Exploring the Effects of Rearing Densities on Epigenetic Modifications in the Zebrafish Gonads

Rearing density directly impacts fish welfare, which, in turn, affects productivity in aquaculture. Previous studies have indicated that high-density rearing during sexual development in fish can induce stress, resulting in a tendency towards male-biased sex ratios in the populations. In recent years, research has defined the relevance of the interactions between the environment and epigenetics playing a key role in the final phenotype. However, the underlying epigenetic mechanisms of individuals exposed to confinement remain elucidated. By using zebrafish (Danio rerio), the DNA methylation promotor region and the gene expression patterns of six genes, namely dnmt1, cyp19a1a, dmrt1, cyp11c1, hsd17b1, and hsd11b2, involved in the DNA maintenance methylation, reproduction, and stress were assessed. Zebrafish larvae were subjected to two high-density conditions (9 and 66 fish/L) during two periods of overlapping sex differentiation of this species (7 to 18 and 18 to 45 days post-fertilization, dpf). Results showed a significant masculinization in the populations of fish subjected to high densities from 18 to 45 dpf. In adulthood, the dnmt1 gene was differentially hypomethylated in ovaries and its expression was significantly downregulated in the testes of fish exposed to high-density. Further, the cyp19a1a gene showed downregulation of gene expression in the ovaries of fish subjected to elevated density, as previously observed in other studies. We proposed dnmt1 as a potential testicular epimarker and the expression of ovarian cyp19a1a as a potential biomarker for predicting stress originated from high densities during the early stages of development. These findings highlight the importance of rearing densities by long-lasting effects in adulthood conveying cautions for stocking protocols in fish hatcheries.

Development of Ovaries and Sex Change in Fish: Bringing Potential into Action

BACKGROUND

Encompassing about half of the 60,000 species of vertebrates, fish display the greatest diversity of sex determination mechanisms among metazoans. As such that phylum offers a unique playground to study the impressive variety of gonadal morphogenetic strategies, ranging from gonochorism, with either genetic or environmental sex determination, to unisexuality, with either simultaneous or consecutive hermaphroditism.

SUMMARY

From the two main types of gonads, the ovaries embrace the important role to produce the larger and non-motile gametes, which is the basis for the development of a future organism. The production of the egg cells is complex and involves the formation of follicular cells, which are necessary for the maturation of the oocytes and the production of feminine hormones. In this vein, our review focuses on the development of ovaries in fish with special emphasis on the germ cells, including those that transition from one sex to the other as part of their life cycle and those that are capable of transitioning to the opposite sex depending on environmental cues.

KEY MESSAGES

Clearly, establishing an individual as either a female or a male is not accomplished by the sole development of two types of gonads. In most cases, that dichotomy, be it final or transient, is accompanied by coordinated transformations across the entire organism, leading to changes in the physiological sex as a whole. These coordinated transformations require both molecular and neuroendocrine networks, but also anatomical and behavioural adjustments. Remarkably, fish managed to tame the ins and outs of sex reversal mechanisms to take the most advantages of changing sex as adaptive strategies in some situations.

Epigenetic Regulation of Phenotypic Sexual Plasticity Inducing Skewed Sex Ratio in Zebrafish

The plasticity of sexual phenotype in response to environmental conditions results in biased sex ratios, and their variation has an effect on population dynamics. Epigenetic modifications can modulate sex ratio variation in species, where sex is determined by genetic and environmental factors. However, the role of epigenetic mechanisms underlying skewed sex ratios is far from being clear and is still an object of debate in evolutionary developmental biology. In this study, we used zebrafish as a model animal to investigate the effect of DNA methylation on sex ratio variation in sex-biased families in response to environmental temperature. Two sex-biased families with a significant difference in sex ratio were selected for genome-wide DNA methylation analysis using reduced representation bisulfite sequencing (RRBS). The results showed significant genome-wide methylation differences between male-biased and female-biased families, with a greater number of methylated CpG sites in testes than ovaries. Likewise, pronounced differences between testes and ovaries were identified within both families, where the male-biased family exhibited a higher number of methylated sites than the female-biased family. The effect of temperature showed more methylated positions in the high incubation temperature than the control temperature. We found differential methylation of many reproduction-related genes (e.g., sox9a, nr5a2, lhx8a, gata4) and genes involved in epigenetic mechanisms (e.g., dnmt3bb.1, dimt1l, hdac11, h1m) in both families. We conclude that epigenetic modifications can influence the sex ratio variation in zebrafish families and may generate skewed sex ratios, which could have a negative consequence for population fitness in species with genotype-environment interaction sex-determining system under rapid environmental changes.

