Hypoxia turns genotypic female medaka fish into phenotypic males

Integrated analysis of transcriptome, translatome and proteome reveals insights into yellow catfish (Pelteobagrus fulvidraco) brain in response to hypoxia

Brain plays a central role in adapting to environmental changes and is highly sensitive to the oxygen level. Although previous studies investigated the molecular response of brain exposure to acute hypoxia in fish, the lack of studies at the translational level hinders further understanding of the regulatory mechanism response to hypoxia from multi-omics levels. Yellow catfish (Pelteobagrus fulvidraco) is an important freshwater aquaculture species; however, hypoxia severely restricts the sustainable development of its breeding industry. In the present study, the transcriptome, translatome, and proteome were integrated to study the global landscapes of yellow catfish brain response to hypoxia. The evidently increased amount of cerebral cortical cells with oedema and pyknotic nuclei has been observed in hypoxia group of yellow catfish. A total of 2750 genes were significantly changed at the translational level. Comparative transcriptional and translational analysis suggested the HIF-1 signaling pathway, autophagy and glycolysis/gluconeogenesis were up-regulated after hypoxia exposure. KEGG enrichment of translational efficiency (TE) differential genes suggested that the lysosome and autophagy were highly enriched. Our result showed that yellow catfish tends to inhibit the TE of genes by increasing the translation of uORFs to adapt to hypoxia. Correlation analysis showed that transcriptome and translatome exhibit higher correlation. In summary, this study demonstrated that hypoxia dysregulated the cerebral function of yellow catfish at the transcriptome, translatome, and proteome, which provides a better understanding of hypoxia adaptation in teleost.

Molecular biological, physiological, cytological, and epigenetic mechanisms of environmental sex differentiation in teleosts: A systematic review

Human activities have been exerting widespread stress and environmental risks in aquatic ecosystems. Environmental stress, including temperature rise, acidification, hypoxia, light pollution, and crowding, had a considerable negative impact on the life histology of aquatic animals, especially on sex differentiation (SDi) and the resulting sex ratios. Understanding how the sex of fish responds to stressful environments is of great importance for understanding the origin and maintenance of sex, the dynamics of the natural population in the changing world, and the precise application of sex control in aquaculture. This review conducted an exhaustive search of the available literature on the influence of environmental stress (ES) on SDi. Evidence has shown that all types of ES can affect SDi and universally result in an increase in males or masculinization, which has been reported in 100 fish species and 121 cases. Then, this comprehensive review aimed to summarize the molecular biology, physiology, cytology, and epigenetic mechanisms through which ES contributes to male development or masculinization. The relationship between ES and fish SDi from multiple aspects was analyzed, and it was found that environmental sex differentiation (ESDi) is the result of the combined effects of genetic and epigenetic factors, self-physiological regulation, and response to environmental signals, which involves a sophisticated network of various hormones and numerous genes at multiple levels and multiple gradations in bipotential gonads. In both normal male differentiation and ES-induced masculinization, the stress pathway and epigenetic regulation play important roles; however, how they co-regulate SDi is unclear. Evidence suggests that the universal emergence or increase in males in aquatic animals is an adaptation to moderate ES. ES-induced sex reversal should be fully investigated in more fish species and extensively in the wild. The potential aquaculture applications and difficulties associated with ESDi have also been addressed. Finally, the knowledge gaps in the ESDi are presented, which will guide the priorities of future research.

Living in a hypoxic world: A review of the impacts of hypoxia on aquaculture

Hypoxia is a harmful result of anthropogenic climate change. With the expansion of global low-oxygen zones (LOZs), many organisms have faced unprecedented challenges affecting their survival and reproduction. Extensive research has indicated that oxygen limitation has drastic effects on aquatic animals, including on their development, morphology, behavior, reproduction, and physiological metabolism. In this review, the global distribution and formation of LOZs were analyzed, and the impacts of hypoxia on aquatic animals and the molecular responses of aquatic animals to hypoxia were then summarized. The commonalities and specificities of the response to hypoxia in aquatic animals in different LOZs were discussed lastly. In general, this review will deepen the knowledge of the impacts of hypoxia on aquaculture and provide more information and research directions for the development of fishery resource protection strategies.

