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

TPT disrupts early embryonic development and glucose metabolism of marine medaka in different salinites

Triphenyltin (TPT) is an organotin compound frequently detected in coastal estuaries, yet studies on TPT's effects in regions with significant salinity fluctuations, such as coastal estuaries, are currently limited. To investigate the toxic effects of TPT under different salinity conditions, this study focused on marine medaka (Oryzias melastigma) embryos. Through early morphological observations, RNA-seq analysis, biochemical marker assays, and qPCR detection, we explored the impact of TPT exposure on the early embryonic development of marine medaka under varying salinities. The study found that TPT exposure significantly increased embryo mortality at salinities of 0 ppt and 30 ppt. RNA-seq analysis revealed that TPT primarily affects glucose metabolism and glycogen synthesis processes in embryos. Under high salinity conditions, TPT may inhibit glucose metabolism by suppressing glycolysis and promoting gluconeogenesis. Furthermore, TPT exposure under different salinities led to the downregulation of genes associated with the insulin signaling pathway (ins, insra, irs2b, pik3ca, pdk1b, akt1, foxo1a), which may be linked to suppressed glucose metabolism and increased embryonic mortality. In summary, TPT exposure under different salinities affects the early development of marine medaka embryos and inhibits glucose metabolism. This study provides additional data to support research on organotin compounds in coastal estuaries.

Neurobehavioral toxicity induced by combined exposure of micro/nanoplastics and triphenyltin in marine medaka (Oryzias melastigma)

Microplastics/nanoplastics (MNPs) inevitably coexist with other pollutants in the natural environment, making it crucial to study the interactions between MNPs and other pollutants as well as their combined toxic effects. In this study, we investigated neurotoxicity in marine medaka (Oryzias melastigma) exposed to polystyrene micro/nanoplastics (PS-MNPs), triphenyltin (TPT), and PS-MNPs + TPT from physiological, behavioral, biochemical, and genetic perspectives. The results showed that marine medaka exposed to 200 ng/L TPT or 200 μg/L PS-NPs alone exhibited some degree of neurodevelopmental deficit, albeit with no significant behavioral abnormalities observed. However, in the PS-MP single exposure group, the average acceleration of short-term behavioral indices was significantly increased by 78.81%, indicating a highly stress-responsive locomotor pattern exhibited by marine medaka. After exposure to PS-MNPs + TPT, the swimming ability of marine medaka significantly decreased. In addition, PS-MNPs + TPT exposure disrupted normal neural excitability as well as activated detoxification processes in marine medaka larvae. Notably, changes in neural-related genes suggested that combined exposure to PS-MNPs and TPT significantly increased the neurotoxic effects observed with exposure to PS-MNPs or TPT alone. Furthermore, compared to the PS-MPs + TPT group, PS-NPs + TPT significantly inhibited swimming behavior and thus exacerbated the neurotoxicity. Interestingly, the neurotoxicity of PS-MPs was more pronounced than that of PS-NPs in the exposure group alone. However, the addition of TPT significantly enhanced the neurotoxicity of PS-NPs compared to PS-MPs + TPT. Overall, the study underscores the combined neurotoxic effects of MNPs and TPT, providing in-depth insights into the ecotoxicological implications of MNPs coexisting with pollutants and furnishing comprehensive data.

Micro(nano)plastics in marine medaka: Entry pathways and cardiotoxicity with triphenyltin