The Utility of Small Fishes for the Genetic Study of Human Age-Related Disorders

Animal models have been used to model human diseases, and among them, small fishes have been highlighted for their usefulness in various ways, such as the low cost of maintenance, ease of genetic modification, small size for easy handling, and strength in imaging studies due to their relative transparency. Recently, the use of turquoise killifish, Nothobranchius furzeri, which is known to exhibit various aging phenotypes in a short period, has attracted attention in research on aging and age-related diseases. However, when using animal models, it is important to keep their genetic background and interspecies differences in mind for translating them into human diseases. In this article, we obtained the gene symbols of protein-coding genes of turquoise killifish, medaka, zebrafish, and humans from NCBI datasets and extracted common shared genes among four species to explore the potential of interspecies translational research and to apply small fish models for human age-related disorders. Common shared protein-coding genes were analyzed with the Reactome Pathway Database to determine the coverage of these genes in each pathway in humans. We applied common shared genes to the Orphanet database to establish a list of human diseases that contain common shared genes among the four species. As examples, the senescence-related pathways and some pathways of human age-related diseases, such as Alzheimer’s disease, Parkinson’s disease, frontotemporal dementia, nonalcoholic fatty liver disease, progeria, hepatocellular carcinoma, and renal cell carcinoma, were extracted from the curated pathway and disease list to discuss the further utility of fish models for human age-related disorders.

Comparative transcriptome analysis of heat-induced domesticated zebrafish during gonadal differentiation

BACKGROUND

The influence of environmental factors, especially temperature, on sex ratio is of great significance to elucidate the mechanism of sex determination. However, the molecular mechanisms by which temperature affects sex determination remains unclear, although a few candidate genes have been found to play a role in the process. In this study, we conducted transcriptome analysis of the effects induced by high temperature on zebrafish during gonad differentiation period.

RESULTS

Totals of 1171, 1022 and 2921 differentially expressed genes (DEGs) between high temperature and normal temperature were identified at 35, 45 and 60 days post-fertilization (dpf) respectively, revealing that heat shock proteins (HSPs) and DNA methyltransferases (DNMTs) were involved in the heat-exposed sex reversal. The Gene Ontology (GO) terms and the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway that were enriched in individuals after heat treatment included Fanconi anemia (FA) pathway, cell cycle, oocyte meiosis and homologous recombination.

CONCLUSIONS

Our study provides the results of comparative transcriptome analyses between high temperature and normal temperature, and reveals that the molecular mechanism of heat-induced masculinization in zebrafish is strongly related to the expression of HSPs and DNMTs and FA pathway during gonad differentiation.

Zebrafish, Medaka and Turquoise Killifish for Understanding Human Neurodegenerative/Neurodevelopmental Disorders

In recent years, small fishes such as zebrafish and medaka have been widely recognized as model animals. They have high homology in genetics and tissue structure with humans and unique features that mammalian model animals do not have, such as transparency of embryos and larvae, a small body size and ease of experiments, including genetic manipulation. Zebrafish and medaka have been used extensively in the field of neurology, especially to unveil the mechanisms of neurodegenerative diseases such as Parkinson's and Alzheimer's disease, and recently, these fishes have also been utilized to understand neurodevelopmental disorders such as autism spectrum disorder. The turquoise killifish has emerged as a new and unique model animal, especially for ageing research due to its unique life cycle, and this fish also seems to be useful for age-related neurological diseases. These small fishes are excellent animal models for the analysis of human neurological disorders and are expected to play increasing roles in this field. Here, we introduce various applications of these model fishes to improve our understanding of human neurological disorders.

Sexual determination in zebrafish

Zebrafish have emerged as a major model organism to study vertebrate reproduction due to their high fecundity and external development of eggs and embryos. The mechanisms through which zebrafish determine their sex have come under extensive investigation, as they lack a definite sex-determining chromosome and appear to have a highly complex method of sex determination. Single-gene mutagenesis has been employed to isolate the function of genes that determine zebrafish sex and regulate sex-specific differentiation, and to explore the interactions of genes that promote female or male sexual fate. In this review, we focus on recent advances in understanding of the mechanisms, including genetic and environmental factors, governing zebrafish sex development with comparisons to gene functions in other species to highlight conserved and potentially species-specific mechanisms for specifying and maintaining sexual fate.

Different influences of anxiety models, environmental enrichment, standard conditions and intraspecies variation (sex, personality and strain) on stress and quality of life in adult and juvenile zebrafish: A systematic review

Antagonist and long-lasting environmental manipulations (EM) have successfully induced or reduced the stress responses and quality of life of zebrafish. For instance, environmental enrichment (EE) generally reduces anxiety-related behaviours and improves immunity, while unpredictable chronic stress (UCS) and aquarium-related stressors generate the opposite effects. However, there is an absence of consistency in outcomes for some EM, such as acute exposure to stressors, social enrichment and some items of structural enrichment. Therefore, considering intraspecies variation (sex, personality, and strain), increasing intervention complexity while improving standardisation of protocols and contemplating the possibility that EE may act as a mild stressor on a spectrum between too much (UCS) and too little (standard conditions) stress intensity or stimulation, would reduce the inconsistencies of these outcomes. It would also help explore the mechanism behind stress resilience and to standardise EM protocols. Thus, this review critically analyses and compares knowledge existing over the last decade concerning environmental manipulations for zebrafish and the influences that sex, strain, and personality may have on behavioural, physiological, and fitness-related responses.