Divergent Carry-Over Effects of Hypoxia during the Early Development of Abalone

After being exposed to environmental stimuli during early developmental stages, some organisms may gain or weaken physiological regulating abilities, which would have long-lasting effects on their performance. Environmental hypoxia events can have significant effects on marine organisms, but for breeding programs and other practical applications, it is important to further explore the long-term physiological effects of early hypoxia exposure in economically significant species. In this study, the Pacific abalone Haliotis discus hannai was exposed to moderate hypoxia (∼4 mg/L) from zygote to trochophora, and the assessments of hypoxia tolerance were conducted on the grow-out stage. The results revealed that juvenile abalones exposed to hypoxia at the early development stages were more hypoxia-tolerant but with slower weight growth, a phenomenon called the trade-off between growth and survival. These phenotypic effects driven by the hypoxia exposure were explained by strong selection of genes involved in signal transduction, autophagy, apoptosis, and hormone regulation. Moreover, long non-coding RNA regulation plays an important role modulating carry-over effects by controlling DNA replication and repair, signal transduction, myocardial activity, and hormone regulation. This study revealed that the ability to create favorable phenotypic differentiation through genetic selection and/or epigenetic regulation is important for the survival and development of aquatic animals in the face of rapidly changing environmental conditions.

Sex determination mechanisms and sex control approaches in aquaculture animals

Aquaculture is one of the most efficient modes of animal protein production and plays an important role in global food security. Aquaculture animals exhibit extraordinarily diverse sexual phenotypes and underlying mechanisms, providing an ideal system to perform sex determination research, one of the important areas in life science. Moreover, sex is also one of the most valuable traits because sexual dimorphism in growth, size, and other economic characteristics commonly exist in aquaculture animals. Here, we synthesize current knowledge of sex determination mechanisms, sex chromosome evolution, reproduction strategies, and sexual dimorphism, and also review several approaches for sex control in aquaculture animals, including artificial gynogenesis, application of sex-specific or sex chromosome-linked markers, artificial sex reversal, as well as gene editing. We anticipate that better understanding of sex determination mechanisms and innovation of sex control approaches will facilitate sustainable development of aquaculture.

A cross-species comparative approach to assessing multi- and transgenerational effects of endocrine disrupting chemicals

A wide range of chemicals have been identified as endocrine disrupting chemicals (EDCs) in vertebrate species. Most studies of EDCs have focused on exposure of both male and female adults to these chemicals; however, there is clear evidence that EDCs have dramatic effects when mature or developing gametes are exposed, and consequently are associated with in multigenerational and transgenerational effects. Several publications have reviewed such actions of EDCs in subgroups of species, e.g., fish or rodents. In this review, we take a holistic approach synthesizing knowledge of the effects of EDCs across vertebrate species, including fish, anurans, birds, and mammals, and discuss the potential mechanism(s) mediating such multi- and transgenerational effects. We also propose a series of recommendations aimed at moving the field forward in a structured and coherent manner.

Influence of hypoxia on biochemical aspects and on expression of genes related to oxygen-homeostasis of the Amazonian cichlid Astronotus ocellatus (Agassiz, 1831)

Variations in dissolved oxygen levels are common in the Amazonian aquatic environments and the aquatic organisms that inhabit these environments developed a variety of adaptive responses to deal with such conditions. Some Amazonian fish species are tolerant to low oxygen levels and the cichlid Astronotus ocellatus is one of the most hypoxia-tolerant species. Herein, we aimed to unveil the biochemical and molecular responses that A. ocellatus presents when submitted to hypoxia. Hypoxia indicators were measured, such as plasma glucose, plasma lactate, hepatic glycogen and relative transcript levels of prolyl hydroxylase 2 (phd2) and hypoxia-inducible factor-1α (hif-1α) in juveniles of approximately 50 g exposed to 1, 3, and 5 hours of hypoxia (0.7 mg O₂.L^-1), followed by 3 hours of recovery in normoxia (6 mg O₂.L^-1). Fish exposed to hypoxia reduced liver glycogen levels within 3 hours of hypoxia, when comparing with 1 hour, and increased plasma glucose and lactate. Under the same condition, phd2 transcripts levels increased in gills, but decreased in liver. In contrast, hypoxia did not affect relative gene expression of hif-1α in both tissues. Based on the transcription pattern of phd2, these results showed that liver and gills of A. ocellatus have different molecular strategies to cope with environmental hypoxia.

