Epigenetics of Social Behaviour

Understanding age and society using natural populations

Ageing affects almost all aspects of life and therefore is an important process across societies, human and non-human animal alike. This article introduces new research exploring the complex interplay between individual-level ageing and demography, and the consequences this interplay holds for the structure and functioning of societies across various natural populations. We discuss how this Special Issue provides a foundation for integrating perspectives from evolutionary biology, behavioural ecology and demography to provide new insights into how ageing shapes individuals' social behaviour and social associations, and how this in turn impacts social networks, social processes (such as disease or information transfer) and fitness. Through examining these topics across taxa, from invertebrates to birds and mammals, we outline how contemporary studies are using natural populations to advance our understanding of the relationship between age and society in innovative ways. We highlight key emerging research themes from this Special Issue, such as how sociality affects lifespan and health, the genetic and ecological underpinnings of social ageing and the adaptive strategies employed by different species. We conclude that this Special Issue underscores the importance of studying social ageing using diverse systems and interdisciplinary approaches for advancing evolutionary and ecological insights into both ageing and sociality more generally.This article is part of the discussion meeting issue 'Understanding age and society using natural populations '.

Serotonin release by parachloroamphetamine in rats with high and low sociability: High prefrontal release capacity in sociable females

BACKGROUND

Social behaviour is the expression of one of the most generally accepted independent dimensions of personality. Serotonergic neurotransmission has been implicated in typical social response and drugs that promote serotonin (5-hydroxytryptamine (5-HT)) release have prosocial effects. By using the social interaction test, we have previously demonstrated sociability as a temperamental trait in male Wistar rats.

AIMS

To assess sociability in male rats of the Sprague-Dawley strain and in female rats of both Wistar and Sprague-Dawley strain, and extracellular levels of 5-HT in rats with high and low sociability (high sociability (HS)- and low sociability (LS)-rats).

METHODS

Social interaction test conducted with different weight-matched partners was used to assess sociability, and in vivo, microdialysis was performed before and after administration of a low dose (2 mg/kg) of parachloroamphetamine (PCA) in the prefrontal cortex, dorsamedial striatum and ventral tegmental area.

RESULTS

Similarly to male Wistar rats, female Wistars and Sprague-Dawley rats of both sexes displayed trait-wise sociability. Male Wistar HS-rats had lower extracellular levels of 5-HT in prefrontal cortex at baseline and after administration of PCA, and higher PCA-induced increase of extracellular 5-HT in ventral tegmental area. In dorsomedial striatum, PCA elicited a comparable increase in extracellular dopamine in HS- and LS-rats, but higher release of 5-HT in HS-rats. Comparison of PCA-induced 5-HT release in prefrontal cortex of male and female Sprague-Dawley rats revealed a larger 5-HT response in female HS-rats.

CONCLUSIONS

5-HT release potential is higher in rats with high expression of sociability trait, whereas some regionally variable differences may be related to relative contributions of social motivation and anxiety in shaping social behaviour.

Fatty Acid Amide Hydrolase and Cannabinoid Receptor Type 1 Genes Regulation is Modulated by Social Isolation in Rats

Social isolation is a state of lack of social connections, involving the modulation of different molecular signalling cascades and associated with high risk of mental health issues. To investigate if and how gene expression is modulated by social experience at the central level, we analyzed the effects of 5 weeks of social isolation in rats focusing on endocannabinoid system genes transcription in key brain regions involved in emotional control. We observed selective reduction in mRNA levels for fatty acid amide hydrolase (Faah) and cannabinoid receptor type 1 (Cnr1) genes in the amygdala complex and of Cnr1 in the prefrontal cortex of socially isolated rats when compared to controls, and these changes appear to be partially driven by trimethylation of Lysine 27 and acetylation of Lysine 9 at Histone 3. The alterations of Cnr1 transcriptional regulation result also directly correlated with those of oxytocin receptor gene. We here suggest that to counteract the effects of SI, it is of relevance to restore the endocannabinoid system homeostasis via the use of environmental triggers able to revert those epigenetic mechanisms accounting for the alterations observed.