Masculinization of Zebrafish Through Partial Depletion of Primordial Germ Cells by Injecting Diluted Morpholino Oligonucleotides into Embryos

The regulation of reproduction in zebrafish, the prime model of fish research, is not fully understood. An efficient tool to gain a better understanding of this complicated process is utilization of severely sex-biased families or groups. Here, we describe a method for partial depletion of primordial germ cells (PGCs) that leads to eventual masculinization of zebrafish. The technique is based on injecting early embryos with diluted morpholino oligonucleotides that temporarily interfere with the production of Dead end (Dnd), an RNA-binding protein essential for PGC survival. In addition, we also propose the use of eviscerated trunk, as a suitable alternative for examining gonadal expression in juvenile zebrafish.

The Role of DNA Methylation Reprogramming during Sex Determination and Transition in Zebrafish

DNA methylation is a prevalent epigenetic modification in vertebrates, and it has been shown to be involved the regulation of gene expression and embryo development. However, it remains unclear how DNA methylation regulates sexual development, especially in species without sex chromosomes. To determine this, we utilized zebrafish to investigate DNA methylation reprogramming during juvenile germ cell development and adult female-to-male sex transition. We reveal that primordial germ cells (PGCs) undergo significant DNA methylation reprogramming during germ cell development, and the methylome of PGCs is reset to an oocyte/ovary-like pattern at 9 days post fertilization (9 dpf). When DNA methyltransferase (DNMT) activity in juveniles was blocked after 9 dpf, the zebrafish developed into females. We also show that Tet3 is involved in PGC development. Notably, we find that DNA methylome reprogramming during adult zebrafish sex transition is similar to the reprogramming during the sex differentiation from 9 dpf PGCs to sperm. Furthermore, inhibiting DNMT activity can prevent the female-to-male sex transition, suggesting that methylation reprogramming is required for zebrafish sex transition. In summary, DNA methylation plays important roles in zebrafish germ cell development and sexual plasticity.

Shedding new light on early sex determination in zebrafish

In contrast to established zebrafish gene annotations, the question of sex determination has still not been conclusively clarified for developing zebrafish, Danio rerio, larvae, 28 dpf or earlier. Recent studies indicate polygenic sex determination (PSD), with the genes being distributed throughout the genome. Early genetic markers of sex in zebrafish help unravel co-founding sex-related differences to apply to human health and environmental toxicity studies. A qPCR-based method was developed for six genes: cytochrome P450, family 17, subfamily A, polypeptide 1 (cyp17a1); cytochrome P450, family 19, subfamily A, polypeptide 1a (cyp19a1a); cytochrome P450, family 19, subfamily A, polypeptides 1b (cyp19a1b); vitellogenin 1 (vtg1); nuclear receptor subfamily 0, group B, member 1 (nr0b1), sry (sex-determining region Y)-box 9b (sox9b) and actin, beta 1 (actb1), the reference gene. Sry-box 9a (Sox9a), insulin-like growth factor 3 (igf3) and double sex and mab-3 related transcription factor 1 (dmrt1), which are also known to be associated with sex determination, were used in gene expression tests. Additionally, Next-Generation-Sequencing (NGS) sequenced the genome of two adult female and male and two juveniles. PCR analysis of adult zebrafish revealed sex-specific expression of cyp17a1, cyp19a1a, vtg1, igf3 and dmrt1, the first four strongly expressed in female zebrafish and the last one highly expressed in male conspecifics. From NGS, nine female and four male-fated genes were selected as novel for assessing zebrafish sex, 28 dpf. Differences in transcriptomes allowed allocation of sex-specific genes also expressed in juvenile zebrafish.

Zebrafish cyp11c1 Knockout Reveals the Roles of 11-ketotestosterone and Cortisol in Sexual Development and Reproduction

Androgen is essential for male development and cortisol is involved in reproduction in fishes. However, the in vivo roles of cortisol and specific androgens such as 11-ketotestosterone (11-KT) in reproductive development need to be described with genetic models. Zebrafish cyp11c1 encodes 11β-hydroxylase, which is essential for the biosynthesis of 11-KT and cortisol. In this study, we generated a zebrafish mutant of cyp11c1 (cyp11c1-/-) and utilized it to clarify the roles of 11-KT and cortisol in sexual development and reproduction. The cyp11c1-/- fish had smaller genital papilla and exhibited defective natural mating but possessed mature gametes and were found at a sex ratio comparable to the wildtype control. The cyp11c1-/- males showed delayed and prolonged juvenile ovary-to-testis transition and displayed defective spermatogenesis at adult stage, which could be rescued by treatment with 11-ketoandrostenedione (11-KA) at certain stages. Specifically, during testis development of cyp11c1-/- males, the expression of insl3, cyp17a1, and amh was significantly decreased, suggesting that 11-KT is essential for the development and function of Leydig cells and Sertoli cells. Further, spermatogenesis-related dmrt1 was subsequently downregulated, leading to insufficient spermatogenesis. The cyp11c1-/- females showed a reduction in egg spawning and a failure of in vitro germinal vesicle breakdown, which could be partially rescued by cortisol treatment. Taken together, our study reveals that zebrafish Cyp11c1 is not required for definite sex differentiation but is essential for juvenile ovary-to-testis transition, Leydig cell development, and spermatogenesis in males through 11-KT, and it is also involved in oocyte maturation and ovulation in females through cortisol.