Developmental aspects of the hypothalamic-pituitary network related to reproduction in teleost fish

The hypothalamic-pituitary-gonadal axis is the main system that regulates reproduction in vertebrates through a complex network that involves different neuropeptides, neurotransmitters, and pituitary hormones. Considering that this axis is established early on life, the main goal of the present work is to gather information on its development and the actions of its components during early life stages. This review focuses on fish because their neuroanatomical characteristics make them excellent models to study neuroendocrine systems. The following points are discussed: i) developmental functions of the neuroendocrine components of this network, and ii) developmental disruptions that may impact adult reproduction. The importance of the components of this network and their susceptibility to external/internal signals that can alter their specific early functions and/or even the establishment of the reproductive axis, indicate that more studies are necessary to understand this complex and dynamic network.

Sex-reversal and Histopathological Assessment of Potential Endocrine-Disrupting Effects of Graphene Oxide on Japanese medaka (Oryzias latipes) Larvae

Sex-ratio is considered as an end point during endocrine disrupting chemicals (EDCs) evaluation. Many fish species including Japanese medaka have XX/XY sex determination mechanism, however, sex reversal (SR) can be induced by external and genetic factors. SR imposed an imbalance in natural sex ratio of a population living in any ecosystem. Considering SR as an end point, we aimed to investigate the potential EDC effects of graphene oxide (GO), a nanocarbon, using Japanese medaka as a model. One-day post-hatch (dph) medaka fries were exposed to GO (2.5, 5.0, 10.0 and 20 mg/L) for 96 h without food, followed by 6 weeks depuration in a GO-free environment with feeding. Phenotypic sex was determined by gonad histology; genotypic sex by genotyping Y-chromosome-specific male sex determining gene, dmy. Our data indicated testes in both XY and XX genotypes, while ovaries were only in XX females. Histopathology of XY and XX testis showed isogenic spermatocysts with active spermatogenesis. Distribution of spermatocytes (SPTs), not the spermatogonium (SPGs), showed enhancement in XY than XX testis. Female phenotypes had single ovary, either in stage 0 or 1. Ovo-testis/testis-ova were absent in XX or XY gonads. GO (2.5-20 mg/L) had inconsistent concentration-dependent effect in both SPGs and SPTs; however, no effect on ovarian follicles. Despite genotypic differences (XY/XX), in the histopathology/histochemistry of liver and kidneys GO effects was found to be minimum. Taken together, present study showed spontaneous induction of SR in some XX genotypes; however, exposure of fasting fries to GO had no apparent EDC effects.

Metabolism and Sex Differentiation in Animals from a Starvation Perspective

Animals determine their sex genetically (GSD: genetic sex determination) and/or environmentally (ESD: environmental sex determination). Medaka (Oryzias latipes) employ a XX/XY GSD system, however, they display female-to-male sex reversal in response to various environmental changes such as temperature, hypoxia, and green light. Interestingly, we found that 5 days of starvation during sex differentiation caused female-to-male sex reversal. In this situation, the metabolism of pantothenate and fatty acid synthesis plays an important role in sex reversal. Metabolism is associated with other biological factors such as germ cells, HPG axis, lipids, and epigenetics, and supplys substances and acts as signal transducers. In this review, we discuss the importance of metabolism during sex differentiation and how metabolism contributes to sex differentiation.

R-spondin1 in loach (Misgurnus anguillicaudatus): Identification, characterization, and analysis of its expression patterns and DNA methylation in response to high-temperature stress

