Whole spectrum of cytochrome P450 genes and molecular responses to water-accommodated fractions exposure in the marine medaka

Genome sequencing, comparative analysis, and gene expression responses of cytochrome P450 genes in Oryzias curvinotus provide insights into environmental adaptation

The mangrove fish (Oryzias curvinotus) serves as a model for researching environmental adaptation and sexual development. To further such research, we sequenced and assembled a high-quality 842 Mb reference genome for O. curvinotus. Comparative genomic analysis revealed 891 expanded gene families, including significantly expanded cytochrome P450 (CYP) detoxification genes known to be involved in xenobiotic defense. We identified 69 O. curvinotus CYPs (OcuCYPs) across 18 families and 10 clans using multiple methods. Extensive RNA-seq and qPCR analysis demonstrated diverse spatiotemporal expression patterns of OcuCYPs by developmental stage, tissue type, sex, and pollutant exposure (17β-estradiol (E2) and testosterone (MT)). Many OcuCYPs exhibited sexual dimorphism in gonads, suggesting reproductive roles in steroidogenesis, while their responsiveness to model toxicants indicates their importance in environmental adaptation through enhanced detoxification. Pathway analysis highlighted expanded CYP genes in arachidonic acid metabolism, drug metabolism, and steroid hormone biosynthesis. This chromosome-level genomic resource provides crucial biological insights to elucidate the functional roles of expanded CYPs in environmental adaptation, sexual development, early life history, and conservation in the anthropogenically impacted mangrove habitats of O. curvinotus. It also enables future ecotoxicology research leveraging O. curvinotus as a pollution sentinel species.

Transcriptome analysis of Sparidentex hasta larvae exposed to water-accommodated fraction of Kuwait crude oil

Anthropogenic activities have been shown to significantly affect marine life. Water pollution and oil spills are particularly deleterious to the fish population, especially during their larval stage. In this study, Sobaity-sea bream Sparidentex hasta (Valenciennes, 1830) larvae were exposed to serial dilutions of water-accommodated fraction of Kuwait crude oil (KCO-WAF) for varying durations (3, 6, 24, 48, 72 or 96 h) in acute exposure regime. Gene expression was assessed using RNA sequencing and validated through RT-qPCR. The RNA sequencing data were aligned to the sequenced genome, and differentially expressed genes were identified in response to treatment with or without KCO-WAF at various exposure times. The highest number of differentially expressed genes was observed at the early time point of 6 h of post-exposure to KCO-WAF. The lowest number of differentially expressed genes were noticed at 96 h of treatment indicating early response of the larvae to KCO-WAF contaminant. The acquired information on the differentially expressed genes was then used for functional and pathway analysis. More than 90% of the differentially expressed genes had a significant BLAST match, with the two most common matching species being Acanthopagrus latus and Sparus aurata. Approximately 65% of the differentially expressed genes had Gene Ontology annotations, whereas > 35% of the genes had KEGG pathway annotations. The differentially expressed genes were found to be enriched for various signaling pathways (e.g., MAPK, cAMP, PI3K-Akt) and nervous system-related pathways (e.g., neurodegeneration, axon guidance, glutamatergic synapse, GABAergic synapse). Early exposure modulated the signaling pathways, while KCO-WAF exposure of larvae for a longer duration affected the neurodegenerative/nervous system-related pathways. RT-qPCR analysis confirmed the differential expression of genes at each time point. These findings provide insights into the underlying molecular mechanisms of the deleterious effects of acute exposure to oil pollution-on marine fish populations, particularly at the early larval stage of Sparidentex hasta.

Cytochrome P450 superfamily in spotted sea bass: Genome-wide identification and expression profiles under trichlorfon and environmental stresses

Cytochrome P450s (CYPs), as one of the most diverse enzyme superfamilies in nature, play critical functions in antioxidant reactions against endogenous and exogenous compounds. In this study, we performed genome-wide characterization of CYP superfamily members and analyzed their expression patterns under several abiotic stresses in spotted sea bass, which is known as an economically important fish species in the Chinese aquaculture industry. A total of 55 CYP genes were identified and divided into 17 families within 10 clans. The analysis of phylogeny, gene structure, and syntenic relationships provided evidence for the evolution of CYP genes and confirmed their annotation and orthology. The expression of CYP genes was examined in the liver during trichlorfon stress using quantitative real-time PCR. The results showed that 20 tested CYP genes displayed significant mRNA expression changes, indicating that they may play crucial roles in the metabolism of trichlorfon and can be potential biomarkers for trichlorfon pollution. Moreover, by screening transcriptomic databases, 10, 3 and 19 CYP genes exhibited differential expression patterns in response to hypoxia, alkalinity and heat stress, respectively. Taken together, this study provided insights into the regulation of CYP genes by toxicological and environmental stresses, laid basis for extensive functional studies of the CYP superfamily in spotted sea bass and other teleost species.

Enantiospecific Uptake and Depuration Kinetics of Chiral Metoprolol and Venlafaxine in Marine Medaka (Oryzias melastigma): Tissue Distribution and Metabolite Formation

The increasing use of chiral pharmaceuticals has led to their widespread presence in the environment. However, their toxicokinetics have rarely been reported. Therefore, the tissue-specific uptake and depuration kinetics of two pairs of pharmaceutical enantiomers, S-(-)-metoprolol versus R-(+)-metoprolol and S-(+)-venlafaxine versus R-(-)-venlafaxine, were studied in marine medaka (Oryzias melastigma) during a 28-day exposure and 14-day clearance period. The toxicokinetics of the studied pharmaceuticals, including uptake and depuration rate constants, depuration half-life (t_1/2), and bioconcentration factor (BCF), were reported for the first time. The whole-fish results demonstrated a higher S- than R-venlafaxine bioaccumulation potential, whereas no significant difference was observed between S- and R-metoprolol. O-desmethyl-metoprolol (ODM) and α-hydroxy-metoprolol (AHM) were the main metoprolol metabolites identified by suspect screening, and the ratios of ODM to AHM were 3.08 and 1.35 for S- and R-metoprolol, respectively. N,O-Didesmethyl-venlafaxine (NODDV) and N-desmethyl-venlafaxine (NDV) were the main venlafaxine metabolites, and the ratios of NODDV to NDV were 1.55 and 0.73 for S- and R-venlafaxine, respectively. The highest tissue-specific BCFs of the four enantiomers were all found in the eyes, meriting in-depth investigation.

