Involvement of estrogens in the process of sex differentiation in two fish species: the rainbow trout (Oncorhynchus mykiss) and a tilapia (Oreochromis niloticus)

Using cell culture systems from the Persian Gulf Arabian yellowfin sea bream, Acanthopagrus arabicus, to assess the effects of dexamethasone on gonad and brain aromatase activity and steroid production

Dexamethasone (DEX) is a synthetic glucocorticoid widely used as pharmaceutical and usually exists in effluents with varying degrees of concentrations. In this study, cultivated Brain, ovary and testis cells from Arabian Sea bream, Acanthopagrus arabicus, were treated by DEX at concentrations of 0, 0.3, 3.0, 30.0 and 300.0 μg/ml for 48 h. The aromatase activity and steroid (17-β-estradiol (E2), progesterone (P) and testosterone (T)) production by cells were measured at 12, 24 and 48 h of the experiment. The results showed that the sensitivity of cultivated ovarian, testicular and brain cells to DEX increased dose dependently. DEX was potent inhibitor of aromatase activity at specially 30.0 and 300.0 μg/ml in the cultivated ovarian and testicular cells at different sampling time. On the other hand, DEX was found to stimulate the aromatase activity of fish brain. DEX also decreased E2, P and T production by cultivated ovarian and testicular cells during the experiment. While, DEX caused an increase in the production of E2 and P by brain cells, which seems logical considering the stimulating effect of this drug on brain aromatase activity. In conclusion, results highlight that DEX is able to change the activity of aromatase, and disrupt the biosynthesis of estrogens and thus affect reproduction in fish.

Determination of the timing of early gonadal differentiation in silver pomfret, Pampus argenteus

Silver pomfret is a species of global significance due to its high nutritional in fisheries sector. To accurately ascertain the timing of sex differentiation mechanism and mRNA level in this species, this study examined gonad morphology and patterns of gene expression related to sex differentiation in males and females from 51 to 180 days post hatch (dph), the temperature of water was maintained at 26 ± 1 ℃. Distinct morphological differentiation of the silver pomfret ovaries, marked by the emergence of primary oocytes, became apparent from 68 dph. By 108 dph, the testes began to differentiate, as evidenced by the appearance of the efferent duct. Early oocytes exhibited a diameter ranged from 0.077 mm to 0.682 mm, with an average diameter of 0.343 ± 0.051 mm. The proportions of various types of germ cells within the testes were subjected to analysis. The localization of Vasa during the early stages of sexual differentiation was a subject to analysis as well. Vasa was predominantly localized within the cytoplasm of gonocyte, peri-nucleolus stage oocytes, primary oocytes and type A spermatogonocytes, indicating that Vasa is involved in the early gonadal differentiation of silver pomfret. The study investigated the expression patterns of dmrt1, gsdf, amh, foxl2, cyp19a1a, cyp11a, sox3 and vasa, all of which are involved in the sex differentiation of teleosts. Among these genes, amh, gsdf, sox3, foxl2, vasa were indentified as crucial contributors to the early gonadal development of silver pomfret. Significant sex-related differences were observed in the expression patterns of amh, dmrt1, gsdf, cyp11a, sox3, cyp19a1a, vasa. This study provides novel insights into the timing of physiological changes associated with the sexual differentiation of silver pomfret. Collectively, the present data indicates that the differentiation of ovaries and testes take place approximately at 68 dph in females and 108 dph in males.

Tilapia, a good model for studying reproductive endocrinology

The Nile tilapia (Oreochromis niloticus), with a system of XX/XY sex determination, is a worldwide farmed fish with a shorter sexual maturation time than that of most cultured fish. Tilapia show a spawning cycle of approximately 14 days and can be artificially propagated in the laboratory all year round to obtain genetically all female (XX) and all male (XY) fry. Its genome sequence has been opened, and a perfect gene editing platform has been established. With a moderate body size, it is convenient for taking enough blood to measure hormone level. In recent years, using tilapia as animal model, we have confirmed that estrogen is crucial for female development because 1) mutation of star2, cyp17a1 or cyp19a1a (encoding aromatase, the key enzyme for estrogen synthesis) results in sex reversal (SR) due to estrogen deficiency in XX tilapia, while mutation of star1, cyp11a1, cyp17a2, cyp19a1b or cyp11c1 affects fertility due to abnormal androgen, cortisol and DHP levels in XY tilapia; 2) when the estrogen receptors (esr2a/esr2b) are mutated, the sex is reversed from female to male, while when the androgen receptors are mutated, the sex cannot be reversed; 3) the differentiated ovary can be transdifferentiated into functional testis by inhibition of estrogen synthesis, and the differentiated testis can be transdifferentiated into ovary by simultaneous addition of exogenous estrogen and androgen synthase inhibitor; 4) loss of male pathway genes amhy, dmrt1, gsdf causes SR with upregulation of cyp19a1a in XY tilapia. Disruption of estrogen synthesis rescues the male to female SR of amhy and gsdf but not dmrt1 mutants; 5) mutation of female pathway genes foxl2 and sf-1 causes SR with downregulation of cyp19a1a in XX tilapia; 6) the germ cell SR of foxl3 mutants fails to be rescued by estrogen treatment, indicating that estrogen determines female germ cell fate through foxl3. This review also summarized the effects of deficiency of other steroid hormones, such as androgen, DHP and cortisol, on fish reproduction. Overall, these studies demonstrate that tilapia is an excellent animal model for studying reproductive endocrinology of fish.

Alternative splicing of histone demethylase Kdm6bb mediates temperature-induced sex reversal in the Nile tilapia

Sex determination in many fish species is remarkably plastic and temperature sensitive. Nile tilapia display a genetic sex-determination system (XX/XY). However, high-temperature treatment during critical thermosensitive periods can induce XX females into XXm pseudo-males, and this phenomenon is termed temperature-induced sex reversal (TISR). To investigate the molecular mechanism of TISR in Nile tilapia, we performed Iso-seq analysis and found a dramatic effect of high temperature on gene alternative splicing (AS). Kdm6bb histone demethylase showed a novel AS at intron 5 that generates Kdm6bb_tv1 transcripts without intron 5 and Kdm6bb_tv2 with intron 5. Kdm6bb_tv1 encodes a full-length protein while Kdm6bb_tv2 encodes a truncated protein. Expression analysis revealed that intron 5 splicing of Kdm6bb is male and gonad biased at larval stage, and only gonad biased at adult stage. High-temperature treatment induced intron 5 splicing in the gonads of XX and XY fish, resulting in increased Kdm6bb_tv1 expression. To directly test the role of Kdm6bb_tv1 in Nile tilapia TISR, we knocked out expression of Kdm6bb_tv1. However, Kdm6bb_tv1-/- homozygous mutants showed embryonic lethality. Overexpression of Kdm6bb_tv1, but not Kdm6bb_tv2, induced sex reversal of XX females into pseudo-males. Overexpression of Kdm6bb_tv1, as with high-temperature treatment, modified the promotor region of Gsdf and Dmrt1 by demethylating the trimethylated lysine 27 of histone 3 (H3K27me3), thereby increasing expression. Collectively, these studies demonstrate that AS of Kdm6bb intron 5 increases the expression of Kdm6bb_tv1, which acts as a direct link between high temperature and activation of Gsdf and Dmrt1 expression, leading to male sex determination.

