Appropriate rearing density in domesticated zebrafish to avoid masculinization: links with the stress response

Direct male development in chromosomally ZZ zebrafish

The genetics of sex determination varies across taxa, sometimes even within a species. Major domesticated strains of zebrafish (Danio rerio), including AB and TU, lack a strong genetic sex determining locus, but strains more recently derived from nature, like Nadia (NA), possess a ZZ male/ZW female chromosomal sex-determination system. AB fish pass through a juvenile ovary stage, forming oocytes that survive in fish that become females but die in fish that become males. To understand mechanisms of gonad development in NA zebrafish, we studied histology and single cell transcriptomics in developing ZZ and ZW fish. ZW fish developed oocytes by 22 days post-fertilization (dpf) but ZZ fish directly formed testes, avoiding a juvenile ovary phase. Gonads of some ZW and WW fish, however, developed oocytes that died as the gonad became a testis, mimicking AB fish, suggesting that the gynogenetically derived AB strain is chromosomally WW. Single-cell RNA-seq of 19dpf gonads showed similar cell types in ZZ and ZW fish, including germ cells, precursors of gonadal support cells, steroidogenic cells, interstitial/stromal cells, and immune cells, consistent with a bipotential juvenile gonad. In contrast, scRNA-seq of 30dpf gonads revealed that cells in ZZ gonads had transcriptomes characteristic of testicular Sertoli, Leydig, and germ cells while ZW gonads had granulosa cells, theca cells, and developing oocytes. Hematopoietic and vascular cells were similar in both sex genotypes. These results show that juvenile NA zebrafish initially develop a bipotential gonad; that a factor on the NA W chromosome, or fewer than two Z chromosomes, is essential to initiate oocyte development; and without the W factor, or with two Z doses, NA gonads develop directly into testes without passing through the juvenile ovary stage. Sex determination in AB and TU strains mimics NA ZW and WW zebrafish, suggesting loss of the Z chromosome during domestication. Genetic analysis of the NA strain will facilitate our understanding of the evolution of sex determination mechanisms.

Direct Male Development in Chromosomally ZZ Zebrafish

The genetics of sex determination varies across taxa, sometimes even within a species. Major domesticated strains of zebrafish ( Danio rerio ), including AB and TU, lack a strong genetic sex determining locus, but strains more recently derived from nature, like Nadia (NA), possess a ZZ male/ZW female chromosomal sex-determination system. AB strain fish pass through a juvenile ovary stage, forming oocytes that survive in fish that become females but die in fish that become males. To understand mechanisms of gonad development in NA zebrafish, we studied histology and single cell transcriptomics in developing ZZ and ZW fish. ZW fish developed oocytes by 22 days post-fertilization (dpf) but ZZ fish directly formed testes, avoiding a juvenile ovary phase. Gonads of some ZW and WW fish, however, developed oocytes that died as the gonad became a testis, mimicking AB fish, suggesting that the gynogenetically derived AB strain is chromosomally WW. Single-cell RNA-seq of 19dpf gonads showed similar cell types in ZZ and ZW fish, including germ cells, precursors of gonadal support cells, steroidogenic cells, interstitial/stromal cells, and immune cells, consistent with a bipotential juvenile gonad. In contrast, scRNA-seq of 30dpf gonads revealed that cells in ZZ gonads had transcriptomes characteristic of testicular Sertoli, Leydig, and germ cells while ZW gonads had granulosa cells, theca cells, and developing oocytes. Hematopoietic and vascular cells were similar in both sex genotypes. These results show that juvenile NA zebrafish initially develop a bipotential gonad; that a factor on the NA W chromosome or fewer than two Z chromosomes is essential to initiate oocyte development; and without the W factor or with two Z doses, NA gonads develop directly into testes without passing through the juvenile ovary stage. Sex determination in AB and TU strains mimics NA ZW and WW zebrafish, suggesting loss of the Z chromosome during domestication. Genetic analysis of the NA strain will facilitate our understanding of the evolution of sex determination mechanisms.

Optimizing zebrafish rearing-Effects of fish density and environmental enrichment

Introduction

Despite its popularity in research, there is very little scientifically validated knowledge about the best practices on zebrafish (Danio rerio) husbandry, which has led to several facilities having their own husbandry protocols. This study was performed to expand knowledge on the effects of enrichment and fish density on the welfare of zebrafish, with hopes of providing a scientific basis for future recommendations and legislations.

Methods

Zebrafish were reared at three different stocking densities, (1, 3 or 6 fish/L), in tanks with or without environmental enrichment. Agonistic behavior was observed twice a week for 9 weeks directly in the housing tanks. Aspects of welfare is known to be reflected in neuroendocrine stress responses. Thus, cortisol secretion in response to lowering the water level was analyzed for each group. In addition, we assessed cortisol secretion in response to confinement and risk-taking behavior (boldness) using the novel tank diving test for individual fish. At termination of the experiment fish were subjected to stress by transfer to a novel environment and brain tissue was sampled for analysis of brain monoaminergic activity.

Results

Fish kept at the lowest density (1 fish/L) showed a significantly higher level of aggression than fish kept at 3 or 6 fish/L. Moreover, fish kept at this low density showed significantly higher cortisol secretion on a group level than fish kept at the higher stocking densities, when subjected to lowering of the water level. Keeping fish at 1 fish/L also had effects on brain monoamines, these fish showing higher brain dopamine concentrations but lower dopamine turnover than fish kept at higher densities. Neither stocking density or enrichment had any clear effects on the behavior of individual fish in the novel tank diving test. However, fish kept at high densities showed lower and more variable growth rates than fish kept at 1 fish/L.

Discussion

Taken together these results suggest that zebrafish should not be kept at a density of 1 fish/L. The optimal stocking density is likely to be in the range of 3-6 fish/L.

