0.75 genetic
identity (Page and Amdam 2007). This suggests
that the bidirectional selection on honeybee foodhoarding captured phenotypic variability that normally is expressed as heterogeneity by genetically
similar individuals.
An analysis of quantitative trait loci that differentiate between high and low pollen-hoarding genotypes showed that epigenetic modulators such as
histones, mSin3A (a core component of a large multiprotein complex that displays histone deacetylase
activity), and a PIWI protein could be, at least
partly, responsible for their physiological and behavioral divide (Hunt et al. 2007). We predicted that the
two genotypes could differ in their patterns of DNA
methylation. Indeed, an analysis of genome-wide
DNA-methylation levels showed differences between
the two genotypes with respect to their patterns of
DNA methylation in the brain (Herb et al., unpublished data). In order to determine whether the two
genotypes also show evidence for DMIMs, which
would support a heritable difference in epigenetic
effects, we analyzed the genome sequences of four
biological replicates per genotype by testing for an
enrichment of C/T transitions at cytosines within
CpG dinucleotides versus all other cytosines. We
found a significant enrichment of C/T transitions
within CpG dinucleotides that was approximately
equal (¼2.59-fold) in both the high and low
pollen-hoarding genotypes (Table 1). We also observed that there is a 38.7% GC content in third
codon positions (3GC) in the honeybee reference
genome, indicating a directional mutational pressure
in the GC to AT direction (Sueoka 1988; Khrustalev
and Barkovsky 2009). However, 81.8% of cytosines
in the third codon position were not contained in
CpG dinucleotides. Thus, it is unlikely that factors
causal to the AT-pressure in the honeybee genome,
besides nucleotide deamination, contributed to the
enrichment of C/T transitions within the CpG dinucleotides that we found. These observations suggest a
connection between DNA methylation and mutations in the two selected genotypes.
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Fig. 5 The effect of DNA methylation induced mutations on
environmentally sensitive methylation patterns. (A) Methylation
patterns (open circles attached to lines) between the parent
(Generation T) and offspring (Generation T þ 1) are erased and
then re-established during development whether or not
the parent generation is methylated. The mechanism that
re-establishes methylation patterns can be sensitive to the
environment. Here, EA and EB represent the normal and new
environmental conditions, respectively. The probability function
P1(E) describes a genotype in which there is no methylation when
the environment is EA, and the chance of methylation is random
(probability ¼ 0.5) in environment EB. After a number of generations S, deamination in the germline may (B) or may not
(C) occur, resulting in a genetic mutation (C ! T transition) that
changes the probability of methylation during development to a
different function P2(E). (D) Scenario (i) represents a case in
which P2(E) is more sensitive to a new environment, leading to a
higher chance of methylation with less environmental change.
Scenario (ii) represents a case in which P2(E) is completely
insensitive to the environment and methylation always occurs.
367
368
K. B. Flores et al.
Table 1 CpG dinucleotides are enriched for C/T transition in bidirectionally selected strains of honeybees
High pollen-hoarding strain
C not within CpG
C within CpG
Low pollen-hoarding strain
C/T
no C/T
C/T
no C/T
22,410
63,758,382
17,381
63,763,411
9148
10,020,985
7102
10,023,031
Remarks and future work
Phenotypic plasticity is a ubiquitous property in
plants and animals that enables a population to
achieve phenotypic variability with respect to environmental change despite genetic uniformity (WestEberhard 2003). Epigenetic mechanisms used in the
developmental program of an organism can be sensitive to the environment; hence epigenetic variation
is expected to occur when phenotypic plasticity is
manifest. Epigenetic mechanisms such as DNA methylation provide a means of extending the flexibility
of the genome by affecting changes to the transcriptome, and to thus increase phenotypic plasticity.