Exploring transcriptional and post-transcriptional epigenetic regulation of crf and 11βhsd2 in rainbow trout brain during chronic social stress

Using dominance hierarchies in juvenile rainbow trout (Oncorhynchus mykiss) as a model of chronic social stress in fish, we explored whether epigenetic transcriptional and post-transcriptional mechanisms are involved in the gene expression of corticotropin-releasing factor (crf) and 11β-hydroxysteroid dehydrogenase (11βhsd2), key factors involved in the regulation of the endocrine stress axis response. In juvenile rainbow trout pairs, subordinate individuals display sustained elevation of circulating cortisol concentrations. Cortisol production is controlled by the hypothalamic-pituitary-interrenal (HPI) axis in fish and initiated by CRF release from the preoptic area (POA). Given that crf is modulated during chronic social stress, and that such stress has been implicated in the epigenetic regulation of crf in other taxa, we probed a role for epigenetic regulation of crf transcript abundance in chronically stressed rainbow trout. We also investigated the regulation of the cortisol-metabolising enzyme 11βhsd2 in the POA, which is upregulated in subordinates. The potential involvement of DNA methylation and microRNAs (miRNAs) in the regulation of crf transcript abundance was investigated during social stress in the POA of fish, as was the potential involvement of miRNAs in 11βhsd2 regulation. Although transcript abundances of crf were elevated in subordinate fish after 4 days, DNA methylation profiles within putative promoter sequences upstream of the crf gene were not significantly affected by chronic stress. An inverse relationship between crf and its predicted posttranscriptional regulator miR-103a-3p in the POA suggests that miRNAs may be involved in mediating the effects of chronic social stress on key components of the endocrine stress axis.

Tet2 acts in the lateral habenula to regulate social preference in mice

The lateral habenula (LHb) has been considered a moderator of social behaviors. However, it remains unknown how LHb regulates social interaction. Here, we show that the hydroxymethylase Tet2 is highly expressed in the LHb. Tet2 conditional knockout (cKO) mice exhibit impaired social preference; however, replenishing Tet2 in the LHb rescues social preference impairment in Tet2 cKO mice. Tet2 cKO alters DNA hydroxymethylation (5hmC) modifications in genes that are related to neuronal functions, as is confirmed by miniature two-photon microscopy data. Further, Tet2 knockdown in the glutamatergic neurons of LHb causes impaired social behaviors, but the inhibition of glutamatergic excitability restores social preference. Mechanistically, we identify that Tet2 deficiency reduces 5hmC modifications on the Sh3rf2 promoter and Sh3rf2 mRNA expression. Interestingly, Sh3rf2 overexpression in the LHb rescues social preference in Tet2 cKO mice. Therefore, Tet2 in the LHb may be a potential therapeutic target for social behavior deficit-related disorders such as autism.

The Epigenetics of Animal Personality

Animal personality, consistent individual differences in behaviour, is an important concept for understanding how individuals vary in how they cope with environmental challenges. In order to understand the evolutionary significance of animal personality, it is crucial to understand the underlying regulatory mechanisms. Epigenetic marks such as DNA methylation are hypothesised to play a major role in explaining variation in phenotypic changes in response to environmental alterations. Several characteristics of DNA methylation also align well with the concept of animal personality. In this review paper, we summarise the current literature on the role that molecular epigenetic mechanisms may have in explaining personality variation. We elaborate on the potential for epigenetic mechanisms to explain behavioural variation, behavioural development and temporal consistency in behaviour. We then suggest future routes for this emerging field and point to potential pitfalls that may be encountered. We conclude that a more inclusive approach is needed for studying the epigenetics of animal personality and that epigenetic mechanisms cannot be studied without considering the genetic background.