Development of epigenetic biomarkers for the identification of sex and thermal stress in fish using DNA methylation analysis and machine learning procedures

The sex ratio is a key ecological demographic parameter crucial for population viability. However, the epigenetic mechanisms operating during gonadal development regulating gene expression and the sex ratio remain poorly understood. Moreover, there is interest in the development of epigenetic markers associated with a particular phenotype or as sentinels of environmental effects. Here, we profiled DNA methylation and gene expression of 10 key genes related to sex development and stress, including steroidogenic enzymes, and growth and transcription factors. We provide novel information on the sex-related differences and on the influence of elevated temperature on these genes in zebrafish, a species with mixed genetic and environmental influences on sex ratios. We identified both positive (e.g., amh, cyp11c and hsd11b2) and negative (e.g., cyp11a1 and dmrt1) correlations in unexposed males, and negative correlation (amh) in exposed females between DNA methylation and gene expression levels. Further, we combined DNA methylation analysis with machine learning procedures and found a series of informative CpGs capable not only of correctly identifying sex (based on cyp19a1a DNA methylation levels) but also of identifying whether males and females had been exposed to abnormally elevated temperature when young (based on amh and foxl2a DNA methylation levels, respectively). This was achieved in the absence of conspicuous morphological alterations of the gonads. These DNA methylation-based epigenetic biomarkers represent molecular resources that can correctly recapitulate past thermal history and pave the way for similar findings in other species to assess potential ecological effects of environmental disturbances in the context of climate change.

Environmentally-induced sex reversal in fish with chromosomal vs. polygenic sex determination

Sex ratio depends on sex determination mechanisms and is a key demographic parameter determining population viability and resilience to natural and anthropogenic stressors. There is increasing evidence that the environment can alter sex ratio even in genetically sex-determined species (GSD), as elevated temperature can cause female-to-male sex reversal (neomales). Alarmingly, neomales are being discovered in natural populations of several fish, amphibian and reptile species worldwide. Understanding the basis of neomale development is important for conservation biology. Among GSD species, it is unknown whether those with chromosomal sex determination (CSD), the most common system, will better resist the influence of high temperature than those with polygenic sex determination (PSD). Here, we compared the effects of elevated temperature in two wild zebrafish strains, Nadia (NA) and Ekkwill (EKW), which have CSD with a ZZ/ZW system, against the AB laboratory strain, which has PSD. First, we uncovered novel sex genotypes and the results showed that, at control temperature, the masculinization rate roughly doubled with the addition of each Z chromosome, while some ZW and WW fish of the wild strains became neomales. Surprisingly, we found that at elevated temperatures WW fish were just as likely as ZW fish to become neomales and that all strains were equally susceptible to masculinization. These results demonstrate that the Z chromosome is not essential for male development and that the dose of W buffers masculinization at the control temperature but not at elevated temperature. Furthermore, at the elevated temperature the testes of neomales, but not of normal males, contained more spermatozoa than at the control temperature. Our results show in an unprecedented way that, in a global warming scenario, CSD species may not necessarily be better protected against the masculinizing effect of elevated temperature than PSD species, and reveal genotype-by-temperature interactions in male sex determination and spermatogenesis.

Improving zebrafish laboratory welfare and scientific research through understanding their natural history

Globally, millions of zebrafish (Danio rerio) are used for scientific laboratory experiments for which researchers have a duty of care, with legal obligations to consider their welfare. Considering the growing use of the zebrafish as a vertebrate model for addressing a diverse range of scientific questions, optimising their laboratory conditions is of major importance for both welfare and improving scientific research. However, most guidelines for the care and breeding of zebrafish for research are concerned primarily with maximising production and minimising costs and pay little attention to the effects on welfare of the environments in which the fish are maintained, or how those conditions affect their scientific research. Here we review the physical and social conditions in which laboratory zebrafish are kept, identifying and drawing attention to factors likely to affect their welfare and experimental science. We also identify a fundamental lack knowledge of how zebrafish interact with many biotic and abiotic features in their natural environment to support ways to optimise zebrafish health and well-being in the laboratory, and in turn the quality of scientific data produced. We advocate that the conditions under which zebrafish are maintained need to become a more integral part of research and that we understand more fully how they influence experimental outcome and in turn interpretations of the data generated.

Endocrine disruption from plastic pollution and warming interact to increase the energetic cost of growth in a fish

Energetic cost of growth determines how much food-derived energy is needed to produce a given amount of new biomass and thereby influences energy transduction between trophic levels. Growth and development are regulated by hormones and are therefore sensitive to changes in temperature and environmental endocrine disruption. Here, we show that the endocrine disruptor bisphenol A (BPA) at an environmentally relevant concentration (10 µgl^-1) decreased fish (Danio rerio) size at 30°C water temperature. Under the same conditions, it significantly increased metabolic rates and the energetic cost of growth across development. By contrast, BPA decreased the cost of growth at cooler temperatures (24°C). BPA-mediated changes in cost of growth were not associated with mitochondrial efficiency (P/O ratios (i.e. adenosine diphosphate (ADP) used/oxygen consumed) and respiratory control ratios) although BPA did increase mitochondrial proton leak. In females, BPA decreased age at maturity at 24°C but increased it at 30°C, and it decreased the gonadosomatic index suggesting reduced investment into reproduction. Our data reveal a potentially serious emerging problem: increasing water temperatures resulting from climate warming together with endocrine disruption from plastic pollution can impact animal growth efficiency, and hence the dynamics and resilience of animal populations and the services these provide.

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.

Sex Determination and Differentiation in Teleost: Roles of Genetics, Environment, and Brain

The fish reproductive system is a complex biological system. Nonetheless, reproductive organ development is conserved, which starts with sex determination and then sex differentiation. The sex of a teleost is determined and differentiated from bipotential primordium by genetics, environmental factors, or both. These two processes are species-specific. There are several prominent genes and environmental factors involved during sex determination and differentiation. At the cellular level, most of the sex-determining genes suppress the female pathway. For environmental factors, there are temperature, density, hypoxia, pH, and social interaction. Once the sexual fate is determined, sex differentiation takes over the gonadal developmental process. Environmental factors involve activation and suppression of various male and female pathways depending on the sexual fate. Alongside these factors, the role of the brain during sex determination and differentiation remains elusive. Nonetheless, GnRH III knockout has promoted a male sex-biased population, which shows brain involvement during sex determination. During sex differentiation, LH and FSH might not affect the gonadal differentiation, but are required for regulating sex differentiation. This review discusses the role of prominent genes, environmental factors, and the brain in sex determination and differentiation across a few teleost species.