Steroidogenic potential of the gonad during sex differentiation in the rice field frog Hoplobatrachus rugulosus (Anura: Dicroglossidae)

Prior studies demonstrated that gonadal differentiation in the rice field frog, Hoplobatrachus rugulosus, was of an undifferentiated type since all individuals had ovaries at complete metamorphosis. However, the steroidogenic potential of the gonad is still unknown. In this study, H. rugulosus were obtained by stimulating fertilization in the laboratory under natural light and temperature conditions. The gonads were collected and their steroidogenic potential was evaluated by determining the expression level of messenger RNA (mRNA) encoding for cytochrome P450 17-hydroxylase/C17-20 lyase (CYP17) and cytochrome P450 aromatase (CYP19) using quantitative real-time RT-PCR and the localization of CYP17 mRNA in tissues by in situ hybridization. The CYP17 mRNA levels in males at 4-11 weeks postmetamorphosis were higher than in female and intersex gonads. This corresponded to their localization in the gonadal tissues, where CYP17 signals were specifically detected in the Leydig cells of the testis at 5-16 weeks postmetamorphosis but was undetectable in all ovary samples. The CYP19 mRNA levels in females at 4-11 weeks postmetamorphosis was higher than in male and intersex gonads, which corresponded with gonadal development, indicating the potential steroidogenic function of the ovary. Based on the present results, the role of CYP17 and CYP19 mRNA in sex differentiation in H. rugulosus may occur after gonadal sex differentiation and the steroidogenic potential of the gonads exhibited a sexual dimorphic pattern. These results provide a crucial basis for further research on the developmental biology in anuran species.

Transcriptome characterization of gonadal sex differentiation in Pacific bluefin tuna, Thunnus orientalis (Temminck et Schlegel)

Tunas (genus Thunnus) are one of the most ecologically and commercially important fish worldwide. To establish a biological basis for reproduction in this globally essential species, we have recently studied crucial reproductive aspects of the Pacific bluefin tuna (T. orientalis; PBT), as a model of tuna species, based on our closed-cycle aquaculture technology. In this study, we clarified the global expression profile of the genes regulating gonadal sex differentiation in PBT, as this developmental process is vital to sexual reproduction. Based on the results of our comparative (RNA-sequencing) and temporal (qRT-PCR) transcriptome analyses using the updated genome dataset, we propose the molecular mechanisms of gonadal sex differentiation in PBT. In female gonads, foxl2 and cyp19a1a (coding aromatase) are expressed at the onset of sex differentiation. Active aromatase-mediated estrogen biosynthesis, which includes positive regulation of cyp19a1a expression by Foxl2, induces ovarian differentiation. By contrast, dmrt1 and gsdf are upregulated in differentiating male gonads lacking active estrogen synthesis. Dmrt1 and Gsdf would mainly promote testicular differentiation. Furthermore, androgen biosynthesis is upregulated in differentiating male gonad. Endogenous androgens may also be vital to testicular differentiation. This study provides the first comprehensive data clarifying the molecular basis for gonadal sex differentiation in tunas.

De Novo Assembly, Characterization and Comparative Transcriptome Analysis of the Gonads of Jade Perch (Scortum barcoo)

Due to the high meat yield and rich nutritional content, jade perch (Scortum barcoo) has become an important commercial aquaculture species in China. Jade perch has a slow growth rate, taking 3-4 years to reach sexual maturity, and has almost no difference in body size between males and females. However, the study of its gonad development and reproduction regulation is still blank, which limited the yield increase. Herein, the gonad transcriptomes of juvenile males and females of S. barcoo were identified for the first time. A total of 107,060 unigenes were successfully annotated. By comparing male and female gonad transcriptomes, a total of 23,849 differentially expressed genes (DEGs) were identified, of which 9517 were downregulated, and 14,332 were upregulated in the testis. In addition, a large number of DEGs involved in sex differentiation, gonadal development and differentiation and gametogenesis were identified, and the differential expression patterns of some genes were further verified using real-time fluorescence quantitative PCR. The results of this study will provide a valuable resource for further studies on sex determination and gonadal development of S. barcoo.

Absence of Testicular Estrogen Leads to Defects in Spermatogenesis and Increased Semen Abnormalities in Male Rabbits

Estrogens are steroid hormones produced by the aromatization of androgens by the aromatase enzyme, encoded by the CYP19A1 gene. Although generally referred to as "female sex hormones", estrogen is also produced in the adult testes of many mammals, including humans. To better understand the function of estrogens in the male, we used the rabbit model which is an important biomedical model. First, the expression of CYP19A1 transcripts was localized mainly in meiotic germ cells. Thus, testicular estrogen appears to be produced inside the seminiferous tubules. Next, the cells expressing ESR1 and ESR2 were identified, showing that estrogens could exert their function on post-meiotic germ cells in the tubules and play a role during sperm maturation, since ESR1 and ESR2 were detected in the cauda epididymis. Then, CRISPR/Cas9 CYP19A1-/- genetically modified rabbits were analyzed. CYP19A1-/- males showed decreased fertility with lower sperm count associated with hypo-spermatogenesis and lower spermatid number. Germ/sperm cell DNA methylation was unchanged, while sperm parameters were affected as CYP19A1-/- males exhibited reduced sperm motility associated with increased flagellar defects. In conclusion, testicular estrogens could be involved in the spermatocyte-spermatid transition in the testis, and in the acquisition of sperm motility in the epididymis.

Identification and functional characterization of gonadotropin -releasing hormone in pompano (Trachinotus ovatus)

Gonadotropin-releasing hormone (GnRH) is an important neuropeptide in the reproductive system. Although GnRH analogues have been used to artificially spawn pompano (Trachinotus sp.), the native forms of GnRH have not been described in this species. In this study three GnRH subtypes [sea bream GnRH (sbGnRH), chicken GnRH-Ⅱ (cGnRH-Ⅱ) and salmon GnRH (sGnRH)] were identified in pompano (Trachinotus ovatus). cgnrh-Ⅱ and sgnrh were mainly expressed in the brain of male and female fish, showing a tissue-specific expression pattern, while sbgnrh was expressed at different transcriptional levels in all tested tissues. In vivo injection experiment showed that sbGnRH significantly increased fsh and lh genes expression in a dose-dependent manner, but a high concentration of sbGnRH could desensitize the expression of lh. High concentrations of cGnRH-Ⅱ and sGnRH could induce the expression of fsh and lh. In addition, the results of in vitro incubation experiments showed that the high concentration of sbGnRH peptide could induce the expression of fsh and lh, while cGnRH-Ⅱ and sGnRH peptides could only induce the expression of fsh. 17β-estradiol (E₂) and 17α-methyltestosterone (MT) significantly inhibited sbgnrh mRNA expression in a dose-dependent manner, but did not affect the expression of cgnrh-Ⅱ and sgnrh mRNA. sbGnRH is the main GnRH subtype in pompano. E₂ and MT can play a negative role in the regulation of sbgnrh. This study provides a theoretical basis for the reproductive endocrinology of pompano.

Estrogenic signaling and sociosexual behavior in wild sex-changing bluehead wrasses, Thalassoma bifasciatum

Estrogenic signaling is an important focus in studies of gonadal and brain sexual differentiation in fishes and vertebrates generally. This study examined variation in estrogenic signaling (1) across three sexual phenotypes (female, female-mimic initial phase [IP] male, and terminal phase [TP] male), (2) during socially-controlled female-to-male sex change, and (3) during tidally-driven spawning cycles in the protogynous bluehead wrasse (Thalassoma bifasciatum). We analyzed relative abundances of messenger RNAs (mRNAs) for the brain form of aromatase (cyp19a1b) and the three nuclear estrogen receptors (ER) (ERα, ERβa, and ERβb) by qPCR. Consistent with previous reports, forebrain/midbrain cyp19a1b was highest in females, significantly lower in TP males, and lowest in IP males. By contrast, ERα and ERβb mRNA abundances were highest in TP males and increased during sex change. ERβa mRNA did not vary significantly. Across the tidally-driven spawning cycle, cyp19a1b abundances were higher in females than TP males. Interestingly, cyp19a1b levels were higher in TP males close (~1 h) to the daily spawning period when sexual and aggressive behaviors rise than males far from spawning (~10-12 h). Together with earlier findings, our results suggest alterations in neural estrogen signaling are key regulators of socially-controlled sex change and sexual phenotype differences. Additionally, these patterns suggest TP male-typical sociosexual behaviors may depend on intermediate rather than low estrogenic signaling. We discuss these results and the possibility that an inverted-U shaped relationship between neural estrogen and male-typical behaviors is more common than presently appreciated.