Identification of sex differences in zebrafish (Danio rerio) brains during early sexual differentiation and masculinization using 17α-methyltestoterone

Sexual behavior in teleost fish is highly plastic. It can be attributed to the relatively few sex differences found in adult brain transcriptomes. Environmental and hormonal factors can influence sex-specific behavior. Androgen treatment stimulates behavioral masculinization. Sex dimorphic gene expression in developing teleost brains and the molecular basis for androgen-induced behavioral masculinization are poorly understood. In this study, juvenile zebrafish (Danio rerio) were treated with 100 ng/L of 17 alpha-methyltestosterone (MT) during sexual development from 20 days post fertilization to 40 days and 60 days post fertilization. We compared brain gene expression patterns in MT-treated zebrafish with control males and females using RNA-Seq to shed light on the dynamic changes in brain gene expression during sexual development and how androgens affect brain gene expression leading to behavior masculinization. We found modest differences in gene expression between juvenile male and female zebrafish brains. Brain aromatase (cyp19a1b), prostaglandin 3a synthase (ptges3a), and prostaglandin reductase 1 (ptgr1) were among the genes with sexually dimorphic expression patterns. MT treatment significantly altered gene expression relative to both male and female brains. Fewer differences were found among MT-treated brains and male brains compared to female brains, particularly at 60 dpf. MT treatment upregulated the expression of hydroxysteroid 11-beta dehydrogenase 2 (hsd11b2), deiodinase, iodothyronine, type II (dio2), and gonadotrophin releasing hormones (GnRH) 2 and 3 (gnrh2 and gnrh3) suggesting local synthesis of 11-ketotestosterone, triiodothyronine, and GnRHs in zebrafish brains which are influenced by androgens. Androgen, estrogen, prostaglandin, thyroid hormone, and GnRH signaling pathways likely interact to modulate teleost sexual behavior.

Changes in fish sex ratio as a basis for regulating endocrine disruptors

Fish sex ratio (SR) is an endpoint potentially indicating both endocrine activity and adversity, essential elements for identifying Endocrine Disrupting Chemicals (EDCs) as required by the EU regulations. Due to different protocols and methods in the literature studies, SR data vary greatly. This study analyses literature SR data and discusses important considerations for using SR data in the regulatory context for the hazard identification, classification, PBT (persistent, bioaccumulative and toxic) assessment, testing, and risk assessment. A total number of 106 studies were compiled for SR of zebrafish, medaka and fathead minnow exposed to 84 chemicals or mixtures. About 53% of literature studies determined SR by methods different from the standard histology method, leading to uncertainty of quantifying SR and differential sensitivity. SR was determined after depuration in 40 papers, which may lead to chemical-induced SR changes reversible to the control. SR was responsive to chemicals with EAS (estrogen, androgen, steoroidogenesis) activity and also to those with thyroid and progesterone activity. Besides, SR was influenced by non-chemical factors, e.g., inbreeding and temperature, leading to difficulty in data interpretation. The ECHA/EFSA/JRC Guidance suggests that SR and gonad histology data can be used for identifying EDCs. Due to reversibility, influence of confounding factors, and responsiveness to chemicals with endocrine activity other than EAS, this study suggests that SR/gonad histology should be combined with certain mode of action evidence for identifying EDCs. Important considerations for using SR data in the identification, classification, PBT assessment, testing, and risk assessment are discussed.

Effects of homozygosity on sex determination in zebrafish Danio rerio

Gynogenetic zebrafish Danio rerio were obtained by activating D. rerio oocytes with UV irradiated common carp Cyprinus carpio sperm and then applying one of four different shocks [two (early) meiotic and two (late) mitotic shocks]. Gynogens produced by three of the shocks survived to maturity. All adult gynogens (n = 52) except one were found to be male. There was no difference in growth rate between the biparental controls and gynogens produced through the most effective shock, thereby eliminating growth rate as a possible cause of the skewed sex ratio. Gynogen males had reduced fertility compared to biparental controls, with about half of gynogens being unable to reproduce through natural spawning (all controls reproduced successfully). Gynogen males that did reproduce gave lower fertilization rates compared with controls. This demonstrates the negative effects of increased homozygosity on male reproductive function. Families sired by meiotic gynogen males were more likely to be female biased (33% of families) compared with families sired by biparental control males (11%). In addition to confirming the polygenic nature of sex determination in D. rerio, these observations suggest that recessive or over-dominant male-determining alleles are present in domesticated D. rerio populations.