Fast life-histories are associated with larger brain size in killifishes

The high energetic demands associated with the vertebrate brain are proposed to result in a trade-off between the pace of life-history and relative brain size. However, because both life-history and brain size also have a strong relationship with body size, any associations between the pace of life-history and relative brain size may be confounded by coevolution with body size. Studies on systems where contrasts in the pace of life-history occur without concordant contrasts in body size could therefore add to our understanding of the potential coevolution between relative brain size and life-history. Using one such system - 21 species of killifish - we employed a common garden design across two ontogenetic stages to investigate the association between relative brain size and the pace of life-history. Contrary to predictions, we found that relative brain size was larger in adult fast-living killifishes, compared to slow-living species. Although we found no differences in relative brain size between juvenile killifishes. Our results suggest that fast- and slow-living killifishes do not exhibit the predicted trade-off between brain size and life-history. Instead, fast and slow-living killifishes could differ in the ontogenetic timing of somatic versus neural growth or inhabit environments that differ considerably in cognitive demands.

Parental selection for growth and early-life low stocking density increase the female-to-male ratio in European sea bass

In European sea bass (Dicentrarchus labrax), as in many other fish species, temperature is known to influence the sex of individuals, with more males produced at relatively high temperatures. It is however unclear to what extent growth or stress are involved in such a process, since temperature is known to influence both growth rate and cortisol production. Here, we designed an experiment aiming at reducing stress and affecting early growth rate. We exposed larvae and juveniles originating from both captive and wild parents to three different treatments: low stocking density, food supplemented with tryptophan and a control. Low stocking density and tryptophan treatment respectively increased and decreased early growth rate. Each treatment influenced the stress response depending on the developmental stage, although no clear pattern regarding the whole-body cortisol concentration was found. During sex differentiation, fish in the low-density treatment exhibited lower expression of gr1, gr2, mr, and crf in the hypothalamus when compared to the control group. Fish fed tryptophan displayed lower crf in the hypothalamus and higher level of serotonin in the telencephalon compared to controls. Overall, fish kept at low density produced significantly more females than both control and fish fed tryptophan. Parents that have been selected for growth for three generations also produced significantly more females than parents of wild origin. Our findings did not allow to detect a clear effect of stress at the group level and rather point out a key role of early sexually dimorphic growth rate in sex determination.

Early social deprivation does not affect cortisol response to acute and chronic stress in zebrafish

Social isolation is a well-established technique for inducing early adversity but, in rodent models, the need of parental care makes it difficult to distinguish the effects of social deprivation from the consequences of nutritional deficiencies. Zebrafish do not require parental care, allowing separation of social deprivation from nutritional deprivation, and have emerged as a promising model to study ontogeny of normal and pathological behaviors relevant for human neuropsychological disorders. Previous reports of life-long isolation in zebrafish showed some consistency with mammalian literature, depicting later social deficits and locomotor hyperactivity. However, unlike reports of higher anxiety and stress behavior in isolated rodents and primates, behavioral responses were tapered in isolated fish. To examine whether life-long developmental isolation has a dampening effect on zebrafish endocrine stress response, we applied stressors to zebrafish siblings that were either isolated or socialized, and compared their whole-body cortisol levels with non-stressed control siblings kept in low-housing densities. Utilizing previously validated paradigms (exposure to novel tank and unpredictable chronic stress), we exposed separate groups (n = 9-14, mixed-sex) of social and isolated zebrafish to acute and chronic stressors and measured their cortisol levels. A univariate ANOVA and post-hoc Tukey's HSD tests confirmed that compared to socially raised control fish, developmental isolation did not increase baseline cortisol levels in zebrafish. Additionally, compared to the non-stressed condition, application of both acute and chronic stressors significantly increased cortisol levels in isolated fish and, to a similar degree, to socially raised fish. Our findings suggest that zebrafish isolation studies may help separate effects of social deprivation from nonsocial aspects of early adversity. These studies further substantiate the use of developmental isolation in zebrafish, particularly with acute and chronic stress paradigms, for modeling neuropsychological disorders.LAY SUMMARYA difficult childhood can make humans react more frequently or severely to later stress and modeling this effect in animals can help explain how and why early stress affects subsequent mental and physical health. Early social isolation does not affect later response to stressful situations in adult zebrafish, providing us with a model of psychiatric disorders that allows separation of effects of poor physical environments (lacking food, shelter, etc.) from poor social environments (lack of appropriate socialization).

Comparison of Juvenile Feed Protocols on Growth and Spawning in Zebrafish

Over the past 2 decades, zebrafish, Danio rerio, have become a mainstream laboratory animal model, yet zebrafish husbandry practices remain far from standardized. Feeding protocols play a critical role in the health, wellbeing, and productivity of zebrafish laboratories, yet they vary significantly between facilities. In this study, we compared our current feeding protocol for juvenile zebrafish (30 dpf to 75 dpf), a 3:1mixture of fish flake and freeze-dried krill fed twice per day with live artemia twice per day (FKA), to a diet of Gemma Micro 300 fed once per day with live artemia once per day (GMA). Our results showed that juvenile EK wild-type zebrafish fed GMA were longer and heavier than juveniles fed FKA. As compared with FKA-fed juveniles, fish fed GMA as juveniles showed better reproductive performance as measured by spawning success, fertilization rate, and clutch size. As adults, fish from both feeding protocols were acclimated to our standard adult feeding protocol, and the long-term effects of juvenile diet were assessed. At 2 y of age, the groups showed no difference in mortality or fecundity. Reproductive performance is a crucial aspect of zebrafish research, as much of the research focuses on the developing embryo. Here we show that switching juvenile zebrafish from a mixture of flake and krill to Gemma Micro 300 improves reproductive performance, even with fewer feedings of live artemia, thus simplifying husbandry practices.

Do the Offspring of Sex Reversals Have Higher Sensitivity to Environmental Perturbations?