Environmentally relevant concentrations of microplastics modulated the immune response and swimming activity, and impaired the development of marine medaka Oryzias melastigma larvae

Microplastics (MPs), due to their impacts on the ecosystem and their integration into the food web either through trophic transfer or ingestion directly from the ambient environment, are an emerging class of environmental contaminants posing a great threat to marine organisms. Most reports on the toxic effects of MPs exclusively focus on bioaccumulation, oxidative stress, pathological damage, and metabolic disturbance in fish. However, the collected information on fish immunity in response to MPs is poorly defined. In particular, little is known regarding mucosal immunity and the role of mucins. In this study, marine medaka (Oryzias melastigma) larvae were exposed to 6.0 µm beads of polystyrene microplastics (PS-MPs) at three environmentally relevant concentrations (10² particles/L, 10⁴ particles/L, and 10⁶ particles/L) for 14 days. The experiment was carried out to explore the developmental and behavioural indices, the transcriptional profiles of mucins, pro-inflammatory, immune, metabolism and antioxidant responses related genes, as well as the accumulation of PS-MPs in larvae. The results revealed that PS-MPs were observed in the gastrointestinal tract, with a concentration- and exposure time-dependent manner. No significant difference in the larval mortality was found between the treatment groups and the control, whereas the body length of larvae demonstrated a significant reduction at 10⁶ particles/L on 14 days post-hatching. The swimming behaviour of the larvae became hyperactive under exposure to 10⁴ and 10⁶ particles/L PS-MPs. In addition, PS-MP exposure significantly up-regulated the mucin gene transcriptional levels of muc7-like and muc13-like, however down-regulated the mucin gene expression levels of heg1, muc2, muc5AC-like and muc13. The immune- and inflammation and metabolism-relevant genes (jak, stat-3, il-6, il-1β, tnf-а, ccl-11, nf-κb, and sod) were significantly induced by PS-MPs at 10⁴ and 10⁶ particles/L compared to the control. Taken together, this study suggests that PS-MPs induced inflammation response and might obstruct the immune functions and retarded the growth of the marine medaka larvae even at environmentally relevant concentrations.

Enantioselective toxic effects of mefentrifluconazole in the liver of adult zebrafish (Danio rerio) based on transcription level and metabolomic profile

Mefentrifluconazole, a new type of chiral triazole fungicide, is widely applied to control a variety of fungal diseases in crops. However, the toxicological effects of mefentrifluconazole on aquatic organisms are unknown, especially at the enantiomer level. In the present study, zebrafish were selected as a typical model for mefentrifluconazole enantiomer exposure. Metabolomic and transcription analyses were performed with 0.01 and 0.10 mg/L mefentrifluconazole and its enantiomers (i.e., rac-mfz/(-)-mfz/(+)-mfz) at 28 days. The ¹H nuclear magnetic resonance (NMR)-based metabolomics analysis showed that 9, 10 and 4 metabolites were changed significantly in the rac-mfz, (+)-mfz and (-)-mfz treatment groups compared with the control group, respectively. The differential metabolites were related to energy metabolism, lipid metabolism and amino acid metabolism. The qRT-PCR analysis revealed that the expression of lipid metabolism-, apoptosis- and CYP-related genes in the livers of female zebrafish in rac-mfz and (+)-mfz was 1.61-108.92 times and 2.37-551.34 times higher than that in (-)-mfz, respectively. The results above indicate that exposure to mefentrifluconazole induced enantioselective liver toxicity in zebrafish. Our study underlined the importance of distinguishing different enantiomers, which will contribute to environmental protection.

Application of a biological multilevel response approach in the copepod Acartia tonsa for toxicity testing of three oil Water Accommodated Fractions

Copepods play a critical role in the marine food webs, being a food source for marine organisms. In this study, we investigated the toxic effects of Water Accommodated Fractions (WAFs) from three types of oil: Naphthenic North Sea crude oil (NNS), Intermediate Fuel Oil (IFO 180) and a commercial Marine Gas Oil (MGO). The WAFs were prepared at 10 °C and 30 PSU (practical salinity unit), and tested on the marine copepod Acartia tonsa at different endpoints and at different levels of biological organization. We determined the median lethal concentrations after 96 h (LC50) and reproduction capabilities were calculated in adult females following seven days of exposure to sublethal WAF doses. The total lipid content was measured in reproductive females using Nile red lipophilic dye after 96 h of WAF exposure. We also measured the transcription levels of genes involved in antioxidant response and xenobiotic biotransformation after short exposure for 48 h. High doses (7% WAF) of MGO affected survival, percentage of fecund females, egg hatching success, and total lipid content. The IFO 180 WAF affected, at medium (20%) and high (40%) doses, the number of fecund females, mortality and produced significant effects on gene expression levels. In conclusion, toxicity assays showed that the WAFs prepared from refined oils were more toxic than crude oil WAF to Acartia tonsa.

The genome of the marine water flea Diaphanosoma celebensis: Identification of phase I, II, and III detoxification genes and potential applications in marine molecular ecotoxicology

To assemble the genome of the marine water flea Diaphanosoma celebensis, a sentinel model for marine environmental monitoring, we constructed a high-quality genome using PromethION and HiSeq 2500 platforms. The total length of the assembled genome was 100.08 Mb, with N50 = 2.56 Mb (benchmarking universal single-copy orthologs, 96.9%) and consisted of 179 scaffolds. A total of 15,427 genes were annotated, and orthologous gene clusters in D. celebensis were analyzed and compared with those of the cladocerans Daphnia magna and Daphnia pulex. In addition, phase I, II, and III detoxification gene families of cytochrome P450s, glutathione S-transferases, and ATP-binding cassette were fully identified and revealed lineage-specific gene loss and/or expansion, suggesting that the evolution of detoxification gene families likely modulates fitness and susceptibility in response to environmental stressors. The study improves our understanding of the detoxification-related gene system and should contribute to future studies of molecular ecotoxicology in cladoceran species and their responses to emerging pollutants.

Development and Evaluation of Olive Flounder cyp1a1-Luciferase Assay for Effective Detection of CYP1A-Inducing Contaminants in Coastal Sediments

Flounders have been widely used as indicator species for monitoring the benthic environment of marine coastal regions owing to their habitat and feeding preferences in or on sandy sediments. Here, a single-step, sensitive, specific, and simple luciferase assay was developed, using the olive flounder cyp1a1 gene, for effective detection of CYP1A-inducing contaminants in coastal sediments. The developed cyp1a1-luciferase assay was highly sensitive to the widely used CYP1A inducers 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), benzo[a]pyrene (B[a]P), and 3,3',4,4',5-pentachlorobiphenyl (PCB 126). In the case of TCDD, significant dose-dependent increases in luciferase activity (0.3-300 ng/L) were detected. The assay was more sensitive to PCB 126 than to B[a]P. The assay also involved the highly sensitive expression of luciferase to extracted mixtures of PCBs and polycyclic aromatic hydrocarbons (PAHs) collected from coastal sediments. PCBs were more capable of cyp1a1 induction in the assay system at small doses than PAHs in environmental samples. Using the cyp1a1-luciferase assay along with water or sediment chemistry will certainly aid in diagnosing CYP1A-inducing contaminants in coastal environments.