The effects of parental carbamazepine and gemfibrozil exposure on sexual differentiation in zebrafish (Danio rerio)

The effects of parental exposure to pharmaceuticals on sexual differentiation in F1 offspring were examined in zebrafish (Danio rerio). Adult zebrafish were exposed to 0 or 10 μg/L of carbamazepine or gemfibrozil for 6 wk and bred in pairwise crosses to generate 7 distinct lineages. Lineages were formed with both parents from the same treatment group or with only one parent exposed, to delineate between maternal and paternal effects. The F1 offspring from each lineage were reared in clean water and sampled at 45 and 60 d post fertilization (dpf). Gonadal differentiation was assessed by histology. The morphological stages of the gonads were converted to a quantitative day-equivalent based on data from offspring of untreated parents sampled from 15 to 75 dpf, which enabled a quantitative statistical analysis on the timing of sexual differentiation. Paternal, but not maternal, exposure to carbamazepine resulted in significantly faster sexual differentiation and a male-biased sex ratio; these effects were not observed when both parents were exposed. Combined paternal and maternal exposure to gemfibrozil resulted in significantly faster sexual differentiation, and paternal, but not maternal, exposure to gemfibrozil led to male-biased sex ratios. The present study demonstrates the ability of parental exposure to pharmaceuticals to disrupt sexual differentiation in the F1 offspring and also shows that effects may be uniquely influenced by which parent was exposed. Environ Toxicol Chem 2018;37:1696-1706. © 2018 SETAC.

Knotting nets: Molecular junctions of interconnecting endocrine axes identified by application of the adverse outcome pathway concept

To be defined as an endocrine disruptor, a substance has to meet several criteria, including the induction of specific adverse effects, a specific endocrine mode of action, and a plausible link between both. The latter criterion in particular might not always be unequivocally determined, especially because the endocrine system consists of diverse endocrine axes. The axes closely interact with each other, and manipulation of one triggers effects on the other. The present review aimed to identify some of the many interconnections between these axes. The focus was on fish, but data obtained in studies on amphibians and mammals were considered if they assisted in closing data gaps, because most of the endocrine mechanisms are evolutionarily conserved. The review includes data both from ecotoxicological studies and on physiological processes and gives information on hormone/hormone receptor interactions or gene transcription regulation. The key events and key event relationships identified provide explanations for unexpected effects on one axis, exerted by substances suspected to act specifically on another axis. Based on these data, several adverse outcome pathway (AOP) segments are identified, describing connections between the hypothalamic-pituitary-gonadal (HPG) and hypothalamic-pituitary-thyroid (HPT) axes, the HPG and hypothalamic-pituitary-adrenal/interrenal (HPA/I) axes, and the HPT and HPA/I axes. Central key events identified across axes were altered aromatase activity as well as altered expression and function of the proteins 11β-hydroxysteroid dehydrogenase (11β-HSD) and steroidogenic acute regulatory (StAR) protein. Substance classes that act on more than one endocrine axis were, for example, goitrogens or aromatase inhibitors. Despite the wealth of information gathered, the present review only provides a few insights into the molecular nets of endocrine axes, demonstrating the complexity of their interconnections. Environ Toxicol Chem 2018;37:318-328. © 2017 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Concentration and timing of application reveal strong fungistatic effect of tebuconazole in a Daphnia-microparasitic yeast model

Given the importance of pollutant effects on host-parasite relationships and disease spread, the main goal of this study was to assess the influence of different exposure scenarios for the fungicide tebuconazole (concentration×timing of application) on a Daphnia-microparasitic yeast experimental system. Previous results had demonstrated that tebuconazole is able to suppress Metschnikowia bicuspidata infection at ecologically-relevant concentrations; here, we aimed to obtain an understanding of the mechanism underlying the anti-parasitic (fungicidal or fungistatic) action of tebuconazole. We exposed the Daphnia-yeast system to four nominal tebuconazole concentrations at four timings of application (according to the predicted stage of parasite development), replicated on two Daphnia genotypes, in a fully crossed experiment. An "all-or-nothing" effect was observed, with tebuconazole completely suppressing infection from 13.5μgl^-1 upwards, independent of the timing of tebuconazole application. A follow-up experiment confirmed that the suppression of infection occurred within a narrow range of tebuconazole concentrations (3.65-13.5μgl^-1), although a later application of the fungicide had to be compensated for by a slight increase in concentration to elicit the same anti-parasitic effect. The mechanism behind this anti-parasitic effect seems to be the inhibition of M. bicuspidata sporulation, since tebuconazole was effective in preventing ascospore production even when applied at a later time. However, this fungicide also seemed to affect the vegetative growth of the yeast, as demonstrated by the enhanced negative effect of the parasite (increasing mortality in one of the host genotypes) at a later time of application of tebuconazole, when no signs of infection were observed. Fungicide contamination can thus affect the severity and spread of disease in natural populations, as well as the inherent co-evolutionary dynamics in host-parasite systems.