The simultaneous presence of micro(nano)plastics (MNPs) and pollutants represents a prevalent environmental challenge that necessitates understanding their combined impact on toxicity. This study examined the distribution of 5 μm (PS-MP5) and 50 nm (PS-NP50) polystyrene plastic particles during the early developmental stages of marine medaka (Oryzias melastigma) and assessed their combined toxicity with triphenyltin (TPT). Results showed that 2 mg/L PS-MP5 and PS-NP50 could adhere to the embryo surface. PS-NP50 can passively enter the larvae and accumulate predominantly in the intestine and head, while PS-MP5 cannot. Nonetheless, both types can be actively ingested by the larvae and distributed in the intestine. 2 mg/L PS-MNPs enhance the acute toxicity of TPT. Interestingly, high concentrations of PS-NP50 (20 mg/L) diminish the acute toxicity of TPT due to their sedimentation properties and interactions with TPT. 200 μg/L PS-MNPs and 200 ng/L TPT affect complement and coagulation cascade pathways and cardiac development of medaka larvae. PS-MNPs exacerbate TPT-induced cardiotoxicity, with PS-NP50 exhibiting stronger effects than PS-MP5, which may be related to the higher adsorption capacity of NPs to TPT and their ability to enter the embryos before hatching. This study elucidates the distribution of MNPs during the early developmental stages of marine medaka and their effects on TPT toxicity, offering a theoretical foundation for the ecological risk assessment of MNPs.

Effects of lipophilic phycotoxin okadaic acid on the early development and transcriptional expression of marine medaka Oryzias melastigma

The lipophilic okadaic acid (OA)-group toxins produced by some species of Dinophysis spp. and Prorocentrum spp. marine dinoflagellates have been frequently and widely detected in natural seawater environments, e.g. 2.1∼1780 ng/L in Spanish sea and 5.63∼27.29 ng/L in the Yellow Sea of China. The toxicological effects of these toxins dissolved in seawater on marine fish is still unclear. Effects of OA on the embryonic development and 1-month old larvae of marine medaka (Oryzias melastigma) were explored and discussed in this study. Significantly increased mortality and decreased hatching rates occurred for the medaka embryos exposed to OA at 1.0 μg/mL. Diverse malformations including spinal curvature, dysplasia and tail curvature were also observed in the embryos exposed to OA and the heart rates significantly increased at 11 d post fertilization. The 96 h LC₅₀ of OA for 1-month old larvae was calculated at 3.80 μg/mL. The reactive oxygen species (ROS) was significantly accumulated in medaka larvae. Catalase (CAT) enzyme activity was significantly increased in 1-month old larvae. Acetylcholinesterase (AChE) activity significantly increased with a dose-dependent pattern in 1-month old larvae. Differentially expressed genes (DEGs) were enriched in 11 KEGG pathways with Q value < 0.05 in 1-month old medaka larvae exposed to OA at 0.38 μg/mL for 96 h, which were mainly related to cell division and proliferation, and nervous system. Most of DEGs involved in DNA replication, cell cycle, nucleotide excision repair, oocyte meiosis, and mismatch repair pathways were significantly up-regulated, while most of DEGs involved in synaptic vesicle cycle, glutamatergic synapse, and long-term potentiation pathways were markedly down-regulated. This transcriptome analysis demonstrated that a risk of cancer developing was possibly caused by OA due to DNA damage in marine medaka larvae. In addition, the neurotoxicity of OA was also testified for marine fish, which potentially cause major depressive disorder (MDD) via the up-regulated expression of NOS1 gene. The genotoxicity and neurotoxicity of OA to marine fish should be paid attention to and explored further in the future.

Effect of thiamethoxam on the behavioral profile alteration and toxicity of adult zebrafish at environmentally relevant concentrations

Thiamethoxam (THM) is a commercial neonicotinoid insecticide with broad-spectrum insecticidal activity. It has been widely detected in the aquatic environment, but its behavioral toxicity on aquatic organisms received limited attention. In this study, adult zebrafish were exposed to THM at three levels (0.1, 10, and 1000 μg/L) for 45 days to investigate its effect on their ecological behavior, histopathology, bioaccumulation, and stress response. The bioconcentration factor in zebrafish brain was significantly higher (p < 0.05) at low concentration of THM (0.1 μg/L) than in other treatment groups. In terms of individual behavior, the locomotor activity, aggregation, and social activity of fish were enhanced after THM exposure, but the memory of the food zone was disturbed and abnormal swimming behavior was observed. THM exposure caused brain tissue necrosis, erythrocyte infiltration, cloudy swelling, and other pathological changes in brain tissue and affected the concentrations of acetylcholinesterase and cortisol related to neurotoxicity. The condition factor and organ coefficients (brain, heart, and intestine) of zebrafish were markedly impacted by THM treatment at 0.1 and 1000 μg/L, respectively. This finding showed that THM was more harmful to fish behavior than lethality, reproduction, and growth, and a behavioral study can be a useful tool for ecological risk assessment.