Benzo[a]pyrene constrains embryo development via oxidative stress induction and modulates the transcriptional responses of molecular biomarkers in the marine medaka Oryzias javanicus

Embryos from the marine medaka fish Oryzias javanicus were treated with eight concentrations of benzo[a]pyrene (BaP) (0.001, 0.01, 0.1, 1, 2, 5, 10, or 20 μg L^--1) after they had been fertilized. Significant mortality and hatching delays were detected in embryos that had been exposed to 10 and 20 μg L^-1 BaP for 4 weeks. The mortality rate after hatching was higher in the medaka that had been previously exposed to > 2 μg L^-1 BaP. Significant elevations in intracellular reactive oxygen species and malondialdehyde contents were measured and the mRNA expressions of the antioxidant defense system genes (gst, sod, cat, and gpx) increased in the embryos exposed to 10 and 20 μg L^-1 BaP for 1 week. The hsp70, ahr, and cyp1a transcriptional responses were also significantly upregulated in the exposed groups after 1 week. The alterations to the in vivo parameters and molecular components suggested that waterborne BaP had a toxic effect on marine medaka embryos. Finally, fin defects, spinal curvature, and cardiac edema were highly induced when the embryos were exposed to > 5 μg L^-1 BaP.

Performance, microbial community and inhibition kinetics of long-term Cu2+ stress on an air-lift nitritation reactor with self-recirculation

Biological nitrogen removal process could be affected due to the presence of heavy metals owing to their toxicity and accumulation in the sludge. In this study, the impact of Cu²⁺ shock on a long-term nitritation operation was investigated in an air-lift reactor with self-recirculation. Both the dynamics of microbial community and inhibition kinetics under Cu²⁺ stress were ascertained. The results showed that Cu²⁺ exerted severe inhibition on nitritation performance of an air-lift reactor (ALR) at 25 mg/L. The corresponding NH₄⁺-N removal efficiency decreased to below 50%, which was mainly due to the variation of microbial community structure, especially the inhibition of nitrifiers like Nitrosomonas (the relative abundance decreased from 30% to 1% after Cu²⁺ inhibition). Kinetic parameters were obtained and compared after fitting the Haldane model. The long-term Cu²⁺ stress on the ALR aggravated the ammonium affinity and the resistance to substrate self-inhibition of the nitritation sludge, but reduced the resistance to Cu²⁺ inhibition. Furthermore, Cu²⁺ acted as uncompetitive inhibitor on nitritation process. Our results provide new insights into the nitritation characteristics under long-term Cu²⁺ stress.

Generation of albino via SLC45a2 gene targeting by CRISPR/Cas9 in the marine medaka Oryzias melastigma

To produce albinism in the marine medaka Oryzias melastigma, we disrupted the solute carrier family 45 (SLC45a2) gene by clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 with a single guide RNA (sgRNA). Selected sgRNAs were able to target a SLC45a2 gene as confirmed by genotyping and heteroduplex mobility assay (HMA). Of the survived embryos after injection, 54.2% and 60.0% embryos exhibited albinism phenotype by sgRNA1 and sgRNA2, respectively. Deep sequencing at the on-target sites showed different insertion and deletion (indel) mutation profiles near the DNA cleavage sites, indicating high efficacy of producing SLC45a2 knock-out mutants by this method. Moreover, HMA at the potential off-target sites revealed that off-target activity would be induced at a low rate, or not induced at all. This albino marine medaka will be a good model for marine molecular ecotoxicology in establishment of diverse in vivo endpoints, and the application of this efficient gene targeting method in the marine medaka would be useful tool for mechanistic approaches.

Rapid recovery of coastal environment and ecosystem to the Hebei Spirit oil spill's impact

The 2007 Hebei Spirit oil spill (HSOS), the largest in the national history, has negatively impacted the entire environment and ecosystem along the west coast of South Korea. Although many studies have reported the damages and impacts from the HSOS, quantitative assessment evaluating the recovery time and status have not been documented. Here, we first address the recovery timeline of the HSOS, by comprehensive analyses of 10-years accumulated data in quantitative manner. Concentrations of residual oils in seawater, sediments, and oysters rapidly dropped to backgrounds in 16, 75, and 33 months, respectively. Also, damaged benthic communities of intertidal and subtidal areas were fully recovered only after ~6 years. The present results collectively indicated unexpectedly fast recovery of the damaged environment and ecosystem from such a huge oil spill. The high tidal mixing (~9 m tidal height) and intensive human cleanup (~1.2 million volunteers) at the initial cleanup period might have contributed to rapid recovery; cf. 4-5 times faster than the Exxon Valdez oil spill. However, potential risk to human health remains unclear. Thus, it is warranted to conduct more in depth epidemiological studies to address chronic health effects associated with the cleanup volunteers as well as the local residents who have been living nearby the oil spill impacted sites.

Differential detoxifying responses to crude oil water-accommodated fraction in Hyallela curvispina individuals from unpolluted and contaminated sites

Sublethal effects of water-accommodated fraction (WAF) from crude oil of Neuquén basin, Northern Patagonia-Argentina, were examined on both antioxidant and detoxification system of Hyalella curvispina adults collected in Los Barreales (LB) lake and in an oil-polluted stream (DS). The effects of WAF exposure during 6, 24 and 48 h were evaluated in the glutathione content (GSH) and glutathione S-transferase (GST), catalase (CAT) and cytochrome P450 (CYP450) activities. Populations from DS and LB showed not only different basal GSH content and enzyme activities but also different behavior to WAF exposure. LB population exposed to WAF showed a significant increase in GSH content, CAT and CYP450 activities, compared to control group. DS population presented high basal levels in CAT and CYP activity compared with LB population, but their response to WAF exposure was minor. Amphipods from DS, chronically exposed to hydrocarbons, were adapted to their environment.

Red tide dinoflagellate Cochlodinium polykrikoides induces significant oxidative stress and DNA damage in the gill tissue of the red seabream Pagrus major

The ichthyotoxic Cochlodinium polykrikoides is one of the most harmful bloom-forming dinoflagellates. In the present study, the economically important red seabream Pagrus major was exposed to sublethal concentrations of C. polykrikoides (i.e., 1,000 and 3,000 cells mL^-1) for 24 h, and the antioxidant defense system and DNA damage dose-specific responses were analyzed during the exposure and additional depuration period (2 h) in the gill tissue. No significant ichthyotoxicity was observed under different light and dark conditions, while significantly lower levels of opercular respiratory rate were measured in the C. polykrikoides-exposed red seabream. Intracellular malondialdehyde (MDA) content increased significantly in the 3,000 cells-exposed gill tissues at 24 h and the increased level was maintained during depuration. Intracellular glutathione (GSH) levels were significantly depleted following exposure to 3,000 cells mL^-1 of C. polykrikoides, but the levels increased significantly in the depuration phase. Overall, significantly higher activity of antioxidant defense system enzymes such as catalase (CAT), superoxide dismutase (SOD), glutathione reductase (GR), and glutathione peroxidase (GPx) were observed in the 3,000 cells mL^-1-exposed gill tissues at 24 h and during depuration. Analysis of the two reliable DNA damage parameters, Olive tail moment and percent tail DNA, showed significantly elevated levels of DNA damage in the 1,000 and 3,000 cells mL^-1-exposed gill tissue. Increases in the activity of the antioxidant defense system and DNA damage may be one of the major mechanisms mediating C. polykrikoides-induced devastation in aquaculture and fisheries.

