Bisphenol A and 17α-ethinylestradiol-induced transgenerational differences in expression of osmoregulatory genes in the gill of medaka (Oryzias latipes)

Methylome profile of medaka eggs and sperm

Eggs and sperm are responsible for the continuation of generations. Following the epigenetic reprogramming of the embryo, core epigenetic information present in the sperm and eggs is transmitted to offspring somatic cells prior to the blastula stage, which specifically influences gene expression in the cells. Differences in the patterns of DNA methylation between the paternal and maternal genomes are critical to regulating allele-specific gene expression in the developing embryo, constituting the basis of genomic imprinting in mammals. While the information on allele-specific epigenetic information has been limited to mammals, it is not clearly understood whether non-mammalian vertebrate gametes possess any sex-specific allelic epigenetic information and whether somatic cells maintain the allele-specific epigenetic information, particularly DNA methylation. To determine the landscape of DNA methylation in paternal and maternal alleles in a non-mammalian vertebrate, we profiled the methylome of egg in medaka fish and compared it with our previously published medaka sperm methylome. We identified a set of gamete-specific differentially methylated regions (DMRs) in the genome- medaka eggs maintained a significantly lower global methylation profile than the sperm. Based on our sequencing depth and data, 10 DMRs were hypermethylated, and 237 DMRs were hypomethylated in the eggs compared to the sperm methylome. Somatic cells in blastula maintained some of those parental gamete-specific DNA methylation profiles. Those DMRs are associated with 70 genes, suggesting that they may have imprinted-like functions and warrant further investigation.

A second hit by PFOS exposure exacerbated developmental defects in medaka embryos with a history of ancestral BPA exposure

Bisphenol-A (BPA), a known endocrine-disrupting chemical (EDC) in plastics and resins, has been found to induce heritable health effects in fish and mammals, affecting directly exposed individuals and indirectly their progenies in subsequent generations. It is not clearly understood if subsequent generations of the BPA-exposed ancestors have increased sensitivity to the second hit by the chemicals of emerging concern. To understand this, the present study examined the effects of developmental exposure to perfluorooctanesulfonic acid (PFOS), which has been a global contaminant recently, in embryos whose ancestors were exposed to BPA. Two lineages of medaka (Oryzias latipes) were established: 1) the BPA lineage in which the F0 generation was exposed to 10 μg/L BPA during early development and 2) the control lineage with no BPA exposure in the F0 generation. These lineages were raised up to the F4 generation without further exposure. The embryos of the F4 generation were exposed to PFOS at 0, 0.002, 0.02, 0.2, 2, and 20 mg/L concentrations. Early developmental defects resulting in mortality, delayed hatching, teratogenic phenotypes, and altered gene expression were examined in both lineages. The expression level of genes encoding DNA methyltransferases and genes responsible for oxidative stress defense were determined. Following environmentally relevant PFOS exposure, organisms with a history of BPA exposure displayed significant changes in all categories of developmental defects mentioned above, including increased expression of genes related to oxidative stress, compared to individuals without BPA exposure. The present study provides initial evidence that a history of ancestral BPA exposure can alter sensitivity to developmental disorders following the second hit by PFOS exposure. The variable of ancestral BPA exposure could be considered in mechanistic, medical, and regulatory toxicology, and can also be applied to holistic environmental equity research.

Medaka liver developed Human NAFLD-NASH transcriptional signatures in response to ancestral bisphenol A exposure

The progression of fatty liver disease to non-alcoholic steatohepatitis (NASH) is a leading cause of death in humans. Lifestyles and environmental chemical exposures can increase the susceptibility of humans to NASH. In humans, the presence of bisphenol A (BPA) in urine is associated with fatty liver disease, but whether ancestral BPA exposure leads to the activation of human NAFLD-NASH-associated genes in the unexposed descendants is unclear. In this study, using medaka fish as an animal model for human NAFLD, we investigated the transcriptional signatures of human NAFLD-NASH and their associated roles in the pathogenesis of the liver of fish that were not directly exposed, but their ancestors were exposed to BPA during embryonic and perinatal development three generations prior. Comparison of bulk RNA-Seq data of the liver in BPA lineage male and female medaka with publicly available human NAFLD-NASH patient data revealed transgenerational alterations in the transcriptional signature of human NAFLD-NASH in medaka liver. Twenty percent of differentially expressed genes (DEGs) were upregulated in both human NAFLD patients and medaka. Specifically in females, among the total shared DEGs in the liver of BPA lineage fish and NAFLD patient groups, 27.69% were downregulated, and 20% were upregulated. Of all DEGs, 52.31% of DEGs were found in ancestral BPA-lineage females, suggesting that NAFLD in females shared the majority of human NAFLD gene networks. Pathway analysis revealed beta-oxidation, lipoprotein metabolism, and HDL/LDL-mediated transport processes linked to downregulated DEGs in BPA lineage males and females. In contrast, the expression of genes encoding lipogenesis-related proteins was significantly elevated in the liver of BPA lineage females only. BPA lineage females exhibiting activation of myc, atf4, xbp1, stat4, and cancerous pathways, as well as inactivation of igf1, suggest their possible association with an advanced NAFLD phenotype. The present results suggest that gene networks involved in the progression of human NAFLD and the transgenerational NAFLD in medaka are conserved and that medaka can be an excellent animal model to understand the development and progression of liver disease and environmental influences in the liver.