Transcriptomic analysis of pituitary in female and male spotted scat (Scatophagus argus) after 17β-estradiol injection

Spotted scat (Scatophagus argus) is a popular species of marine fish cultured in China. It shows normal sexual growth dimorphism. Female spotted scat grows quicker and bigger than males. Growth and reproduction are the most important traits in aquaculture. In vertebrates, the pituitary gland occupies an important position in the growth and reproduction axis. Estrogen is involved in regulating growth and reproduction in the pituitary gland in an endocrine fashion. Transcriptome sequencing of the pituitary was performed in female and male fish at 6 h after 17β-estradiol injection (4.0 μg E₂/g body weight, BW). Compared with the pituitary of female and male groups, 144 and 64 genes [|log₂^{(fold change)}| ≥ 1.0 and false discovery rate (FDR) < 0.05] were significantly differentially expressed in E₂-injected females and males, respectively (p < 0.05). Of these, 59 and 48 were up-regulated, and 85 and 16 were down-regulated. According to the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) pathway analyses, DEGs were involved in signal pathways, such as growth, reproduction, oocyte meiosis and steroid biosynthesis. Of these, estrogen affected the expression of some sex steroid synthesis and receptor genes in the pituitary gland through feedback, such as hsd17b7, pgr and cyp19a1b, regulating the reproductive activities. Besides, some growth-related genes, such as gap43, junbb, mstn2 and insm1a responded to estrogen. E₂ might affect the expression level of gh mRNA by regulating the expression levels of growth-related genes. Our results provide a theoretical basis for studying the molecular mechanism of growth and reproduction regulation at the pituitary level of spotted scat responded to E₂.

Cyp17a1 is Required for Female Sex Determination and Male Fertility by Regulating Sex Steroid Biosynthesis in Fish

In teleost fish, sex steroids are involved in sex determination, sex differentiation, and fertility. Cyp17a1 (Cytochrome P450 family 17 subfamily A member 1) is thought to play essential roles in fish steroidogenesis. Therefore, to further understand its roles in steroidogenesis, sex determination, and fertility in fish, we constructed a cyp17a1 gene mutant in Nile tilapia (Oreochromis niloticus). In XX fish, mutation of the cyp17a1 gene led to a female-to-male sex reversal with a significant decline in 17β-estradiol (E2) and testosterone (T) production, and ectopic expression of male-biased markers (Dmrt1 and Gsdf) in gonads from the critical window of sex determination. Sex reversal was successfully rescued via T or E2 administration, and ovarian characteristics were maintained after termination of E2 supplementation in the absence of endogenous estrogen production in cyp17a1-/- XX fish. Likewise, deficiencies in T and 11-ketotestosterone (11-KT) production in both cyp17a1-/- XX sex-reversed males and cyp17a1-/- XY mutants resulted in meiotic initiation delays, vas deferens obstruction and sterility due to excessive apoptosis and abnormal mitochondrial morphology. However, 11-KT treatment successfully rescued the dysspermia to produce normal sperm in cyp17a1-/- male fish. Significant increases in gonadotropic hormone (gth) and gth receptors in cyp17a1-/- mutants may excessively upregulate steroidogenic gene expression in Leydig cells through a feedback loop. Taken together, our findings demonstrate that Cyp17a1 is indispensable for E2 production, which is fundamental for female sex determination and differentiation in XX tilapia. Additionally, Cyp17a1 is essential for T and 11-KT production, which further promotes spermatogenesis and fertility in XY males.

The genome of New Zealand trevally (Carangidae: Pseudocaranx georgianus) uncovers a XY sex determination locus

BACKGROUND

The genetic control of sex determination in teleost species is poorly understood. This is partly because of the diversity of mechanisms that determine sex in this large group of vertebrates, including constitutive genes linked to sex chromosomes, polygenic constitutive mechanisms, environmental factors, hermaphroditism, and unisexuality. Here we use a de novo genome assembly of New Zealand silver trevally (Pseudocaranx georgianus) together with sex-specific whole genome sequencing data to detect sexually divergent genomic regions, identify candidate genes and develop molecular makers.

RESULTS

The de novo assembly of an unsexed trevally (Trevally_v1) resulted in a final assembly of 579.4 Mb in length, with a N50 of 25.2 Mb. Of the assembled scaffolds, 24 were of chromosome scale, ranging from 11 to 31 Mb in length. A total of 28,416 genes were annotated after 12.8 % of the assembly was masked with repetitive elements. Whole genome re-sequencing of 13 wild sexed trevally (seven males and six females) identified two sexually divergent regions located on two scaffolds, including a 6 kb region at the proximal end of chromosome 21. Blast analyses revealed similarity between one region and the aromatase genes cyp19 (a1a/b) (E-value < 1.00E-25, identity > 78.8 %). Males contained higher numbers of heterozygous variants in both regions, while females showed regions of very low read-depth, indicative of male-specificity of this genomic region. Molecular markers were developed and subsequently tested on 96 histologically-sexed fish (42 males and 54 females). Three markers amplified in absolute correspondence with sex (positive in males, negative in females).

CONCLUSIONS

The higher number of heterozygous variants in males combined with the absence of these regions in females support a XY sex-determination model, indicating that the trevally_v1 genome assembly was developed from a male specimen. This sex system contrasts with the ZW sex-determination model documented in closely related carangid species. Our results indicate a sex-determining function of a cyp19a1a-like gene, suggesting the molecular pathway of sex determination is somewhat conserved in this family. The genomic resources developed here will facilitate future comparative work, and enable improved insights into the varied sex determination pathways in teleosts. The sex marker developed in this study will be a valuable resource for aquaculture selective breeding programmes, and for determining sex ratios in wild populations.

Endocrine disruption in the sub Antarctic fish Patagonotothen tessellata (Perciformes, Notothenidae) from Beagle Channel associated to anthropogenic impact

Situated in the sub-Antarctic region, Beagle Channel represents a unique marine ecosystem due to the connection between the Pacific and the Atlantic Oceans, and its proximity to the Antarctic Peninsula. Ushuaia city, the biggest settlement on the channel, exerts an increasing anthropogenic pressure by discharges of urban and industrial effluents. In the present work, we use Patagonotothen tessellata, one of the most abundant and widespread species in the channel, as a bioindicator species in order to evidence anthropic impact from Ushuaia Bay and surrounding areas. We first analyzed and characterized real time gene expression of androgen receptor, estrogen receptor and different forms of vitellogenin (VTG), under laboratory conditions. This was achieved by induction with estradiol of P. tessellata males. Then, the selected genes were used as biomarkers for an environmental biomonitoring study. Morphometric indices and circulating sex steroids (estradiol and testosterone) were also quantified in male fish collected from different sites. The qPCR analysis showed that vtgAb form is more inducible than vtgAa or vtgC forms after estrogen induction. The field survey revealed the up-regulation of vtgAb and the androgen receptor in fish from sites with higher anthropogenic influence. Sex steroids followed seasonal variations according to their reproductive cycle, with higher levels of estradiol and testosterone in winter and summer seasons. The use of biomarkers such as gene expression of VTG demonstrates that fish from Ushuaia Bay are likely to be exposed to endocrine disrupting compounds. To our knowledge, this research is the first attempt to assess the endocrine disruption associated to anthropic impact in a widespread fish of the Beagle Channel and contributes to a better understanding of the reproductive physiology of sub Antarctic ichthyofauna.

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.