Waterborne manganese modulates immunity, biochemical, and antioxidant parameters in the blood of red seabream and black rockfish

Immunotoxic effects of manganese (Mn) were investigated in the blood of the economically important marine fish, red seabream (Pagrus major) and black rockfish (Sebastes schlegelii) when exposed to different concentrations of Mn (0, 0.5, 1, and 2 mg L^-1) for 14 days. During exposure, the levels of alternative complement activity in both fish were significantly lowered at 2 mg L^-1 of Mn of exposure. Lysozyme activity was significantly decreased in black rockfish in all concentrations of Mn after 14 days, while in red seabream, the decrease was significant with concentrations of 1 and 2 mg L^-1 of Mn after 7 and 14 days of exposure. A significantly low level was observed only in the 2 mg L^-1-exposed red seabream on day 14 of exposure. The concentrations of hemoglobin, red blood cells, white blood cells, and total serum proteins were significantly decreased in both fish under exposure to 1 and 2 mg L^-1 of Mn, while cortisol, alanine transferase, aspartate transaminase, and alkaline phosphatase levels were significantly increased compared to the levels of control groups. No significant change was found in serum glucose and albumin except in red seabream exposed to 2 mg L^-1 of Mn for 14 days. The responses of the antioxidant defense system were significantly induced in both fish after exposure to 1 and 2 mg L^-1 of Mn on day 7 and 14 of exposure. Taken together, alterations of these parameters suggest the immunotoxicity of waterborne Mn produced by the modulation of hematological components and the induction of oxidative stress in the blood of these marine fish.

Adverse effects of two pharmaceuticals acetaminophen and oxytetracycline on life cycle parameters, oxidative stress, and defensome system in the marine rotifer Brachionus rotundiformis

To investigate the adverse effect of two widely used pharmaceuticals, paracetamol (acetaminophen [APAP]) and oxytetracycline (OTC) on the marine rotifer Brachionus rotundiformis (B. rotundiformis), the animals were exposed to various environmentally-relevant concentrations. Up to date, acetaminophen and oxytetracycline have been considered as toxic, if used above threshold concentration, i.e. overdosed. However, this study demonstrated these two pharmaceuticals even at low concentration (i.e., μg/L scale) elicited oxidative stress through the generation of reactive oxygen species (ROS) along with the increased glutathione S-transferase activity, despite no-observed effect in in-vivo population growth. To validate the adverse effects of the two pharmaceuticals at relatively low concentrations, mRNA expression analysis was performed of the entire set of genes encoding 26 cytochrome P450s (CYPs) of phase I and 19 glutathione S-transferases (GSTs) of phase II of the rotifer B. rotundiformis. The mRNA expression analysis suggested specific genes CYP3045A2 and GSTσ1, GSTσ4, and GSTω1 take part in detoxification of APAP and OTC, resulting in no significant changes in the population growth and undetermined no observed effect concentration (NOEC) in the marine rotifer B. rotundiformis.

RNA-Seq-based transcriptome profiling and expression of 16 cytochrome P450 genes in the benzo[α]pyrene-exposed estuarine copepod Eurytemora affinis

The calanoid copepod Eurytemora affinis is one of the most abundant estuarine species and is considered to be an ideal candidate species for ecotoxicological research. An RNA-Seq-based transcriptome was developed from whole bodies of this species. Among 142,442 contigs of the de novo assembly by Trinity, 48,480 open reading frame (ORF) contigs were found using TransDecoder. A total of 17,762 genes were identified by BLAST analysis, which covers about 75% of the annotated genes in the E. affinis genome. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that most annotated sequences were related to metabolism pathways, including xenobiotic biodegradation. Using transcriptome data, we identified putative transcripts related to xenobiotic processing genes including phase I enzymes, phase II enzymes, transporters, and transcription factors. To understand the CYP-mediated detoxification metabolism of xenobiotics, we measured the transcriptional levels of 16 CYPs (within full sequences) of E. affinis in response to benzo[α]pyrene (B[α]P). Most Ea-CYP genes were significantly down- and/or up-regulated (P < 0.05) in response to B[α]P, suggesting that Ea-CYP genes are likely involved in detoxification (mainly in biotransformation of xenobiotics) with particular genes, demonstrating significant upregulation or downregulation compared to others, as shown in other copepod model species (e.g. Tigriopus japonicus and Paracyclopina nana). This study will provide insight into the potential role of E. affinis in response to various toxic or xenobiotic chemicals in the marine environment.

The genome of the marine medaka Oryzias melastigma

Marine medaka (Oryzias melastigma) is considered to be a useful fish model for marine and estuarine ecotoxicology studies and has good potential for field-based population genomics because of its geographical distribution in Asian estuarine and coastal areas. In this study, we present the first whole-genome draft of O. melastigma. The genome assembly consists of 8,602 scaffolds (N50 = 23.737 Mb) and a total genome length of 779.4 Mb. A total of 23,528 genes were predicted, and 12,670 gene families shared with three teleost species (Japanese medaka, mangrove killifish and zebrafish) were identified. Genome analyses revealed that the O. melastigma genome is highly heterozygous and contains a large number of repeat sequences. This assembly represents a useful genomic resource for fish scientists.

Identification of 74 cytochrome P450 genes and co-localized cytochrome P450 genes of the CYP2K, CYP5A, and CYP46A subfamilies in the mangrove killifish Kryptolebias marmoratus

BACKGROUND

The mangrove killifish Kryptolebias marmoratus is the only vertebrate that reproduces by self-fertilizing and is an important model species in genetics and marine ecotoxicology. Using whole-genome and transcriptome sequences, we identified all members of the cytochrome P450 (CYP) family in this model teleost and compared them with those of other teleosts.

RESULTS

A total of 74 cytochrome P450 genes and one pseudogene were identified in K. marmoratus. Phylogenetic analysis indicated that the CYP genes in clan 2 were most expanded, while synteny analysis with other species showed orthologous relationships of CYP subfamilies among teleosts. In addition to the CYP2K expansions, five tandem duplicated gene copies of CYP5A were observed. These features were unique to K. marmoratus.

CONCLUSIONS

These results shed a light on CYP gene evolution, particularly the co-localized CYP2K, CYP5A, and CYP46A subfamilies in fish. Future studies of CYP expression could identify specific endogenous and exogenous environmental factors that triggered the evolution of tandem CYP duplication in K. marmoratus.