With a well-understood function in mammals, R-spondin1 (Rspo1) is an important regulator of ovarian development via the Wnt/β-catenin pathway. Rspo1 deficiency causes retardation of ovarian development in XX fish, and increases Rspo1 function induces femininity and sex reversal in XY fish. In this study, Rspo1 was successfully cloned from loach (Misgurnus anguillicaudatus), and its expression profile was analyzed. The full-length cDNA of Misgurnus anguillicaudatus Rspo1 (MaRspo1) comprised 1322 bp and included an open reading frame (ORF) of 795 bp, which encoded a predicted polypeptide measuring 264 amino acids in length. Phylogenetic and gene structure analyses showed a highly conserved sequence of MaRspo1 (identical to the Rspo1 genes of other species), consisting of an N-terminal signal peptide (SP), two furin-like cysteine-rich domains (FU1 and FU2), a thrombospondin type 1 repeat (TSP1) and a C-terminal region. Real-time PCR revealed the female-biased expression profile of MaRspo1, with the highest expression level among tested tissues detected in ovary. Investigation of MaRspo1 expression levels throughout the early development stage (10-60 days post hatching) under three temperature treatments (25 °C, 28 °C, and 31 °C) revealed significantly differential expression of MaRspo1 among the three temperature groups, with decreased MaRspo1 expression in the high-temperature (31 °C) group. The results of DNA methylation analysis indicated that exposure to high temperature during early development can increase the average promoter methylation level of MaRspo1 in both females and males. Taken together, the results of this study provide the basis for the further investigation of the molecular mechanism of Rspo1 in response to temperature.

Assessment of reproductive and developmental effects of graphene oxide on Japanese medaka (Oryzias latipes)

Due to its unique properties, graphene oxide (GO) has potential for biomedical and electronic applications, however environmental contamination including aquatic ecosystem is inevitable. Moreover, potential risks of GO in aquatic life are inadequately explored. Present study was designed to evaluate GO as an endocrine disrupting chemical (EDC) using the model Japanese medaka (Oryzias latipes). GO was injected intraperitoneally (25-200 μg/g) once to breeding pairs and continued pair breeding an additional 21 days. Eggs laid were analyzed for fecundity and the fertilized eggs were evaluated for developmental abnormalities including hatching. Histopathological evaluation of gonads, liver, and kidneys was made 21 days post-injection. LD50 was found to be sex-dependent. Fecundity tended to reduce in a dose-dependent manner during early post-injection days; however, the overall evaluation showed no significant difference. The hatchability of embryos was reduced significantly in the 200 μg/g group; edema (yolk and cardiovascular) and embryo-mortality remained unaltered. Histopathological assessment identified black particles, probably agglomerated GO, in the gonads of GO-treated fish. However, folliculogenesis in stromal compartments of ovary and the composition of germinal elements in testis remained almost unaltered. Moreover, granulosa and Leydig cells morphology did not indicate any significant EDC-related effects. Although liver and kidney histopathology did not show GO as an EDC, some GO-treated fish accumulated proteinaceous fluid in hepatic vessels and induced hyperplasia in interstitial lymphoid cells (HIL) located in kidneys. GO agglomerated in medaka gonads after 21-days post-injection. However, gonad histopathology including granulosa and Leydig cells alterations were associated with GO toxicity rather than EDC effects.

Activation of stress response axis as a key process in environment-induced sex plasticity in fish

The determination of sex is an important hallmark in the life cycle of organisms, in which the fate of gonads and then the individual sex are defined. In gonochoristic teleost fish, this process is characterized by a high plasticity, considering that in spite of genotypic sex many environmental factors can cause shifts from one to another molecular pathway, resulting in organisms with mismatching genotypic and phenotypic sexes. Interestingly, in most instances, both female-to-male or male-to-female sex-reversed individuals develop functional gonads with normal gametogenesis and respective progenies with full viability. The study of these mechanisms is being spread to other non-model species or to those inhabiting more extreme environmental conditions. Although water temperature is an important mechanism involved in sex determination, there are other environmental stressors affected by the climate change which are also implicated in stress response-induced masculinization in fish. In this regard, the brain has emerged as the transducer of the environment input that can influence the gonadal fate. Furthermore, the evaluation of other environmental stressors or their synergic effect on sex determination at conditions that simulate the natural environments is growing gradually. Within such scope, the concerns related to climate change impacts rely on the fact that many of biotic and abiotic parameters reported to affect sex ratios are expected to increase concomitantly as a result of increased greenhouse gas emissions and, particularly worrying, many of them are related to male bias in the populations, such as high temperature, hypoxia, and acidity. These environmental changes can also generate epigenetic changes in sex-related genes affecting their expression, with implications on sex differentiation not only of exposed individuals but also in following generations. The co-analysis of multi-stressors with potential inter- and transgenerational effects is essential to allow researchers to perform long-term predictions on climate change impacts in wild populations and for establishing highly accurate monitoring tools and suitable mitigation strategies.