Sex determination systems in vertebrates vary along a continuum from genetic (GSD) to environmental sex determination (ESD). Individuals that show a sexual phenotype opposite to their genotypic sex are called sex reversals. Aside from genetic elements, temperature, sex steroids, and exogenous chemicals are common factors triggering sex reversal, a phenomenon that may occur even in strict GSD species. In this paper, we review the literature on instances of sex reversal in fish, amphibians, reptiles, birds, and mammals. We focus on the offspring of sex-reversed parents in the instances that they can be produced, and show that in all cases studied the offspring of these sex-reversed parents exhibit a higher sensitivity to environmental perturbations than the offspring of non-sex-reversed parents. We suggest that the inheritance of this sensitivity, aside from possible genetic factors, is likely to be mediated by epigenetic mechanisms such as DNA methylation, since these mechanisms are responsive to environmental cues, and epigenetic modifications can be transmitted to the subsequent generations. Species with a chromosomal GSD system with environmental sensitivity and availability of genetic sex markers should be employed to further test whether offspring of sex-reversed parents have greater sensitivity to environmental perturbations. Future studies could also benefit from detailed whole-genome data in order to elucidate the underlying molecular mechanisms. Finally, we discuss the consequences of such higher sensitivity in the context of global climate change.

Zebrafish male differentiation: Do all testes go through a "juvenile ovary" stage?

Zebrafish (Danio rerio) studies describe before the onset of mature gonads differentiation all individuals go through a "juvenile ovary" stage. However, the sequential events of the early zebrafish gonad differentiation are still not described in full detail and recent works indicate that some individuals never form a "juvenile ovary" structure. Therefore, the present study aimed to confirm the existence of two processes of zebrafish male differentiation. For this purpose, every two days between 20 and 30 days post-fertilization (dpf) zebrafish were collected for a stereological analysis of the differentiating gonads. The histological evaluation showed that prior to 22 dpf, zebrafish gonads were still undifferentiated. At 24 dpf, some individuals started to present a "juvenile ovary" and from 26 to 30 dpf, it was possible to discern two processes of gonad development. The majority of the individuals (80 %) developed a "juvenile ovary", while in the remaining (20 %) it was not possible to detect this structure. The results of the present study show the existence of two distinct processes of zebrafish male gonad development, indicating that not all individuals go through the "juvenile ovary" stage.

Plasticity of the skeleton and skeletal deformities in zebrafish (Danio rerio) linked to rearing density

The teleost zebrafish (Danio rerio), an established model for human skeletal diseases, is reared under controlled conditions with defined parameters for temperature and photoperiod. Studies aimed at defining the proper rearing density have been performed with regard to behavioural and physiological stress response, sex ratio and reproduction. Studies concerning the effect of rearing density on the skeletal phenotype are lacking. This study analyses the response of the skeleton to different rearing densities and describes the skeletal deformities. Wild-type zebrafish were reared up to 30 dpf (days post-fertilization) in a common environment. From 30 to 90 dpf, animals were reared at three different densities: high density (HD), 32 fish l^-1 ; medium density (MD), 8 fish l^-1 and low density (LD), 2 fish l^-1 . Animals at 30 and 90 dpf were collected and whole-mount stained with Alizarin red S to visualize mineralized tissues. The entire skeleton was analysed for meristic counts and 172 types of deformities. The results showed that the rearing density significantly influenced the specimens' average standard length, which decreased with the increase in the rearing density. Differences in meristic counts among the three groups were not observed. Rearing density-independent malformations affected the ribs, neural arches and the spines of the abdominal region, as well as vertebrae of the caudal complex. The HD group showed the highest number of deformities per specimen, the highest number of observed types of deformities and, together with the MD group, the highest frequency of specimens affected by severe deformities. In particular, the HD group showed deformities affecting arches, spines and vertebral centra in the caudal region of the vertebral column. This study provides evidence of an effect of the rearing density on the development of different skeletal phenotypes.

Aquatic Models: Water Quality and Stability and Other Environmental Factors

The use of aquatic animals in ecotoxicology, genetic, and biomedical research has grown immensely in recent years, especially due to the increased use of zebrafish in the laboratory setting. Because water is the primary environment of most aquatic species, the composition and management of this water is paramount to ensuring their health and welfare. In this publication, we will describe the important variables in water quality that can influence animal health and research results, using the zebrafish model for detailed specifics of optimal conditions. Wherever possible, recommendations are provided to reduce the potential impact of poor or highly variable water quality, and standards are given which can be used as institutional goals to maximize animal health and welfare and reduce research variability. It is increasingly important that authors of publications describing work done using aquatic models characterize water quality and other environmental conditions of the animal environment so that the work can be repeated and understood in context of these important factors. It is clear that there are a great many extrinsic factors which may influence research outcomes in the aquatics model laboratory setting, and consequently, an increased level of funding will be essential to support continued research exploring these and other important husbandry conditions. References from a large body of literature on this subject are provided.

The Influence of Behavioral, Social, and Environmental Factors on Reproducibility and Replicability in Aquatic Animal Models

The publication of reproducible, replicable, and translatable data in studies utilizing animal models is a scientific, practical, and ethical necessity. This requires careful planning and execution of experiments and accurate reporting of results. Recognition that numerous developmental, environmental, and test-related factors can affect experimental outcomes is essential for a quality study design. Factors commonly considered when designing studies utilizing aquatic animal species include strain, sex, or age of the animal; water quality; temperature; and acoustic and light conditions. However, in the aquatic environment, it is equally important to consider normal species behavior, group dynamics, stocking density, and environmental complexity, including tank design and structural enrichment. Here, we will outline normal species and social behavior of 2 commonly used aquatic species: zebrafish (Danio rerio) and Xenopus (X. laevis and X. tropicalis). We also provide examples as to how these behaviors and the complexity of the tank environment can influence research results and provide general recommendations to assist with improvement of reproducibility and replicability, particularly as it pertains to behavior and environmental complexity, when utilizing these popular aquatic models.