Reproductive toxic effects of chronic exposure to bisphenol A and its analogues in marine medaka (Oryzias melastigma)

As awareness of BPA's health risks has increased, many countries and regions have implemented strict controls on its use. Consequently, bisphenol analogues like BPF and BPAF are being increasingly used as substitutes. However, these compounds are also becoming increasingly prevalent in the environment due to production, use and disposal processes. The oceans act as a repository for various pollutants, and recent studies have revealed the extensive presence of bisphenols (BPs, including BPA, BPF, BPAF, etc.) in the marine environment, posing numerous health hazards to marine wildlife. Nevertheless, the reproductive toxicity of these chemicals on marine fish is not comprehensively comprehended yet. Thus, the histological features of the gonads and the gene expression profiles of HPG (Hypothalamic-Pituitary-Gonadal) axis-related genes in marine medaka (Oryzias melastigma) were studied after exposure to single and combined BPs for 70 days. The effects of each exposure group on spawning, embryo fertilization, and hatching in marine medaka were also assessed. Furthermore, the impacts of each exposure group on the genes related to methylation in the F2 and F3 generations were consistently investigated. BPs exposure was found to cause follicular atresia, irregular oocytes, and empty follicles in the ovary; but no significant lesions in the testis were observed. The expression of several HPG axis genes, including cyp19b, 17βhsd, 3βhsd, and fshr, resulted in significant changes compared to the control group. The quantity of eggs laid and fertilization rate decreased in all groups treated with BPs, with the BPAF-treated group showing a notable reduction in the number of eggs laid. Additionally, the hatching rate showed a more significant decline in the BPF-treated group. The analysis of methylated genes in the offspring of bisphenol-treated groups revealed significant changes in the expression of genes including amh, dnmt1, dnmt3ab, mbd2, and mecp2, indicating a potential transgenerational impact of bisphenols on phenotype through epigenetic modifications. Overall, the potential detrimental impact of bisphenol on the reproduction of marine medaka emphasizes the need for caution in considering the use of BPAF and BPF as substitutes.

Effects of salinity on behavior and reproductive toxicity of BPA in adult marine medaka

Salinity is an important environmental factor influencing the toxicity of chemicals. Bisphenol A (BPA) is an environmental endocrine disruptor with adverse effects on aquatic organisms, such as fish. However, the influence of salinity on the biotoxicity of BPA and the underlying mechanism are unclear. In this study, we exposed marine medaka (Oryzias melastigma) to BPA at different salinities (0 psμ, 15 psμ, and 30 psμ) for 70days to investigate the toxic effects. At 0 psμ salinity, BPA had an inhibitory effect on the swimming behavior of female medaka. At 15 psμ salinity, exposure to BPA resulted in necrotic cells in the ovaries but not on the spermatozoa. In addition, BPA exposure changed the transcript levels of genes related to the nervous system (gap43, elavl3, gfap, mbpa, and α-tubulin) and the hypothalamic-pituitary-gonadal (HPG) axis (fshr, lhr, star, arα, cyp11a, cyp17a1, cyp19a, and erα); the expression changes differed among salinity levels. These results suggest that salinity influences the adverse effects of BPA on the nervous system and reproductive system of medaka. These results emphasize the importance of considering the impact of environmental factors when carrying out ecological risk assessment of pollutants.