Evaluation of potential environmental toxicity of polymeric nanomaterials and surfactants

Nanomaterials have gained huge importance in various fields including nanomedicine. Nanoformulations of drugs and nanocarriers are used to increase pharmaceutical potency. However, it was seen that polymeric nanomaterials can cause negative effects. Thus, it is essential to identify nanomaterials with the least adverse effects on aquatic organisms. To determine the toxicity of polymeric nanomaterials, we investigated the effects of poly(lactic-co-glycolid) acid (PLGA), Eudragit® E 100 and hydroxylpropyl methylcellulose phthalate (HPMCP) on zebrafish embryos using the fish embryo toxicity test (FET). Furthermore, we studied Cremophor® RH40, Cremophor® A25, Pluronic® F127 and Pluronic® F68 applied in the generation of nanoformulations to identify the surfactant with minimal toxic impact. The order of ecotoxicty was HPMCP < PLGA < Eudragit® E100 and Pluronic® F68 < Pluronic® F127 < Cremophor® RH40 < Cremophor® A25. In summary, HPMCP and Pluronic® F68 displayed the least toxic impact, thus suggesting adequate environmental compatibility for the generation of nanomedicines.

Transgenic Medaka Identify Embryonic Periods Sensitive to Disruption of Sex Determination

Gonadal development in medaka (Oryzias latipes) is dependent on the synergy between estrogens and androgens. Disruption of steroid hormone levels can lead to ovo-testis. To determine the sensitive windows for hormonally induced sex reversal in medaka, we developed a novel 42sp50-GFP_ChgH-GFP transgenic medaka line, allowing the identification of female gonadal tissue by fluorescence present in developing oocytes. Germinal transgenesis resulted in a stable line exhibiting a strong green fluorescent protein signal constitutively in the ovaries and in the liver in response to estrogens. The sensitivity of this line to disruption of sex determination following 16-d chronic exposures was in the nanograms per liter range. To identify the developmental period sensitive to exogenous agents, fry were exposed to 24-h pulses of high concentrations of 17β-estradiol (E2) or 5α-dihydrotestosterone (DHT) at various time points between days postfertilization (dpf) 0 and 12. Evaluation of phenotype followed by genotyping at 16 dpf revealed sensitivity to E2 between 1 and 8 dpf as well as 2 periods of susceptibility to DHT between 0 and 1 dpf and 4 and 8 dpf. No phenotypic sex reversal was detected after exposure to DHT or E2 on 11 or 12 dpf. The observed effects persisted to at least 24 dpf. The identified sensitive embryonic time periods for disruption of sex determination will aid future research on sex determination and the development of screening assays using early embryonic life stages. Environ Toxicol Chem 2020;39:842-851. © 2020 SETAC.

Interpretation of sexual secondary characteristics (SSCs) in regulatory testing for endocrine activity in fish

Secondary sexual characteristics (SSCs) are important features that have evolved in many fish species because of inter-individual competition for mates. SSCs are crucial not only for sexual selection, but also for other components of the reproductive process and parental care. Externally, they are especially clear in males (for instance, tubercles, fatpad, anal finnage, colouration) but are also externally present in the females (for instance, ovipositor). These characters are under hormonal control and as such there has been much interest in incorporating them as measures in fish test methods to assess the potential endocrine activity of chemicals. Here we describe the external SSCs in typical laboratory test species for endocrine testing - fathead minnow (Pimephales promelas), Japanese medaka (Oryzias latipes), zebrafish (Danio rerio) and the three-spined stickleback (Gasterosteus aculeatus L.). We also provide some examples and discuss the utility of SSC responses to the endocrine activity of chemicals in the field and the laboratory. This paper is not aimed to provide a comprehensive review of SSCs in fish but presents a view on the assessment of SSCs in regulatory testing. Due to the current regulatory importance of establishing an endocrine mode-of-action for chemicals, we also consider other, non-endocrine factors that may lead to SSC responses in fish. We conclude with recommendations for how the assessment of SSCs in fish could be usefully incorporated into the endocrine hazard and risk assessment of chemicals.