Molecular Characterization and Expression of Cytochrome P450 Aromatase in Atlantic Croaker Brain: Regulation by Antioxidant Status and Nitric Oxide Synthase During Hypoxia Stress

We have previously shown that nitric oxide synthase (NOS, an enzyme) is significantly increased during hypoxic stress in Atlantic croaker brains and modulated by an antioxidant (AOX). However, the influence of NOS and AOX on cytochrome P450 aromatase (AROM, CYP19a1, an enzyme) activity on vertebrate brains during hypoxic stress is largely unknown. In this study, we characterized brain AROM (bAROM, CYP19a1b) cDNA in croaker and examined the interactive effects of hypoxia and a NOS-inhibitor or AOX on AROM activity. The amino acid sequence of croaker bAROM cDNA is highly homologous (76–80%) to other marine teleost bAROM cDNAs. Both real-time PCR and Northern blot analyses showed that bAROM transcript (size: ∼2.8 kb) is highly expressed in the preoptic-anterior hypothalamus (POAH). Hypoxia exposure (dissolved oxygen, DO: 1.7 mg/L for 4 weeks) caused significant decreases in hypothalamic AROM activity, bAROM mRNA and protein expressions. Hypothalamic AROM activity and mRNA levels were also decreased by pharmacological treatment with N-ethylmaleimide (NEM, an alkylating drug that modifies sulfhydryl groups) of fish exposed to normoxic (DO: ∼6.5 mg/L) conditions. On the other hand, treatments with Nω-nitro-L-arginine methyl ester (NAME, a competitive NOS-inhibitor) or vitamin-E (Vit-E, a powerful AOX) prevented the downregulation of hypothalamic AROM activity and mRNA levels in hypoxic fish. Moreover, NAME and Vit-E treatments also restored gonadal growth in hypoxic fish. Double-labeled immunohistochemistry results showed that AROM and NOS proteins are co-expressed with NADPH oxidase (generates superoxide anion) in the POAH. Collectively, these results suggest that the hypoxia-induced downregulation of AROM activity in teleost brains is influenced by neuronal NOS activity and AOX status. The present study provides, to the best of our knowledge, the first evidence of restoration of AROM levels in vertebrate brains by a competitive NOS-inhibitor and potent AOX during hypoxic stress.

A Transient Hermaphroditic Stage in Early Male Gonadal Development in Little Yellow Croaker, Larimichthys polyactis

Animal taxa show remarkable variability in sexual reproduction, where separate sexes, or gonochorism, is thought to have evolved from hermaphroditism for most cases. Hermaphroditism accounts for 5% in animals, and sequential hermaphroditism has been found in teleost. In this study, we characterized a novel form of the transient hermaphroditic stage in little yellow croaker (Larimichthys polyactis) during early gonadal development. The ovary and testis were indistinguishable from 7 to 40 days post-hatching (dph). Morphological and histological examinations revealed an intersex stage of male gonads between 43 and 80 dph, which consist of germ cells, somatic cells, efferent duct, and early primary oocytes (EPOs). These EPOs in testis degenerate completely by 90 dph through apoptosis yet can be rescued by exogenous 17-β-estradiol. Male germ cells enter the mitotic flourishing stage before meiosis is initiated at 180 dph, and they undergo normal spermatogenesis to produce functional sperms. This transient hermaphroditic stage is male-specific, and the ovary development appears to be normal in females. This developmental pattern is not found in the sister species Larimichthys crocea or any other closely related species. Further examinations of serum hormone levels indicate that the absence of 11-ketotestosterone and elevated levels of 17-β-estradiol delineate the male intersex gonad stage, providing mechanistic insights on this unique phenomenon. Our research is the first report on male-specific transient hermaphroditism and will advance the current understanding of fish reproductive biology. This unique gonadal development pattern can serve as a useful model for studying the evolutionary relationship between hermaphroditism and gonochorism, as well as teleost sex determination and differentiation strategies.

Effects of 17β-estradiol on early gonadal development and expression of genes implicated in sexual differentiation of a South American teleost, Astyanax altiparanae

Gonadal sex differentiation in teleost fish shows greater plasticity as compared to other vertebrates, as it can be influenced by a variety of factors such as exogenous sex steroids. Exogenous estrogens, such as 17β-estradiol (E2), can induce feminization when administered during early embryonic development. However, the mechanisms underlying the E2-induced feminization are not fully understood, especially in Neotropical species. Therefore, the aim of this study was to evaluate the effects of E2 administration on the phenotypic sex characteristics, histological assessment of the gonads, and the expression of selected genes in Astyanax altiparanae exposed to dietary E2 prior to gonadal differentiation. At 4 days post-hatch (dph), groups of 30-40 undifferentiated larvae were fed with a diet containing varying amounts of E2 for 28 days, and fish were sampled at 90 dph. Previous studies revealed that ovary formation in A. altiparanae occurred at 58 dph, whereas the first sign of testis formation was found at 73 dph. In relation to the control, E2 exposure increased the proportion of phenotypic females in 120% and 148.4% for 4 and 6 mg E2/Kg, respectively. However, histological analysis revealed that treatments did not affect gonadal sex ratio between males and females, but induced intersex (testis-ova) in the group treated with 6 mg E2/Kg food. Treatment with E2 also altered gonadal transcript levels of a selected number of genes implicated in sexual differentiation. Males overexpressed dmrt1, sox9 and amh following E2 treatment as compared to control. Females showed increased mRNA levels of dmrt1 and sox9, which might be related to the down-regulation of cyp19a1a after E2 exposure. In summary, E2 exposure during early gonadal development affected male secondary characteristics without changing the gonadal sex ratio, and altered expression of genes implicated in sexual differentiation.

Amh dominant expression in Sertoli cells during the testicular differentiation and development stages in the olive flounder Paralichthys olivaceus

The olive flounder Paralichthys olivaceus, an important marine fish, shows gender differences in growth. The mechanism on its gonadal differentiation direction affected with exogenous factors still needs to be clarified. The anti-Müllerian hormone (amh) gene is involved in fish testicular differentiation and maintenance. The aim of this study was to investigate the expression of the flounder amh in tissues and the gonads. The quantitative expression analysis results showed that it was highly expressed in the testis, especially in the testis at stages I - IV (P < 0.05). Also, amh was detected in Sertoli cells surrounding spermatogonia and peripheral seminiferous lobule of the testis with in situ hybridization (ISH) and immunohistochemistry (IHC). During the differentiation period, the amh expression in the testis of the tamoxifen treatment group (100 ppm) was higher than that in the ovary of the 17β-estradiol (E2, 5 ppm) group, and the expression levels of amh during process of the male differentiation in the tamoxifen group were higher than those in the 17ɑ-methyltestosterone (MT, 5 ppm) group (P < 0.05). ISH results also exhibited that amh was expressed in the somatic cells that surrounded the germ cells of juvenile flounder similar to adult ones. Furthermore, the flounder gonads in the tamoxifen group maintained more germ cells and somatic cells than those in the MT group from 20 to 80 mm total length (TL). Especially, at 60 and 80 mm TL, the numbers of germ and somatic cells in the tamoxifen group were significantly higher than those in the MT group (P < 0.05). In summary, amh might initiate the process of testicular differentiation, and is involved in the early development and maintenance of testis.

Climate change impacts on fish reproduction are mediated at multiple levels of the brain-pituitary-gonad axis

Anthropogenic emissions of carbon dioxide in the atmosphere have generated rapid variations in atmospheric composition which drives major climate changes. Climate change related effects include changes in physico-chemical proprieties of sea and freshwater, such as variations in water temperature, salinity, pH/pCO₂ and oxygen content, which can impact fish critical physiological functions including reproduction. In this context, the main aim of the present review is to discuss how climate change related effects (variation in water temperature and salinity, increases in duration and frequency of hypoxia events, water acidification) would impact reproduction by affecting the neuroendocrine axis (brain-pituitary-gonad axis). Variations in temperature and photoperiod regimes are known to strongly affect sex differentiation and the timing and phenology of spawning period in several fish species. Temperature mainly acts at the level of gonad by interfering with steroidogenesis, (notably on gonadal aromatase activity) and gametogenesis. Temperature is also directly involved in the quality of released gametes and embryos development. Changes in salinity or water acidification are especially associated with reduction of sperm quality and reproductive output. Hypoxia events are able to interact with gonad steroidogenesis by acting on the steroids precursor cholesterol availability or directly on aromatase action, with an impact on the quality of gametes and reproductive success. Climate change related effects on water parameters likely influence also the reproductive behavior of fish. Although the precise mechanisms underlying the regulation of these effects are not always understood, in this review we discuss different hypothesis and propose future research perspectives.

