5 Hormonal Sex Control and its Application to Fish Culture

Steroid hormones in the aquatic environment

Steroid hormones are extremely important natural hormones in all vertebrates. They control a wide range of physiological processes, including osmoregulation, sexual maturity, reproduction and stress responses. In addition, many synthetic steroid hormones are in widespread and general use, both as human and veterinary pharmaceuticals. Recent advances in environmental analytical chemistry have enabled concentrations of steroid hormones in rivers to be determined. Many different steroid hormones, both natural and synthetic, including transformation products, have been identified and quantified, demonstrating that they are widespread aquatic contaminants. Laboratory ecotoxicology experiments, mainly conducted with fish, but also amphibians, have shown that some steroid hormones, both natural and synthetic, can adversely affect reproduction when present in the water at extremely low concentrations: even sub-ng/L. Recent research has demonstrated that mixtures of different steroid hormones can inhibit reproduction even when each individual hormone is present at a concentration below which it would not invoke a measurable effect on its own. Limited field studies have supported the conclusions of the laboratory studies that steroid hormones may be environmental pollutants of significant concern. Further research is required to identify the main sources of steroid hormones entering the aquatic environment, better describe the complex mixtures of steroid hormones now known to be ubiquitously present, and determine the impacts of environmentally-realistic mixtures of steroid hormones on aquatic vertebrates, especially fish. Only once that research is completed can a robust aquatic risk assessment of steroid hormones be concluded.

Characteristics and Cryopreservation of Semen of Sex-Reversed Females of Salmonid Fish

Sex reversal has been used as a breeding strategy by salmonid fish to produce genetically and phenotypically single sex populations. Production of all-female fish has great importance for the creation of monosex female triploids of salmonid fish, which are valued for their sterility, lack of female maturation, and larger commercial size. Among salmonids, the majority of rainbow trout (Oncorhynchus mykiss) production is based on all-female production with a high proportion of all-female triploid production in Europe. The main aim of this review is to present the recent knowledge regarding sex-reversed females (SRFs) of salmonid fish. We discuss the methods of sex reversal as well as their effects on the morphology and histology of the reproductive tract. We focus on the characteristics of SRF semen as well as the factors determining semen quality. The lower quality of SRF sperm compared to that of normal males has resulted in the need for the artificial maturation of semen. Most importantly, methods of semen storage-both short-term and long-term (cryopreservation)-that can improve hatchery operations are presented with the special emphasis on recent progress in development of efficient cryopreservation procedures and use of cryopreserved semen in hatchery practice. Moreover, we also address the emerging knowledge concerning the proteomic investigations of salmonid sperm, focusing primarily on the proteomic comparison of normal male and SRF testicular semen and presenting changes in SRF rainbow trout sperm proteome after in vitro incubation in artificial seminal plasma.

Fish reproductive biology - Reflecting on five decades of fundamental and translational research

Driven by the broad diversity of species and physiologies and by reproduction-related bottlenecks in aquaculture, the field of fish reproductive biology has rapidly grown over the last five decades. This review provides my perspective on the field during this period, integrating fundamental and applied developments and milestones. Our basic understanding of the brain-pituitary-gonadal axis led to overcoming the failure of farmed fish to ovulate and spawn in captivity, allowing us to close the fish life cycle and establish a predictable, year-round production of eggs. Dissecting the molecular and hormonal mechanisms associated with sex determination and differentiation drove technologies for producing better performing mono-sex and reproductively-sterile fish. The growing contingent of passionate fish biologists, together with the availability of innovative platforms such as transgenesis and gene editing, as well as new models such as the zebrafish and medaka, have generated many discoveries, also leading to new insights of reproductive biology in higher vertebrates including humans. Consequently, fish have now been widely accepted as vertebrate reproductive models. Perhaps the best testament of the progress in our discipline is demonstrated at the International Symposia on Reproductive Physiology of Fish (ISRPF), at which our scientific family has convened every four years since the grandfather of the field, the late Ronald Billard, organized the inaugural 1977 meeting in Paimpont, France. As the one person who has been fortunate enough to attend all of these meetings since their inception, I have witnessed first-hand the astounding evolution of our field as we capitalized on the molecular and biotechnological revolutions in the life sciences, which enabled us to provide a higher resolution of fish reproductive and endocrine processes, answer more questions, and dive into deeper comprehension. Undoubtedly, the next (five) decades will be similarly exciting as we continue to integrate physiology with genomics, basic and translational research, and the small fish models with the aquacultured species.

17α-ethynylestradiol prevents the natural male-to-female sex change in gilthead seabream (Sparus aurata L.)

Exposure to 17α-ethynylestradiol (EE2, 5 μg/g food) impairs some reproductive events in the protandrous gilthead seabream and a short recovery period does not allow full recovery. In this study, spermiating seabream males in the second reproductive cycle (RC) were fed a diet containing 5 or 2.5 μg EE2/g food for 28 days and then a commercial diet without EE2 for the remaining RC. Individuals were sampled at the end of the EE2 treatment and then at the end of the RC and at the beginning of the third RC, 146 and 333 days after the cessation of treatment, respectively. Increased hepatic transcript levels of the gene coding for vitellogenin (vtg) and plasma levels of Vtg indicated both concentrations of EE2 caused endocrine disruption. Modifications in the histological organization of the testis, germ cell proliferation, plasma levels of the sex steroids and pituitary expression levels of the genes coding for the gonadotropin β-subunits, fshβ and lhβ were detected. The plasma levels of Vtg and most of the reproductive parameters were restored 146 days after treatments. However, although 50% of the control fish underwent sex reversal as expected at the third RC, male-to female sex change was prevented by both EE2 concentrations.

Effect of short-term intermittent exposure to waterborne estradiol on the reproductive physiology of the round goby (Neogobius melanostomus)

The objective of this study was to determine how the short-term exposure to a supraphysiological concentration of waterborne 17β-estradiol (E₂) influences on melatonin (Mel) and thyroxine (T₄) concentrations in plasma and E₂ and 11-ketotestosterone (11-KT) concentrations in plasma and gonads in both sexes of round goby (Neogobius melanostomus) during the pre-spawning, spawning, late spawning and non-spawning phases. The experimental protocol was based on short-term, repeated exposures of fish to a supraphysiological dose of waterborne E₂. Mel level was unchanged on exposure to E₂ during the investigated phases, and its role in determining a time frame for spawning in both sexes of round goby seems to be stable in those conditions. T₄ and sex steroids (E₂ and 11-KT) were sensitive to the exposure of E₂, and those changes influence gonads by accelerating oocyte development, ovulation and regression and inhibiting spermatogenesis in this species. The results demonstrate that the physiological responses of fish in all investigated phases were altered over a short window of exposure, indicating that short-term exposure to a supraphysiological dose of E₂ may impact fish in the wild. Furthermore, round goby can be recommended as a very suitable model for studying endocrine disruptors, which is sensitive to even short exposure to E₂.

Production of WW males lacking the masculine Z chromosome and mining the Macrobrachium rosenbergii genome for sex-chromosomes

The cultivation of monosex populations is common in animal husbandry. However, preselecting the desired gender remains a major biotechnological and ethical challenge. To achieve an efficient biotechnology for all-female aquaculture in the economically important prawn (Macrobrachium rosenbergii), we achieved – for the first time – WW males using androgenic gland cells transplantation which caused full sex-reversal of WW females to functional males. Crossing the WW males with WW females yielded all-female progeny lacking the Z chromosome. We now have the ability to manipulate – by non-genomic means – all possible genotype combinations (ZZ, WZ and WW) to retain either male or female phenotypes and hence to produce monosex populations of either gender. This calls for a study of the genomic basis underlying this striking sexual plasticity, questioning the content of the W and Z chromosomes. Here, we report on the sequencing of a high-quality genome exhibiting distinguishable paternal and maternal sequences. This assembly covers ~ 87.5% of the genome and yielded a remarkable N50 value of ~ 20 × 10⁶ bp. Genomic sex markers were used to initiate the identification and validation of parts of the W and Z chromosomes for the first time in arthropods.