Expression profiles of amhy and major sex-related genes during gonadal sex differentiation and their relation with genotypic and temperature-dependent sex determination in pejerrey Odontesthes bonariensis

To shed light on the mechanisms of and interactions of GSD and TSD in pejerrey, we investigated how the transcriptional profiles of amhy and amha are affected by feminizing (17 °C) and masculinizing (29 °C) temperatures during the critical period of sex determination/differentiation and their relation with the expression profiles of AMH receptor type II (amhrII), gonadal aromatase (cyp19a1a), and 11 beta-hydroxysteroid dehydrogenase 2 (hsd11b2). Careful consideration of the results of this study and all information currently available for this species, including similar analyzes for an intermediate, mixed-sex promoting temperature (25 °C), suggests a model for genotypic/temperature-dependent sex determination and gonadal sex differentiation that involves a) cyp19a1a-dependent, developmentally-programmed ovarian development as the default state that becomes self-sustaining in the absence of a potent and timely masculinizing stimulus, b) early, developmentally-programmed amhy expression and high temperature as masculinization signals that antagonize the putative female pathway by suppressing cyp19a1a expression, c) increasing stress response, cortisol, and the synthesis of the masculinizing androgen 11-keto-testosterone via hsd11b2 with increasing temperature that is important for masculinization in both genotypes but particularly so in XX individuals, and d) an endocrine network with positive/negative feedback mechanisms that ensure fidelity of the male/female pathway once started. The proposed model, albeit tentative and non-all inclusive, accounts for the continuum of responses, from all-females at low temperatures to all-males at high temperatures and for the balanced-, genotype-linked sex ratios obtained at intermediate temperatures, and therefore supports the coexistence of TSD and GSD in pejerrey across the range of viable temperatures for this species.

Hypoxia-inducible transcription factors in fish: expression, function and interconnection with the circadian clock

The hypoxia-inducible transcription factors are key regulators for the physiological response to low oxygen availability. In vertebrates, typically three Hif-α isoforms, Hif-1α, Hif-2α and Hif-3α, are expressed, each of which, together with Hif-1β, may form a functional heterodimer under hypoxic conditions, controlling expression of hundreds of genes. A teleost-specific whole-genome duplication complicates the analysis of isoform-specific functions in fish, but recent studies suggest that the existence of paralogues of a specific isoform opens up the possibility for a subfunctionalization. In contrast to during development inside the uterus, fish eggs are freely accessible and studies analyzing Hif expression in fish embryos during development have revealed that Hif proteins are not only controlling the hypoxic response, but are also crucial for proper development and organ differentiation. Significant advances have been made in our knowledge about tissue-specific functions of Hif proteins, especially with respect to gill or gonadal tissue. The hypoxia signalling pathway is known to be tightly and mutually intertwined with the circadian clock in zebrafish and mammals. Recently, a mechanistic explanation for the hypoxia-induced dampening of the transcriptional clock was detected in zebrafish, including also metabolically induced alterations of cellular redox signalling. In turn, MAP kinase-mediated H2O2 signalling modulates the temporal expression of Hif-1α protein, similar to the redox regulation of the circadian clock itself. Once again, the zebrafish has emerged as an excellent model organism with which to explore these specific functional aspects of basic eukaryotic cell biology.

Transcriptomic analysis reveals transgenerational effect of hypoxia on the neural control of testicular functions

There are over 400 hypoxic zones in the ocean worldwide. Both laboratory and field studies have shown that hypoxia causes endocrine disruption and reproductive impairments in vertebrates. More importantly, our recent study discovered that parental (F0) hypoxia exposure resulted in the transgenerational impairment of sperm quality in the F2 generation through the epigenetic regulation of germ cells. In the present study, we aim to test the hypothesis that the brain, as the major regulator of the brain-pituitary-gonad (BPG) axis, is also involved in the observed transgenerational effect. Using comparative transcriptomic analysis on brain tissues of marine medaka Oryzias melastigma, 45 common differentially expressed genes caused by parental hypoxia exposure were found in the hypoxic group of the F0 and F2 generations, and the transgenerational groups of the F2 generation. The bioinformatic analysis on this deregulated gene cluster further highlighted the possible involvement of the brain in the transgenerational effect of hypoxia on testicular structure, including abnormal morphologies of the epididymis and the seminal vesicle, and degeneration of the seminiferous tubule. This finding is concordant to the result of hematoxylin and eosin staining, which showed the reduction of testicular lobular diameter in the F0 and F2 generations. Our study demonstrated for the first time the involvement of the brain in the transgenerational effect of hypoxia.