Biological mechanisms matter in contemporary wildlife conservation

Given limited resources for wildlife conservation paired with an urgency to halt declines and rebuild populations, it is imperative that management actions are tactical and effective. Mechanisms are about how a system works and can inform threat identification and mitigation such that conservation actions that work can be identified. Here, we call for a more mechanistic approach to wildlife conservation and management where behavioral and physiological tools and knowledge are used to characterize drivers of decline, identify environmental thresholds, reveal strategies that would restore populations, and prioritize conservation actions. With a growing toolbox for doing mechanistic conservation research as well as a suite of decision-support tools (e.g., mechanistic models), the time is now to fully embrace the concept that mechanisms matter in conservation ensuring that management actions are tactical and focus on actions that have the potential to directly benefit and restore wildlife populations.

Allele-specific cis-regulatory methylation of the gene for vasoactive intestinal peptide in white-throated sparrows

White-throated sparrows (Zonotrichia albicollis) offer a unique opportunity to connect genotype with behavioral phenotype. In this species, a rearrangement of the second chromosome is linked with territorial aggression; birds with a copy of this "supergene" rearrangement are more aggressive than those without it. The supergene has captured the gene VIP, which encodes vasoactive intestinal peptide, a neuromodulator that drives aggression in other songbirds. In white-throated sparrows, VIP expression is higher in the anterior hypothalamus of birds with the supergene than those without it, and expression of VIP in this region predicts the level of territorial aggression regardless of genotype. Here, we aimed to identify epigenetic mechanisms that could contribute to differential expression of VIP both in breeding adults, which exhibit morph differences in territorial aggression, and in nestlings, before territorial behavior develops. We extracted and bisulfite-converted DNA from samples of the hypothalamus in wild-caught adults and nestlings and used high-throughput sequencing to measure DNA methylation of a region upstream of the VIP start site. We found that the allele inside the supergene was less methylated than the alternative allele in both adults and nestlings. The differential methylation was attributed primarily to CpG sites that were shared between the alleles, not to polymorphic sites, which suggests that epigenetic regulation is occurring independently of the genetic differentiation within the supergene. This work represents an initial step toward understanding how epigenetic differentiation inside chromosomal inversions leads to the development of alternative behavioral phenotypes.

Variations in cost of transport and their ecological consequences: a review

Movement is essential in the ecology of most animals, and it typically consumes a large proportion of individual energy budgets. Environmental conditions modulate the energetic cost of movement (cost of transport, COT), and there are pronounced differences in COT between individuals within species and across species. Differences in morphology affect COT, but the physiological mechanisms underlying variation in COT remain unresolved. Candidates include mitochondrial efficiency and the efficiency of muscle contraction-relaxation dynamics. Animals can offset increased COT behaviourally by adjusting movement rate and habitat selection. Here, we review the theory underlying COT and the impact of environmental changes on COT. Increasing temperatures, in particular, increase COT and its variability between individuals. Thermal acclimation and exercise can affect COT, but this is not consistent across taxa. Anthropogenic pollutants can increase COT, although few chemical pollutants have been investigated. Ecologically, COT may modify the allocation of energy to different fitness-related functions, and thereby influence fitness of individuals, and the dynamics of animal groups and communities. Future research should consider the effects of multiple stressors on COT, including a broader range of pollutants, the underlying mechanisms of COT and experimental quantifications of potential COT-induced allocation trade-offs.

Insect Epigenetic Mechanisms Facing Anthropogenic-Derived Contamination, an Overview

Currently, the human species has been recognized as the primary species responsible for Earth's biodiversity decline. Contamination by different chemical compounds, such as pesticides, is among the main causes of population decreases and species extinction. Insects are key for ecosystem maintenance; unfortunately, their populations are being drastically affected by human-derived disturbances. Pesticides, applied in agricultural and urban environments, are capable of polluting soil and water sources, reaching non-target organisms (native and introduced). Pesticides alter insect's development, physiology, and inheritance. Recently, a link between pesticide effects on insects and their epigenetic molecular mechanisms (EMMs) has been demonstrated. EMMs are capable of regulating gene expression without modifying genetic sequences, resulting in the expression of different stress responses as well as compensatory mechanisms. In this work, we review the main anthropogenic contaminants capable of affecting insect biology and of triggering EMMs. EMMs are involved in the development of several diseases in native insects affected by pesticides (e.g., anomalous teratogenic reactions). Additionally, EMMs also may allow for the survival of some species (mainly pests) under contamination-derived habitats; this may lead to biodiversity decline and further biotic homogenization. We illustrate these patterns by reviewing the effect of neonicotinoid insecticides, insect EMMs, and their ecological consequences.