Sex-specific expression of the human NAFLD-NASH transcriptional signatures in the liver of medaka with a history of ancestral bisphenol A exposure

The progression of fatty liver disease to non-alcoholic steatohepatitis (NASH) is a leading cause of death in humans. Lifestyles and environmental chemical exposures can increase the susceptibility of humans to NASH. In humans, the presence of bisphenol A (BPA) in urine is associated with fatty liver disease, but whether ancestral BPA exposure leads to the activation of human NAFLD-NASH-associated genes in the unexposed descendants is unclear. In this study, using medaka fish as an animal model for human NAFLD, we investigated the transcriptional signatures of human NAFLD-NASH and their associated roles in the pathogenesis of the liver of fish who were not directly exposed but their ancestors were exposed to BPA during embryonic and perinatal development three generations prior. Comparison of bulk RNA-Seq data of the liver in BPA lineage male and female medaka with publicly available human NAFLD-NASH patient data revealed transgenerational alterations in the transcriptional signature of human NAFLD-NASH in medaka liver. Twenty percent of differentially expressed genes (DEGs) were upregulated in both human NAFLD patients and medaka. Specifically in females, among the total shared DEGs in the liver of BPA lineage fish and NAFLD patient groups, 27.69% DEGs were downregulated and 20% DEGs were upregulated. Off all DEGs, 52.31% DEGs were found in ancestral BPA-lineage females, suggesting that NAFLD in females shared majority of human NAFLD gene networks. Pathway analysis revealed beta-oxidation, lipoprotein metabolism, and HDL/LDL-mediated transport processes linked to downregulated DEGs in BPA lineage males and females. In contrast, the expression of genes encoding lipogenesis-related proteins was significantly elevated in the liver of BPA lineage females only. BPA lineage females exhibiting activation of myc, atf4, xbp1, stat4, and cancerous pathways, as well as inactivation of igf1, suggest their possible association with an advanced NAFLD phenotype. The present results suggest that gene networks involved in the progression of human NAFLD and the transgenerational NAFLD in medaka are conserved and that medaka can be an excellent animal model to understand the development and progression of liver disease and environmental influences in the liver.

Effects of Acute and Developmental Exposure to Bisphenol S on Chinese Medaka (Oryzias sinensis)

Bisphenol S (BPS), one of the substitutes for bisphenol A (BPA), is widely used in various commodities. The BPS concentrations in surface water have gradually increased in recent years, making it a predominant bisphenol analogue in the aquatic environment and raising concerns about its health and ecological effects on aquatic organisms. For this study, we conducted a 96 h acute toxicity test and a 15-day developmental exposure test to assess the adverse effects of BPS exposure in Chinese medaka (Oryzias sinensis), a new local aquatic animal model. The results indicate that the acute exposure of Chinese medaka embryos to BPS led to relatively low toxicity. However, developmental exposure to BPS was found to cause developmental abnormalities, such as decreased hatching rate and body length, at 15 dpf. A transcriptome analysis showed that exposure to different concentrations of bisphenol S often induced different reactions. In summary, environmental concentrations of BPS can have adverse effects on the hatching and physical development of Chinese medaka, and further attention needs to be paid to the potential toxicity of environmental BPS.

The PCOS-NAFLD Multidisease Phenotype Occurred in Medaka Fish Four Generations after the Removal of Bisphenol A Exposure

As a heterogeneous reproductive disorder, polycystic ovary syndrome (PCOS) can be caused by genetic, diet, and environmental factors. Bisphenol A (BPA) can induce PCOS and nonalcoholic fatty liver disease (NAFLD) due to direct exposure; however, whether these phenotypes persist in future unexposed generations is not currently understood. In a previous study, we observed that transgenerational NAFLD persisted in female medaka for five generations (F4) after exposure to an environmentally relevant concentration (10 μg/L) of BPA. Here, we demonstrate PCOS in the same F4 generation female medaka that developed NAFLD. The ovaries contained immature follicles, restricted follicular progression, and degenerated follicles, which are characteristics of PCOS. Untargeted metabolomic analysis revealed 17 biomarkers in the ovary of BPA lineage fish, whereas transcriptomic analysis revealed 292 genes abnormally expressed, which were similar to human patients with PCOS. Metabolomic-transcriptomic joint pathway analysis revealed activation of the cancerous pathway, arginine-proline metabolism, insulin signaling, AMPK, and HOTAIR regulatory pathways, as well as upstream regulators esr1 and tgf signaling in the ovary. The present results suggest that ancestral BPA exposure can lead to PCOS phenotypes in the subsequent unexposed generations and warrant further investigations into potential health risks in future generations caused by initial exposure to EDCs.