Marine copepod cytochrome P450 genes and their applications for molecular ecotoxicological studies in response to oil pollution

Recently, accidental spills of heavy oil have caused adverse effects in marine organisms. Oil pollution can induce damages on development and reproduction, linking with detrimental effects on diverse molecular levels of genes and proteins in plankton and fish. However, most information was mainly focused on marine vertebrates and consequently, limited information was available in marine invertebrates. Furthermore, there is still a lack of knowledge bridging in vivo endpoints with the functional regulation of cytochrome P450 (CYP) genes in response to oil spill pollution in marine invertebrates. In this paper, adverse effects of oil spill pollution in marine invertebrates are summarized with the importance of CYP genes as a potential biomarker, applying for environmental monitoring to detect oil spill using marine copepods.

Assessing the toxicity of triphenyltin to different life stages of the marine medaka Oryzias melastigma through a series of life-cycle based experiments

Toxic effects of triphenyltin (TPT) to different life stages of the marine medaka Oryzias melastigma were investigated through a series of life-cycle based exposure experiments. In embryo stage, TPT exposure could elevate the heartbeat rate at Day 6-8 post-fertilization and increase the expression levels of five heart development related genes (i.e., ATPase, COX2, BMP4, GATA4 and NKX2.5). In larval stage, TPT shortened the body length at ≥10μg/L and suppressed the swimming activity of the fish larvae at Day 1 post-hatching at 50μg/L. In reproductive stage, TPT exposure resulted in a male-biased sex ratio (2μg/L) and reduced the gonadosomatic index (GSI) in females (≥ 0.1μg/L), which might in turn lead to a decline in their population fitness. The reproductive stage of O. melastigma was more sensitive to TPT than other stages, while the GSI of female medaka was the most sensitive endpoint.

The use of rockfish Sebastiscus marmoratus as a sentinel species to assess petroleum hydrocarbons pollution: A case study in Quanzhou Bay, China

To monitor the biological effects of marine pollution, choosing a native fish species and establishing suitable biomarkers are required. In this study, the full-length cDNA of cyp1a1 was cloned from Sebastiscus marmoratus (SM-CYP1A1). Then the dose-response and time-course induction of hepatic CYP1A1 mRNA by the crude oil water-soluble fraction (WSF) were determined. Subsequently, SM-CYP1A1 mRNA was applied to investigate the biological effect of petroleum hydrocarbon pollution in Quanzhou Bay, China. The transcription levels of hepatic CYP1A1 were significantly elevated in fish caged in the polluted sites for 2weeks compared with those of the reference site, which were correlated with the concentrations of petroleum hydrocarbon and polycyclic aromatic hydrocarbon (PAH) in the surface seawaters. The results suggest that S. marmoratus is a potential sentinel organism to monitor marine pollutants and the hepatic CYP1A1 mRNA can serve as a sensitive biomarker to organic xenobiotics in aquatic environments.

Genome-wide identification of 31 cytochrome P450 (CYP) genes in the freshwater rotifer Brachionus calyciflorus and analysis of their benzo[α]pyrene-induced expression patterns

While marine invertebrate cytochrome P450 (CYP) genes and their roles in detoxification mechanisms have been studied, little information is available regarding freshwater rotifer CYPs and their functions. Here, we used genomic sequences and RNA-seq databases to identify 31 CYP genes in the freshwater rotifer Brachionus calyciflorus. The 31 Bc-CYP genes with a few tandem duplications were clustered into CYP 2, 3, 4, mitochondrial, and 46 clans with two marine rotifers Brachionus plicatilis and Brachionus koreanus. To understand the molecular responses of these 31 Bc-CYP genes, we also examined their expression patterns in response to benzo[α]pyrene (B[α]P). Three Bc-CYP genes (Bc-CYP3044B3, Bc-CYP3049B4, Bc-CYP3049B6) were significantly upregulated (P<0.05) in response to B[α]P, suggesting that these CYP genes can be involved in detoxification in response to B[α]P exposure. These genes might be useful as biomarkers of B[α]P exposure in B. calyciflorus. Overall, our findings expand the repertoire of known CYPs and shed light on their potential roles in xenobiotic detoxification in rotifers.

Genome-wide identification of 52 cytochrome P450 (CYP) genes in the copepod Tigriopus japonicus and their B[α]P-induced expression patterns

Cytochrome P450s (CYPs) are enzymes with a heme-binding domain that are found in all living organisms. CYP enzymes have important roles associated with detoxification of xenobiotics and endogenous compounds (e.g. steroids, fatty acids, and hormones). Although CYP enzymes have been reported in several invertebrates, including insects, little is known about copepod CYPs. Here, we identified the entire repertoire of CYP genes (n=52) from whole genome and transcriptome sequences of the benthic copepod Tigriopus japonicus, including a tandem duplication (CYP3026A3, CYP3026A4, CYP3026A5), and examined patterns of gene expression over various developmental stages and in response to benzo[α]pyrene (B[α]P) exposure. Through phylogenetic analysis, the 52 T. japonicus CYP genes were assigned to five distinct clans: CYP2 (22 genes), CYP3 (19 genes), CYP4 (two genes), CYP20 (one gene), and mitochondrial (eight genes). Developmental stage and gender-specific expression patterns of the 52 T. japonicus CYPs were analyzed. CYP3022A1 was constitutively expressed during all developmental stages. CYP genes in clans 2 and 3 were induced in response to B[α]P, suggesting that these differentially modulated CYP transcripts are likely involved in defense against exposure to B[α]P and other pollutants. This study enhances our understanding of the repertoire of CYP genes in copepods and of their potential role in development and detoxification in copepods.

An evaluation of the endocrine disruptive potential of crude oil water accommodated fractions and crude oil contaminated surface water to freshwater organisms using in vitro and in vivo approaches

Knowledge regarding the potential impacts of crude oil on endocrine signaling in freshwater aquatic vertebrates is limited. The expression of selected genes as biomarkers for altered endocrine signaling was studied in African clawed frog, Xenopus laevis, tadpoles and juvenile Mozambique tilapia, Oreochromis mossambicus, exposed to weathered bunker and unweathered refinery crude oil water accommodated fractions (WAFs). In addition, the expression of the aforementioned genes was quantified in X. laevis tadpoles exposed to surface water collected from the proximity of an underground oil bunker. The (anti)estrogenicity and (anti)androgenicity of crude oil, crude oil WAFs, and surface water were furthermore evaluated using recombinant yeast. Thyroid hormone receptor beta expression was significantly down-regulated in X. laevis in response to both oil WAF types, whereas a further thyroid linked gene, type 2 deiodinase, was up-regulated in O. mossambicus exposed to a high concentration of bunker oil WAF. In addition, both WAFs altered the expression of the adipogenesis-linked peroxisome proliferator-activated receptor gamma in X. laevis. The crude oil and WAFs exhibited antiestrogenic and antiandrogenic activity in vitro. However, O. mossambicus androgen receptor 2 was the only gene, representing the reproductive system, significantly affected by WAF exposure. Estrogenicity, antiestrogenicity, and antiandrogenicity were detected in surface water samples; however, no significant changes were observed in the expression of any of the genes evaluated in X. laevis exposed to surface water. The responses varied among the 2 model organisms used, as well as among the 2 types of crude oil. Nonetheless, the data provide evidence that crude oil pollution may lead to adverse health effects in freshwater fish and amphibians as a result of altered endocrine signaling. Environ Toxicol Chem 2017;36:1330-1342. © 2016 SETAC.