Long-term exposure to waterborne nonylphenol alters reproductive physiological parameters in economically important marine fish

Low concentrations of nonylphenol (NP) in aquatic environment can induce drastic effects on the endocrine system in animals. In this study, we examined the modulatory effects of NP on reproductive and physiological parameters in juveniles of the red seabream and black rockfish following waterborne NP exposure (0, 1, 10, and 50 μg L^-1) for 60 days. In red seabream exposed to 50 μg L^-1 NP, plasma levels of 17β-estradiol (E2) and 11-ketotestosterone (11-KT) were significantly lower at 30 and 60 days, while E2 levels were slightly higher in 10 μg L^-1-exposed individuals at day 30. Similarly, significantly lower levels of E2 and 11-KT were observed in 10 and 50 μg L^-1-exposed black rockfish at 60 days, whereas the E2 level was higher in 1 μg L^-1-exposed individuals at day 30. After exposure to NP, plasma and mRNA levels of vitellogenin (VTG) were significantly higher in both species at 30 and 60 days, similar to the inducible effects from synthetic estrogen. Plasma cortisol levels were significantly elevated by relatively higher concentrations of NP (10 and 50 μg L^-1) at 30 and 60 days. Finally, 60 days of exposure of 50 μg L^-1 NP significantly decreased the gonadosomatic index (GSI) and increased the hepatosomatic index (HSI) in both species. The results obtained from this study provide an evidence of the endocrine disrupting potential of waterborne NP on early stages of economically important marine fish. The NP-triggered endocrine modulation can induce effects on the development of reproductive and metabolic organs in fish species.

THE OPTIMAL IMPLEMENTATION OF THETROJAN Y CHROMOSOMEERADICATION STRATEGY OF AN INVASIVE SPECIES

Invasive aquatic species continue to be a persistent problem around the world. The Trojan Y Chromosome (TYC) eradication strategy has recently been developed to help fight the problem in aquatic systems by targeting only the invasive species, sparing native marine stock. It involves rearing genetically modified samples of the invasive species and introducing them into the environment to alter the sex ratio of the invasive population. The paper is devoted to finding the optimal implementation of the TYC eradication strategy of an invasive species as well as a modified, potentially more cost-effective strategy. The modified TYC strategy (MTYC) eliminates one round of exposure to sex hormones compared to the TYC strategy. After introducing both strategies, the optimal control problems for each are formulated. The two strategies are compared through numerical simulations. Our results illustrate that the MTYC strategy, with lower implementation costs, is a better strategy option when trying to minimize the overall effective cost in most scenarios.

Gonadogenesis and annual reproductive cycles of an endangered cyprinid fish, the lake minnow Eupallasella percnurus (Pallas, 1814)

In this study, gonadogenesis, the effect of temperature (15, 20 and 25°C) on sex differentiation, and annual changes in the gonads of mature lake minnow Eupallasella percnurus (Pallas, 1814) were determined. The lake minnow was found to be a primary gonochoristic fish species, where gonads are formed directly in the ovaries or testes. The morphological differentiation of gonads was initiated 35days post hatch (DPH) when two types of gonadal anlages were visible: a pear-shaped gonad attached by a single mesentery string and a spindle-shaped gonad attached on both sides to the peritoneum. Gonadogenesis occurred faster in females than in males, with the first previtellogenic oocytes and ovarian lamellae being already observed in 45 DPH fish. In males, cytological differentiation occurred approximately 85 DPH, when the fish reached an average body weight of more than 400mg. No significant effect of rearing temperature on sex ratio in lake minnow juveniles was observed. The proportion of males and females was similar (close to 1:1) in all of the thermal-treated groups, although there were effects of temperature on the final sizes of fish. Histological examination of wild, mature lake minnow ovaries during the annual cycle (from May to February the following year) showed asynchronous oocyte maturation. The testes were characteristic of multi-batch spawning fish. Quantitative dominance of spermatids and mature spermatozoa in May was observed, while the presence of primary and secondary spermatocytes in all other periods was confirmed. These changes were also reflected in the seasonal variation in the gonado-somatic index in both sexes, with the highest mean values of 11.2% (females) and 4.0% (males) in May, which were found to be significantly different to all other periods. The data presented in this study provide an important contribution to our understanding of the biology and reproductive strategy of the endangered lake minnow.

The Reversible Sex of Gonochoristic Fish: Insights and Consequences

Fish sex reversal is a means to understand sex determination and differentiation, but it is also used to control sex in aquaculture. This review discusses sex reversal in gonochoristic fish, with the coexistence of genetic and environmental influences. The different periods of fish sensitivity to sex reversal treatments are presented with the mechanisms implicated. The old players of sex differentiation are revisited with transcriptome data and loss of function studies following hormone- or temperature-induced sex reversal. We also discuss whether cortisol is the universal mediator of sex reversal in fish due to its implication in ovarian meiosis and 11KT increase. The large plasticity in fish for sex reversal is also evident in the brain, with a reversibility existing even in adulthood. Studies on epigenetics are presented, since it links the environment, gene expression, and sex reversal, notably the association of DNA methylation in sex reversal. Manipulations with exogenous factors reverse the primary sex in many fish species under controlled conditions, but several questions arise on whether this can occur under wild conditions and what is the ecological significance. Cases of sex reversal in wild fish populations are shown and their fitness and future perspectives are discussed.

Ovarian differentiation and development in cachara Pseudoplatystoma fasciatum

One thousand five hundred cachara or tiger shovelnose catfish Pseudoplatystoma fasciatum, obtained from induced reproduction, were used to determine the onset of ovarian differentiation and development and to record the main characteristics of this process. Samples were collected from 0 to 240 days post-fertilization (dpf) and the results classified into stages I-XII. Ovarian formation was histologically detected for the first time when juveniles measured mean ± s.d. 51·5 ± 8·3 mm total length (LT ) at 39-45 dpf (stages I-V), with intense somatic cell proliferation originating in the ovarian cavity. Both LT and age of fish had a positive correlation (P < 0·001) with ovarian differentiation, but LT showed a greater correlation (r(2)  = 0·95) than age (r(2)  = 0·85), especially during the initial stages of development. From stages VI to VII, the ovarian cavity was enlarged and undifferentiated oogonia were present. At stage VIII, small projections formed in the ovarian stroma towards the ventral region of the gonad (future ovarian lamellae) and the basal membrane and differentiated oogonia nests could be seen. At stages IX and X, the germ cells entered meiosis and folliculogenesis was completed by stages XI and XII, which can be considered late in comparison to other Siluriformes. This study has demonstrated that ovarian differentiation in P. fasciatum begins with an intense proliferation of squamous epithelial cells (somatic cells) during the early stages of development and that sex inversion protocols could, thus, be applied successfully before this period. Furthermore, the results have demonstrated that both size and age can influence gonad differentiation and development in this species.

Timing reproduction in teleost fish: cues and mechanisms

Fish comprise half of extant vertebrate species and use a rich variety of reproductive strategies that have yielded insights into the basic mechanisms that evolved for sex. To maximize the chances of fertilization and survival of offspring, fish species time reproduction to occur at optimal times. For years, ethologists have performed painstaking experiments to identify sensory inputs and behavioral outputs of the brain during mating. Here we review known mechanisms that generate sexual behavior, focusing on the factors that govern the timing of these displays. The development of new technologies, including high-throughput sequencing and genome engineering, has the potential to provide novel insights into how the vertebrate brain consummates mating at the appropriate time.

Lampreys as Diverse Model Organisms in the Genomics Era

Lampreys, one of the two surviving groups of ancient vertebrates, have become important models for study in diverse fields of biology. Lampreys (of which there are approximately 40 species) are being studied, for example, (a) to control pest sea lamprey in the North American Great Lakes and to restore declining populations of native species elsewhere; (b) in biomedical research, focusing particularly on the regenerative capability of lampreys; and (c) by developmental biologists studying the evolution of key vertebrate characters. Although a lack of genetic resources has hindered research on the mechanisms regulating many aspects of lamprey life history and development, formerly intractable questions are now amenable to investigation following the recent publication of the sea lamprey genome. Here, we provide an overview of the ways in which genomic tools are currently being deployed to tackle diverse research questions and suggest several areas that may benefit from the availability of the sea lamprey genome.