Breeding Zebrafish: A Review of Different Methods and a Discussion on Standardization

In recent years, a rapidly increasing number of scientific papers have been published that utilize zebrafish (Danio rerio) as an alternative model organism in the study of a wide range of biological phenomena from cancer to behavior. This is, in large part, due to the prolific nature, relative ease of maintenance, and sufficiently high genetic homology of zebrafish to humans. With the surge of zebrafish use in animal research, the variations in methodologies of breeding and husbandry of this species have also increased. Investigators usually focus on the development and implementation of rigorous laboratory control that is specific to their studies. We suggest that the same scrutiny and attention may be required for the methods of breeding and housing of zebrafish. This article reviews a variety of zebrafish husbandry and breeding techniques and conditions employed around the world. It discusses factors ranging from numerous aspects of rearing/housing conditions through the sex ratio of the breeding group to the composition of the diet of zebrafish that may vary across laboratories. It provides some feedback on the potential pros and cons of the different methods. It argues that there is a substantial need for systematic analysis of these methods, that is, the effects of environmental factors on zebrafish health and breeding. It also discusses the question as to whether some degree of standardization of these methods is needed to enhance cross-laboratory comparability of results.

Heat-induced masculinization in domesticated zebrafish is family-specific and yields a set of different gonadal transcriptomes

Understanding environmental influences on sex ratios is important for the study of the evolution of sex-determining mechanisms and for evaluating the effects of global warming and chemical pollution. Fishes exhibit sexual plasticity, but the underlying mechanisms of environmental effects on their reproduction are unclear even in the well-established teleost research model, the zebrafish. Here we established the conditions to study the effects of elevated temperature on zebrafish sex. We showed that sex ratio response to elevated temperature is family-specific and typically leads to masculinization (female-to-male sex reversal), resulting in neomales. These results uncovered genotype-by-environment interactions that support a polygenic sex determination system in domesticated (laboratory) zebrafish. We found that some heat-treated fish had gene expression profiles similar to untreated controls of the same sex, indicating that they were resistant to thermal effects. Further, most neomales had gonadal transcriptomes similar to that of regular males. Strikingly, we discovered heat-treated females that displayed a normal ovarian phenotype but with a "male-like" gonadal transcriptome. Such major transcriptomic reprogramming with preserved organ structure has never been reported. Juveniles were also found to have a male-like transcriptome shortly after exposure to heat. These findings were validated by analyzing the expression of genes and signaling pathways associated with sex differentiation. Our results revealed a lasting thermal effect on zebrafish gonads, suggesting new avenues for detection of functional consequences of elevated temperature in natural fish populations in a global warming scenario.

Finding clues to the riddle of sex determination in zebrafish

How sex is determined has been one of the most intriguing puzzles in biology since antiquity. Although a fundamental process in most metazoans, there seems to be myriad of ways in which sex can be determined - from genetic to environmental sex determination. This variation is limited mainly to upstream triggers with the core of sex determination pathway being conserved. Zebrafish has gained prominence as a vertebrate model system to study development and disease. However, very little is known about its primary sex determination mechanism. Here we review our current understanding of the sex determination in zebrafish. Zebrafish lack identifiable heteromorphic sex chromosomes and sex is determined by multiple genes, with some influence from the environment. Recently, chromosome 4 has been identified as sex chromosome along with few sex-linked loci on chromosomes 5 and 16. The identities of candidate sex-linked genes, however, have remained elusive. Sex in zebrafish is also influenced by the number of meiotic oocytes in the juvenile ovary, which appear to instruct retention of the ovarian fate. The mechanism and identity of this instructive signal remain unknown. We hypothesize that sex in zebrafish is a culmination of combinatorial effects of the genome, germ cells and the environment with inputs from epigenetic factors translating the biological meaning of this interaction.

Knockout of Zebrafish Ovarian Aromatase Gene (cyp19a1a) by TALEN and CRISPR/Cas9 Leads to All-male Offspring Due to Failed Ovarian Differentiation

Sexual or gonadal differentiation is a complex event and its mechanism remains elusive in teleosts. Despite its complexity and plasticity, the process of ovarian differentiation is believed to involve gonadal aromatase (cyp19a1a) in nearly all species studied. However, most data concerning the role of aromatase have come from gene expression analysis or studies involving pharmacological approaches. There has been a lack of genetic evidence for the importance of aromatase in gonadal differentiation, especially the timing when the enzyme starts to exert its effect. This is due to the lack of appropriate loss-of-function approaches in fish models for studying gene functions. This situation has changed recently with the development of genome editing technologies, namely TALEN and CRISPR/Cas9. Using both TALEN and CRISPR/Cas9, we successfully established three mutant zebrafish lines lacking the ovarian aromatase. As expected, all mutant fish were males, supporting the view that aromatase plays a critical role in directing ovarian differentiation and development. Further analysis showed that the ovarian aromatase did not seem to affect the formation of so-called juvenile ovary and oocyte-like germ cells; however, it was essential for further differentiation of the juvenile ovary into the true ovary.