Triphenyltin exposure induced abnormal morphological colouration in adult male guppies (Poecilia reticulata)

Fish morphological colouration is essential for their survival and reproduction success; however, it is vulnerable to environmental factors, such as pollutants. Triphenyltin (TPT) is widespread in aquatic ecosystems, and its impacts on fish have been problematic. Therefore, the purpose of this study was to investigate the effects of TPT at environment-related concentrations (0, 1, 10 and 100 ng Sn/L) on morphological colouration in male guppies (Poecilia reticulata). The results showed that TPT exposure affected both orange/red and dark morphological colouration in guppies. The faded orange/red colouration might be related to the decrease of coloured pteridine and Pts (6-Pyruvoyltetrahydropterin Synthase) expression. In addition, TPT exposure induced melanogenesis, however, much melanin was distributed diffusely in the skin and did not seem to form a spot pattern, giving the fish a dull appearance. According to the skin transcriptional profiles, the changes of dark morphological colouration might be related to the changes in genes related to the functions of melanosome components (Gpnmb, Slc45a2 and Tyr), construction (Ap3d1, Fig4, Hps3, Hps5, Lyst, Rabggta, Txndc5 and Vps33a), and transport (Rab27a). Additionally, genes related to the regulation of melanogenesis (Atrn and Pomc) and system effects (Atox1, Atp6ap2, Atp6v1f, Atp6v1h, Rpl24, Rps19 and Rps20) might also be involved in the molecular mechanisms of abnormal morphological colouration induced by TPT. The present study provides crucial data on the molecular basis of abnormal morphological colouration in fish exposed to TPT and underscores the importance of toxicological studies of the effects of pollutants in aquatic environments on fish morphological colouration.

Transcriptomics reveal triphenyltin-induced molecular toxicity in the marine mussel Perna viridis

Triphenyltin (TPT) is widely used as an active ingredient in antifouling paints and fungicides, and continuous release of this highly toxic endocrine disruptor has caused serious pollution to coastal marine ecosystems and organisms worldwide. Using bioassays and transcriptome sequencing, this study comprehensively investigated the molecular toxicity of TPT chloride (TPTCl) to the marine mussel Perna viridis which is a commercially important species and a common biomonitor for marine pollution in Southeast Asia. Our results indicated that TPTCl was highly toxic to adult P. viridis, with a 96-h LC₁₀ and a 96-h EC₁₀ at 18.7 μg/L and 2.7 μg/L, respectively. A 21-day chronic exposure to 2.7 μg/L TPTCl revealed a strong bioaccumulation of TPT in gills (up to 36.48 μg/g dry weight) and hepatopancreas (71.19 μg/g dry weight) of P. viridis. Transcriptome analysis indicated a time course dependent gene expression pattern in both gills and hepatopancreas. Higher numbers of differentially expressed genes were detected at Day 21 (gills: 1686 genes; hepatopancreas: 1450 genes) and at Day 28 (gills: 628 genes; hepatopancreas: 238 genes) when compared with that at Day 7 (gills: 104 genes, hepatopancreas: 112 genes). Exposure to TPT strongly impaired the endocrine system through targeting on nuclear receptors and putative steroid metabolic genes. Moreover, TPT widely disrupted cellular functions, including lipid metabolism, xenobiotic detoxification, immune response and endoplasmic-reticulum-associated degradation expression, which might have caused the bioaccumulation of TPT in the tissues and aggregation of peptides and proteins in cells that further activated the apoptosis process in P. viridis. Overall, this study has advanced our understanding on both ecotoxicity and molecular toxic mechanisms of TPT to marine mussels, and contributed empirical toxicity data for risk assessment and management of TPT contamination.