Molecular cloning of Pcc-dmrt1s and their specific expression patterns in Pengze crucian carp (Carassius auratus var. Pengze) affected by 17α-methyltestosterone

Dmrt1, an important transcription factor associated with testicular differentiation, is conserved among teleost, which could also be detected in ovaries. In the present study, three isoforms of Pcc-dmrt1s (Pcc-dmrt1a, Pcc-dmrt1b and Pcc-dmrt1c) resulting from alternative splicing of the dmrt1 gene were cloned and characterized in the triploid gynogenetic fish, the Pengze crucian carp. Their mRNA expression profiling was investigated in juvenile developmental stages, tissues of the adult fish, and the juveniles under 84.2 ng/L 17α-methyltestosterone (MT) treatments. Results showed that their putative proteins shared high identities to Dmrt1 in cyprinid fish species. Gene expression profiling in the developmental stages showed that all the three target genes had a highest/lowest expression at 56/40 days post-hatching (dph), respectively. The period of 40 dph appeared to be a key time during the process of the ovary development of Pengze crucian carp. The tissue distribution results indicated that Pcc-dmrt1s were predominantly expressed in hepatopancreas, brain, spleen and ovary of the female fish. MT significantly increased the mRNA expression of Pcc-dmrt1a (all 4-week exposures) and Pcc-dmrt1b (except for week 2), while repressed Pcc-dmrt1c transcripts at all exposure period except for week 2. MT extremely significant repressed cyp19a1a transcripts for 1 week. The present study indicated that MT could influence the ovary development of Pengze crucian carp by disturbing gene expressions of Pcc-dmrt1s and cyp19a1a. Furthermore, the present study will be of great significance to broaden the understanding of masculinizing pathway during ovary development in gynogenetic teleost.

Analysis of the Trojan Y-Chromosome eradication strategy for an invasive species

The Trojan Y-Chromosome (TYC) strategy, an autocidal genetic biocontrol method, has been proposed to eliminate invasive alien species. In this work, we analyze the dynamical system model of the TYC strategy, with the aim of studying the viability of the TYC eradication and control strategy of an invasive species. In particular, because the constant introduction of sex-reversed trojan females for all time is not possible in practice, there arises the question: What happens if this injection is stopped after some time? Can the invasive species recover? To answer that question, we perform a rigorous bifurcation analysis and study the basin of attraction of the recovery state and the extinction state in both the full model and a certain reduced model. In particular, we find a theoretical condition for the eradication strategy to work. Additionally, the consideration of an Allee effect and the possibility of a Turing instability are also studied in this work. Our results show that: (1) with the inclusion of an Allee effect, the number of the invasive females is not required to be very low when the introduction of the sex-reversed trojan females is stopped, and the remaining Trojan Y-Chromosome population is sufficient to induce extinction of the invasive females; (2) incorporating diffusive spatial spread does not produce a Turing instability, which would have suggested that the TYC eradication strategy might be only partially effective, leaving a patchy distribution of the invasive species.

Molecular cloning and characterization of amh, dax1 and cyp19a1a genes and their response to 17α-methyltestosterone in Pengze crucian carp

The proteins encoded by amh, dax1 and cyp19a1a play important roles in gonad differentiation. Their functions have been far less studied in teleosts. In this study, the full-length cDNAs of amh, dax1 and cyp19a1a were cloned and characterized in a triploid gynogenic fish, the Pengze crucian carp. Their expression profilings in juvenile development, adult tissues and juveniles exposed to 100 ng/L 17α-methyltestosterone (MT) were investigated. Results showed that their putative proteins shared high identities to their counterparts in cyprinid fish species, respectively. The tissue distribution results indicated that amh and cyp19a1a were predominantly expressed in the ovary and dax1 was dominantly expressed in the liver. Gene profiling in the developmental stages showed that all the three target genes had a consistent highest expression at 48 days post hatching (dph). The period of 48 dph appeared to be a key time during the process of the gonad development of Pengze crucian carp. 100 ng/L MT significantly increased the mRNA expression of amh at 2- and 4-week exposures and enhanced dax1 and cyp19a1a at 6-week exposure. The present study indicated that MT could influence the gonad development in Pengze crucian carp by disturbing sex-differentiation associated gene expression. Furthermore, the present study will be of great significance to broaden the understanding of molecular mechanisms of the physiological processes of reproduction in fish.

The sexually dimorphic on the Y-chromosome gene (sdY) is a conserved male-specific Y-chromosome sequence in many salmonids

All salmonid species investigated to date have been characterized with a male heterogametic sex-determination system. However, as these species do not share any Y-chromosome conserved synteny, there remains a debate on whether they share a common master sex-determining gene. In this study, we investigated the extent of conservation and evolution of the rainbow trout (Oncorhynchus mykiss) master sex-determining gene, sdY (sexually dimorphic on the Y-chromosome), in 15 different species of salmonids. We found that the sdY sequence is highly conserved in all salmonids and that sdY is a male-specific Y-chromosome gene in the majority of these species. These findings demonstrate that most salmonids share a conserved sex-determining locus and also strongly suggest that sdY may be this conserved master sex-determining gene. However, in two whitefish species (subfamily Coregoninae), sdY was found both in males and females, suggesting that alternative sex-determination systems may have also evolved in this family. Based on the wide conservation of sdY as a male-specific Y-chromosome gene, efficient and easy molecular sexing techniques can now be developed that will be of great interest for studying these economically and environmentally important species.

A combined strategy involving Sanger and 454 pyrosequencing increases genomic resources to aid in the management of reproduction, disease control and genetic selection in the turbot (Scophthalmus maximus)

Background

Genomic resources for plant and animal species that are under exploitation primarily for human consumption are increasingly important, among other things, for understanding physiological processes and for establishing adequate genetic selection programs. Current available techniques for high-throughput sequencing have been implemented in a number of species, including fish, to obtain a proper description of the transcriptome. The objective of this study was to generate a comprehensive transcriptomic database in turbot, a highly priced farmed fish species in Europe, with potential expansion to other areas of the world, for which there are unsolved production bottlenecks, to understand better reproductive- and immune-related functions. This information is essential to implement marker assisted selection programs useful for the turbot industry.

Results

Expressed sequence tags were generated by Sanger sequencing of cDNA libraries from different immune-related tissues after several parasitic challenges. The resulting database (“Turbot 2 database”) was enlarged with sequences generated from a 454 sequencing run of brain-hypophysis-gonadal axis-derived RNA obtained from turbot at different development stages. The assembly of Sanger and 454 sequences generated 52,427 consensus sequences (“Turbot 3 database”), of which 23,661 were successfully annotated. A total of 1,410 sequences were confirmed to be related to reproduction and key genes involved in sex differentiation and maturation were identified for the first time in turbot (AR, AMH, SRY-related genes, CYP19A, ZPGs, STAR FSHR, etc.). Similarly, 2,241 sequences were related to the immune system and several novel key immune genes were identified (BCL, TRAF, NCK, CD28 and TOLLIP, among others). The number of genes of many relevant reproduction- and immune-related pathways present in the database was 50–90% of the total gene count of each pathway. In addition, 1,237 microsatellites and 7,362 single nucleotide polymorphisms (SNPs) were also compiled. Further, 2,976 putative natural antisense transcripts (NATs) including microRNAs were also identified.

Conclusions

The combined sequencing strategies employed here significantly increased the turbot genomic resources available, including 34,400 novel sequences. The generated database contains a larger number of genes relevant for reproduction- and immune-associated studies, with an excellent coverage of most genes present in many relevant physiological pathways. This database also allowed the identification of many microsatellites and SNP markers that will be very useful for population and genome screening and a valuable aid in marker assisted selection programs.