The thermal environment as a moderator of social evolution

Animal sociality plays a crucial organisational role in evolution. As a result, understanding the factors that promote the emergence, maintenance, and diversification of animal societies is of great interest to biologists. Climate is among the foremost ecological factors implicated in evolutionary transitions in social organisation, but we are only beginning to unravel the possible mechanisms and specific climatic variables that underlie these associations. Ambient temperature is a key abiotic factor shaping the spatio-temporal distribution of individuals and has a particularly strong influence on behaviour. Whether such effects play a broader role in social evolution remains to be seen. In this review, we develop a conceptual framework for understanding how thermal effects integrate into pathways that mediate the opportunities, nature, and context of social interactions. We then implement this framework to discuss the capacity for temperature to initiate organisational changes across three broad categories of social evolution: group formation, group maintenance, and group elaboration. For each category, we focus on pivotal traits likely to have underpinned key social transitions and explore the potential for temperature to affect changes in these traits by leveraging empirical examples from the literature on thermal and behavioural ecology. Finally, we discuss research directions that should be prioritised to understand the potentially constructive and/or destructive effects of future warming on the origins, maintenance, and diversification of animal societies.

Alterations in gene expression due to chronic lead exposure induce behavioral changes

Lead (Pb) is a pollutant commonly found in the environment, despite the implementation of public health policies intended to remove it. Due to its chemical characteristics as a divalent ion, Pb interacts with cells, enzymes, and tissues, causing pathological, physical, and behavioral alterations. Recent biotechnological advances have helped us to understand the mechanisms underlying the damage caused by Pb in human populations and in experimental models, and new evidence on the epigenetic alterations caused by exposition to environmental Pb is available. It is known that Pb exposure impacts on behavior (causing aggressiveness, anxiety, and depression), leading to learning deficit and locomotor activity alterations, and its presence has been linked with the abnormal release of neurotransmitters and other biochemical changes involved in these disorders. Still, further reductionist studies are required to determine the effects of Pb exposure on DNA and protein expression and understand the processes underlying the diseases caused by Pb. This will also indicate possible therapeutic targets to offset the negative effects of the heavy metal. By elucidating the epigenetic changes involved, it would be possible to manipulate them and propose novel therapeutic approaches in this area. This review is aimed to provide an overview of studies that link Pb exposure to behavioral changes, as well as biochemical and epigenetic alterations at a neurotransmitter level, considering the importance of this metal in behavior abnormalities.

Inter- and intrasex habitat partitioning in the highly dimorphic southern elephant seal

Partitioning resources is a key mechanism for avoiding intraspecific competition and maximizing individual energy gain. However, in sexually dimorphic species it is difficult to discern if partitioning is due to competition or the different resource needs of morphologically distinct individuals. In the highly dimorphic southern elephant seal, there are intersexual differences in habitat use; at Iles Kerguelen, males predominantly use shelf waters, while females use deeper oceanic waters. There are equally marked intrasexual differences, with some males using the nearby Kerguelen Plateau, and others using the much more distant Antarctic continental shelf (~2,000 km away). We used this combination of inter and intrasexual behavior to test two hypotheses regarding habitat partitioning in highly dimorphic species. (a) that intersexual differences in habitat use will not appear until the seals diverge in body size and (b) that some habitats have higher rates of energy return than others. In particular, that the Antarctic shelf would provide higher energy returns than the Kerguelen Shelf, to offset the greater cost of travel. We quantified the habitat use of 187 southern elephant seals (102 adult females and 85 subadult males). The seals in the two groups were the same size (~2.4 m) removing the confounding effect of body size. We found that the intersexual differences in habitat use existed before the divergence in body size. Also, we found that the amount of energy gained was the same in all of the major habitats. This suggests that the use of shelf habitats by males is innate, and a trade-off between the need to access the large benthic prey available on shelf waters, against the higher risk of predation there. Intrasexual differences in habitat use are another trade-off; although there are fewer predators on the Antarctic shelf, it is subject to considerable interannual fluctuations in sea-ice extent. In contrast, the Kerguelen Plateau presents more consistent foraging opportunities, but contains higher levels of predation. Habitat partitioning in this highly dimorphic species is therefore the result of complex interplay of life history strategies, environmental conditions and predation pressure.