Influence of salinity on physiological development and zinc toxicity in the marine medaka Oryzias melastigma

To determine whether the marine medaka Oryzias melastigma is a suitable model organism for evaluating the effects of environmental chemicals on marine teleosts, we examined the effect of salinity on physiological development and zinc toxicity. Growth as measured by total body length was significantly lower in fresh water compared to brackish water. Reproductive success was also significantly reduced in fresh water, although we observed cells in the pituitary producing gonadotropins such as Gpa (common glycoprotein hormone α), Fshb (follicle stimulating hormone β), and Lhb (luteinizing hormone β) at all salinities. These results indicate that O. melastigma is adaptable to various salinities from fresh to seawater, and brackish water is best for physiological processes including growth performance and reproduction. When zinc was dissolved in saltwater, a white precipitate formed immediately, and the dissolved concentration decreased in the supernatant and increased at precipitate. We performed zinc toxicity tests on early life stage and adult stage in fresh water, brackish water, and seawater. Among adults, the lowest observed effect concentration for mortality in freshwater (15.3 mg/L) was lower than in brackish water (>48 mg/L) or seawater (>48 mg/L). Similarly, among embryos and larvae, the lowest observed effect concentration for mortality in freshwater (4.8 mg/L) was lower than in brackish water (48 mg/L) or seawater (48 mg/L). These results highlight the importance of using marine organisms to evaluate the ecological effects of marine pollutants.

Microplastics decrease the toxicity of triphenyl phosphate (TPhP) in the marine medaka (Oryzias melastigma) larvae

Plastics have been recognized as a serious threat to the environment. Besides their own toxicity, microplastics can interact with other environmental pollutants, acting as carriers and potentially modulating their toxicity. In this study, the toxicity of polystyrene (PS) microplastic fragments (plain PS; carboxylated PS, PS-COOH and aminated PS, PS-NH₂) and triphenyl phosphate (TPhP) (an emerging organophosphate flame retardant) at the environmentally relevant concentrations to the marine medaka (Oryzias melastigma) larvae was investigated. Larvae were exposed to 20 μg/L of microplastic fragments or 20 and 100 μg/L of TPhP or a combination of both for 7 days. The results showed that the three microplastics did not affect the larval locomotor activity. For TPhP, the larval moving duration and distance moved were significantly decreased by the TPhP exposure, with a maximum decrease of 43.5% and 59.4% respectively. Exposure to 100 μg/L TPhP respectively down-regulated the expression levels of sine oculis homeobox homologue 3 (six3) and short wavelength-sensitive type 2 (sws2) by 19.1% and 41.7%, suggesting that TPhP might disturb eye development and photoreception and consequently the low locomotor activity in the larvae. Interestingly, during the binary mixture exposure, the presence of PS, PS-COOH or PS-NH₂ reversed the low locomotor activity induced by 100 μg/L TPhP to the normal level. Relative to the larvae from the 100 μg/L TPhP group, the movement duration and distance moved were increased by approximately 60% and 100%, respectively, in the larvae from the TPhP + PS, TPhP + PS-COOH and TPhP + PS-NH₂ groups. However, the gene expression profiles were distinct among the fish from the TPhP + PS, TPhP + PS-COOH and TPhP + PS-NH₂ groups, implying different mechanisms underlying the reversal of the locomotor activity. The findings in this study challenge the general view that microplastics aggravate the toxicity of the adsorbed pollutants, and help better understand the environmental risk of microplastic pollution.

Influence of triphenyltin on morphologic abnormalities and the thyroid hormone system in early-stage zebrafish (Danio rerio)

In the present study, we assessed the negative effects of triphenyltin (TPT) on zebrafish (Danio rerio) by exposing embryos and early-stage larvae to various concentrations of TPT from 2 h after fertilization (haf) until 30 days after hatching (dah). Whether test groups were fed or fasted during ecotoxicity studies using fish models has varied historically, and whether this experimental condition influences test results is unknown. Here, we confirmed that the lethal concentration of TPT to embryo and early-stage larvae (i.e., 3 dah or younger) showed in fed (lowest observed effect concentration (LOEC); 6.34 μg/L) and fasted (LOEC; 6.84 μg/L) groups. In addition, 84% and 100% of the larvae in the 2.95 and 6.64 μg/L exposure groups, respectively, had uninflated swim bladders; all affected larvae died within 9 dah. This finding suggests that morphologic abnormalities in early larval zebrafish are useful as endpoints for predicting the lethality of chemical substances after hatching. We then assessed the expression of several genes in the thyroid hormone pathway, which regulates swim bladder development in many fish species, including zebrafish. Larvae exposed to 6.64 μg/L TPT showed significant increases in the mRNA expression levels of thyroid hormone receptor α (trα) and trβ but not of thyroid stimulating hormone β subunit. These findings suggest that TPT disrupts the thyroid system in zebrafish.