Using ovarian and brain cell culture from the Mullet, Liza klunzingeri, to assess the inhibitory effects of benzo[a]pyrene on aromatase activity

We assessed the toxic effects of benzo[a]pyrene (BaP) on cell viability, aromatase (Aro) activity and steroid production using ovarian and brain cell cultures obtained from Mullet, Liza klunzingeri. The brain and ovary were minced and digested, and the cells were suspended in Leibovitz's L-15 medium supplemented with 15% and 20% fetal bovine serum. The cell suspensions were seeded on 25-cm² cell-culture flasks at 1 × 10⁶ cells/mL and incubated at 25 °C for 2 weeks. A BaP concentration of 10^-5 mol/L was accepted as the half-maximal inhibitory concentration. Ovarian and brain cells were exposed to different concentrations of BaP [0 (control), 10^-6 , 2 × 10^-6 , 3 × 10^-6 mol/L] and incubated at 30 °C. At different sampling times (0, 12, 24 and 48 h) 40 ng/10⁵ cells of 1,4,6-androstatriene-3,17-dione (ATD) was added to each well. Aro activity, 17β-estradiol (E2) and ATD production were determined. The sensitivity of the cultivated ovarian and brain cells to BaP increased dose dependently. BaP was a potent inhibitor of Aro activity at 2 × 10^-6 and 3 × 10^-6 mol/L, both in the cultivated brain and ovarian cells at different sampling times, with 10^-6 mol/L BaP found to be the least potent Aro inhibitor. E2 production decreased from cultivated ovarian and brain cells treated by different concentrations of BaP. In conclusion, BaP is able to change the activity of Aro and disrupt the biosynthesis of estrogens, and thus affects reproduction in fish.

Roles of estrogens in fish sexual plasticity and sex differentiation

Fish sex could be reversed at the undifferentiated stage of gonad by administration of exogenous estrogen (E2) or blockade of endogenous estrogen synthesis with aromatase inhibitors, which is designated as primary sex reversal (PSR). Recent studies have well demonstrated that gonochoristic fish maintain their sexual plasticity after sex determination/differentiation. The differentiated ovary could be transdifferentiated into functional testis, and vice versa, the differentiated testis could be transdifferentiated into ovary. By analyzing these two secondary sex reversal (SSR) models, it was found that induction of male-to-female sex reversal initiates from dorsal (near the blood vessel) to the ventral, while induction of female-to-male sex reversal initiates from the ventral to dorsal. Down regulation of endogenous estrogen is the prerequisite for the ovarian transdifferentiation. However, exogenous estrogen alone is not sufficient for inducing differentiated testis to ovary. Administration of E2 and simultaneous blockage of androgen synthesis could induce testicular transdifferentiation. Therefore, endogenous estrogen is critical for the ovarian differentiation/maintenance and androgen is critical for testicular maintenance. Recently, genetic studies with genome editing technologies also showed that disruption of Cyp19a1a induced testicular development, indicating that cyp19a1a is the key gene essential for estrogen synthesis and ovary differentiation/maintenance. Knockout of male pathway genes or overexpression of female pathway genes could up-regulate cyp19a1a expression and increase estrogen level so as to promote ovary. Conversely, knockout of female pathway genes or overexpression of male pathway genes could down-regulate cyp19a1a expression and decrease estrogen level so as to promote testis (transgenic or knockout sex reversal, TSR). Epigenetic regulation of cyp19a1a play a critical role in natural sex reversal (NSR), but its relation with PSR, SSR and TSR needs further detailed investigations. In all, these studies further highlighted the important roles of endogenous estrogens in fish sex differentiation/maintenance.

Expression profiles of dmrts and foxls during gonadal development and sex reversal induced by 17α-methyltestosterone in the orange-spotted grouper

The orange-spotted grouper, Epinephelus coioides, is a marine protogynous hermaphrodite fish of commercial importance. There are many examples of sex change species among marine fish, but the molecular basis for the sex change is still unknown. Gonadal expression patterns of the dmrts and foxls genes in E. coioides have pointed to sexual dimorphism in this species and it has been shown that mRNA levels of dmrts and foxls to vary significantly during reproduction cycles. The steroid 17α-methyltestosterone was used to induce sex reversal in these fish, during which dmrts and foxls levels changed significantly and subsequently reverted to normal when 17α-methyltestosterone was withdrawn. Interestingly, the expression of dmrt2b and dmrt3 was not affected by this steroid. We speculate that the role of foxl2 in reproduction may be conserved via regulation of early differentiation of the ovary by the hypothalamus-pituitary-gonad axis, and dmrt2 may have a significant role in premature ovarian differentiation and maintenance in E. coioides. dmrt1 and foxl3 played a role in the development of the testes and are believed to be potential male regulatory genes.

Temporal evaluation of estrogenic endocrine disruption markers in smallmouth bass (Micropterus dolomieu) reveals seasonal variability in intersex

A reconnaissance project completed in 2009 identified intersex and elevated plasma vitellogenin in male smallmouth bass inhabiting the Missisquoi River, VT. In an attempt to identify the presence and seasonality of putative endocrine disrupting chemicals or other factors associated with these observations, a comprehensive reevaluation was conducted between September 2012 and June 2014. Here, we collected smallmouth bass from three physically partitioned reaches along the river to measure biomarkers of estrogenic endocrine disruption in smallmouth bass. In addition, polar organic chemical integrative samples (POCIS) were deployed to identify specific chemicals associated with biological observations. We did not observe biological differences across reaches indicating the absence of clear point source contributions to the observation of intersex. Interestingly, intersex prevalence and severity decreased in a stepwise manner over the timespan of the project. Intersex decreased from 92.8% to 28.1%. The only significant predictor of intersex prevalence was year of capture, based on logistic regression analysis. The mixed model of fish length and year-of-capture best predicted intersex severity. Intersex severity was also significantly different across late summer and early spring collections indicating seasonal changes in this metric. Plasma vitellogenin and liver vitellogenin Aa transcript abundance in males did not indicate exposure to estrogenic endocrine disrupting chemicals at any of the four sample collections. Analysis of chemicals captured by the POCIS as well as results of screening discrete water samples or POCIS extracts did not indicate the contribution of appreciable estrogenic chemicals. It is possible that unreported changes in land-use activity have ameliorated the problem, and our observations indicate recovery. Regardless, this work clearly emphasizes that single, snap shot sampling for intersex may not yield representative data given that the manifestation of this condition within a population can change dramatically over time.

The unusual rainbow trout sex determination gene hijacked the canonical vertebrate gonadal differentiation pathway

Evolutionary novelties require rewiring of transcriptional networks and/or the evolution of new gene functions. Sex determination (SD), one of the most plastic evolutionary processes, requires such novelties. Studies on the evolution of vertebrate SD revealed that new master SD genes are generally recruited from genes involved in the downstream SD regulatory genetic network. Only a single exception to this rule is currently known in vertebrates: the intriguing case of the salmonid master SD gene (sdY), which arose from duplication of an immune-related gene. This exception immediately posed the question of how a gene outside from the classical sex differentiation cascade could acquire its function as a male SD gene. Here we show that SdY became integrated in the classical vertebrate sex differentiation cascade by interacting with the Forkhead box domain of the female-determining transcription factor, Foxl2. In the presence of Foxl2, SdY is translocated to the nucleus where the SdY:Foxl2 complex prevents activation of the aromatase (cyp19a1a) promoter in cooperation with Nr5a1 (Sf1). Hence, by blocking a positive loop of regulation needed for the synthesis of estrogens in the early differentiating gonad, SdY disrupts a preset female differentiation pathway, consequently allowing testicular differentiation to proceed. These results also suggest that the evolution of unusual vertebrate master sex determination genes recruited from outside the classical pathway like sdY is strongly constrained by their ability to interact with the canonical gonadal differentiation pathway.