Tissue bioaccumulation patterns, xenobiotic biotransformation and steroid hormone levels in Atlantic salmon (Salmo salar) fed a diet containing perfluoroactane sulfonic or perfluorooctane carboxylic acids

In the present study, groups of juvenile Atlantic salmon (Salmo salar) were fed gelatine capsules containing fish-food spiked with PFOA or PFOS (0.2 mg kg(-1) fish) and solvent (methanol). The capsules were given at days 0, 3 and 6. Blood, liver and whole kidney samples were collected prior to exposure (no solvent control), and at days 2, 5, 8 and 14 after exposure (Note: that day 14 after exposure is equal to 7d recovery period). We report on the differences in the tissue bioaccumulation patterns of PFOS and PFOA, in addition to tissue and compound differences in modulation pattern of biotransformation enzyme genes. We observed that the level of PFOS and PFOA increased in the blood, liver and kidney during the exposure period. Different PFOS and PFOA bioaccumulation patterns were observed in the kidney and liver during exposure- and after the recovery periods. Particularly, after the recovery period, PFOA levels in the kidney and liver tissues were almost at the control level. On the contrary, PFOS maintained an increase with tissue-specific differences, showing a higher bioaccumulation potential (also in the blood), compared with PFOA. While PFOS and PFOA produced an apparent time-dependent increase in kidney CYP3A, CYP1A1 and GST expression, similar effects were only temporary in the liver, significantly increasing at sampling day 2. PFOA and PFOS exposure resulted in significant decreases in plasma estrone, testosterone and cortisol levels at sampling day 2, and their effects differed with 17α-methyltestostrerone showing significant decrease by PFOA (also for cholesterol) and increase by PFOS. PFOA significantly increased estrone and testosterone, and no effects were observed for cortisol, 17α-methyltestosterone and cholesterol at sampling day 5. Overall, the changes in plasma steroid hormone levels parallel changes in CYP3A mRNA levels. Given that there are no known studies that have demonstrated such tissue differences in bioaccumulation patterns with associated differences in toxicological responses in any fish species or lower vertebrate, the present findings provide some potential insights and basis for a better understanding of the possible mechanisms of PFCs toxicity that need to be studied in more detail.

Sex differentiation and sex change in the protandrous black porgy, Acanthopagrus schlegeli

Protandrous black porgy fish, Acanthopagrus schlegeli, have a striking life cycle with a male sex differentiation at the juvenile stage and male-to-female sex change at 3 years of age. We had characterized the sex differentiation and sex change in this species by the integrative approaches of histology, endocrine and molecular genetics. The fish differentiated in gonad at the age around 4-months and the gonad further developed with a bisexual gonad for almost for 3 years and sex change at 3 year of age. An antagonistic relationship in the testicular and ovarian tissues was found during the development of the gonadal tissue. Male- (such as sf-1, dmrt1, dax-1 and amh) and female- (such as wnt4, foxl2 and cyp19a1a) promoting genes were associated with testicular and ovarian development, respectively. During gonadal sex differentiation, steroidogenic pathway and estrogen signaling were also highly expressed in the brain. The increased expression of sf-1 and wnt4, cyp19a1a in ovarian tissue and decreased expression of dax-1 in the ovarian tissue may play important roles in sex change from a male-to-female. Endocrine factors such as estradiol and luteinizing hormone may also involve in the natural sex change. Estradiol induced the expression of female-promoting genes and resulted in the precocious sex change in black porgy. Our series of studies shed light on the sex differentiation and sex change in protandrous black porgy and other animals.

Molecular characterization of cyp11a1 and cyp11b1 and their gene expression profile in pejerrey (Odontesthes bonariensis) during early gonadal development

Sex steroids are known to be involved in gonadal differentiation in fish, but whether androgens are early mediators of testis differentiation remains unclear. We studied the sex-related developmental variations in the gene expression of two key enzymes involved in steroids and androgen synthesis (cyp11a1 and cyp11b1) in trunks and isolated gonads of pejerrey (Odontesthes bonariensis) larvae during and after the sex determination period. Also, and in order to have a better characterization of this process we studied the expression of Sertoli (dmrt1, amh, sox9) and Leydig (nr5a1 or sf-1) cell markers as well as a gene with higher expression in females (cyp19a1a). No clear differences were observed in the expression of cyp11a1 and cyp11b1 during the temperature-sensitive window in the trunk of pejerrey larvae. Nevertheless, a clear increase of cyp11b1 was observed in isolated gonads taken from fish reared at the male producing temperature. In these gonads we also confirmed the trends of genes with higher expression in males (dmrt1, amh) and females (cyp19a1a) as previously described in larval trunks of pejerrey. Our results showed that the expression of cyp11b1 was positively associated with the morphological differentiation of the testis. Nevertheless the involvement of 11-oxygenated androgens during the temperature-sensitive window could not be clearly established.

Female-specific increase in primordial germ cells marks sex differentiation in threespine stickleback (Gasterosteus aculeatus)

Gonadal sex differentiation is increasingly recognized as a remarkably plastic process driven by species-specific genetic or environmental determinants. Among aquatic vertebrates, gonadal sex differentiation is a frequent endpoint in studies of endocrine disruption with little appreciation of underlying developmental mechanisms. Work in model organisms has highlighted the diversity of master sex-determining genes rather than uncovering any broad similarities prompting the highly conserved developmental decision of testes versus ovaries. Here we use molecular genetic markers of chromosomal sex combined with traditional histology to examine the transition of the bipotential gonads to ovaries or testes in threespine stickleback (Gasterosteus aculeatus). Serially-sectioned threespine stickleback fry were analyzed for qualitative and quantitative indications of sexual differentiation, including changes in gonadal morphology, number of germ cells and the incidence of gonadal apoptosis. We show that threespine stickleback sampled from anadromous and lacustrine populations are differentiated gonochorists. The earliest sex-specific event is a premeiotic increase in primordial germ cell number followed by a female-specific spike in apoptosis in the undifferentiated gonad of genetic females. The data suggest that an increase in PGC number may direct the undifferentiated gonad toward ovarian differentiation.

Sex hormone concentrations and gonad histology in brown trout (Salmo trutta) exposed to 17beta-estradiol and bisphenol A

The impact of 17beta-estradiol (E2) and bisphenol A (BPA) on steroid hormone levels and gonad development in brown trout (Salmo trutta) was determined. Exposure took place from 0 to 63 days post-fertilisation (dpf) and gonad development was followed till 400 dpf. The onset of xenoestrogen metabolism was examined by measurements of whole body concentrations of bisphenol A (BPA) and its conjugation product bisphenol A glucuronic acid (BPAGA). Exposure to 500 ng E2/l led to an increase in E2 levels in the embryos and fry while 10 ng E2/l did not. Metabolic conversion of BPA to BPAGA began during the first weeks of embryonic development. Few consistent effects were found on the sex differentiation of the brown trout. Only one intersex fish (4.5%) was found among male fish at 400 dpf exposed to 500 ng E2/l. Females with male germ cells among the normally developing oocytes were observed in all groups (in up to 50% of the female fish, independently of exposure regime). The fact that exposure to 500 ng E2/l only caused subtle effects in a small number of individuals indicates that exposure during early life stages results in little to no induction of endocrine disruption in brown trout.

Early aspects of gonadal sex differentiation in Xenopus tropicalis with reference to an antero-posterior gradient

In an effort to contribute to the development of Xenopus tropicalis as an amphibian model system, we carried out a detailed histological analysis of the process of gonadal sex differentiation and were able to find evidence that gonadal differentiation in X. tropicalis follows an antero-posterior gradient. Although the main reason for the presence of a gradient of sex differentiation is still unknown, this gradient enabled us to define the early events that signal ovarian and testicular differentiation and to identify the undifferentiated gonad structure. Given the various advantages of this emerging model, our work paves the way for experiments that should contribute to our understanding of the dynamics and mechanisms of gonadal sex differentiation in amphibians.