Environmental occurrence and ecotoxicity of aquaculture-derived plastic leachates

Plastic products such as fishing nets and foam buoys have been widely used in aquaculture. To enhance the desirable characteristics of the final equipment, plastic gear for aquaculture is mixed with a wide range of additives. Recent studies have shown that additives could be leached out to the environment with a long-term use of aquaculture plastics, forming aquaculture-derived plastic leachates. It should be emphasized that some leachates such as phthalic acid esters (PAEs) and organophosphate esters (OPEs) are endocrine disruptors, which could increase the exposure risk of aquatic products and subsequently display potential threats to human health via food chain. However, systematic studies on the release, occurrence, bioaccumulation, and toxic effects of aquaculture-derived plastic leachates are missing, overlooking their potential sources and ecotoxicological risks in aquatic environments. We have reviewed and compared the concentrations of major plastic leachates in the water environment and organisms of global aquaculture and non-farmed areas, confirming that aquaculture leachate is an important source of contaminants in the environment. Moreover, the toxic effects of aquaculture-derived plastic additives and the related mechanisms are summarized with fish as a representative, revealing their potential health risk. In addition, we proposed current challenges and future research needs, which provides scientific guidance for the use and management of plastic products in aquaculture industries.

Transcriptional Alterations Induced by Delta-9 Tetrahydrocannabinol in the Brain and Gonads of Adult Medaka

With the legalization of marijuana smoking in several states of the United States and many other countries for medicinal and recreational use, the possibility of its release into the environment cannot be overruled. Currently, the environmental levels of marijuana metabolites are not monitored on a regular basis, and their stability in the environment is not well understood. Laboratory studies have linked delta 9-tetrahydrocannabinol (Δ⁹-THC) exposure with behavioral abnormalities in some fish species; however, their effects on endocrine organs are less understood. To understand the effects of THC on the brain and gonads, we exposed adult medaka (Oryzias latipes, Hd-rR strain, both male and female) to 50 ug/L THC for 21 days spanning their complete spermatogenic and oogenic cycles. We examined transcriptional responses of the brain and gonads (testis and ovary) to Δ⁹-THC, particularly molecular pathways associated with behavioral and reproductive functions. The Δ⁹-THC effects were more profound in males than females. The Δ⁹-THC-induced differential expression pattern of genes in the brain of the male fish suggested pathways to neurodegenerative diseases and pathways to reproductive impairment in the testis. The present results provide insights into endocrine disruption in aquatic organisms due to environmental cannabinoid compounds.

Ancestral BPA exposure caused defects in the liver of medaka for four generations

Environmental chemicals can induce liver defects in experimental animals due to their direct and acute exposure. It is not clear whether environmental chemical exposures result in the transgenerational passage of liver defects in subsequent generations living in an uncontaminated environment. Bisphenol A (BPA), a plasticizer chemical, has been ubiquitous in the environment in the recent decade. Every organism is exposed to this chemical at some point during its lifetime. Literature suggests that direct BPA exposure can result in several metabolic diseases, including non-alcoholic fatty liver disease (NAFLD). Despite the phasing out of BPA from several consumer goods, it is unclear whether ancestral BPA exposure causes liver health problems in the unexposed future generations. Here, we demonstrate an advanced stage of NAFLD in the grandchildren (F2 generation) of medaka fish (Oryzias latipes) due to embryonic BPA exposure in the grandparental generation (F0), which persists for five generations (F4) even in the absence of BPA. The severity of transgenerational NAFLD phenotype included steatosis together with perisinusoidal fibrosis and apoptosis of hepatocytes. Adult females developed more severe histopathological conditions in the liver than males. Genes encoding enzymes involved in lipolytic pathways were significantly decreased. The present results suggest that ancestral BPA exposure can result in transgenerational metabolic diseases that can persist for five generations and that the NAFLD trait is sexually dimorphic. Given that ancestral BPA exposure can lead to altered metabolic health outcomes in the subsequent unexposed generations, the development of the methods and strategies to mitigate the transgenerational onset of metabolic diseases seem imperative to protect future generations.

