DNA methylation differs extensively between strains of the same geographical origin and changes with age in Daphnia magna

Exploring the ageing methylome in the model insect, Nasonia vitripennis

BACKGROUND

The ageing process is a multifaceted phenomenon marked by the gradual deterioration of cellular and organismal functions, accompanied by an elevated susceptibility to diseases. The intricate interplay between genetic and environmental factors complicates research, particularly in complex mammalian models. In this context, simple invertebrate organisms have been pivotal, but the current models lack detectable DNA methylation limiting the exploration of this critical epigenetic ageing mechanism. This study introduces Nasonia vitripennis, the jewel wasp, as an innovative invertebrate model for investigating the epigenetics of ageing. Leveraging its advantages as a model organism and possessing a functional DNA methylation system, Nasonia emerges as a valuable addition to ageing research.

RESULTS

Whole-genome bisulfite sequencing unveiled dynamic alterations in DNA methylation, with differentially methylated CpGs between distinct time points in both male and female wasps. These changes were associated with numerous genes, enriching for functions related to telomere maintenance, histone methylation, and mRNA catabolic processes. Additionally, other CpGs were found to be variably methylated at each timepoint. Sex-specific effects on epigenetic entropy were observed, indicating differential patterns in the loss of epigenetic stability over time. Constructing an epigenetic clock containing 19 CpGs revealed a robust correlation between epigenetic age and chronological age.

CONCLUSIONS

Nasonia vitripennis emerges as a promising model for investigating the epigenetics of ageing, shedding light on the intricate dynamics of DNA methylation and their implications for age-related processes. This research not only expands the repertoire of ageing models but also opens avenues for deeper exploration of epigenetic mechanisms in the context of ageing.

Comparative analysis of animal lifespan

Comparative studies of aging are a promising approach to identifying general properties of and processes leading to aging. While to date, many comparative studies of aging in animals have focused on relatively narrow species groups, methodological innovations now allow for studies that include evolutionary distant species. However, comparative studies of aging across a wide range of species that have distinct life histories introduce additional challenges in experimental design. Here, we discuss these challenges, highlight the most pressing problems that need to be solved, and provide suggestions based on current approaches to successfully carry out comparative aging studies across the animal kingdom.

DNA methylation clocks for clawed frogs reveal evolutionary conservation of epigenetic aging

To address how conserved DNA methylation-based epigenetic aging is in diverse branches of the tree of life, we generated DNA methylation data from African clawed frogs (Xenopus laevis) and Western clawed frogs (Xenopus tropicalis) and built multiple epigenetic clocks. Dual species clocks were developed that apply to both humans and frogs (human-clawed frog clocks), supporting that epigenetic aging processes are evolutionary conserved outside mammals. Highly conserved positively age-related CpGs are located in neural-developmental genes such as uncx, tfap2d as well as nr4a2 implicated in age-associated disease. We conclude that signatures of epigenetic aging are evolutionary conserved between frogs and mammals and that the associated genes relate to neural processes, altogether opening opportunities to employ Xenopus as a model organism to study aging.

DNA methylation extends lifespan in the bumblebee Bombus terrestris

Epigenetic alterations are a primary hallmark of ageing. In mammals, age-related epigenetic changes alter gene expression profiles, disrupt cellular homeostasis and physiological functions and, therefore, promote ageing. It remains unclear whether ageing is also driven by epigenetic mechanisms in invertebrates. Here, we used a pharmacological hypomethylating agent (RG108) to evaluate the effects of DNA methylation (DNAme) on lifespan in an insect-the bumblebee Bombus terrestris. RG108 extended mean lifespan by 43% and induced the differential methylation of genes involved in hallmarks of ageing, including DNA damage repair and chromatin organization. Furthermore, the longevity gene sirt1 was overexpressed following the treatment. Functional experiments demonstrated that SIRT1 protein activity was positively associated with lifespan. Overall, our study indicates that epigenetic mechanisms are conserved regulators of lifespan in both vertebrates and invertebrates and provides new insights into how DNAme is involved in the ageing process in insects.