Effects of 17alpha-methyltestosterone exposure on steroidogenesis and cyclin-B mRNA expression in previtellogenic oocytes of Atlantic cod (Gadus morhua)

Steroid hormone (estrogens and androgens) synthesis and regulation involve a large number of enzymes and potential biochemical pathways. In the context of these biochemical pathways, it is believed that the true rate-limiting step in acute steroid production is the movement of cholesterol across the mitochondrial membrane by the steroidogenic acute regulatory (StAR) protein and the subsequent conversion to pregnenolone by cytochrome P450-mediated side-chain cleavage (P450scc) enzyme. Oocyte development is a complex process that is triggered by the maturation-promoting factor (MPF) involving cyclin-B as a regulatory factor. In the present study, we evaluated the endocrine effects of 17alpha-methyltestosterone (MT) on steroidogenic pathways of Atlantic cod (Gadus morhua), using an in vitro previtellogenic oocyte culture technique that is based on an agarose floating method. Tissue was cultured in a humidified incubator at 10 degrees C for 1, 5, 10 and 20 days with different concentrations of the synthetic androgen MT (0 (control), 1, 10, 100 and 1000 microM) dissolved in ethanol (0.3%). Gene expressions for StAR, P450scc, aromatase-alpha (P450aromA) and cyclin-B were detected using validated real-time PCR with specific primer pairs. Cellular localization of the StAR protein and P450scc were performed using the immunohistochemical technique with antisera prepared against synthetic peptide for both proteins. Steroid hormones (estradiol-17beta: E2 and testosterone: T) levels were estimated using enzyme immunoassay. Our data showed significant concentration-specific increase (at day 1 and 5) and decrease (at day 10 and 20) of the StAR mRNA expression after exposure to MT. P450scc expression showed a MT concentration-specific decrease during the exposure periods and cyclin-B mRNA expression was decreased in MT concentration-dependent manner at days 10 and 20 (reaching almost total inhibition after exposure to 1000 microM MT). MT exposure produced variable effects on the P450aromA mRNA expression that can be described as concentration-specific increase (day 1) and decrease (days 5 and 10). Cellular localization of the StAR protein and P450scc demonstrated their expression mainly in ovarian follicular cells. MT produced an apparent concentration-and time-dependent increase of E2 and T levels. Thus, the present study reveals some novel effects of pharmaceutical endocrine disruptor on the development of previtellogenic oocytes in cod. The impaired steroidogenesis and hormonal imbalance reported in the present study may have potential consequences for the vitellogenic process and overt fecundity in teleosts.

Androgen receptors in a cichlid fish, Astatotilapia burtoni: structure, localization, and expression levels

Androgens are an important output of the hypothalamic-pituitary-gonadal (HPG) axis that controls reproduction in all vertebrates. In male teleosts two androgens, testosterone and 11-ketotestosterone, control sexual differentiation and development in juveniles and reproductive behavior in adults. Androgenic signals provide feedback at many levels of the HPG axis, including the hypothalamic neurons that synthesize and release gonadotropin-releasing hormone 1 (GnRH1), but the precise cellular site of androgen action in the brain is not known. Here we describe two androgen receptor subtypes, ARalpha and ARbeta, in the cichlid Astatotilapia burtoni and show that these subtypes are differentially located throughout the adult brain in nuclei known to function in the control of reproduction. ARalpha was expressed in the ventral part of the ventral telencephalon, the preoptic area (POA) of the hypothalamus and the ventral hypothalamus, whereas ARbeta was more widely expressed in the dorsal and ventral telencephalon, the POA, and the ventral and dorsal hypothalamus. We provide the first evidence in any vertebrate that the GnRH1-releasing neurons, which serve as the central control point of the HPG axis, express both subtypes of AR. Using quantitative real-time PCR, we show that A. burtoni AR subtypes have different expression levels in adult tissue, with ARalpha showing significantly higher expression than ARbeta in the pituitary, and ARbeta expressed at a higher level than ARalpha in the anterior and middle brain. These data provide important insight into the role of androgens in regulating the vertebrate reproductive axis.

Molecular cloning, expression and phylogenetic analyses of parvalbumin in tilapia, Oreochromis mossambicus

The gene expression of parvalbumin (Pvalb), a high-affinity calcium-binding protein and the major fish allergen, was significantly increased in the tilapia fry treated with methyltestosterone (MT) as examined using a subtractive hybridization assay. Using the real-time quantitative PCR, we further confirmed the increased Pvalb expression in the MT-treated tilapia fry. The 568 base pairs (bp) tilapia Pvalb (tPvalb) cDNA clone was fully sequenced and found to contain a coding region of 330 bp, which encodes a 108 amino acids protein with a molecular weight of 11,370.5 and an calculated isoelectric point of 4.56. The predicted secondary structure of tPvalb is comprised of seven alpha helices. It contains two characteristic EF-hand calcium-binding motifs, one PKC and five casein kinase II consensus phosphorylation sites. The tPvalb is highly homologous to the selected fish Pvalbs at a similarity ranging from 53% to 80%. The phylogenetic tree analysis showed that the tPvalb is closest to the Scomber japonicus Pvalb. The tPvalb was found to express in the heart, muscle, gill, kidney, brain and ovary of adult fish by RT-PCR analysis. In situ hybridization also revealed that the tPvalb was highly expressed in the hypothalamus and sarcoplasmic reticulum. A tPvalb glutathione S-transferase (GST) fusion protein was generated and digested by thrombin to remove the GST moiety. Further Western analysis showed that the tPvalb protein was cross-reacted to an anti-rat Pvalb antibody. Those results suggest that Pvalb is evolutionally conserved in tilapia.

In vivo exposure of carp to graded concentrations of bisphenol A

A wide range of environmental pollutants commonly termed endocrine disrupters (ED) can mimic steroid hormones causing adverse health effects. Recent studies showed that bisphenol A (BPA) together with other estrogenic chemicals, may be responsible for the disrupting effects observed in fish of the middle River Po. The present study has investigated the potential role of BPA in inducing this kind of effects and to this aim, 1-year-old (k1) common carp were exposed for 14 days to graded concentrations of BPA (1, 10, 100, and 1000microg/L). Histological alterations of gonads were described and compared to vitellogenin (VTG) and sex steroid levels. In carp males, BPA caused severe alterations of testis structure starting from 1microg/L. Several specimens lost the typical lobular structure showing spermatogenic cysts intermingled with free spermatozoa often degenerating into the lumen. Oocyte atresia was observed starting from 1micro BPA/L, concerning 57.1% of females at the highest concentration (1000microg BPA/L). A few carp from the same highest treatment also showed intersexuality (27%). Oestradiol-17beta (E2) significantly decreased both in 1 and 10microg BPA/L exposed carp reaching again values not significantly different from control in 1000microg BPA/L. At this concentration, a significant reduction of testosterone (T) was observed in both males and females. The mean plasma concentration of VTG significantly increased in 1000microg BPA/L exposed carp, even if a growing number of responsive carp was observed starting from the environmental concentrations (1 and 10microg BPA/L).

Antiestrogens inhibit xenoestrogen-induced brain aromatase activity but do not prevent xenoestrogen-induced feminization in Japanese medaka (Oryzias latipes)

In fish, exposure to estrogen or estrogen-mimicking chemicals (xenoestrogens) during a critical period of development can irreversibly invert sex differentiation. In medaka, a male-to-female reversal upon exposure to a xenoestrogen is accompanied by an increase in brain aromatase expression and activity. However, whether this increase is the direct cause of sex reversal is unknown. In this study we further examined the role brain aromatase plays in genesis of developmental abnormalities in response to endocrine-disrupting chemicals (EDCs). Further, the effects of a mixture of apparent antagonistic environmentally relevant EDCs on development were examined to determine if their combined actions could lessen each other's impacts. To this end, hatchling medaka were subjected in a 2-week flow-through immersion exposure to an estrogen mimic [dichlorodiphenyltrichloroethane (o,p -DDT)] and to pharmaceutical [fadrozole (FAD)] and environmental aromatase inhibitors [tributyltin (TBT)] alone and in combination. Brain aromatase expression and enzyme activity were measured on exposure days 5, 9, and 14 by real-time reverse-transcriptase polymerase chain reaction and tritiated water release assay, respectively. We recorded sex reversals at sexual maturity by examining the phenotypic and genotypic sex of d-rR-strain medaka. Results indicate that FAD and TBT inhibit aromatase activity in o,p -DDT-treated fish but do not prevent feminization, indicating that increased brain aromatase activity is not critical to EDC-induced male-to-female sex inversion. The observation that estradiol biosynthesis inhibitors do not block the effect of the xenoestrogen suggests that in the environment, exposure to seemingly antagonistic EDCs does not necessarily lessen the harmful impacts of these compounds.