Expression analysis of growth differentiation factor 9 (Gdf9/gdf9), anti-müllerian hormone (Amh/amh) and aromatase (Cyp19a1a/cyp19a1a) during gonadal differentiation of the zebrafish, Danio rerio

In the zebrafish, no sex-determining gene has been identified, while some sex-related genes, such as cyp19a1a and amh, show sexually dimorphic expression. Interestingly, most of these genes are expressed in the somatic cells. With increasing evidence suggesting roles of germ cells in gonadal differentiation, there is an increasing interest in the factors released by the germ cells for the bidirectional communication between the two compartments. We have reported that Gdf9/gdf9 is an oocyte-specific factor in the zebrafish, similar to that of mammals. Whether and how Gdf9 is involved in gonadal differentiation is unknown. In this study, we compared the expression levels of gdf9, cyp19a1a, and amh among several other sex-related genes in the gonads before, during, and after sex differentiation. The expression of gdf9 started in the gonads before sex differentiation, and its level surged in the differentiated ovary. Its expression pattern was similar to that of cyp19a1a, but reciprocal to amh expression. Using recombinant zebrafish Gdf9 (rzfGdf9), we further showed that Gdf9 significantly suppressed the expression of amh while increased that of activin beta subunits (inhbaa and inhbb) in vitro. Although gdf9 and cyp19a1a showed co-expression during gonadal differentiation, we only observed a slight but not significant response of cyp19a1a to rzfGdf9. Knocking down the expression of gdf9 and cyp19a1a with vivo-morpholinos caused a male-skewed sex ratio. Our data suggested that Gdf9 is likely involved in promoting oocyte/ovary differentiation in the zebrafish and it may act by suppressing amh expression, at least partly, in the somatic cells.

Sex hormone-binding globulin b expression in the rainbow trout ovary prior to sex differentiation

Salmonids have two sex hormone-binding globulin (Shbg) paralogs. Shbga is mainly expressed in the liver, while Shbgb is secreted by the granulosa cells of the rainbow trout ovary. Coexpression of shbgb and the gonadal aromatase cyp19a1a mRNAs been observed in granulosa cells, suggesting a physiological coordination between Shbgb expression and estrogen synthesis. As estrogens are essential for female sex determination in the fish ovary, we propose that Shbgb participates in early ovarian differentiation, either by binding with estrogen or through another mechanism that remains to be discovered. To elucidate this potential role, monosex populations of female trout were studied during the molecular ovarian differentiation period (28-56 dpf). shbgb mRNA expression was measured using qPCR and compared with expression of genes for other ovarian markers (cyp19a1a, foxl2, follistatin, and estrogen receptors). shbgb transcript expression was detected during the final stages of embryonic development (21-26 dpf) and during molecular ovarian differentiation (32-52 dpf) after hatching (which occurred at 31 dpf). In situ hybridization localized shbgb transcription to the undifferentiated ovary at 42 dpf, and shbgb and cyp19a1a mRNA showed similar expression patterns. These results suggest that Shbgb is involved in early ovarian differentiation, supporting an important role for the salmonid shbgb gene in sex determination.

Differential expression of subunits of 20β-hydroxysteroid dehydrogenase during gametogenesis in rainbow trout (Oncorhychus mykiss)

The patterns of expression of two subunits of 20β-hydroxysteroid dehydrogenase (20β-HSD), key enzyme involved in the biosynthesis and activation of steroid hormones, were examined in rainbow trout by using a combination of quantitative real-time PCR and in-situ hybridization. The expression of targeted genes was examined in mRNA extracted from different tissues at different gonadal stages in male and female trout. Both subunits of 20β-HSD were found to be widely distributed in tissues. The highest expression of 20β-HSD A was found in intestine followed by skin, stomach, liver and gills, whereas, the highest expression of 20β-HSD B was observed in stomach followed by head kidney, ovary - at late vitellogenesis stage- and trunk kidney. In ovarian tissue 20β-HSD A was highly expressed in mature oocytes, and the highest expression of 20β-HSD B was in ovary at late vitellogenesis stage. There were no differences in the level of expression of either subunit among groups of rainbow trout at different stages of maturational competence. In male fish, 20β-HSD A was highly expressed in testis stage I in contrast to 20β-HSD B which was highly expressed in testis stage VIII. In situ- hybridization results showed that the 20β-HSD gene was highly expressed in gastrointestinal organs, while only slightly expressed in the gonadal tissue of fish at stage 62day-post-fertilization (dpf). Overall, the results confirm the ubiquitous presence of 20β-HSD among tissues in rainbow trout with relatively minor fluctuations in expression associated with reproductive cycles which collectively suggests a wider metabolic role of these enzymes than just an association with the synthesis of control hormones for reproduction.

Blockage of androgen and administration of estrogen induce transdifferentiation of testis into ovary

Induction of sex reversal of XY fish has been restricted to the sex undifferentiated period. In the present study, differentiated XY tilapia were treated with trilostane (TR), metopirone (MN) and glycyrrhetinic acid (GA) (inhibitor of 3β-HSD, Cyp11b2 and 11β-HSD, respectively) alone or in combination with 17β-estradiol (E2) from 30 to 90 dah (days after hatching). At 180 dah, E2 alone resulted in 8.3%, and TR, MN and GA alone resulted in no secondary sex reversal (SSR), whereas TR + E2, MN + E2 and GA + E2 resulted in 88.3, 60.0 and 46.7% of SSR, respectively. This sex reversal could be rescued by simultaneous administration of 11-ketotestosterone (11-KT). Compared with the control XY fish, decreased serum 11-KT and increased E2 level were detected in SSR fish. Immunohistochemistry analyses revealed that Cyp19a1a, Cyp11b2 and Dmrt1 were expressed in the gonads of GA + E2, MN + E2 and TR + E2 SSR XY fish at 90 dah, but only Cyp19a1a was expressed at 180 dah. When the treatment was applied from 60 to 120 dah, TR + E2 resulted in 3.3% of SSR, MN + E2 and GA + E2 resulted in no SSR. These results demonstrated that once 11-KT was synthesized, it could antagonize E2-induced male-to-female SSR, which could be abolished by simultaneous treatment with the inhibitor of steroidogenic enzymes. The upper the enzyme was located in the steroidogenic pathway, the higher SSR rate was achieved when it was inhibited as some of the precursors, such as androstenedione, testosterone and 5α-dihydrotestosterone, could act as androgens. These results highlight the key role of androgen in male sex maintenance.

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.

Transcriptome analysis of the gonads of olive flounder (Paralichthys olivaceus)

Olive flounder (Paralichthys olivaceus) is an economically important cultured marine fish in China, Korea, and Japan. Controlling and managing the breeding of olive flounder in captivity is an imperative step toward obtaining a sustainable supply of this fish in aquaculture production systems. Therefore, investigation on the molecular regulatory mechanism of gonadal development and gametogenesis in this species is of great significance in aquaculture. Furthermore, identification of the expression profile of numerous sex-related genes is the first step to primarily understand such molecular regulatory mechanism. Six female and six male gonads obtained from 2-year-old olive flounders were sequenced using Illumina, which produced 6.68 and 6.24 GB data for ovary and testis, respectively. The reads were mapped to the draft genome of olive flounder, and then the reads per kilobase per million (FPKM) for each gene were calculated. The female-/male-biased expressed genes were investigated based on the FPKM values. Overall, 3086 female-biased and 5048 male-biased genes were screened out. GO enrichment analysis showed that the GO terms "male meiosis," "gamete generation," "fertilization," "spermatogenesis," and "germ plasma" were enriched in male-biased genes. In addition, the GO terms "cell morphogenesis involved in differentiation," "embryonic morphogenesis," "plasma membrane," "steroid hormone receptor activity," and "aromatase activity" were enriched in female-biased genes. Moreover, 373,369 single nucleotide polymorphisms and 32,993 indels were identified in the transcriptome. This work is the largest collection of gonad transcriptome data for olive flounder and provides an extensive resource for future gonadal development and gametogenesis molecular biology studies in this species.