The hologenome of Daphnia magna reveals possible DNA methylation and microbiome-mediated evolution of the host genome

Properties that make organisms ideal laboratory models in developmental and medical research are often the ones that also make them less representative of wild relatives. The waterflea Daphnia magna is an exception, by both sharing many properties with established laboratory models and being a keystone species, a sentinel species for assessing water quality, an indicator of environmental change and an established ecotoxicology model. Yet, Daphnia's full potential has not been fully exploited because of the challenges associated with assembling and annotating its gene-rich genome. Here, we present the first hologenome of Daphnia magna, consisting of a chromosomal-level assembly of the D. magna genome and the draft assembly of its metagenome. By sequencing and mapping transcriptomes from exposures to environmental conditions and from developmental morphological landmarks, we expand the previously annotates gene set for this species. We also provide evidence for the potential role of gene-body DNA-methylation as a mutagen mediating genome evolution. For the first time, our study shows that the gut microbes provide resistance to commonly used antibiotics and virulence factors, potentially mediating Daphnia's environmental-driven rapid evolution. Key findings in this study improve our understanding of the contribution of DNA methylation and gut microbiota to genome evolution in response to rapidly changing environments.

Genome-wide DNA methylation patterns in bumble bee (Bombus vosnesenskii) populations from spatial-environmental range extremes

Unraveling molecular mechanisms of adaptation to complex environments is crucial to understanding tolerance of abiotic pressures and responses to climatic change. Epigenetic variation is increasingly recognized as a mechanism that can facilitate rapid responses to changing environmental cues. To investigate variation in genetic and epigenetic diversity at spatial and thermal extremes, we use whole genome and methylome sequencing to generate a high-resolution map of DNA methylation in the bumble bee Bombus vosnesenskii. We sample two populations representing spatial and environmental range extremes (a warm southern low-elevation site and a cold northern high-elevation site) previously shown to exhibit differences in thermal tolerance and determine positions in the genome that are consistently and variably methylated across samples. Bisulfite sequencing reveals methylation characteristics similar to other arthropods, with low global CpG methylation but high methylation concentrated in gene bodies and in genome regions with low nucleotide diversity. Differentially methylated sites (n = 2066) were largely hypomethylated in the northern high-elevation population but not related to local sequence differentiation. The concentration of methylated and differentially methylated sites in exons and putative promoter regions suggests a possible role in gene regulation, and this high-resolution analysis of intraspecific epigenetic variation in wild Bombus suggests that the function of methylation in niche adaptation would be worth further investigation.

Lipofuscin, amyloids, and lipid peroxidation as potential markers of aging in Daphnia

Accumulation of autofluorescent waste products, amyloids, and products of lipid peroxidation (LPO) are important hallmarks of aging. Until now, these processes have not been documented in Daphnia, a convenient model organism for longevity and senescence studies. We conducted a longitudinal cohort study of autofluorescence and Congo Red (CR) fluorescent staining for amyloids in four clones of D. magna. Additionally, we used a single time point cross-sectional common garden experiment within a single clone in which autofluorescence and BODIPY C11 fluorescence were measured. We observed a robust increase in autofluorescent spots that show diagnostic co-staining by Sudan Black indicating lipofuscin aggregates, particularly in the upper body region. There was also a significant clone-by-age interaction indicating that some genotypes accumulated lipofuscins faster than others. Contrary to predictions, CR fluorescence and lipid peroxidation did not consistently increase with age. CR fluorescence demonstrated a slight non-monotonous relationship with age, achieving the highest values at intermediate ages, possibly due to elimination of physiological heterogeneity in our genetically uniform cohorts. LPO demonstrated a significant ovary status-by-age interaction, decreasing with age when measured in Daphnia with full ovaries (late phase ovarian cycle) and showing no significant trend or slight increase with age when measured during the early phase in the ovarian cycle.

The maternal effects of dietary restriction on Dnmt expression and reproduction in two clones of Daphnia pulex