Genetic Determinism Exists for the Global DNA Methylation Rate in Sheep

Recent studies showed that epigenetic marks, including DNA methylation, influence production and adaptive traits in plants and animals. So far, most studies dealing with genetics and epigenetics considered DNA methylation sites independently. However, the genetic basis of the global DNA methylation rate (GDMR) remains unknown. The main objective of the present study was to investigate genetic determinism of GDMR in sheep. The experiment was conducted on 1,047 Romane sheep allocated into 10 half-sib families. After weaning, all the lambs were phenotyped for global GDMR in blood as well as for production and adaptive traits. GDMR was measured by LUminometric Methylation Analysis (LUMA) using a pyrosequencing approach. Association analyses were conducted on some of the lambs (n = 775) genotyped by using the Illumina OvineSNP50 BeadChip. Blood GDMR varied among the animals (average 70.7 ± 6.0%). Female lambs had significantly higher GDMR than male lambs. Inter-individual variability of blood GDMR had an additive genetic component and heritability was moderate (h² = 0.20 ± 0.05). No significant genetic correlation was found between GDMR and growth or carcass traits, birthcoat, or social behaviors. Association analyses revealed 28 QTLs associated with blood GDMR. Seven genomic regions on chromosomes 1, 5, 11, 17, 24, and 26 were of most interest due to either high significant associations with GDMR or to the relevance of genes located close to the QTLs. QTL effects were moderate. Genomic regions associated with GDMR harbored several genes not yet described as being involved in DNA methylation, but some are already known to play an active role in gene expression. In addition, some candidate genes, CHD1, NCO3A, KDM8, KAT7, and KAT6A have previously been described to be involved in epigenetic modifications. In conclusion, the results of the present study indicate that blood GDMR in domestic sheep is under polygenic influence and provide new insights into DNA methylation genetic determinism.

Using zebrafish (Danio rerio) models to understand the critical role of social interactions in mental health and wellbeing

Social behavior represents a beneficial interaction between conspecifics that is critical for maintaining health and wellbeing. Dysfunctional or poor social interaction are associated with increased risk of physical (e.g., vascular) and psychiatric disorders (e.g., anxiety, depression, and substance abuse). Although the impact of negative and positive social interactions is well-studied, their underlying mechanisms remain poorly understood. Zebrafish have well-characterized social behavior phenotypes, high genetic homology with humans, relative experimental simplicity and the potential for high-throughput screens. Here, we discuss the use of zebrafish as a candidate model organism for studying the fundamental mechanisms underlying social interactions, as well as potential impacts of social isolation on human health and wellbeing. Overall, the growing utility of zebrafish models may improve our understanding of how the presence and absence of social interactions can differentially modulate various molecular and physiological biomarkers, as well as a wide range of other behaviors.

Mapping the past, present and future research landscape of paternal effects

BACKGROUND

Although in all sexually reproducing organisms an individual has a mother and a father, non-genetic inheritance has been predominantly studied in mothers. Paternal effects have been far less frequently studied, until recently. In the last 5 years, research on environmentally induced paternal effects has grown rapidly in the number of publications and diversity of topics. Here, we provide an overview of this field using synthesis of evidence (systematic map) and influence (bibliometric analyses).