Critical roles of the ddx5 gene in zebrafish sex differentiation and oocyte maturation

DEAD-box helicase 5 (Ddx5) functions as an ATP-dependent RNA helicase and as a transcriptional coactivator for several transcription factors; however, the developmental function of the ddx5 gene in vertebrates is not fully understood. We found that the zebrafish ddx5 gene was expressed in developing gonads. Using the genome editing technology transcription activator-like effector nuclease, we established a ddx5-disrupted zebrafish and examined the morphological phenotypes of the mutant. We found that the majority of ddx5-deficient mutants developed as fertile males with normal testes and a small number of ddx5-deficient mutants developed as infertile females with small ovaries. Apoptotic cell death at 31 days post fertilization was increased in thick immature gonads (presumptive developing ovaries) of the ddx5-deficient mutant compared to those of heterozygous wild-type fish, while the number of apoptotic cells in thin immature gonads (presumptive developing testes) was comparable between the mutant and wild-type animals. Histological analysis revealed that ovaries of adult ddx5-deficient females had fewer vitellogenic oocytes and a larger number of stage I and II oocytes. The amount of cyclic adenosine monophosphate in the ddx5-deficient ovaries was high compared to that of wild-type ovaries, presumably leading to the mitotic arrest of oocyte maturation. Therefore, the ddx5 gene is dispensable for testis development, but it is essential for female sex differentiation and oocyte maturation in zebrafish.

Capn3 depletion causes Chk1 and Wee1 accumulation and disrupts synchronization of cell cycle reentry during liver regeneration after partial hepatectomy

Recovery of liver mass to a healthy liver donor by compensatory regeneration after partial hepatectomy (PH) is a prerequisite for liver transplantation. Synchronized cell cycle reentry of the existing hepatocytes after PH is seemingly a hallmark of liver compensatory regeneration. Although the molecular control of the PH-triggered cell cycle reentry has been extensively studied, little is known about how the synchronization is achieved after PH. The nucleolus-localized protein cleavage complex formed by the nucleolar protein Digestive-organ expansion factor (Def) and cysteine proteinase Calpain 3 (Capn3) has been implicated to control wounding healing during liver regeneration through selectively cleaving the tumor suppressor p53 in the nucleolus. However, whether the Def-Capn3 complex participates in regulating the synchronization of cell cycle reentry after PH is unknown. In this report, we generated a zebrafish capn3b null mutant (capn3b^∆19∆14). The homozygous mutant was viable and fertile, but suffered from a delayed liver regeneration after PH. Delayed liver regeneration in capn3b^∆19∆14 was due to disruption of synchronized cell proliferation after PH. Mass spectrometry (MS) analysis of nuclear proteins revealed that a number of negative regulators of cell cycle are accumulated in the capn3b^∆19∆14 liver after PH. Moreover, we demonstrated that Check-point kinase 1 (Chk1) and Wee1, two key negative regulators of G2 to M transition, are substrates of Capn3. We also demonstrated that Chk1 and Wee1 were abnormally accumulated in the nucleoli of amputated capn3b^∆19∆14 liver. In conclusion, our findings suggest that the nucleolar-localized Def-Capn3 complex acts as a novel regulatory pathway for the synchronization of cell cycle reentry, at least partially, through inactivating Chk1 and Wee1 during liver regeneration after PH.

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.

Crowding stress during the period of sex determination causes masculinization in pejerrey Odontesthes bonariensis, a fish with temperature-dependent sex determination

The pejerrey is an atherinopsid species from South America that presents a combination of genotypic and environmental (temperature-dependent) sex determination whereby low and high temperatures induce feminization and masculinization, respectively. Masculinization involves a heat-induced stress response leading to increased circulating cortisol and androgens. We tested whether crowding would elicit a similar response as high temperature and affect the sex ratios of pejerrey. Larvae with XX and XY genotypes were reared at 15, 62 and 250 larvae/L in 0.4, 1.6, and 6.4 L containers during a period considered critical for sex determination at 25 °C, a mixed-sex promoting temperature. Fish were analysed at 3-7 weeks for whole-body cortisol and 11-ketotestosterone (11-KT) titer and hydroxy-steroid dehydrogenase (hsd11b2) mRNA transcript abundance, and after completion of gonadal sex differentiation (10-14 weeks) for determination of phenotypic and genotypic sex mismatches. Crowding was associated with depressed growth, higher cortisol and 11-KT titers, increased hsd11b2 transcription, and increased frequency of masculinization compared to intermediate and/or low rearing densities. Perceived crowding (by rearing in containers with mirror-finish, reflecting walls) also caused masculinization. These results suggest the possibility that other environmental factors besides temperature can also affect sex determination in pejerrey and that a stress response leading to increased cortisol and androgen levels, which is potentially perceived by the brain, may be a common feature among different forms of environmental sex determination in this species.

A Drive to Driven Model of Mapping Intraspecific Interaction Networks

Community ecology theory suggests that an individual's phenotype is determined by the phenotypes of its coexisting members to the extent at which this process can shape community evolution. Here, we develop a mapping theory to identify interaction quantitative trait loci (QTL) governing inter-individual dependence. We mathematically formulate the decision-making strategy of interacting individuals. We integrate these mathematical descriptors into a statistical procedure, enabling the joint characterization of how QTL drive the strengths of ecological interactions and how the genetic architecture of QTL is driven by ecological networks. In three fish full-sib mapping experiments, we identify a set of genome-wide QTL that control a range of societal behaviors, including mutualism, altruism, aggression, and antagonism, and find that these intraspecific interactions increase the genetic variation of body mass by about 50%. We showcase how the interaction QTL can be used as editors to reconstruct and engineer new social networks for ecological communities.