Adverse Effects, Expression of the Bk-CYP3045C1 Gene, and Activation of the ERK Signaling Pathway in the Water Accommodated Fraction-Exposed Rotifer

To examine the deleterious effects of the water accommodated fraction (WAF) of crude oil, the growth curve, fecundity, and lifespan of the monogonont rotifer (Brachionus koreanus) were measured for 24 h in response to three different doses (0.2×, 0.4×, and 0.8×) of WAFs. A higher dose of WAFs significantly reduced the fecundity and lifespan. A rotifer 32K microarray chip showed that the Bk-CYP3045C1 gene had the highest expression. Of the 25 entire CYP genes, the Bk-CYP3045C1 gene showed a significant expression for different doses and times in response to WAFs and chemical components of WAFs (naphthalene and phenanthrene); also, glutathione S-transferase genes, ABC transporter, and other genes showed dose responses upon exposure to 80% WAF over time. Different doses of WAFs increased the oxidative stress with an induction of reactive oxygen species (ROS) and a depletion of glutathione (GSH). Exposure to WAFs did not show toxic effects on survivability in B. koreanus; however, toxicity to WAFs was shown when piperonyl butoxide, a potent inhibitor of cytochrome P450 (CYP) enzymes, was added. This toxicity was dose-dependent. After WAFs exposure, p-ERK was activated over time in response to WAFs, which suggests that WAFs can be activated by the p-ERK signaling pathway.

Transcriptomic response to water accommodated fraction of crude oil exposure in the gill of Japanese flounder, Paralichthys olivaceus

Illumina-based RNA-seq was used to determine the short-term transcriptomic responses of Paralichthys olivaceus gill to an environmentally relevant level of water accommodated fraction (WAF) of crude oil. 213,979 transcripts and 128,482 unigenes were obtained. Differential expression analysis revealed that 1641 and 2142 genes were significantly up- and down-regulated. Enrichment analysis identified a set of GO terms and putative pathways involved in the response of P. olivaceus to WAF exposure. Analysis of the transcripts revealed the effective protective mechanisms of P. olivaceus to reduce the toxic effects of WAF. Moreover, WAF exposure induced the metabolism of energy substrates, and downstream pathway genes were modified to provide protection against toxic damage. Transcripts analysis demonstrated that the genes involved in circadian rhythm signaling were regulated in gills of P. olivaceus exposed to WAF. These results provide insights into the mechanisms of WAF-induced toxicity in fishes and into the WAF-sensitive biomarkers in P. olivaceus.

Omics of the marine medaka (Oryzias melastigma) and its relevance to marine environmental research

In recent years, the marine medaka (Oryzias melastigma), also known as the Indian medaka or brackish medaka, has been recognized as a model fish species for ecotoxicology and environmental research in the Asian region. O. melastigma has several promising features for research, which include a short generation period (3-4 months), daily spawning, small size (3-4 cm), transparent embryos, sexual dimorphism, and ease of mass culture in the laboratory. There have been extensive transcriptome and genome studies on the marine medaka in the past decade. Such omics data can be useful in understanding the signal transduction pathways of small teleosts in response to environmental stressors. An omics-integrated approach in the study of the marine medaka is important for strengthening its role as a small fish model for marine environmental studies. In this review, we present current omics information about the marine medaka and discuss its potential applications in the study of various molecular pathways that can be targets of marine environmental stressors, such as chemical pollutants. We believe that this review will encourage the use of this small fish as a model species in marine environmental research.

Marine medaka ATP-binding cassette (ABC) superfamily and new insight into teleost Abch nomenclature

The ABC gene family is recognized as one of the largest gene families in all kingdoms of life. Although many genes involved in the ABC superfamily have been annotated from several fish species, information on large sets of the ABC superfamily and their evolutionary characterization are still unclear. In the marine medaka Oryzias melastigma, 50 ABC transporters were identified with bioinformatics-aided in silico analyses, and their full-length cDNA sequences were characterized. Phylogenetic analysis revealed that they could be classified into the eight subfamilies (A–H) that include all members of all ABC subfamilies. Interestingly, several teleosts’ Abcg members were closely clustered with Abch members in a distinctive clade. The abch gene was also observed in the coelacanth and the spotted gar, suggesting that this gene was retained from a bilaterian ancestor and that a gene loss event recently occurred in the tetrapod lineage. In teleosts, the nomenclature of previously annotated abcg genes should be considered carefully, as they form a distinctive clade with the marine medaka abch subfamily and other teleost abch genes, but not with the members of the Abcg subfamily.

Significance of adverse outcome pathways in biomarker-based environmental risk assessment in aquatic organisms

In environmental risk assessments (ERA), biomarkers have been widely used as an early warning signal of environmental contamination. However, biomarker responses have limitation due to its low relevance to adverse outcomes (e.g., fluctuations in community structure, decreases in population size, and other similar ecobiologically relevant indicators of community structure and function). To mitigate these limitations, the concept of adverse outcome pathways (AOPs) was developed. An AOP is an analytical, sequentially progressive pathway that links a molecular initiating event (MIE) to an adverse outcome. Recently, AOPs have been recognized as a potential informational tool by which the implications of molecular biomarkers in ERA can be better understood. To demonstrate the utility of AOPs in biomarker-based ERA, here we discuss a series of three different biological repercussions caused by exposure to benzo(a)pyrene (BaP), silver nanoparticles (AgNPs), and selenium (Se). Using mainly aquatic invertebrates and selected vertebrates as model species, we focus on the development of the AOP concept. Aquatic organisms are suitable bioindicator species whose entire lifespans can be observed over a short period; moreover, these species can be studied on the molecular and population levels. Also, interspecific differences between aquatic organisms are important to consider in an AOP framework, since these differences are an integral part of the natural environment. The development of an environmental pollutant-mediated AOP may enable a better understanding of the effects of environmental pollutants in different scenarios in the diverse community of an ecosystem.