Gonadal morphogenesis and sex differentiation in intraovarian embryos of the viviparous fishZoarces viviparus (Teleostei, Perciformes, Zoarcidae): A histological and ultrastructural study

It is essential to know the timing and process of normal gonadal differentiation and development in the specific species being investigated in order to evaluate the effect of exposure to endocrine-disrupting chemicals on these processes. In the present study gonadal sex differentiation and development were investigated in embryos of a viviparous species of marine fish, the eelpout, Zoarces viviparus, during their intraovarian development (early September to January) using light and electron microscopy. In both sexes of the embryos at the time of hatching (September 20) the initially undifferentiated paired bilobed gonad contains primordial germ cells. In the female embryos, ovarian differentiation, initiated 14 days posthatch (dph), is characterized by the initial formation of the endoovarian cavity of the single ovary as well as by the presence of some early meiotic oocytes in a chromatin-nucleolus stage. By 30 dph, the endoovarian cavity has formed. By 44 dph and onward, the ovary and the oocytes grow in size and at 134 dph, just prior to birth, the majority of the oocytes are at the perinucleolar stage of primary growth and definitive follicles have formed. In the presumptive bilobed testis of the male embryos, the germ cells (spermatogonia), in contrast to the germ cells of the ovary, remain quiescent and do not enter meiosis during intraovarian development. However, other structural (somatic) changes, such as the initial formation of the sperm duct (30 dph), the presence of blood vessels in the stromal areas of the testis (30 dph), and the appearance of developing testicular lobules (102 dph), indicate testicular differentiation. Ultrastructually, the features of the primordial germ cells, oogonia, and spermatogonia are similar, including nuage, mitochondria, endoplasmic reticulum, and Golgi complexes.

Sex Reversal in the Medaka Oryzias latipes by Brief Exposure of Early Embryos to Estradiol-17β

To induce sex reversal of male to female, freshly-fertilized eggs of the S-rR strain medaka (Oryzias latipes) were immersed in saline containing estradiol-17beta (E2) in different concentrations for various durations until hatching. Results of the present experiment showed that the immersion duration in 1 microg/ml E2 to induce 100% reversal of sex differentiation in the genotypic males was enough only for one day (24 hr) post-fertilization (dpf) and that treatment with E2 for 1 dpf resulted in a dose-dependent manner with the maximum sex reversal of 100% at 1 microg/ml. To ascertain early developmental periods efficacious for inducing sex reversal, additional brief immersion treatments of eggs with E2 were further performed individually for four different early developmental periods (Stages 4-9, 10-12, 13-15 and 16-18) within 1 dpf. As a result, induction of sex reversal was observed in all these short immersion periods without any restricted efficacy. Between both experimental and control groups treated with or without E2 for 1 dpf, differences in the number of germ cells in a gonad were compared in newly-hatched fry. It was found that gonads of the genotypic males (XY) treated with E2 revealed the female type which contained many germ cells with much dividing activity. These data suggest that a possible switch mechanism that exogenous E2 could trigger to change the genetic cascades involved in sex determination upon fertilization exists in early developmental stages.

Where does gonadal sex differentiation begin? Gradient of histological sex differentiation in the gonads of pejerrey, Odontesthes bonariensis (Pisces, Atherinidae)

This study investigated the possibility that the histological process of gonadal sex differentiation in pejerrey (Odontesthes bonariensis), a fish with marked temperature-dependent sex determination (TSD), occurs through a predictable gradient of differentiation as opposed to simultaneous or random differentiation throughout the gonad. For this purpose, fish reared at 17 degrees, 24 degrees, and 29 degrees C from hatching were sampled weekly for 11 weeks, fixed, and prepared for histological observation of serial cross-sections of the gonads. The thermal manipulation and sampling procedure ensured the availability of males and females at various degrees of gonadal sex differentiation. The location of the differentiated area(s) was estimated in the right and left gonads of 17 females and 14 males selected among the available specimens so as to represent increasing degrees of differentiation. The analysis revealed that sex differentiation followed a gradient from the anterior to posterior areas of the gonads regardless of sex. Furthermore, plotting of the degree of sex differentiation in the right gonad as a function of the degree of differentiation of the left gonad clearly showed that sex differentiation only begins in the right gonad when 10-30% of the length of the left gonad has already differentiated. The mean rostral edge of the differentiated areas in females was 9% and 10.8% for the left and right gonads, respectively, while for males these values were 7.3% and 7.0%, respectively. Thus, it was established that ovarian and testicular differentiation in pejerrey follow both a cephalocaudal and a left-to-right gradient. Possible explanations for this gradient and its relevance for TSD in pejerrey, that is, as a mechanism to prevent discrepant differentiation of male and female features within the same gonad, are discussed.

Brain aromatase in Japanese medaka (Oryzias latipes): Molecular characterization and role in xenoestrogen-induced sex reversal

In female fish estrogen is required for the development of primary and secondary sex characteristics and is derived from the aromatization of androgens by aromatase. There are two isoforms of aromatase in several teleost species, brain and ovarian. The objective of this study was two-fold: clone and sequence the coding and promoter region of brain aromatase in medaka, and determine the effects of exposure to an environmental estrogen (o,p-DDT) on sex determination and brain aromatase transcription and activity. The brain aromatase coding sequence was obtained by reverse transcription polymerase chain reaction (RT-PCR) and PCR-based genomic DNA walking was used to clone the promoter of the brain aromatase gene. The promoter sequence revealed potential binding sites for the estrogen receptor and for transcription factors involved in primary neurogenesis and sex determination. Medaka fry were exposed to increasing o,p-DDT concentrations (0-5.5 microg/L) from days 1 to 15 after hatch and brain aromatase expression and activity were measured on days 5, 9, and 14. A complete male-to-female sex reversal occurred at 5.5 microg/L o,p-DDT and aromatase activity and expression data showed a significant five-fold increase at this concentration at day 14. This information suggests that brain aromatase is involved in the abnormal sexual differentiation of fish treated with xenoestrogens.

Gonadal development and sex differentiation in the cichlid fishCichlasoma dimerus (Teleostei, perciformes): A light- and electron-microscopic study

Although the overall pattern and timing of gonadal sex differentiation have been established in a considerable number of teleosts, the ultrastructure of early stages of gonadal development is not well documented. In this study, gonads from larval and juvenile stages of laboratory-reared Cichlasoma dimerus were examined at the light-microscopic and ultrastructural levels. This freshwater species adapts easily to captivity and spawns with high frequency during 8 months of the year, providing an appropriate model for developmental studies. Larvae and juveniles were kept at a water temperature of 26.5 +/- 1 degrees C and a 12:12 hour photoperiod. Gonadal development was documented from 14-100 days postfertilization, covering the period of histologically discernible sex differentiation. Gonadal tissue was processed according to standard techniques for light and electron microscopy. C. dimerus, a perciform teleost, is classified as a differentiated gonochorist, in which an indifferent gonad develops directly into a testis or ovary. On day 14, the gonadal primordium consists of a few germ cells surrounded by enveloping somatic cells. Ovarian differentiation precedes testicular differentiation, as usual in teleost fishes. The earliest signs of differentiation, detected from day 42 onward, include the onset of meiotic activity in newly formed oocytes, which is soon accompanied by increased oogonial mitotic proliferation and the somatic reorganization of the presumptive ovary. The ovarian cavity is completely formed by day 65. Numerous follicles containing perinucleolar oocytes are observed by day 100. In contrast, signs of morphological differentiation in the presumptive testis are not observed until day 72. By day 100, the unrestricted lobular organization of the testis is evident. The latest stage of spermatogenesis observed by this time of testicular development is spermatocyte II.