The inheritance of epigenetic marks induced by environmental variation in a previous generation is broadly accepted as a mediator of phenotypic plasticity. Transgenerational effects linking maternal experiences to changes in morphology, gene expression, and life history of successive generations are known across many taxa. While the number of studies linking epigenetic variation to ecological maternal effects is increasing rapidly, few if any attempts have been made to investigate molecular mechanisms governing epigenetic functions in the context of ecologically relevant maternal effects. Daphnia make an ideal model for investigating molecular epigenetic mechanisms and ecological maternal effects because they will reproduce asexually in the lab. Daphnia are also known to have strong maternal effects, involving a variety of traits and environmental variables. Using two clones of Daphnia pulex, we investigated the plasticity of life history and DNA methyltransferase (Dnmt) gene expression with respect to food limitation within and across generations. We found strong evidence of genotypic variation of responses of life history and Dnmt expression to low food diets, both within and across generations. In general, effects of offspring diet were larger than either the direct maternal effect or offspring-maternal environment interactions, but the direction of the maternal effect was usually in the opposite direction of the within-generation effect. For both life history and Dnmt expression, we also found that when offspring had low food, effects of the maternal environment were stronger than when offspring had high food.

Genetic and epigenetic regulation of growth, reproduction, disease resistance and stress responses in aquaculture

Major progress has been made with genomic and genetic studies in aquaculture in the last decade. However, research on epigenetic regulation of aquaculture traits is still at an early stage. It is apparent that most, if not all, aquaculture traits are regulated at both genetic and epigenetic levels. This paper reviews recent progress in understanding of genetic and epigenetic regulation of important aquaculture traits such as growth, reproduction, disease resistance, and stress responses. Although it is challenging to make generalized statements, DNA methylation is mostly correlated with down-regulation of gene expression, especially when at promoters and enhancers. As such, methylation of growth factors and their receptors is negatively correlated with growth; hypomethylation of genes important for stress tolerance is correlated with increased stress tolerance; hypomethylation of genes important for male or female sex differentiation leads to sex differentiation into males or females, respectively. It is apparent that environmental regulation of aquaculture traits is mediated at the level of epigenetic regulation, and such environment-induced epigenetic changes appeared to be intergenerationally inherited, but evidences for transgenerational inheritance are still limited.

Daphnia magna egg piRNA cluster expression profiles change as mothers age

BACKGROUND

PiRNAs prevent transposable elements wreaking havoc on the germline genome. Changes in piRNA expression over the lifetime of an individual may impact on ageing through continued suppression, or release, of transposable element expression. We identified piRNA producing clusters in the genome of Daphnia magna by a combination of bioinformatic methods, and then contrasted their expression between parthenogenetically produced eggs representing maternally-deposited germline piRNAs of young (having their 1st clutch) and old (having their 5th clutch) mothers. Results from eggs were compared to cluster expression in three generations of adults.

RESULTS

As for other arthropods, D. magna encodes long uni-directionally transcribed non-coding RNAs consisting of fragmented transposable elements which account for most piRNAs expressed. Egg tissues showed extensive differences between clutches from young mothers and those from old mothers, with 578 and 686 piRNA clusters upregulated, respectively. Most log fold-change differences for significant clusters were modest, however. When considering only highly expressed clusters, there was a bias towards 1st clutch eggs at 41 upregulated versus eight clusters in the eggs from older mothers. F0 generation differences between young and old mothers were fewer than eggs, as 179 clusters were up-regulated in young versus 170 old mothers. This dropped to 31 versus 22 piRNA clusters when comparing adults in the F1 generation, and no differences were detected in the F3 generation. Inter-generational losses of differential piRNA cluster were similar to that observed for D. magna micro-RNA expression.

CONCLUSIONS

Little overlap in differentially expressed clusters was found between adults containing mixed somatic and germline (ovary) tissues and germ-line representing eggs. A cluster encompassing a Tudor domain containing gene important in the piRNA pathway was upregulated in the eggs from old mothers. We hypothesise that regulation of this gene could form part of a feedback loop that reduces piRNA pathway activity explaining the reduced number of highly-expressed clusters in eggs from old mothers.

Potential differences in chitin synthesis ability cause different sensitivities to diflubenzuron among three strains of Daphnia magna