Identification of the Full 46 Cytochrome P450 (CYP) Complement and Modulation of CYP Expression in Response to Water-Accommodated Fractions of Crude Oil in the Cyclopoid Copepod Paracyclopina nana

The 46 cytochrome P450 (CYP) gene superfamily was identified in the marine copepod Paracyclopina nana after searching an RNA-seq database and comparing it with other copepod CYP gene families. To annotate the 46 Pn-CYP genes, a phylogenetic analysis of CYP genes was performed using a Bayesian method. Pn-CYP genes were separated into five different clans: CYP2, CYP3, CYP20, CYP26, and mitochondrial. Among these, the principal Pn-CYP genes involved in detoxification were identified by comparing them with those of the copepod Tigriopus japonicus and were examined with respect to their responses to exposure to a water-accommodated fraction (WAF) of crude oil and to the alkylated forms of two polycyclic aromatic hydrocarbons (PAHs; phenanthrene and fluorene). The expression of two Pn-CYP3027 genes (CYP3027F1 and CYP3027F2) was increased in response to WAF exposure and also was upregulated in response to the two alkylated PAHs. In particular, Pn-CYP3027F2 showed the most notable increase in response to 80% WAF exposure. These two responsive CYP genes (Pn-CYP3027F1 and CYP3027F2) were also phylogenetically clustered into the same clade of the WAF- and alkylated PAH-specific CYP genes of the copepod T. japonicus, suggesting that these CYP genes would be those chiefly involved in detoxification in response to WAF exposure in copepods. In this paper, we provide information on the copepod P. nana CYP gene superfamily and also speculate on its potential role in the detoxification of PAHs in marine copepods. Despite the nonlethality of WAF, Pn-CYP3027F2 was rapidly and significantly upregulated in response to WAF that may serve as a useful biomarker of 40% or higher concentration of WAF exposure. This paper will be helpful to better understand the molecular mechanistic events underlying the metabolism of environmental toxicants in copepods.

Unexpected interaction with dispersed crude oil droplets drives severe toxicity in Atlantic haddock embryos

The toxicity resulting from exposure to oil droplets in marine fish embryos and larvae is still subject for debate. The most detailed studies have investigated the effects of water-dissolved components of crude oil in water accommodated fractions (WAFs) that lack bulk oil droplets. Although exposure to dissolved petroleum compounds alone is sufficient to cause the characteristic developmental toxicity of crude oil, few studies have addressed whether physical interaction with oil micro-droplets are a relevant exposure pathway for open water marine speices. Here we used controlled delivery of mechanically dispersed crude oil to expose pelagic embryos and larvae of a marine teleost, the Atlantic haddock (Melanogrammus aeglefinus). Haddock embryos were exposed continuously to two different concentrations of dispersed crude oil, high and low, or in pulses. By 24 hours of exposure, micro-droplets of oil were observed adhering and accumulating on the chorion, accompanied by highly elevated levels of cyp1a, a biomarker for exposure to aromatic hydrocarbons. Embryos from all treatment groups showed abnormalities representative of crude oil cardiotoxicity at hatch (5 days of exposure), such as pericardial and yolk sac edema. Compared to other species, the frequency and severity of toxic effects was higher than expected for the waterborne PAH concentrations (e.g., 100% of larvae had edema at the low treatment). These findings suggest an enhanced tissue uptake of PAHs and/or other petroleum compounds from attached oil droplets. These studies highlight a novel property of haddock embryos that leads to greater than expected impact from dispersed crude oil. Given the very limited number of marine species tested in similar exposures, the likelihood of other species with similar properties could be high. This unanticipated result therefore has implications for assessing the ecological impacts of oil spills and the use of methods for dispersing oil in the open sea.

Tissue-specific transcriptome assemblies of the marine medaka Oryzias melastigma and comparative analysis with the freshwater medaka Oryzias latipes

Background

The marine medaka Oryzias melastigma has been demonstrated as a novel model for marine ecotoxicological studies. However, the lack of genome and transcriptome reference has largely restricted the use of O. melastigma in the assessment of in vivo molecular responses to environmental stresses and the analysis of biological toxicity in the marine environment. Although O. melastigma is believed to be phylogenetically closely related to Oryzias latipes, the divergence between these two species is still largely unknown. Using Illumina high-throughput RNA sequencing followed by de novo assembly and comprehensive gene annotation, we provided transcriptomic resources for the brain, liver, ovary and testis of O. melastigma. We also investigated the possible extent of divergence between O. melastigma and O. latipes at the transcriptome level.

Results

More than 14,000 transcripts across brain, liver, ovary and testis in marine medaka were annotated, of which 5880 transcripts were orthologous between O. melastigma and O. latipes. Tissue-enriched genes were identified in O. melastigma, and Gene Ontology analysis demonstrated the functional specificity of the annotated genes in respective tissue. Lastly, the identification of marine medaka-enriched transcripts suggested the necessity of generating transcriptome dataset of O. melastigma.

Conclusions

Orthologous transcripts between O. melastigma and O. latipes, tissue-enriched genes and O. melastigma-enriched transcripts were identified. Genome-wide expression studies of marine medaka require an assembled transcriptome, and this sequencing effort has generated a valuable resource of coding DNA for a non-model species. This transcriptome resource will aid future studies assessing in vivo molecular responses to environmental stresses and those analyzing biological toxicity in the marine environment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1325-7) contains supplementary material, which is available to authorized users.

Effects of benzo[a]pyrene on whole cytochrome P450-involved molecular responses in the marine medaka Oryzias melastigma

Despite being a strong toxicant for aquatic ecosystems, the effect of benzo[a]pyrene (B[a]P) on whole cytochrome P450 (CYP) biotransformation mechanisms has not been deeply investigated in aquatic organisms. To understand the mode of action of B[a]P on CYP molecular responses in fish, we analyzed the full spectrum of cyp genes and the activities of enzymes that are involved in detoxification and antioxidant defense systems after exposure to different concentrations of B[a]P over different time courses in the marine medaka, Oryzias melastigma. Upon B[a]P exposure, we found significant downregulation of cyp genes associated with steroidogenesis with decreased concentrations of actual hormones including estradiol (E2) and testosterone (11-KT), indicating that B[a]P-treated groups were closely associated with the dysfunction of hormone synthesis in a dose-dependent manner. In addition, B[a]P exposure strongly influenced transcriptional levels of antioxidant-related genes and their enzyme activities. Based on these results, we suggest that B[a]P induced the CYPs-involved systematic biotransformation mechanism with oxidative stress in the juvenile marine medaka, resulting in changes of endogenous hormonal levels and transcriptional levels of several steroidogenic metabolism-related CYPs.