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We develop a new theory for complex-trait mapping by integrating behavioral ecology

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This theory can characterize how QTL drive cooperation or competition in populations

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It can also illustrate how the activation of QTL is driven by ecological interactions

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The new theory leverages interdisciplinary studies of genetics, ecology, and evolution

Ferredoxin 1b Deficiency Leads to Testis Disorganization, Impaired Spermatogenesis, and Feminization in Zebrafish

The roles of steroids in zebrafish sex differentiation, gonadal development, and function of the adult gonad are poorly understood. Herein, we used ferredoxin 1b (fdx1b) mutant zebrafish to explore such processes. Fdx1b is an essential electron-providing cofactor to mitochondrial steroidogenic enzymes, which are crucial for glucocorticoid and androgen production in vertebrates. Fdx1b-/- zebrafish mutants develop into viable adults in which concentrations of androgens and cortisol are significantly reduced. Adult fdx1b-/- mutant zebrafish display predominantly female secondary sex characteristics but may possess either ovaries or testes, confirming that androgen signaling is dispensable for testicular differentiation in this species, as previously demonstrated in androgen receptor mutant zebrafish. Adult male fdx1b-/- mutant zebrafish exhibit reduced characteristic breeding behaviors and impaired sperm production, resulting in infertility in standard breeding scenarios. However, eggs collected from wild-type females can be fertilized by the sperm of fdx1b-/- mutant males by in vitro fertilization. The testes of fdx1b-/- mutant males are disorganized and lack defined seminiferous tubule structure. Expression of several promale and spermatogenic genes is decreased in the testes of fdx1b-/- mutant males, including promale transcription factor sox9a and spermatogenic genes igf3 and insl3. This study establishes an androgen- and cortisol-deficient fdx1b zebrafish mutant as a model for understanding the effects of steroid deficiency on sex development and reproductive function. This model will be particularly useful for further investigation of the roles of steroids in spermatogenesis, gonadal development, and regulation of reproductive behavior, thus enabling further elucidation of the physiological consequences of endocrine disruption in vertebrates.

Zebrafish: Housing and husbandry recommendations

This article provides recommendations for the care of laboratory zebrafish (Danio rerio) as part of the further implementation of Annex A to the European Convention on the protection of vertebrate animals used for experimental and other scientific purposes, EU Commission Recommendation 2007/526/EC and the fulfilment of Article 33 of EU Directive 2010/63, both concerning the housing and care of experimental animals. The recommendations provide guidance on best practices and ranges of husbandry parameters within which zebrafish welfare, as well as reproducibility of experimental procedures, are assured.

Husbandry procedures found today in zebrafish facilities are numerous. While the vast majority of these practices are perfectly acceptable in terms of zebrafish physiology and welfare, the reproducibility of experimental results could be improved by further standardisation of husbandry procedures and exchange of husbandry information between laboratories. Standardisation protocols providing ranges of husbandry parameters are likely to be more successful and appropriate than the implementation of a set of fixed guidance values neglecting the empirically successful daily routines of many facilities and will better reflect the wide range of environmental parameters that characterise the natural habitats occupied by zebrafish.

A joint working group on zebrafish housing and husbandry recommendations, with members of the European Society for Fish Models in Biology and Medicine (EUFishBioMed) and of the Federation of European Laboratory Animal Science Associations (FELASA) has been given a mandate to provide guidelines based on a FELASA list of parameters, ‘Terms of Reference’.

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.

What makes a bad egg? Egg transcriptome reveals dysregulation of translational machinery and novel fertility genes important for fertilization

BACKGROUND

Egg quality can be defined as the egg ability to be fertilized and subsequently develop into a normal embryo. Previous research has shed light on factors that can influence egg quality. Large gaps however remain including a comprehensive view of what makes a bad egg. Initial development of the embryo relies on maternally-inherited molecules, such as transcripts, deposited in the egg during its formation. Bad egg quality is therefore susceptible to be associated with alteration or dysregulation of maternally-inherited transcripts. We performed transcriptome analysis on a large number (N = 136) of zebrafish egg clutches, each clutch being split to monitor developmental success and perform transcriptome analysis in parallel. We aimed at drawing a molecular portrait of the egg in order to characterize the relation between egg transcriptome and developmental success and to subsequently identify new candidate genes involved in fertility.

RESULTS

We identified 66 transcript that were differentially abundant in eggs of contrasted phenotype (low or high developmental success). Statistical modeling using partial least squares regression and genetics algorithm demonstrated that gene signatures from transcriptomic data can be used to predict developmental success. The identity and function of differentially expressed genes indicate a major dysregulation of genes of the translational machinery in poor quality eggs. Two genes, otulina and slc29a1a, predominantly expressed in the ovary and dysregulated in poor quality eggs were further investigated using CRISPR/Cas9 mediated genome editing. Mutants of each gene revealed remarkable subfertility whereby the majority of their eggs were unfertilizable. The Wnt pathway appeared to be dysregulated in the otulina mutant-derived eggs.

CONCLUSIONS

Here we show that egg transcriptome contains molecular signatures, which can be used to predict developmental success. Our results also indicate that poor egg quality in zebrafish is associated with a dysregulation of (i) the translational machinery genes and (ii) novel fertility genes, otulina and slc29a1a, playing an important role for fertilization. Together, our observations highlight the diversity of the possible causes of egg quality defects and reveal mechanisms of maternal origin behind the lack of fertilization and early embryonic failures that can occur under normal reproduction conditions.

The Adaptive Sex in Stressful Environments

The impact of early stress on juvenile development has intrigued scientists for decades, but the adaptive significance of such effects remains an ongoing debate. This debate has largely ignored some characteristics of the offspring, such as their sex, despite strong evolutionary and demographic implications of sex-ratio variation. We review recent studies that examine associations between glucocorticoids (GCs), the main class of stress hormones, and offspring sex. Whereas exposure to GCs at around the time of sex determination in fish consistently produces males, the extent and direction of sex-ratio bias in response to stress vary in reptiles, birds, and mammals. We propose proximate and ultimate explanations for most of these trends.