Early life co-exposures to a real-world PAH mixture and hypoxia result in later life and next generation consequences in medaka (Oryzias latipes)

Acute effects of individual and complex mixtures of polycyclic aromatic hydrocarbons (PAHs) are well documented in vertebrate species. Hypoxia in fish reduces metabolic rate and reproduction. However, less is known about the later life consequences stemming from early-life exposure to PAHs or hypoxia, particularly their co-exposure. To address this, medaka (Oryzias latipes) embryos were exposed to a complex PAH mixture sediment extract from the Elizabeth River, VA (ERSE) at concentrations of 0.1, 0.5, or 1.0% or to one of three different hypoxia scenarios: continuous, nocturnal, or late stage embryogenesis hypoxia. Co-exposures with 0.1% ERSE and each of the hypoxia scenarios were conducted. Results included decreased survival with ERSE, hatching delays with hypoxia, and higher occurrences of deformities with each. The continuous hypoxia scenario caused the most significant changes in all endpoints. These early-life exposures altered later-life growth, impaired reproductive capacity, and reduced the quality of their offspring. ERSE alone resulted in a female-biased sex ratio while continuous or nocturnal hypoxia produced significantly greater numbers of males; and co-exposure produced an equal sex ratio. Exposure to a PAH mixture and hypoxia during early life stages has meaningful later-life and next generational consequences.

Characterization and functional analysis of hypoxia-inducible factor HIF1α and its inhibitor HIF1αn in tilapia

Hypoxia is a major cause of fish morbidity and mortality in the aquatic environment. Hypoxia-inducible factors are very important modulators in the transcriptional response to hypoxic stress. In this study, we characterized and conducted functional analysis of hypoxia-inducible factor HIF1α and its inhibitor HIF1αn in Nile tilapia (Oreochromis niloticus). By cloning and Sanger sequencing, we obtained the full length cDNA sequences for HIF1α (2686bp) and HIF1αn (1308bp), respectively. The CDS of HIF1α includes 15 exons encoding 768 amino acid residues and the CDS of HIF1αn contains 8 exons encoding 354 amino acid residues. The complete CDS sequences of HIF1α and HIF1αn cloned from tilapia shared very high homology with known genes from other fishes. HIF1α show differentiated expression in different tissues (brain, heart, gill, spleen, liver) and at different hypoxia exposure times (6h, 12h, 24h). HIF1αn expression level under hypoxia is generally increased (6h, 12h, 24h) and shows extremely highly upregulation in brain tissue under hypoxia. A functional determination site analysis in the protein sequences between fish and land animals identified 21 amino acid sites in HIF1α and 2 sites in HIF1αn as significantly associated sites (α = 0.05). Phylogenetic tree-based positive selection analysis suggested 22 sites in HIF1α as positively selected sites with a p-value of at least 95% for fish lineages compared to the land animals. Our study could be important for clarifying the mechanism of fish adaptation to aquatic hypoxia environment.

Bmp15 Is an Oocyte-Produced Signal Required for Maintenance of the Adult Female Sexual Phenotype in Zebrafish

Although the zebrafish is a major model organism, how they determine sex is not well understood. In domesticated zebrafish, sex determination appears to be polygenic, being influenced by multiple genetic factors that may vary from strain to strain, and additionally can be influenced by environmental factors. However, the requirement of germ cells for female sex determination is well documented: animals that lack germ cells, or oocytes in particular, develop exclusively as males. Recently, it has been determined that oocytes are also required throughout the adult life of the animal to maintain the differentiated female state. How oocytes control sex differentiation and maintenance of the sexual phenotype is unknown. We therefore generated targeted mutations in genes for two oocyte produced signaling molecules, Bmp15 and Gdf9 and here report a novel role for Bmp15 in maintaining adult female sex differentiation in zebrafish. Females deficient in Bmp15 begin development normally but switch sex during the mid- to late- juvenile stage, and become fertile males. Additionally, by generating mutations in the aromatase cyp19a1a, we show that estrogen production is necessary for female development and that the function of Bmp15 in female sex maintenance is likely linked to the regulation of estrogen biosynthesis via promoting the development of estrogen-producing granulosa cells in the oocyte follicle.