Ecotoxicity testing of crustaceans using Daphnia magna has been implemented in the chemical management systems of various countries. While the chemical sensitivity of D. magna varies depending on genetically different clonal lineages, the strain used in ecotoxicity tests, including the acute immobilization test (OECD TG202), has not been specified. We hypothesized that comprehensive gene expression profiles could provide useful information on phenotypic differences among strains, including chemical sensitivity. To test this hypothesis, we performed mRNA sequencing on three different strains (NIES, England, and Clone 5) of D. magna under culture conditions. The resulting expression profile of the NIES strain was clearly different compared to the profiles of the other two strains. Gene ontology (GO) enrichment analysis suggested that chitin metabolism was significantly enriched in the NIES strain compared to that in the England strain. Consistent with the GO analysis, evidence of high levels of chitin metabolism in the NIES strain were observed across multiple levels of biological organization, such as expression of chitin synthase genes, chitin content, and chitinase activity, which suggested that the different strains would exhibit different sensitivities to chemicals used to inhibit chitin synthesis. We found that among all strains, the NIES strain was more tolerant to diflubenzuron, a chitin synthesis inhibitor, with a 14-fold difference in the 48 h-EC₅₀ value for the acute immobilization test compared to the England strain. The present study demonstrates that the differences among strains in chitin metabolism may lead to sensitivity difference to diflubenzuron, and serves as a case study of the usefulness of comprehensive gene expression profiles in finding sensitivity differences.

Differential DNA methylation across environments has no effect on gene expression in the eastern oyster

It has been hypothesized that environmentally induced changes to gene body methylation could facilitate adaptive transgenerational responses to changing environments. We compared patterns of global gene expression (Tag-seq) and gene body methylation (reduced representation bisulfite sequencing) in 80 eastern oysters Crassostrea virginica from six full-sib families, common gardened for 14 months at two sites in the northern Gulf of Mexico that differed in mean salinity. At the time of sampling, oysters from the two sites differed in mass by 60% and in parasite loads by nearly two orders of magnitude. They also differentially expressed 35% of measured transcripts. However, we observed differential methylation at only 1.4% of potentially methylated loci in comparisons between individuals from these different environments, and little correspondence between differential methylation and differential gene expression. Instead, methylation patterns were largely driven by genetic differences among families, with a PERMANOVA analysis indicating nearly a two orders of magnitude greater number of genes differentially methylated between families than between environments. An analysis of CpG observed/expected values (CpG O/E) across the C. virginica genome showed a distinct bimodal distribution, with genes from the first cluster showing the lower CpG O/E values, greater methylation and higher and more stable gene expression, while genes from the second cluster showed lower methylation, and lower and more variable gene expression. Taken together, the differential methylation results suggest that only a small portion of the C. virginica genome is affected by environmentally induced changes in methylation. At this point, there is little evidence to suggest that environmentally induced methylation states would play a leading role in regulating gene expression responses to new environments.

DNA methylation differs extensively between strains of the same geographical origin and changes with age in Daphnia magna

BACKGROUND

Patterns of methylation influence lifespan, but methylation and lifespan may also depend on diet, or differ between genotypes. Prior to this study, interactions between diet and genotype have not been explored together to determine their influence on methylation. The invertebrate Daphnia magna is an excellent choice for testing the epigenetic response to the environment: parthenogenetic offspring are identical to their siblings (making for powerful genetic comparisons), they are relatively short lived and have well-characterised inter-strain life-history trait differences. We performed a survival analysis in response to caloric restriction and then undertook a 47-replicate experiment testing the DNA methylation response to ageing and caloric restriction of two strains of D. magna.

RESULTS

Methylated cytosines (CpGs) were most prevalent in exons two to five of gene bodies. One strain exhibited a significantly increased lifespan in response to caloric restriction, but there was no effect of food-level CpG methylation status. Inter-strain differences dominated the methylation experiment with over 15,000 differently methylated CpGs. One gene, Me31b, was hypermethylated extensively in one strain and is a key regulator of embryonic expression. Sixty-one CpGs were differentially methylated between young and old individuals, including multiple CpGs within the histone H3 gene, which were hypermethylated in old individuals. Across all age-related CpGs, we identified a set that are highly correlated with chronological age.

CONCLUSIONS

Methylated cytosines are concentrated in early exons of gene sequences indicative of a directed, non-random, process despite the low overall DNA methylation percentage in this species. We identify no effect of caloric restriction on DNA methylation, contrary to our previous results, and established impacts of caloric restriction on phenotype and gene expression. We propose our approach here is more robust in invertebrates given genome-wide CpG distributions. For both strain and ageing, a single gene emerges as differentially methylated that for each factor could have widespread phenotypic effects. Our data showed the potential for an epigenetic clock at a subset of age positions, which is exciting but requires confirmation.