Ethical considerations in fish research

Fishes are used in a wide range of scientific studies, from conservation research with potential benefits to the species used to biomedical research with potential human benefits. Fish research can take place in both laboratories and field environments and methods used represent a continuum from non-invasive observations, handling, through to experimental manipulation. While some countries have legislation or guidance regarding the use of fish in research, many do not and there exists a diversity of scientific opinions on the sentience of fish and how we determine welfare. Nevertheless, there is a growing pressure on the scientific community to take more responsibility for the animals they work with through maximising the benefits of their research to humans or animals while minimising welfare or survival costs to their study animals. In this review, we focus primarily on the refinement of common methods used in fish research based on emerging knowledge with the aim of improving the welfare of fish used in scientific studies. We consider the use of anaesthetics and analgesics and how we mark individuals for identification purposes. We highlight the main ethical concerns facing researchers in both laboratory and field environments and identify areas that need urgent future research. We hope that this review will help inform those who wish to refine their ethical practices and stimulate thought among fish researchers for further avenues of refinement. Improved ethics and welfare of fishes will inevitably lead to increased scientific rigour and is in the best interests of both fishes and scientists.

Evidence that fertility trades off with early offspring fitness as males age

Models of ageing predict that sperm function and fertility should decline with age as sperm are exposed to free radical damage and mutation accumulation. However, theory also suggests that mating with older males should be beneficial for females because survival to old age is a demonstration of a male's high genetic and/or phenotypic quality. Consequently, declines in sperm fitness may be offset by indirect fitness benefits exhibited in offspring. While numerous studies have investigated age-based declines in male fertility, none has taken the integrated approach of studying age-based effects on both male fertility and offspring fitness. Here, using a cohort-based longitudinal study of zebrafish (Danio rerio), we report a decline in male mating success and fertility with male age but also compensating indirect benefits. Using in vitro fertilization, we show that offspring from older males exhibit superior early survival compared to those from their youngest counterparts. These findings suggest that the high offspring fitness observed for the subset of males that survive to an old age (approx. 51% in this study) may represent compensating benefits for declining fertility with age, thus challenging widely held views about the fitness costs of mating with older males.

Effects of environmental enrichment on survivorship, growth, sex ratio and behaviour in laboratory maintained zebrafish Danio rerio

Environmental enrichment involves increasing the complexity of a fish's environment in order to improve welfare. Researchers are legally obliged to consider the welfare of laboratory animals and poor welfare may result in less robust data in experimental science. Laboratory zebrafish Danio rerio are usually kept in bare aquaria for ease of husbandry and, despite being a well-studied species, little is known about how laboratory housing affects their welfare. This study shows that environmental enrichment, in the form of the addition of gravel substratum and plants into the tank, affects survivorship, growth and behaviour in laboratory-maintained D. rerio. Larvae reared in enriched tanks had significantly higher survivorship compared with larvae reared in bare tanks. Effects of the tank conditions on growth were more variable. Females from enriched tanks had a higher body condition than females maintained in bare tanks, but intriguingly this was not the case for males, where the only difference was a more variable body condition in males maintained in bare tanks. Sex ratio in the rearing tanks did not differ between treatments. Resource monopolisation was higher for fish in enriched tanks than for those in bare tanks. Fish from enriched tanks displayed lower levels of behaviours associated with anxiety compared with fish from bare tanks when placed into a novel environment. Thus, this study demonstrates differences in welfare for D. rerio maintained under different environmental conditions with enhancements in welfare more commonly associated with tank enrichment.

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.

G Protein-Coupled Estrogen Receptor Is Not Required for Sex Determination or Ovary Function in Zebrafish

Estrogens regulate vertebrate development and function through binding to nuclear estrogen receptors α and β (ERα and ERβ) and the G protein-coupled estrogen receptor (GPER). Studies in mutant animal models demonstrated that ERα and ERβ are required for normal ovary development and function. However, the degree to which GPER signaling contributes to ovary development and function is less well understood. Previous studies using cultured fish oocytes found that estradiol inhibits oocyte maturation in a GPER-dependent manner, but whether GPER regulates oocyte maturation in vivo is not known. To test the hypothesis that GPER regulates oocyte maturation in vivo, we assayed ovary development and function in gper mutant zebrafish. We found that homozygous mutant gper embryos developed into male and female adults with normal sex ratios. Adult mutant fish exhibited normal secondary sex characteristics and fertility. Additionally, mutant ovaries were histologically normal. We observed no differences in the number of immature versus mature oocytes in mutant versus wild-type ovaries from both young and aged adults. Furthermore, expression of genes associated with sex determination and ovary function was normal in gper mutant ovaries compared with wild type. Our findings suggest that GPER is not required for sex determination, ovary development, or fertility in zebrafish.

Zebrabase: An Intuitive Tracking Solution for Aquatic Model Organisms

Small fish species, such as zebrafish and medaka, are increasingly gaining popularity in basic research and disease modeling as a useful alternative to rodent model organisms. However, the tracking options for fish within a facility are rather limited. In this study, we present an aquatic species tracking database, Zebrabase, developed in our zebrafish research and breeding facility that represents a practical and scalable solution and an intuitive platform for scientists, fish managers, and caretakers, in both small and large facilities. Zebrabase is a scalable, cross-platform fish tracking database developed especially for fish research facilities. Nevertheless, this platform can be easily adapted for a wide variety of aquatic model organisms housed in tanks. It provides sophisticated tracking, reporting, and management functions that help keep animal-related records well organized, including a QR code functionality for tank labeling. The implementation of various user roles ensures a functional hierarchy and customized access to specific functions and data. In addition, Zebrabase makes it easy to personalize rooms and racks, and its advanced statistics and reporting options make it an excellent tool for creating periodic reports of animal usage and productivity. Communication between the facility and the researchers can be streamlined by the database functions. Finally, Zebrabase also features an interactive breeding history and a smart interface with advanced visualizations and intuitive color coding that accelerate the processes.