Crude oil exposure results in oxidative stress-mediated dysfunctional development and reproduction in the copepod Tigriopus japonicus and modulates expression of cytochrome P450 (CYP) genes

In this study, we investigated the effects of the water-accommodated fraction (WAF) of crude oil on the development and reproduction of the intertidal copepod Tigriopus japonicus through life-cycle experiments. Furthermore, we investigated the mechanisms underlying the toxic effects of WAF on this benthic organism by studying expression patterns of cytochrome P450 (CYP) genes. Development of T. japonicus was delayed and molting was interrupted in response to WAF exposure. Hatching rate was also significantly reduced in response to WAF exposure. Activities of antioxidant enzymes such as glutathione S-transferase (GST), glutathione reductase (GR), and catalase (CAT) were increased by WAF exposure in a concentration-dependent manner. These results indicated that WAF exposure resulted in oxidative stress, which in turn was associated with dysfunctional development and reproduction. To evaluate the involvement of cytochrome P450 (CYP) genes, we cloned the entire repertoire of CYP genes in T. japonicus (n=52) and found that the CYP genes belonged to five different clans (i.e., Clans 2, 3, 4, mitochondrial, and 20). We then examined expression patterns of these 52 CYP genes in response to WAF exposure. Three TJ-CYP genes (CYP3024A2, CYP3024A3, and CYP3027C2) belonging to CYP clan 3 were significantly induced by WAF exposure in a time- and concentration-dependent manner. We identified aryl hydrocarbon responsive elements (AhRE), xenobiotic responsive elements (XREs), and metal response elements (MRE) in the promoter regions of these three CYP genes, suggesting that these genes are involved in detoxification of toxicants. Overall, our results indicate that WAF can trigger oxidative stress and thus induce dysfunctional development and reproduction in the copepod T. japonicus. Furthermore, we identified three TJ-CYP genes that represent potential biomarkers of oil pollution.

Using combined bio-omics methods to evaluate the complicated toxic effects of mixed chemical wastewater and its treated effluent

Mixed chemical wastewaters (MCWW) from industrial park contain complex mixtures of trace contaminants, which cannot be effectively removed by wastewater treatment plants (WWTP) and have become an unignored threat to ambient environment. However, limited information is available to evaluate the complicated toxic effects of MCWW and its effluent from wastewater treatment plant (WTPE) from the perspective of bio-omics. In this study, mice were exposed to the MCWW and WTPE for 90 days and distinct differences in the hepatic transcriptome and serum metabolome were analyzed by digital gene expression (DGE) and proton nuclear magnetic resonance ((1)H-NMR) spectra, respectively. Our results indicated that disruption of lipid metabolism in liver and hepatotoxicity were induced by both MCWW and WTPE exposure. WTPE is still a health risk to the environment, which is in need of more attention. Furthermore, we demonstrated the potential ability of bio-omics approaches for evaluating toxic effects of MCWW and WTPE.

Development of a promising fish model (Oryzias melastigma) for assessing multiple responses to stresses in the marine environment

With the increasing number of contaminants in the marine environment, various experimental organisms have been “taken into labs” by investigators to find the most suitable environmentally relevant models for toxicity testing. The marine medaka, Oryzias melastigma, has a number of advantages that make it a prime candidate for these tests. Recently, many studies have been conducted on marine medaka, especially in terms of their physiological, biochemical, and molecular responses after exposure to contaminants and other environmental stressors. This review provides a literature survey highlighting the steady increase of ecotoxicological research on marine medaka, summarizes the advantages of using O. melastigma as a tool for toxicological research, and promotes the utilization of this organism in future studies.

Exposure to DEHP and MEHP from hatching to adulthood causes reproductive dysfunction and endocrine disruption in marine medaka (Oryzias melastigma)

Concern has increased regarding the adverse effects of di-(2-ethylhexyl)-phthalate (DEHP) on reproduction. However, limited information is available on the effects of DEHP in marine organisms. The aim of the present study was to examine whether long-term exposure to DEHP and its active metabolite mono-(2-ethylhexyl)-phthalate (MEHP) disrupts endocrine function in marine medaka (Oryzias melastigma). Marine medaka larvae were exposed to either DEHP (0.1 and 0.5mg/L) or MEHP (0.1 and 0.5mg/L) for 6 months, and the effects on reproduction, sex steroid hormones, liver vitellogenin (VTG), gonad histology and the expression of genes involved in the hypothalamic-pituitary-gonad (HPG) axis were investigated. Exposure to DEHP, but not MEHP, from hatching to adulthood accelerated the start of spawning and decreased the egg production of exposed females. Moreover, exposure to both DEHP and MEHP resulted in a reduction in the fertilization rate of oocytes spawned by untreated females paired with treated males. A significant increase in plasma 17β-estradiol (E2) along with a significant decrease in testosterone (T)/E2 ratios was observed in males, which was accompanied by the upregulation of ldlr, star, cyp17a1, 17βhsd, and cyp19a transcription in the testis. Increased concentrations of T and E2 were observed in females, which was consistent with the upregulation of ldlr. The expression of brain gnrhr2, fshβ, cyp19b and steroid hormone receptor genes also corresponded well with hormonal and reproductive changes. The liver VTG level was significantly increased after DEHP and MEHP exposure in males. DEHP induced histological changes in the testes and ovaries: the testes displayed a reduced number of spermatozoa, and the ovaries displayed an increased number of atretic follicles. In addition, the tissue concentrations of MEHP, MEHHP and MEOHP in DEHP-exposed groups were much higher than those in MEHP-exposed groups, and there were no dose- or sex-specific effects. Thus, DEHP exerts more obvious toxic effects compared with MEHP. There were some commonalities in the toxic effects and molecular mechanisms of DEHP and MEHP, suggesting that some of the toxic effects of DEHP may be induced by both DEHP itself and DEHP metabolites (including MEHP). Taken together, these results indicate that exposure to DEHP and MEHP from hatching to adulthood causes endocrine disruption with sex-specific effects in marine medaka, with males being more sensitive than females.

Expression pattern of entire cytochrome P450 genes and response of defensomes in the benzo[a]pyrene-exposed monogonont rotifer Brachionus koreanus

Cytochrome P450 (CYP) proteins are involved in the first line of detoxification mechanism against diverse polycyclic aromatic hydrocarbons (PAHs) including benzo[a]pyrene (B[a]P). In aquatic invertebrates, there is still a lack of knowledge on the CYP genes involved in the molecular response to B[a]P exposure due to limited gene information. In this study, we cloned the entire 25 CYP genes in the monogonont rotifer Brachionus koreanus with the aid of next generation sequencing (NGS) technologies and analyzed their transcript profiles with a real-time RT-PCR array to better understand B[a]P-triggered molecular response over different time courses. As a result, B[a]P exposure induced CYP2/3-involved detoxification mechanisms and defensome, including phase II detoxification and antioxidant systems with a modulation of the chaperone heat shock protein (hsp) expression but did not change expression of other CYP clans in B. koreanus . Therefore, we found that B[a]P induced a strong detoxification mechanism to overcome detrimental effects of B[a]P associated with B[a]P-induced growth retardation as a trade-off in fitness costs. Also, this approach revealed that the entire CYP profiling can be a way of providing a better understanding on the mode of action of B[a]P in B. koreanus with respect to molecular defense metabolism.