The juvenile three-spined stickleback (Gasterosteus aculeatus L.) as a model organism for endocrine disruption. I. Sexual differentiation

Juvenile three-spined stickleback (Gasterosteus aculeatus L.) is introduced as a unique model organism for both androgenic and oestrogenic endocrine action. Intersex is often used as an indicator for disruption of sexual differentiation in fish exposed to different kinds of effluents from human activities. In wild fish it has exclusively been reported in terms of feminisation due to xenoestrogens in the environment. The assumption that the intersex individuals are feminised genetic males can only be proven by genetic sex identification of the intersexual individuals. Intersex and gonadal sex reversal were induced in three-spined sticklebacks by treatment with natural and synthetic steroid hormones. Juvenile sticklebacks were exposed to three nominal concentrations of 17 beta-oestradiol (E2); i.e. 0.01, 1.0 and 10.0 microg/L; which were administered to the water either continuously from hatching to the end of the experiment (39-58 days post hatch), during the first 2 weeks after hatching only, from 14 days after hatching onwards, or during the chorionated embryo stage until hatching. Other groups were exposed to 17 alpha-ethinylestradiol (EE2) at 0.05 microg/L and 17 alpha-methyltestosterone (MT) at 1.0 microg/L (nominal concentrations). MT was applied continuously, during the first 2 weeks post hatch only, or from 14 days after hatching onwards. Gonad histology was examined and the genetic sex was identified with male sex-linked PCR markers. Treatment with oestrogens caused feminisation at the two highest E2 concentrations and with EE2. Exposure to E2 before hatching had no effect. Intersexual individuals from oestrogen treatments were genetic males. The genetic sex marker identified apparent total reversal of the gonad type of genetic males. Treatment with MT did not reveal a clear picture, since intersex was observed in both genetic females and males. MT also caused severe testis abnormalities, mainly the development of large branched cavities with unidentified origin. The process of sex differentiation is most sensitive to the influence of external steroids during the first 2 weeks after hatching. A lower incidence of intersex could also be induced in sticklebacks exposed from 14 days after hatching by E2 treatment, but not with MT. The combination of gonad histopathology with genetic sex identification in juvenile sticklebacks is suggested as a tool for detecting endocrine disruption in laboratory studies, and might become very useful in field surveys.

Aromatase modulation alters gonadal differentiation in developing zebrafish (Danio rerio)

This study investigated whether gonadal sex differentiation of zebrafish (Danio rerio) is susceptible to compounds that interfere with cytochrome P450 aromatase (P450arom). Treatment of zebrafish during the period of gonadal differentiation with either the non-steroidal aromatase inhibitor fadrozole or 17alpha-methyltestosterone (MT) changed gonad morphological differentiation and altered the pattern of P450arom gene (CYP19) expression. Application of fadrozole (500 microg/g of food) between days 35 and 71 post-fertilisation (pf) resulted in 100% masculinisation, i.e. the gonads of all individuals examined ( n = 40) showed testicular morphology. At the same time, fadrozole treatment suppressed gonadal CYP19A (gonad-derived CYP19 gene) mRNA expression, as assessed by means of semi-quantitative RT-PCR. After termination of fadrozole treatment at day 71 pf and subsequent rearing of zebrafish under control conditions until 161 days pf, the gonads of all individuals still displayed testicular morphology. Gonadal CYP19A expression, however, showed a dimorphic pattern, with 14 out of 22 individuals having low CYP19A mRNA levels similar to those found in testes of control fish, while eight fish showed high, ovary-like levels of gonadal CYP19A mRNA. MT treatment (10 microg/l) during the period of gonadal differentiation (days 35 to 71 pf) resulted in phenological feminisation, i.e., all fish examined (n = 28) showed an ovarian gonadal morphology. While gonadal CYP19A expression was suppressed compared to control fish, cerebral CYP19B (brain-derived CYP19 gene) mRNA expression was increased in 71-day-old MT-treated fish. The results from this study provide evidence that exogenous compounds interfering with the P450arom system are able to disrupt, even persistently, gonadal sex differentiation of the protogynic zebrafish.

Identification and characterization of 17 beta-hydroxysteroid dehydrogenases in the zebrafish, Danio rerio

The 17 beta-hydroxysteroid dehydrogenases (17 beta-HSDs) are key enzymes in the final steps of steroid hormone synthesis. 17beta-HSD type 1 (HSD17B1) catalyzes the reduction of estrone to estradiol, while type 3 (HSD17B3) performs the conversion of androstenedione to testosterone. Here we present a functional genomics study of putative candidates of these enzymes in the zebrafish. By an in silico screen of zebrafish EST databases we identified three candidate homologs for both HSD17B1 and HSD17B3. Phylogenetic analysis, unique expression patterns (RT-PCR) during embryogenesis and adulthood, as well as activity measurements revealed that one of the HSD17B1 candidates is the zebrafish homolog, while the other two are paralogous photoreceptor-associated retinol dehydrogenases. All three HSD17B3 candidate genes showed nearly identical, ubiquitous expressions in embryogenesis and adult tissues and were identified to be paralogs of HSD17B12 and a yet uncharacterized putative steroid dehydrogenase. Phylogenetic analysis shows that HSD17B3 and HSD17B12 are descendants from a common ancestor.

Gonad development and vitellogenin production in zebrafish (Danio rerio) exposed to ethinylestradiol and methyltestosterone

In a partial life-cycle test, the impact of 17alpha-ethinylestradiol (EE2) and 17alpha-methyltestosterone (MT) on juvenile zebrafish was evaluated by use of vitellogenin measurements and gonadal development. Exposure to EE2 (1-25 ng/l) resulted in a dose-dependent increase in vitellogenin production starting at 2 ng/l. Significant changes in sex ratios in female direction were detected at 1 ng/l, with complete sex reversal taking place after exposure to 2 ng/l. No intersex fish were observed after exposure to EE2. Exposure to MT resulted in decreased vitellogenin concentrations. Complete sex reversal was detected in all MT concentrations used (26-1000 ng/l). A large proportion of intersex fish was observed after exposure to 1000 ng MT/l. The period of gonadal sex reversal in non-exposed zebrafish was also studied. The main morphological features of the transformation of ovaries into testis were observed 4-5 weeks after hatching.

Developmental genetics and physiology of sex differentiation in vertabrates

The involvement of the Y chromosome in sex determination was determined by the development and the application of techniques for karyotyping the mammalian chromosome in 1960s. There were many reports on the particular region of the Y chromosome, such as histocompatibility (H-Y) antigen, bandit krait minor satellite (Bkm) the zinc finger Y gene (ZFY) and the sex-determining region of the Y chromosome (SRY) which were believed to be the testis determining factors. However, converging experimental evidence have indicated that the sex determining region of the Y chromosome (sry) is the testis determining factor (TDF) in mammalian species since sex is determined genetically at the time of fertilization in these species. In non-mammalian vertebrates especially in fishes, amphibians and reptiles, genotypic sex can be overridden by the external application of steroid hormones and temperature. In this review paper, after reviewing the complex literature on the molecular and biochemical mechanisms of sex determination and differentiation in all vertebrates, the potential danger of environmentally induced sex determination will be focused on.