Approaches for predicting effects of unintended environmental exposure to an endocrine active pharmaceutical, tamoxifen

Tamoxifen is an endocrine-active pharmaceutical (EAP) that is used world-wide. Because tamoxifen is a ubiquitous pharmaceutical and interacts with estrogen receptors, a case study was conducted with this compound to (1) determine effects on reproductive endpoints in a nontarget species (i.e., a fish), (2) compare biologically-active metabolites across species, (3) assess whether in vitro assays predict in vivo results, and (4) investigate metabolomic profiles in tamoxifen-treated fish to better understand the biological mechanisms of tamoxifen toxicity. In reproductive assays, tamoxifen exposure caused a significant reduction in egg production and significantly increased ovarian aromatase activity in spawning adult cunner fish (Tautogolabrus adspersus). In plasma from tamoxifen-exposed cunner, the predominant metabolite was 4-hydroxytamoxifen, while in rats it was N-desmethyltamoxifen. Because 4-hydroxytamoxifen is a more biologically active metabolite than N-desmethyltamoxifen, this difference could result in a different level of risk for the two species. The results of in vitro assays with fish hepatic microsomes to assess tamoxifen metabolism did not match in vivo results, indicating probable differences in excretion of tamoxifen metabolites in fish compared with rats. For the first time, a complete in vitro characterization of the metabolism of tamoxifen using fish microsomes is presented. Furthermore, a metabolomic investigation of cunner gonad extracts demonstrates that tamoxifen alters the biochemical profile in this nontarget species. Understanding the consequence of tamoxifen exposure in nontarget species, and assessing the discrepancies between sex- and species-mediated endpoints, is a step toward understanding how to accurately assess the risks posed by EAPs, such as tamoxifen, in the aquatic environment. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 1834-1850, 2016.

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

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

Simultaneous exposure to estrogen and androgen resulted in feminization and endocrine disruption

Estrogen, which is synthesized earlier in females than androgen in males, is critical for sex determination in non-mammalian vertebrates. However, it remains unknown that what would happen to the gonadal phenotype if estrogen and androgen were administrated simultaneously. In this study, XY and XX tilapia fry were treated with the same dose of 17α-methyltestosterone (MT) and 17β-estradiol (E2) alone and in combination from 0 to 30 days after hatching. Treatment of XY fish with E2 resulted in male to female sex reversal, while treatment of XX fish with MT resulted in female to male sex reversal. In contrast, simultaneous treatment of XX and XY fish with MT and E2 resulted in female, but with cyp11b2 and cyp19a1a co-expressed in the ovary. Serum 11-ketotestosteron level of the MT and E2 simultaneously treated XX and XY female was similar to that of the XY control, while serum E2 level of these two groups was similar to that of the XX control. Transcriptomic cluster analysis revealed that the MT and E2 treated XX and XY gonads clustered into the same branch with the XX control. However a small fraction of genes, which showed disordered expression, may be associated with stress response. These results demonstrated that estrogen could maintain the female phenotype of XX fish and feminize XY fish even in the presence of androgen. Simultaneous treatment with estrogen and androgen up-regulated the endogenous estrogen and androgen synthesis, and resulted in disordered gene expression and endocrine disruption in tilapia.

Comprehensive transcriptomic analysis of the process of gonadal sex differentiation in the turbot (Scophthalmus maximus)

The turbot is a flatfish with a ZW/ZZ sex determination system but with a still unknown sex determining gene(s), and with a marked sexual growth dimorphism in favor of females. To better understand sexual development in turbot we sampled young turbot encompassing the whole process of gonadal differentiation and conducted a comprehensive transcriptomic study on its sex differentiation using a validated custom oligomicroarray. Also, the expression profiles of 18 canonical reproduction-related genes were studied along gonad development. The expression levels of gonadal aromatase cyp19a1a alone at three months of age allowed the accurate and early identification of sex before the first signs of histological differentiation. A total of 56 differentially expressed genes (DEG) that had not previously been related to sex differentiation in fish were identified within the first three months of age, of which 44 were associated with ovarian differentiation (e.g., cd98, gpd1 and cry2), and 12 with testicular differentiation (e.g., ace, capn8 and nxph1). To identify putative sex determining genes, ∼4.000 DEG in juvenile gonads were mapped and their positions compared with that of previously identified sex- and growth-related quantitative trait loci (QTL). Although no genes mapped to the previously identified sex-related QTLs, two genes (foxl2 and 17βhsd) of the canonical reproduction-related genes mapped to growth-QTLs in linkage group (LG) 15 and LG6, respectively, suggesting that these genes are related to the growth dimorphism in this species.

Morphology, sex steroid level and gene expression analysis in gonadal sex reversal of triploid female (XXX) rainbow trout (Oncorhynchus mykiss)

In non-mammalian vertebrates, estrogens and expressions of cyp19a1 and foxl2 play critical roles in maintaining ovary differentiation and development, while dmrt1 and sox9 are male-specific genes in testicular differentiation and are highly conserved. In order to deeply understand the morphological change, sex steroids level and molecular mechanism of triploid female gonadal reversal in rainbow trout, we studied the ovary morphology, tendency of estradiol-17β (E2) and testosterone (T) levels and the relative expressions of dmrt1, cyp19a1, sox9 and foxl2 in juvenile and adult fish. Our results demonstrated that the development of triploid female gonads in rainbow trout went through arrested development, oocytes dedifferentiation, ovary reconstruction and sex reversal finally. During early gonadal development (154-334 days post-fertilization), the expressions of foxl2 and cyp19a1 increased linearly, while expressions of dmrt1 and sox9 were extremely suppressed, and E2 level was higher, while T level was lower. During the mid-to-late period of triploid female gonadal development (574-964 days post-fertilization), the expressions of dmrt1 and sox9 remained high and were very close to the quantity of diploid male genes, and T levels were even reaching diploid male plasma concentrations, while expressions of cyp19a1 and foxl2 were decreased, leading to decrease in E2 level. We realized that the development model of rainbow trout triploid female gonads was extremely rare, and the regulatory mechanism was very special. Genes involved in gonadal development and endogenous estrogens are pivotal factors in fish natural sex reversal.

Molecular identification and expression of the Foxl2 gene during gonadal sex differentiation in northern snakehead Channa argus

Channa argus is one of the most commercially important fish species in China. Studies show that males of C. argus grow faster than females at the same age. In order to explore the sex differentiation mechanism of C. argus, we isolated the full length of the sex-related gene Foxl2 cDNA and analysed its expression patterns during gonadal sex differentiation. Alignment of known Foxl2 amino acid sequences from vertebrates confirmed the conservation of the Foxl2 open reading frame, especially the forkhead domain and C-terminal region. Quantitative RT-PCR revealed that Foxl2 is predominantly expressed in brain, pituitary, gill and ovary, with its highest level in ovary but low levels in testis and other tissues, reflecting a potential role for Foxl2 in the brain-pituitary-gonad axis in C. argus. Our ontogenetic stage data showed that C. argus Foxl2 expression was significantly upregulated from 1 to 11 days posthatching (dph) and that the initiation of expression preceded the first anatomical ovarian differentiation (27 dph), suggesting that Foxl2 might play a potential role in early gonadal sex differentiation in C. argus. In addition, the Foxl2 protein was primarily located in granulosa cells surrounding the oocytes of mature C. argus, implying that Foxl2 may have a basic function in granulosa cell differentiation and the maintenance of oocytes.