Disruption of apoptosis pathways involved in zebrafish gonad differentiation by 17α-ethinylestradiol and fadrozole exposures

Zebrafish (Danio rerio) sex determination seems to involve genetic factors (GSD) but also environmental factors (ESD), such as endocrine disrupting chemicals (EDCs) that are known to mimic endogenous hormones and disrupt gonad differentiation. Apoptosis has also been proposed to play a crucial role in zebrafish gonad differentiation. Nevertheless, the interactions between EDCs and apoptosis have received little attention. Thus, this study aimed to assess if and which apoptotic pathways are involved in zebrafish gonad differentiation and how EDCs may interfere with this process. With these purposes, zebrafish were exposed to 17α-ethinylestradiol (EE2, 4ng/L) and fadrozole (Fad, 50μg/L) from 2h to 35days post-fertilization (dpf). Afterwards, a gene expression analysis by qRT-PCR and a stereological analysis, based on systematic sampling and protein immunohistochemistry, were performed. The death receptors (FAS; TRADD), anti-apoptotic (BCL-2; MDM2), pro-apoptotic (CASP-2 and -6) and cell proliferation (BIRC5/survivin; JUN) genes and proteins were evaluated. In general, apoptosis was inhibited in females through the involvement of anti-apoptotic pathways, while in males apoptosis seemed to be crucial to the failure of the "juvenile ovary" development and the induction of testes transformation. The JUN protein was shown to be necessary in juvenile ovaries, while the BIRC5 protein seemed to be involved in zebrafish spermatogenesis. Both EDCs, EE2 and Fad, increased the apoptosis stimulus in zebrafish gonad. It was noticed that the few females that were resistant to Fad-induced sex reversal had increased anti-apoptotic factor levels, while males exposed to EE2 showed increased pro-apoptotic genes/proteins and were more advanced in gonad differentiation. Overall, our findings show that apoptosis pathways are involved in zebrafish gonad differentiation and that EDCs can disrupt this process.

Effects of 17α-ethinylestradiol at different water temperatures on zebrafish sex differentiation and gonad development

In the current climate change scenario, studies combining effects of water contaminants with environmental parameters, such as temperature, are essential to predict potentially harmful impacts on aquatic organisms. In zebrafish (Danio rerio), sex determination seems to have a polygenic genetic basis, which can be secondarily influenced by environmental factors, such as temperature and endocrine disrupting chemicals (EDCs). The present study aimed to evaluate the effects of the EDC 17α-ethinylestradiol (EE2), a potent synthetic estrogen, on zebrafish sex differentiation and gonad development at different water temperatures. Therefore, zebrafish raised at three distinct water temperatures (23, 28 or 33±0.5°C), were exposed to 4ng/L of EE2, from 2hours to 60days post-fertilization (dpf). Subsequently, a quantitative (stereological) assessment of zebrafish gonads was performed, at 35 and 60dpf, to identify alterations on gonadal development and differentiation. The results show that low temperature delayed general growth of zebrafish, as well as gonad differentiation and maturation, while high temperature induced an opposite effect. Moreover, sex ratio was skewed toward males when zebrafish were exposed to the high temperature. In general, EE2 exposure promoted gonad maturation in both genders, independently of the temperature. However, at the high temperature condition, exposure to EE2 induced a delay in the male gonad development, with some individuals still showing differentiating gonads at 60dpf. The findings of this study support the notion that zebrafish has a genetic sex determination mechanism highly sensitive to environmental factors and show that it is essential to study the effects of water contaminants at different climate scenarios in order to understand potential future impacts on organisms.

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.

Early environment influences later performance in fishes

Conditions fish encounter during embryogenesis and early life history can leave lasting effects not only on morphology, but also on growth rate, life-history and behavioural traits. The ecology of offspring can be affected by conditions experienced by their parents and mother in particular. This review summarizes such early impacts and their ecological influences for a variety of teleost species, but with special reference to salmonids. Growth and adult body size, sex ratio, egg size, lifespan and tendency to migrate can all be affected by early influences. Mechanisms behind such phenotypically plastic impacts are not well known, but epigenetic change appears to be one central mechanism. The thermal regime during development and incubation is particularly important, but also early food consumption and intraspecific density can all be responsible for later life-history variation. For behavioural traits, early experiences with effects on brain, sensory development and cognition appear essential. This may also influence boldness and other social behaviours such as mate choice. At the end of the review, several issues and questions for future studies are given.