Here, we suggest that, by causing increased mutability, DNA methylation links this flexibility with evolutionary processes, culminating in selectable genetic
variability. Interestingly, recent work has also shown
that DNA hypo-methylation is associated with a
higher frequency of homologous recombination and
genomic instability (Li et al. 2012). Thus, it is possible that environmentally-induced hyper-methylation or hypo-methylation of DNA could lead to a
higher mutation rate.
The evolutionary role of phenotypic plasticity mediated by DNA methylation remains unclear because
patterns of DNA methylation mostly are reset in the
gametes of plants and mammals, and in the primordial germ cells (PGCs) of mammals (Feng et al. 2010;
Law and Jacobsen 2010). However, despite this limitation, we reviewed several mechanisms whereby
methylation patterns are transferred transgenerationally. The assessment of these mechanisms will be facilitated by a clearer understanding of how piRNAs
are generated, direct the placement of DNA methylation, and whether they are transferred to eggs or
embryos. The transgenerational transference of functional DNA methylation has the potential for
contributing to short-term adaptation to environmental changes that cause variation in the methylomes of offspring. This effect can vary ever more
dramatically in a population that exhibits genetic
variance in master regulators of the machinery of
DNA methylation and thus shows a broad range in
sensitivity toward environmental perturbations. The
resulting variable epigenetic response could confer
positive fitness effects with respect to environmental
change if it increases the rate at which alternative
phenotypes that are only manifested during periods
of environmental change become genetically fixed
(West-Eberhard 2003). For example, in the genotypes
of pollen-hoarding honeybees, it is possible that genetic differences in key epigenetic regulators, such as
PIWI proteins, could have led to a difference in
overall DNA methylation and thereby to variability
in phenotypes such as pollen-hoarding behavior. In
this scenario, a difference in behavioral phenotype is
then repeatedly selected upon over generations and,
over time, leads to DMIMs.
It remains to be uncovered precisely how DMIMs
aid in the adaptability of an organism to its environment. We argue that DMIMs could decouple the
developmental response from the environment by
changing the likelihood that the functional effect of
DNA methylation will occur without environmental
extremes or by fixing genetic changes that replace the
effect of DNA methylation. Thereby, DNA methylation may play a role in evolutionary adaptation
due to the increased mutability induced in genomic
regions where it is used.
Future research targeted at the fixation and reversibility of DNA methylation will be needed in order
to shed more light on the question of how changes
in the environment relate to epigenetic patterns
and adaptability of organisms. Because of their
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Compared with the number of C/T transitions at cytosine’s not contained in CpG dinucleotides, there are 2.59 times more C/T transitions at
CpG dinucleotides in honeybee genotypes that were artificially selected for high or low pollen-hoarding behavior [14]. This enrichment of C/T
transitions is statistically significant by Fisher’s exact test in each of the two genotypes (P52.2e16). We re-sequenced the whole genomes of
four individuals from each of the low and high pollen-hoarding strains of honeybees, using deep sequencing (HiSeq 2000, Illumina, San Diego,
CA). Reads generated from deep sequencing were aligned to Amel_2.0 with Bowtie (version 0.12.7) with default options (Langmead et al.
2009). To determine the presence of genetic transitions between the high and low strains, we performed a standard case/control association
analysis using PLINK (version 1.07) (Purcell et al. 2007; Purcell 2010); default options and a P-value cutoff of 0.05 were used to infer the
presence of a significant genetic difference. This resulted in a total of 401,804 significant genetic differences. For each genotype, we tallied the
number of C/T transitions by counting the number of significant genetic differences in which the genotype was called T and the reference
genome was C. Similarly, C/T transitions were counted on the opposite strand if a G/A transition was found on the positive strand.
Role of methylation of DNA
Acknowledgments
We thank Sabine Deviche and the ASU School of
Life Sciences Vislab for their help with the figures,
and David Galbraith and Erik Rasmussen for helpful
comments and suggestions.
Funding
The Research Council of Norway (nos 180504,
191699, and 213976); the PEW Charitable Trust;
and National Institute on Aging (NIA P01
AG22500) (to G.V.A.).
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