RESULTS

We find that motivations for studies into paternal effects are diverse. For example, from the ecological and evolutionary perspective, paternal effects are of interest as facilitators of response to environmental change and mediators of extended heredity. Medical researchers track how paternal pre-fertilization exposures to factors, such as diet or trauma, influence offspring health. Toxicologists look at the effects of toxins. We compare how these three research guilds design experiments in relation to objects of their studies: fathers, mothers and offspring. We highlight examples of research gaps, which, in turn, lead to future avenues of research.

CONCLUSIONS

The literature on paternal effects is large and disparate. Our study helps in fostering connections between areas of knowledge that develop in parallel, but which could benefit from the lateral transfer of concepts and methods.

Long-Term Impact of Social Isolation and Molecular Underpinnings

Prolonged periods of social isolation can have detrimental effects on the physiology and behavior of exposed individuals in humans and animal models. This involves complex molecular mechanisms across tissues in the body which remain partly identified. This review discusses the biology of social isolation and describes the acute and lasting effects of prolonged periods of social isolation with a focus on the molecular events leading to behavioral alterations. We highlight the role of epigenetic mechanisms and non-coding RNA in the control of gene expression as a response to social isolation, and the consequences for behavior. Considering the use of strict quarantine during epidemics, like currently with COVID-19, we provide a cautionary tale on the indiscriminate implementation of such form of social isolation and its potential damaging and lasting effects in mental health.

FKBP5: A Key Mediator of How Vertebrates Flexibly Cope with Adversity

Flexibility in the regulation of the hypothalamic–pituitary–adrenal (HPA) axis is an important mediator of stress resilience as it helps organisms adjust to, avoid, or compensate for acute and chronic challenges across changing environmental contexts. Glucocorticoids remain the favorite metric from medicine to conservation biology to attempt to quantify stress resilience despite the skepticism around their consistency in relation to individual health, welfare, and fitness. We suggest that a cochaperone molecule related to heat shock proteins and involved in glucocorticoid receptor activity, FKBP5, may mediate HPA flexibility and therefore stress resilience because it affects how individuals can regulate glucocorticoids and therefore capacitates their abilities to adjust phenotypes appropriately to prevailing, adverse conditions. Although the molecule is well studied in the biomedical literature, FKBP5 research in wild vertebrates is limited. In the present article, we highlight the potential major role of FKBP5 as mediator of HPA axis flexibility in response to adversity in humans and lab rodents.

The relevance of a rodent cohort in the Consortium on Individual Development

One of the features of the Consortium on Individual Development is the existence of a rodent cohort, in parallel with the human cohorts. Here we give an overview of the current status. We first elaborate on the choice of rat and mouse models mimicking early life adverse or beneficial conditions during development. We performed a systematic literature search on early life adversity and adult social behavior to address the status quo. Next, we describe the behavioral tasks we used and designed to examine behavioral control and social competence in rodents. The results so far indicate that manipulation of the environment in the first postnatal week only subtly affects social behavior. Stronger effects were seen in the model that targeted early adolescence; once adult, these rats are characterized by increased attention, a higher degree of impulsiveness and reduced social interest in peers. Many experiments in our rodent models with tightly controlled conditions were inspired by findings in human cohorts, and now allow in-depth mechanistic investigations. Vice versa, some of the findings in rodents are currently followed up by dedicated investigations in the human cohorts. This exemplifies the added value of animal investigations in a consortium encompassing primarily human developmental cohorts.

Epigenomics and gene regulation in mammalian social systems

Social epigenomics is a new field of research that studies how the social environment shapes the epigenome and how in turn the epigenome modulates behavior. We focus on describing known gene-environment interactions (GEIs) and epigenetic mechanisms in different mammalian social systems. To illustrate how epigenetic mechanisms integrate GEIs, we highlight examples where epigenetic mechanisms are associated with social behaviors and with their maintenance through neuroendocrine, locomotor, and metabolic responses. We discuss future research trajectories and open questions for the emerging field of social epigenomics in nonmodel and naturally occurring social systems. Finally, we outline the technological advances that aid the study of epigenetic mechanisms in the establishment of GEIs and vice versa.