Potassium perchlorate effects on primordial germ cells of developing medaka larvae

Perchlorate is a chemical compound commonly used in military artillery and equipment. It has been detected in drinking water, air, soil, and breast milk. Exposure of humans to perchlorate can occur in the theater of war and areas adjacent to military training grounds. A high concentration of perchlorate has been found to affect reproduction in vertebrates, including fish. However, whether environmental concentrations of perchlorate can affect primordial germ cells (PGCs), the founders of sperm and eggs, is not clearly understood. In the present study, we examined the effects of 0, 10, 100, and 1000 μg/L potassium perchlorate exposure on the embryonic development of medaka and their PGCs. Perchlorate exposure delayed hatching time, reduced heartbeat, inhibited migration of PGCs, and increased developmental deformities in the larvae. The 10 and 20 mg/L concentrations of perchlorate were lethal to embryos, whereas vitamin C co-treatment (1 mg/L) completely blocked perchlorate-induced mortality. RNA-seq analysis of isolated PGCs showed a non-linear pattern in expression profiles of differentially altered genes. Significantly upregulated genes were found in PGCs from the 10 and 1000 μg/L groups, whereas the 100 μg/L groups showed the highest number of significantly downregulated genes. Gene ontology analysis predicted differentially expressed genes to be involved in proteolysis, metabolic processes, peptides activity, hydrolase activity, and hormone activity. Among the cellular components, extracellular, intracellular, sarcoplasmic, and 6-phosphofructokinase and membrane-bounded processes were affected. Ingenuity Pathway Analysis of PGC transcriptomes revealed thyroid hormone signaling to be affected by all concentrations of perchlorate. The present results suggested that perchlorate affected the development of medaka larvae and vitamin C was able to ameliorate perchlorate-induced embryo mortality. Additionally, perchlorate altered the global transcriptional network in PGCs in a non-linear fashion suggesting its potential effects on developing germ cells and fertility.

Bisphenol A and 17α-ethinylestradiol-induced transgenerational gene expression differences in the brain-pituitary-testis axis of medaka, Oryzias latipes†

Endocrine disrupting chemicals (EDCs), such as bisphenol A (BPA) and 17α-ethinylestradiol (EE2), can have far reaching health effects, including transgenerational abnormalities in offspring that never directly contacted either chemical. We previously reported reduced fertilization rates and embryo survival at F2 and F3 generations caused by 7-day embryonic exposure (F0) to 100 μg/L BPA or 0.05 μg/L EE2 in medaka. Crossbreeding of fish in F2 generation indicated subfertility in males. To further understand the mechanisms underlying BPA or EE2-induced adult onset and transgenerational reproductive defects in males, the present study examined the expression of genes regulating the brain-pituitary-testis (BPT) axis in the same F0 and F2 generation male medaka. Embryonic exposure to BPA or EE2 led to hyperactivation of brain and pituitary genes, which are actively involved in reproduction in adulthood of the F0 generation male fish, and some of these F0 effects continued to the F2 generation (transgenerational effects). Particularly, the F2 generation inherited the hyperactivated state of expression for kisspeptin (kiss1 and kiss2) and their receptors (kiss1r and kiss2r), and gnrh and gnrh receptors. At F2 generation, expression of DNA methyltransferase 1 (dnmt1) decreased in brain of the BPA treatment lineage, while EE2 treatment lineage showed increased dnmt3bb expression. Global hypomethylation pattern was observed in the testis of both F0 and F2 generation fish. Taken together, these results demonstrated that BPA or EE2-induced transgenerational reproductive impairment in the F2 generation was associated with alterations of reproductive gene expression in brain and testis and global DNA methylation in testis.

Distinct expression patterns of seven crucial microRNAs during early embryonic development in medaka (Oryzias latipes)

MicroRNAs (i.e. miRNAs) are small non-coding RNAs that play essential modulation roles in embryonic development in vertebrates. Paternal and maternal miRNAs contribute to the development of post-fertilization embryo and zygotic genome activation. The pattern of expression and their roles in embryonic development of medaka are not clearly understood. The present study, therefore, examined a temporal expression of seven miRNAs, ola-let-7a, ola-miR-202-3p, ola-miR-126-3p, ola-miR-122, ola-miR-92a, ola-miR-125a-3p and ola-miR-430a in sperm, oocytes, and embryos during early developmental stages. Three unique expression patterns of miRNAs were observed. ola-let7a, ola-miR-202-3p and ola-miR-126-3p showed both paternal and maternal expression, and ola-miR-122, ola-miR-92a, ola-miR-125a-3p showed maternal expression only. The expression of six out of seven miRNAs significantly decreased after maternal-zygotic transition (MZT), whereas ola-miR-430a expression initiated only after MZT. The temporal dynamic expression of these miRNAs suggests their potential roles in early embryogenesis and genome-zygotic activation in medaka.