Toxicity and non-harmful effects of the soya isoflavones, genistein and daidzein, in embryos of the zebrafish, Danio rerio

Based on the assumed oestrogenic and apoptotic properties of soya isoflavones (genistein, daidzein), and following the current OECD test-guidelines and principle of 3Rs, we have studied the potential toxicity of phytochemicals on the zebrafish embryos test (ZFET). For this purpose, zebrafish embryos at 2-3 h post-fertilisation (hpf) were exposed to both soya isoflavones (from 1.25 mg/L to 20 mg/L) and assayed until 96 hpf. Lethal and sub-lethal endpoints (mortality, hatching rates and malformations) were estimated in the ZFET, which was expanded to potential gene expression markers, determining the lowest observed effect (and transcriptional) concentrations (LOEC, LOTEC), and the no-observable effect (and transcriptional) concentrations (NOEC, NOTEC). The results revealed that genistein is more toxic (LC50-96 hpf: 4.41 mg/L) than daidzein (over 65.15 mg/L). Both isoflavones up-regulated the oestrogen (esrrb) and death receptors (fas) and cyp1a transcript levels. Most thyroid transcript signals were up-regulated by genistein (except for thyroid peroxidase/tpo), and the hatching enzyme (he1a1) was exclusively up-regulated by daidzein (from 1.25 mg/L onwards). The ZFET proved suitable for assessing toxicant effects of both isoflavones and potential disruptions (i.e. oestrogenic, apoptotic, thyroid, enzymatic) during the embryogenesis and the endotrophic larval period.

Guidance for the identification of endocrine disruptors in the context of Regulations (EU) No 528/2012 and (EC) No 1107/2009

This Guidance describes how to perform hazard identification for endocrine-disrupting properties by following the scientific criteria which are outlined in Commission Delegated Regulation (EU) 2017/2100 and Commission Regulation (EU) 2018/605 for biocidal products and plant protection products, respectively.

Contested Paradigm in Raising Zebrafish (Danio rerio)

The current body of work on rearing larval/juvenile zebrafish is based on (1) utilization of freshwater and (2) diurnal light/dark cycle, (3) provision of live feed at modest density, and (4) culture in high visibility environment. We challenged these rearing approaches by maintaining zebrafish under constant light for 46-48 days (days postfertilization [dpf]), while securing continuous feeding in high turbidity and saline (1.8-2.1 parts per thousand) environment for the experiment's duration, allowing 24 h feeding/growth of fish from first exogenous feeding to maturation. There was no evidence of negative effects on zebrafish larvae behavior, growth, survival, and life cycle duration at constant illumination when food was continuously available. Zebrafish were stocked at high initial density (100 larvae/L) in a static system and fed high densities of rotifers (Brachionus plicatilis) (200-400/mL) from 6 to 12 dpf. Fish density was then reduced by 50% and two diet treatments, live rotifers or brine shrimp (Artemia) nauplii (10/mL), followed. Fish were reared on these two diets until first maturation. Performance of adult zebrafish fed live rotifer followed by Artemia nauplii diet was the highest recorded in the literature after 42 dpf, 250 ± 29 (males) and 430 ± 5 mg (females). Use of these rearing conditions, during the entire life cycle, until reproduction, resulted in the shortest ever recorded generation time (from egg to egg) of 43-45 dpf and fertilization rate (1 dpf) of 80.3%-94%.

Treatment with a DNA methyltransferase inhibitor feminizes zebrafish and induces long-term expression changes in the gonads

BACKGROUND

The role of epigenetic modifications such as DNA methylation during vertebrate sexual development is far from being clear. Using the zebrafish model, we tested the effects of one of the most common DNA methyltransferase (dnmt) inhibitor, 5-aza-2'-deoxycytidine (5-aza-dC), which is approved for the treatment of acute myeloid leukaemia and is under active investigation for the treatment of solid tumours. Several dose-response experiments were carried out during two periods, including not only the very first days of development (0-6 days post-fertilization, dpf), as done in previous studies, but also, and as a novelty, the period of gonadal development (10-30 dpf).

RESULTS

Early treatment with 5-aza-dC altered embryonic development, delayed hatching and increased teratology and mortality, as expected. The most striking result, however, was an increase in the number of females, suggesting that alterations induced by 5-aza-dC treatment can affect sexual development as well. Results were confirmed when treatment coincided with gonadal development. In addition, we also found that the adult gonadal transcriptome of 5-aza-dC-exposed females included significant changes in the expression of key reproduction-related genes (e.g. cyp11a1, esr2b and figla), and that several pro-female-related pathways such as the Fanconi anaemia or the Wnt signalling pathways were downregulated. Furthermore, an overall inhibition of genes implicated in epigenetic regulatory mechanisms (e.g. dnmt1, dicer, cbx4) was also observed.

CONCLUSIONS

Taken together, our results indicate that treatment with a DNA methylation inhibitor can also alter the sexual development in zebrafish, with permanent alterations of the adult gonadal transcriptome, at least in females. Our results show the importance of DNA methylation for proper control of sexual development, open new avenues for the potential control of sex ratios in fish (aquaculture, population control) and call attention to possibly hidden long-term effects of dnmt therapy when used, for example, in the treatment of prepuberal children affected by some types of cancer.

Histological and transcriptomic effects of 17α-methyltestosterone on zebrafish gonad development

BACKGROUND

Sex hormones play important roles in teleost ovarian and testicular development. In zebrafish, ovarian differentiation appears to be dictated by an oocyte-derived signal via Cyp19a1a aromatase-mediated estrogen production. Androgens and aromatase inhibitors can induce female-to-male sex reversal, however, the mechanisms underlying gonadal masculinisation are poorly understood. We used histological analyses together with RNA sequencing to characterise zebrafish gonadal transcriptomes and investigate the effects of 17α-methyltestosterone on gonadal differentiation.

RESULTS

At a morphological level, 17α-methyltestosterone (MT) masculinised gonads and accelerated spermatogenesis, and these changes were paralleled in masculinisation and de-feminisation of gonadal transcriptomes. MT treatment upregulated expression of genes involved in male sex determination and differentiation (amh, dmrt1, gsdf and wt1a) and those involved in 11-oxygenated androgen production (cyp11c1 and hsd11b2). It also repressed expression of ovarian development and folliculogenesis genes (bmp15, gdf9, figla, zp2.1 and zp3b). Furthermore, MT treatment altered epigenetic modification of histones in zebrafish gonads. Contrary to expectations, higher levels of cyp19a1a or foxl2 expression in control ovaries compared to MT-treated testes and control testes were not statistically significant during early gonad development (40 dpf).

CONCLUSION

Our study suggests that both androgen production and aromatase inhibition are important for androgen-induced gonadal masculinisation and natural testicular differentiation in zebrafish.