Interpretation of fish biomarker data for identification, classification, risk assessment and testing of endocrine disrupting chemicals

Chemical induced changes in fish biomarkers vitellogenin (VTG), secondary sex characteristics (SSC), and sex ratio indicate modes/mechanisms of action (MOAs) of EAS (estrogen, androgen and steroidogenesis) pathways. These biomarkers could be used for defining MOAs and the causal link between MOAs and adverse effects in fish for the identification of endocrine disrupting chemicals (EDCs). This paper compiled data sets of 150 chemicals for VTG, 57 chemicals for SSC and 38 chemicals for sex ratio in fathead minnow, medaka and zebrafish. It showed 1) changes in fish biomarkers can indicate the MOAs as anticipated; 2) in addition to EAS pathways, chemicals with non-EAS pathways induced changes in fish biomarkers; 3) responses of fish biomarkers did not always follow the anticipated patterns of EAS pathways. These responses may result from the interaction of chemical-induced multiple MOAs and confounding factors like fish diet, infection, culture conditions, general toxicity and stress response. The complex response of fish biomarkers to a chemical of interest requires EDC testing at multiple biological levels. Interpretation of fish biomarker data should be combined with relevant information at different biological levels, which is critical for defining chemical specific MOAs. The utility of fish biomarker data for identification, classification, PBT assessment, risk assessment, and testing of EDCs in the regulatory context was discussed. This paper emphasizes the importance of fish biomarker data in the regulatory context, a weight of evidence approach for the interpretation of fish biomarker data and the need for defining levels of evidence for the identification of EDCs.

The genomic and genetic toolbox of the teleost medaka (Oryzias latipes)

The Japanese medaka, Oryzias latipes, is a vertebrate teleost model with a long history of genetic research. A number of unique features and established resources distinguish medaka from other vertebrate model systems. A large number of laboratory strains from different locations are available. Due to a high tolerance to inbreeding, many highly inbred strains have been established, thus providing a rich resource for genetic studies. Furthermore, closely related species native to different habitats in Southeast Asia permit comparative evolutionary studies. The transparency of embryos, larvae, and juveniles allows a detailed in vivo analysis of development. New tools to study diverse aspects of medaka biology are constantly being generated. Thus, medaka has become an important vertebrate model organism to study development, behavior, and physiology. In this review, we provide a comprehensive overview of established genetic and molecular-genetic tools that render medaka fish a full-fledged vertebrate system.

Wild sex in zebrafish: loss of the natural sex determinant in domesticated strains

Sex determination can be robustly genetic, strongly environmental, or genetic subject to environmental perturbation. The genetic basis of sex determination is unknown for zebrafish (Danio rerio), a model for development and human health. We used RAD-tag population genomics to identify sex-linked polymorphisms. After verifying this "RAD-sex" method on medaka (Oryzias latipes), we studied two domesticated zebrafish strains (AB and TU), two natural laboratory strains (WIK and EKW), and two recent isolates from nature (NA and CB). All four natural strains had a single sex-linked region at the right tip of chromosome 4, enabling sex genotyping by PCR. Genotypes for the single nucleotide polymorphism (SNP) with the strongest statistical association to sex suggested that wild zebrafish have WZ/ZZ sex chromosomes. In natural strains, "male genotypes" became males and some "female genotypes" also became males, suggesting that the environment or genetic background can cause female-to-male sex reversal. Surprisingly, TU and AB lacked detectable sex-linked loci. Phylogenomics rooted on D. nigrofasciatus verified that all strains are monophyletic. Because AB and TU branched as a monophyletic clade, we could not rule out shared loss of the wild sex locus in a common ancestor despite their independent domestication. Mitochondrial DNA sequences showed that investigated strains represent only one of the three identified zebrafish haplogroups. Results suggest that zebrafish in nature possess a WZ/ZZ sex-determination mechanism with a major determinant lying near the right telomere of chromosome 4 that was modified during domestication. Strains providing the zebrafish reference genome lack key components of the natural sex-determination system but may have evolved variant sex-determining mechanisms during two decades in laboratory culture.