Effects of waterborne exposure of octylphenol and oestrogen on pregnant viviparous eelpout (Zoarces viviparus) and her embryos in ovario

Exposure to oestrogenic chemicals (xeno-oestrogens) may have severe effects on embryonic development. The present study investigates whether the oestrogenic endocrine disruptor 4-tert-octylphenol (4-tOP) or 17β-oestradiol (E2) is accumulated in the viviparous fish the eelpout (Zoarces viviparus) and transferred to the embryos in ovario and subsequently affects embryonic development, including gonadal differentiation. Pregnant eelpouts were exposed to nominal concentrations of 25 μgl-1 or 100 μgl-1 4-tOP (OP25 or OP100,respectively) or 0.5 μgl-1 E2 in water. During 4-tOP exposure, the compound accumulated in both plasma and ovarian fluid in a concentration-dependent manner. In the mother fish, the oestrogenic biomarkers, vitellogenin (Vtg) in plasma, Vtg mRNA in liver and oestrogen-binding activity in liver, were all induced by 4-tOP (and by E2) at an actual concentration of 14 μgl-1. E2 and 4-tOP were examined for their potency to disturb the maternal—foetal trophic relationship by disturbing the physiology of the ovary and by changing the distribution of essential nutrients normally transported to embryos during pregnancy. After exposure to E2 or 4-tOP, calcium was depleted from the ovarian fluid and the level of free amino acids available in maternal plasma was decreased. A marked overall effect on ovarian components, including the ovarian sac, ovarian fluid and embryonic mass, was evident. Embryonic growth was significantly decreased, which might in part be attributed to disturbances of the maternal—foetal trophic relationship. Marked inductions of Vtg mRNA and Vtg protein, determined by RT-PCR and immunohistochemistry, respectively, were found in embryos from the OP100 group — the only group to show considerable accumulation of an oestrogenic compound in the ovarian fluid. A different pattern of gonadal development was found in embryos from the OP100 group compared with embryos from the control, OP25 or E2 groups, in which approximately 50% had normal ovaries and 50% had normal presmptive male gonads. In the OP100 group,46% had normal ovaries but, in contrast to controls, only 22% had normal presumptive male gonads, whereas the remaining 32% had abnormal male gonads with structures resembling the endo-ovarian cavity of a female gonad. As oestrogen receptor (ER) expression was detected by in situhybridisation in early differentiating gonads, these effects could be mediated by direct interaction of the xeno-oestrogens with gonadal ER. In conclusion,this study indicates that the xeno-oestrogen 4-tOP can be transferred from the water via the mother fish to the ovarian fluid and can subsequently disturb the maternal—foetal trophic relationship and cause severe effects on embryonic development, including gonadal differentiation in ovario.

Early estrogen exposure induces abnormal development of Fundulus heteroclitus

Many chemicals released into the environment exhibit estrogenic activity, having the potential to disrupt development and the functioning of the endocrine system. In order to establish a model system to study the effects of such environmental chemicals on aquatic animals, we examined the effects of a natural estrogen, 17 beta-estradiol (E(2)), on early development of Fundulus heteroclitus. Embryos of F. heteroclitus were reared in seawater containing 10(-10), 10(-8), and 10(-6) M E(2) throughout the experiment. Hatching and survival rates decreased in a dose-dependent manner, and fry treated with 10(-6) M E(2) and 10(-8) M E(2) were dead by two weeks and 12 weeks after hatching, respectively. More than 85% of fry treated with 10(-8) M E(2) showed malformations: i.e., eye extrusion, crooked vertebral column, faded lateral-stripe pattern eight weeks after hatching. Body weight and head and body lengths were significantly reduced in E(2)-treated fry when compared to controls. Ossification was not completed in vertebrae, cranial bones, and other bones in fry treated with 10(-8) M E(2) even 12 weeks after hatching. Sex ratio of control fry was 57% male and 43% female, whereas fry treated with 10(-8) M E(2) were 100% female eight weeks after hatching. The present results demonstrate that exogenous estrogen induced death of embryos and fry, malformations, sex reversal, and incomplete ossification of vertebrae and cranial bones, which would result in shorter body and head lengths and in malformed vertebrae leading to a hunchback condition.

Effects of long-term nonylphenol exposure on gonadal development and biomarkers of estrogenicity in juvenile rainbow trout Oncorhynchus mykiss

Environmental pollutants with estrogenic activity including nonylphenol (NP) have the potential to alter gonadal development and reproduction of wild fish. To investigate the estrogenic action of environmentally relevant concentrations of NP, rainbow trout (Oncorhynchus mykiss) were continuously exposed during the embryonic, larval and juvenile life stage to 1.05 and 10.17 microg/l NP for 1 year, and sexual differentiation, vitellogenin (VG), VG mRNA, and zona radiata protein (ZRP) expression were examined after that period. The applied NP concentrations did not affect mortality and hatching rates, and did not have an influence on the body weight of 1-year-old fish. No occurrence of testis-ova was observed and sex-ratios of NP exposed groups of fish were unchanged when compared with control groups. The induction of VG and ZRP expression was a more sensitive reaction to the presence of NP than the formation of testis-ova and the reversal of sex. Increased VG expression in trout liver occurred already at 1.05 microg/l NP, whereas VG mRNA levels, quantified by competitive RT-PCR, were not significantly elevated in NP exposed fish. ZRP contents were significantly higher at 10.17 microg/l NP. Since induction of VG did not occur in all fish exposed to 1 or 10 microg/l NP and ZRP induction did not occur in all fish exposed to 10 microg/l, some individuals may be more affected by exposure to NP than others. This study demonstrates that NP concentrations typically found in sewage treatment effluents and some rivers do not affect sexual differentiation in rainbow trout, but induce VG and ZRP expression in the liver of exposed fish.

Effects of waterborne exposure to 4-nonylphenol on plasma sex steroid and vitellogenin concentrations in sexually mature male carp (Cyprinus carpio)

4-Nonylphenol (NP) has been shown to elicit estrogenic responses both in vivo and in vitro. The mechanism by which NP exerts estrogenic and other endocrine-modulating effects in vivo remains unclear, however. The goal of this study was to evaluate the ability of NP to elicit estrogenic responses through indirect mechanisms of action involving the modulation of endogenous steroid hormone concentrations. Sexually mature male common carp (Cyprinus carpio) were exposed to aqueous NP concentrations ranging from <0.05 to 5.4 microg NP/l for 28-31 d. Approximately 0.5-3.5 ppm of NP was detected in pooled plasma samples or tissue samples from the carp studied. NP exposure did not significantly increase plasma concentrations of 17beta-estradiol (E2), testosterone (T) or vitellogenin (VTG). Excluding outliers, plasma E2 concentrations ranged from <175 to 700 pg E2/ml. T concentrations ranged from 940 to 24,700 pg T/ml plasma. The greatest VTG concentration detected was 52 microg/ml. One-third of the plasma samples tested contained <1 microg VTG/ml. Overall, the results of this study did not support the hypothesis that exposure to waterborne NP can modulate concentrations of steroid hormones in the plasma of sexually mature male carp. The results did, however, raise a number of questions regarding the utility of estradiol equivalent (EEQ) estimates as a means of predicting in vivo effects of estrogenic substances. Furthermore, they provide information regarding the concentrations and variability of E2, T, and VTG in the plasma of sexually mature male carp, which may aid in design and interpretation of future studies.

Steroid enzyme gene expressions during natural and androgen-induced gonadal differentiation in the rainbow trout, Oncorhynchus mykiss

In fish, according to Yamamoto's model, androgens would drive testis differentiation and estrogens ovarian differentiation. In order to study the implication of steroid enzymes in rainbow trout gonadal differentiation, we examined the expression of some steroid enzyme genes during natural differentiation (cholesterol side chain cleavage = P450scc, 17-hydroxylase/lyase = P450c17, 3beta-hydroxysteroid dehydrogenase = 3betaHSD) and androgen-induced differentiation (P450scc, P450c17, 3betaHSD, aromatase = P450aro, and 11beta-hydroxylase = P45011beta). Expressions of P450scc, 3betaHSD, and P450c17 were all detected in male and female gonads at 55 days post-fertilization (dpf), i.e., two weeks before histological differentiation. There were no differences in their expression level respective to the sex. The androgen treatment was carried out by administration of 11beta-hydroxyandrostenedione (11betaOHDelta4) in genetic all-female populations and the resulting sex ratios were found to be 100% male even at a low dosage of 1 mg/kg of food. Following 11betaOHDelta4 treatment, only the expression of P450c17 was found to be sustained when compared with the female untreated control. In contrast, P450scc was clearly up-regulated and 3betaHSD and P450aro down-regulated by the androgen treatment. P45011beta gene expression remained low in gonads of androgen-treated females, as it did in control untreated females. These results together demonstrate that steroidogenesis in rainbow trout is potentially active in pre-differentiating gonads of both sexes, and that one of the masculinizing actions of androgens in the species may be to down-regulate the female-specific gonadal P450aro gene expression. However, in vivo androgen treatment in genetic females does not induce the same pattern of steroid gene expression as in genetic males. These data suggest that exogenous androgens might induce a male differentiation process with P450aro inhibition being one of the steps required. However, this process would not involve endogenously produced 11-oxygenated androgens.