Transcriptome analysis of testis reveals the effects of developmental exposure to bisphenol a or 17α-ethinylestradiol in medaka (Oryzias latipes)

Endocrine disrupting chemicals (EDCs) can induce abnormalities in organisms via alteration of molecular pathways and subsequent disruption of endocrine functions. Bisphenol A (BPA) and 17α-ethinylestradiol (EE2) are ubiquitous EDCs in the environment. Many aquatic organisms, including fish, are often exposed to varying concentrations of BPA and EE2 throughout their lifespan. Both BPA and EE2 can activate estrogenic signaling pathways and cause adverse effects on reproduction via alteration of pathways associated with steroidogenesis. However, transcriptional pathways that are affected by chronic exposure to these two ubiquitous environmental estrogens during embryonic, larval, and juvenile stages are not clearly understood. In the present study, we examined transcriptional alterations in the testis of medaka fish (Oryzias latipes) chronically exposed to a low concentration of BPA or EE2. Medaka were exposed to BPA (10 μg/L) or EE2 (0.01 μg/L) from 8 h post-fertilization (as embryos) to adulthood 50 days post fertilization (dpf), and transcriptional alterations in the testis were examined by RNA sequencing (RNA-seq). Transcriptomic profiling revealed 651 differentially expressed genes (DEGs) between BPA-exposed and control testes, while 1475 DEGs were found between EE2-exposed and control testes. Gene ontology (GO) analysis showed a significant enrichment of "intracellular receptor signaling pathway", "response to steroid hormone" and "hormone-mediated signaling pathway" in the BPA-induced DEGs, and of "cilium organization", "microtubule-based process" and "organelle assembly" in the EE2-induced DEGs. Pathway analysis showed significant enrichment of "integrin signaling pathway" in both treatment groups, and of "cadherin signaling pathway", "Alzheimer disease-presenilin pathway" in EE2-induced DEGs. Single nucleotide polymorphism (SNP) and insertion-deletion (Indel) analysis found no significant differences in mutation rates with either BPA or EE2 treatments. Taken together, global gene expression differences in testes of medaka during early stages of gametogenesis were responsive to chronic BPA and EE2 exposure.

The dynamics of DNA methylation during epigenetic reprogramming of primordial germ cells in medaka (Oryzias latipes)

Primordial germ cells (PGCs) are precursors of eggs and sperm. How the PGCs epigenetically reprogram during early embryonic development in fish is currently unknown. Here we generated a series of PGC methylomes using whole genome bisulfite sequencing across key stages from 8 days post fertilization (dpf) to 25 dpf coinciding with germ cell sex determination and gonadal sex differentiation in medaka (Oryzias latipes) to elucidate the dynamics of DNA methylation during epigenetic reprogramming in germ cells. Our high-resolution DNA methylome maps show a global demethylation taking place in medaka PGCs in a two-step strategy. The first step occurs between the blastula and 8-dpf stages, and the second step occurs between the 10-dpf and 12-dpf stages. Both demethylation processes are global, except for CGI promoters which remain hypomethylated throughout the stage of PGC specification. De novo methylation proceeded at 25-dpf stage with the process in male germ cells superseding female germ cells. Gene expression analysis showed that tet2 maintains high levels of expression during the demethylation stage, while dnmt3ba expression increases during the de novo methylation stage during sexual fate determination in germ cells. The present results suggest that medaka PGCs undergo a bi-phasic epigenetic reprogramming process. Global erasure of DNA methylation marks peaks at 15-dpf and de novo methylation in male germ cells takes precedence over female germ cells at 25 dpf. Results also provide important insights into the developmental window of susceptibility to environmental stressors for multi- and trans-generational health outcomes in fish.

DNA methylation reprogramming in medaka fish, a promising animal model for environmental epigenetics research

DNA methylation is a major epigenetic modification that undergoes dramatic changes in two epigenetic reprogramming windows during development: first in preimplantation embryos and second in primordial germ cell (PGC) specification. In both windows, DNA methylation patterns are reprogrammed genome-wide, and the majority of inherited methylation marks are erased, generating cells with broad developmental potential. Recent studies reported that the reprogramming of genome methylation in medaka is similar to human and mouse, suggesting that medaka may serve as a suitable biomedical model for comparative studies focused on the epigenetic and transgenerational inheritance of phenotypic traits. In this mini review, we will discuss how somatic and germ cells in post-fertilization stage embryos are epigenetically reprogrammed in mammals and fishes with a particular focus on DNA methylation dynamics.