Gonad differentiation in zebrafish is regulated by the canonical Wnt signaling pathway

Zebrafish males undergo a "juvenile ovary-to-testis" gonadal transformation process. Several genes, including nuclear receptor subfamily 5, group A (nr5a) and anti-Müllerian hormone (amh), and pathways such as Tp53-mediated germ-cell apoptosis have been implicated in zebrafish testis formation. However, our knowledge of the regulation of this complex process is incomplete, and much remains to be investigated about the molecular pathways and network of genes that control it. Using a microarray-based analysis of transforming zebrafish male gonads, we demonstrated that their transcriptomes undergo transition from an ovary-like pattern to an ovotestis to a testis-like profile. Microarray results also validated the previous histological and immunohistochemical observation that there is high variation in the duration and extent of commitment to the juvenile ovary phase among individuals. Interestingly, global gene expression profiling of diverging zebrafish juvenile ovaries and transforming ovotestes revealed that some members of the canonical Wnt/beta-catenin signaling pathway were differentially expressed between these two phases. To investigate whether Wnt/beta-catenin signaling plays a role in zebrafish gonad differentiation, we used the Tg (hsp70l:dkk1b-GFP)w32 line to inhibit Wnt/beta-catenin signaling during gonad differentiation. Activation of dkk1b-GFP expression by heat shock resulted in an increased proportion of males and corresponding decrease in gonadal aromatase gene (cyp19a1a) expression. The Wnt target gene, lymphocyte enhancer binding factor 1 (lef1), was also down-regulated in the process. Together, these results provide the first functional evidence that, similarly to mammals, Wnt/beta-catenin signaling is a "pro-female" pathway that regulates gonad differentiation in zebrafish.

Zebrafish sex: a complicated affair

In this review, we provide a detailed overview of studies on the elusive sex determination (SD) and gonad differentiation mechanisms of zebrafish (Danio rerio). We show that the data obtained from most studies are compatible with polygenic sex determination (PSD), where the decision is made by the allelic combinations of several loci. These loci are typically dispersed throughout the genome, but in some teleost species a few of them might be located on a preferential pair of (sex) chromosomes. The PSD system has a much higher level of variation of SD genotypes both at the level of gametes and the sexual genotype of individuals, than that of the chromosomal sex determination systems. The early sexual development of zebrafish males is a complicated process, as they first develop a ‘juvenile ovary’, that later undergoes a transformation to give way to a testis. To date, three major developmental pathways were shown to be involved with gonad differentiation through the modulation of programmed cell death. In our opinion, there are more pathways participating in the regulation of zebrafish gonad differentiation/transformation. Introduction of additional powerful large-scale genomic approaches into the analysis of zebrafish reproduction will result in further deepening of our knowledge as well as identification of additional pathways and genes associated with these processes in the near future.

Environmental stress-induced testis differentiation: androgen as a by-product of cortisol inactivation

This review deals with the gonadal masculinization induced by thermal stress in fish with focus on the action of 11β-hydroxysteroid dehydrogenase (11β-HSD) as this mechanism key transducer. High temperatures have been reported to produce male-skewed sex ratios in several species with TSD (temperature-dependent sex determination), and in some of them, this process was reported to be associated with high levels of cortisol, the hormone-related stress in vertebrates, during early gonad development. In addition, in pejerrey larvae reared at high-masculinizing temperatures, 11-ketotestosterone (11-KT), the main and most potent androgen in fish, was also detected at high levels. In testicular explants, cortisol induced the synthesis of 11-KT, suggesting that its synthesis could be under the control of the stress axis at the time of gonadal fate determination. 11β-HSD is one of the enzymes shared by the glucocorticoid and androgen pathways; this enzyme converts cortisol to cortisone and also participates in the finals steps of the synthesis of the 11-oxigenated androgens. Based on these data and literature information, here we propose that the masculinization induced by thermal stress can be considered as a consequence of cortisol inactivation and the concomitant synthesis of 11-KT and discussing this as a possible mechanism of masculinization induced by different types of environmental stressors.

Multiple sex-associated regions and a putative sex chromosome in zebrafish revealed by RAD mapping and population genomics

Within vertebrates, major sex determining genes can differ among taxa and even within species. In zebrafish (Danio rerio), neither heteromorphic sex chromosomes nor single sex determination genes of large effect, like Sry in mammals, have yet been identified. Furthermore, environmental factors can influence zebrafish sex determination. Although progress has been made in understanding zebrafish gonad differentiation (e.g. the influence of germ cells on gonad fate), the primary genetic basis of zebrafish sex determination remains poorly understood. To identify genetic loci associated with sex, we analyzed F(2) offspring of reciprocal crosses between Oregon *AB and Nadia (NA) wild-type zebrafish stocks. Genome-wide linkage analysis, using more than 5,000 sequence-based polymorphic restriction site associated (RAD-tag) markers and population genomic analysis of more than 30,000 single nucleotide polymorphisms in our *ABxNA crosses revealed a sex-associated locus on the end of the long arm of chr-4 for both cross families, and an additional locus in the middle of chr-3 in one cross family. Additional sequencing showed that two SNPs in dmrt1 previously suggested to be functional candidates for sex determination in a cross of ABxIndia wild-type zebrafish, are not associated with sex in our AB fish. Our data show that sex determination in zebrafish is polygenic and that different genes may influence sex determination in different strains or that different genes become more important under different environmental conditions. The association of the end of chr-4 with sex is remarkable because, unique in the karyotype, this chromosome arm shares features with known sex chromosomes: it is highly heterochromatic, repetitive, late replicating, and has reduced recombination. Our results reveal that chr-4 has functional and structural properties expected of a sex chromosome.