Review on endocrine disrupting toxicity of triphenyltin from the perspective of species evolution: Aquatic, amphibious and mammalian

Triphenyltin (TPT) is widely used as a plastic stabilizer, insecticide and the most common fungicide in antifouling coatings. This paper reviewed the main literature evidences on the morphological and physiological changes of animal endocrine system induced by TPT, with emphasis on the research progress of TPT metabolism, neurological and reproductive regulation in animal endocrine system. Similar to tributyltin (TBT), the main effects of TPT on the potential health risks of 25 species of animals, from aquatic animals to mammals, are not only related to exposure dose and time, but also to age, sex and exposed tissue/cells. Moreover, current studies have shown that TPT can directly damage the endocrine glands, interfere with the regulation of neurohormones on endocrine function, and change hormone synthesis and/or the bioavailability (i.e., in the retinoid X receptor and peroxisome proliferator-activated receptor gamma RXR-PPARγ) in target cells. Importantly, TPT can cause biochemical and morphological changes of gonads and abnormal production of steroids, both of which are related to reproductive dysfunction, for example, the imposex of aquatic animals and the irregular estrous cycle of female mammals or spermatogenic disorders of male animals. Therefore, TPT should indeed be regarded as a major endocrine disruptor, which is essential for understanding the main toxic effects on different tissues and their pathogenic effects on endocrine, metabolism, neurological and reproductive dysfunction.

Tissue distribution of triphenyltin compounds in marine teleost fishes

Continuous release of the highly toxic triphenyltin compounds (TPT) from antifouling paints and fungicides has caused serious pollution to urbanized coastal marine environments worldwide since the 1960s. Using gas-chromatography mass-spectrometry (GC-MS), this study investigated the distribution profile of TPT in 15 types of tissues of four marine teleost fish species collected from Hong Kong waters. Concentrations of TPT in various tissues had a significant positive correlation with protein contents in the tissues (r = 0.346, p < 0.001) and, to a lesser extent with lipid contents (r = 0.169, p =  0.020). Highest concentrations of TPT were consistently found in liver, ranging from 1074.9 to 3443.7 ng/g wet weight; whereas fish scales always contained the least concentration of TPT in all species, ranging from 10.4 to 48.5 ng/g wet weight. Through mass balance models and regression analyses, muscle tissues were found to contribute most to the total TPT body burden, and the average TPT concentration of both dorsal and ventral muscles was identified as the best predictor for estimating TPT burden in the entire fish. Hence, further investigations of bioaccumulation and biomagnification of TPT in fishes should adopt this modelling approach in estimating its total body burden in individual fish.

Life stage-dependent toxicities of 1-ethyl-3-methylimidazolium bromide on Caenorhabditis elegans

Ionic liquids (ILs) are considered as extracting solvents in soil remediation. However, they can be pollutants themselves, and their own toxicities are of concerns. Notably, organisms were exposed to pollutants at random life stages in actual environmental exposure scenario, which is different from the set-up of one uniform life stage in usual experiment designs. The influence of life stages on ILs toxicities will provide essential information on their actual environmental risks. In the present study, effects of 1-ethyl-3-methylimidazolium bromide ([C₂mim]Br) were measured on C. elegans with egg exposure and adult exposure. In egg exposure, [C₂mim]Br increased the lifespan, stimulated initial reproduction and inhibited the total reproduction. Biochemical indices including oxidative stress, antioxidant responses and oxidative damage were further measured to explore the toxicity mechanisms. Results showed that [C₂mim]Br significantly stimulated O₂^-· as the oxidative stress and superoxide dismutase (SOD) as the antioxidant defense. In adult exposure, [C₂mim]Br inhibited initial reproduction, total reproduction and lifespan. Biochemical results showed that [C₂mim]Br significantly stimulated H₂O₂ and oxidized glutathione (GSSG). The overall findings demonstrated that [C₂mim]Br caused life stage-dependent toxicities on C. elegans. Future studies are still needed for the detailed mechanisms.

Microplastics negatively impact embryogenesis and modulate the immune response of the marine medaka Oryzias melastigma

Microplastic (MP) pollution is an emerging contaminant in aquatic environments worldwide. Nonetheless, the developmental toxicity of MPs in the early life stages of fish and the mechanisms involved are not yet fully understood. The present study investigated the effects of different concentrations of polystyrene (PS) MPs on the early development of the marine model fish the medaka Oryzias melastigma. Our results showed that waterborne exposure to PS MPs significantly delayed the hatching time, altered the heartbeat and decreased the hatching rate of embryos. Furthermore, the genes involved in cardiac development, encoding for embryo-hatching enzymes, as well as inflammatory responses were significantly upregulated. The transcriptome results showed that mainly the pathways involved in metabolism, immune response, genetic information processing and diseases were significantly enriched. These results demonstrate that PS MPs negatively impact embryogenesis and the immune response of O. melastigma.

Generation and application of a Tg(cyp1a:egfp) transgenic marine medaka (Oryzias melastigma) line as an in vivo assay to sensitively detect dioxin-like compounds in the environment

Large-range environmental pollution by dioxin and dioxin-like compounds (DLCs) is becoming a serious problem. To establish an in vivo method for the detection of DLCs in seawater, a Tg(cyp1a-12DRE:egfp) transgenic marine medaka (Oryzias melastigma) line was first developed with the modified cyp1a-12DRE promoter driving enhanced green fluorescent protein (EGFP) expression using Tol2 transgenesis technology. With increasing concentrations of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), the EGFP fluorescence intensity increased significantly. The Tg(cyp1a-12DRE:egfp) medaka possessed high sensitivity (limit of detection of 1 ng/L TCDD) and specificity and low background. This transgenic line is capable of detecting DLCs in environmental seawater in which the concentration of DLCs is at least 0.12207 ng/L TCDD after sample enrichment. The fluorescence-toxic equivalency (TEQ) values from EGFP intensity were closely correlated with the chemical-TEQ values obtained from chemical analyses. Furthermore, the Tg(cyp1a-12DRE:egfp) medaka can directly detect DLCs in seawater samples after a serious pollution accident and screen unknown aryl hydrocarbon receptor (AhR) agonists for risk assessment. For the first time, a convenient method has been established that sensitively and specifically responds to DLCs using the Tg(cyp1a-12DRE:egfp) marine medaka, which could be a highly efficient tool for detecting seawater DLCs in the future.

Hemolysis associated toxicities of benthic dinoflagellates from Hong Kong waters

Benthic dinoflagellates produce a diverse range of phycotoxins, which are responsible for intoxication events in marine fauna. This study assessed the hemolysis associated toxicities of six species of benthic dinoflagellates from the genera Coolia, Fukuyoa, Amphidinium and Prorocentrum. Results demonstrated that Amphidinium carterae, Coolia tropicalis and Fukuyoa ruetzleri were the three most toxic species, while Prorocentrum cf. lima did not have significant hemolytic effect. Grouper samples (Cephalopholis boenak) were more tolerant to the hemolytic algae than the blackhead seabream (Acanthopagrus schlegelii), with decreased heart rate and blood flow being observed in medaka larvae after exposure to toxic algal extracts. LC-MS/MS analysis detected a gambierone analogue called 44-methylgambierone produced by the C. tropicalis isolate. This analogue was also detected in the F. ruetzleri isolate. This study provided new information on the hemolysis associated toxicities of local toxic benthic dinoflagellates, which contributes to better understanding of their emerging threats to marine fauna and reef systems in Hong Kong.

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.

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.