Effects of 17α-Methyltestosterone and Aromatase Inhibitor Letrozole on Sex Reversal, Gonadal Structure, and Growth in Yellow Catfish Pelteobagrus fulvidraco

Monosex populations are in demand in many fish species with sexual dimorphism, e.g., better growth performance, higher gonad value, superior ornamental value. From the point of view of research, a monosex population is one of the best materials for investigating sex-determining mechanisms, sex differentiation, and sex-linked markers. Sex reversal of females (phenotypic reversal from XX female to XX male) is the first step in all-female production in species with an XX/XY system for sex determination. In the present study, masculinization of yellow catfish, a species with XX/XY sex determination, was investigated by oral administration of various doses of 17α-methyltestosterone (MT) or an aromatase inhibitor (AI) letrozole (LZ); effects on survival, growth performance, sex ratio, and changes in gonadal structure were evaluated. Three doses (20, 50, and 100 mg kg(-1) diet) of oral MT or LZ were administered to fry from 10 days post-hatching (DPH) to 59 DPH. Oral administration of MT at all doses did not significantly change the ratio of males (45.8%, 33.3%, and 50.0% respectively) compared to the control group (37.5%), while yielding intersex fish at all doses (4.2% to 8.3%). Oral administration of LZ produced a significantly higher proportion of males in all doses (75.5%, 83.3%, and 75.0%, respectively). Additionally, the lowest dose of LZ improved the growth of treated fish compared to the control, and all doses of LZ enhanced spermatogenesis in treated males.

Molecular pathways associated with the intersex condition in rainbow darter (Etheostoma caeruleum) following exposures to municipal wastewater in the Grand River basin, ON, Canada. Part B

Rainbow darter (Etheostoma caeruleum; RBD) is a small benthic fish found in North America. This species is sensitive to sewage effluent, and intersex is found in up to 80% of males in near-field areas in the Grand River, Ontario, Canada. To learn more about the molecular signaling cascades associated with intersex, a developed customized oligonucleotide microarray (4×180 K) using next generation sequencing was developed to characterize the transcriptome in the gonad of male and female RBD. Gene expression profiling was performed in males and females from both a reference site and a polluted site. Males with and without intersex condition from the areas closest to effluent outfalls were compared to males and females from a reference site. Microarray analysis revealed that there was increased mRNA abundance for genes associated with oogenesis in intersex males (i.e. the presence of eggs within the testis), and a decrease in mRNA abundance for genes associated with spermatid development. In females exposed to effluent, cell processes related with hatching and ovulation were down-regulated, and genes involved in immune responses were increased in abundance. In the non-intersex males exposed to effluent, cell processes such as sperm cell adhesion were decreased at the transcript level relative to males from the reference site. Microarray analysis revealed that heat shock proteins (HSP) were significantly increased in non-intersex males exposed to effluent; however, HSPs were not differentially expressed in intersex males exposed to the effluent. Genes involved in sex differentiation (sox9, foxl2 and dmrt1) and reproduction (esr1, esrb, ar, vtg, cyp19a1 and cyp11a) were measured in males, females, and intersex individuals. Consistent with the intersex condition, many transcripts showed an intermediate expression level in intersex males when compared to phenotypic males and females. This study improves our knowledge regarding the molecular pathways that underlie the intersex condition and develops a suite of qPCR bioassays in RBD that are able to discriminate pollutant-exposed males without intersex from those males with intersex. Part A of this study reports on the effects of municipal wastewater effluents (MWWEs) on RBD in the Grand River and demonstrates that there are disruptions in higher level endpoints that include altered steroid levels. Here we develop a new tool for assessing and monitoring the intersex condition in RBD in polluted natural environments and begin to characterize gene networks that are associated with the condition.

Artificial masculinization in tilapia involves androgen receptor activation

Estrogens have a pivotal role in natural female sexual differentiation of tilapia while lack of steroids results in testicular development. Despite the fact that androgens do not participate in natural sex differentiation, synthetic androgens, mainly 17-α-methyltestosterone (MT) are effective in the production of all-male fish in aquaculture. The sex inversion potency of synthetic androgens may arise from their androgenic activity or else as inhibitors of aromatase activity. The current study is an attempt to differentiate between the two alleged activities in order to evaluate their contribution to the sex inversion process and aid the search for novel sex inversion agents. In the present study, MT inhibited aromatase activity, when applied in vitro as did the non-aromatizable androgen dihydrotestosterone (DHT). In comparison, exposure to fadrozole, a specific aromatase inhibitor, was considerably more effective. Androgenic activity of MT was evaluated by exposure of Sciaenochromis fryeri fry to the substance and testing for the appearance of blue color. Flutamide, an androgen antagonist, administered concomitantly with MT, reduced the appearance of the blue color and the sex inversion potency of MT in a dose-dependent manner. In tilapia, administration of MT, fadrozole or DHT resulted in efficient sex inversion while flutamide reduced the sex inversion potency of all three compounds. In the case of MT and DHT the decrease in sex inversion efficiency caused by flutamide is most likely due to the direct blocking of the androgen binding to its cognate receptor. The negative effect of flutamide on the efficiency of the fadrozole treatment may indicate that the masculinizing activity of fadrozole may be attributed to excess, un-aromatized, androgens accumulated in the differentiating gonad. The present study shows that when androgen receptors are blocked, there is a reduction in the efficiency of sex inversion treatments. Our results suggest that in contrast to natural sex differentiation, during sex inversion treatments, androgens, either endogenous or exogenous, participate in inducing testicular differentiation.

Differential expression analysis of genes involved in high-temperature induced sex differentiation in Nile tilapia

Nowadays, high temperature effects on the molecular pathways during sex differentiation in teleosts need to be deciphered. In this study, a systematic differential expression analysis of genes involved in high temperature-induced sex differentiation was done in the Nile tilapia gonad and brain. Our results showed that high temperature caused significant down-regulation of CYP19A1A in the gonad of both sexes in induction group, and FOXL2 in the ovary of the induction group. The expressions of GTHα, LHβ and ERα were also significantly down-regulated in the brain of both sexes in the induction and recovery groups. On the contrary, the expression of CYP11B2 was significantly up-regulated in the ovary, but not in the testis in both groups. Spearman rank correlation analysis showed that there are significant correlations between the expressions of CYP19A1A, FOXL2, or DMRT1 in the gonads and the expression of some genes in the brain. Another result in this study showed that high temperature up-regulated the expression level of DNMT1 in the testis of the induction group, and DNMT1 and DNMT3A in the female brain of both groups. The expression and correlation analysis of HSPs showed that high temperature action on tilapia HSPs might indirectly induce the expression changes of sex differentiation genes in the gonads. These findings provide new insights on TSD and suggest that sex differentiation related genes, heat shock proteins, and DNA methylation genes are new candidates for studying TSD in fish species.

Oestrogen and insulin-like growth factors during the reproduction and growth of the tilapia Oreochromis niloticus and their interactions

Oestrogens and insulin-like growth factors (Igfs) play both a central role in the regulation of reproduction and growth and can interact especially in species showing a clear-cut sex-linked growth dimorphism (SGD) like in tilapia. Aromatase is essential in ovarian differentiation and oogenesis since it controls oestrogen synthesis. During tilapia sex differentiation, aromatase cyp19a1a expression increases from 9 days post-fertilization (dpf), resulting in high oestradiol level. High temperature, exogenous androgens or aromatase inhibitors override genetic sex differentiation inducing testes development through the suppression of cyp19a1a gene expression and aromatase activity. Supplementation with 17ß-oestradiol (E2) of gonadectomized juveniles induced a sustained and higher E2 plasma level than in intact or gonadectomized controls and both sexes showed reduced growth. Juvenile and mature females treated with the aromatase inhibitor 1,4,6-androstatriene-3,17-dione had 19% lower E2 plasma level compared to controls and they showed a 32% increased growth after 28 days of treatment. Altogether, these data suggest that E2 inhibits female growth leading to the SGD. Regarding Igf-1, mRNA and peptide appeared in liver at ∼ 4 dpf and then in organs involved in growth and metabolism, indicating a role in early growth, metabolism and organogenesis. Gonad igf-1 showed an early expression and the peptide could be detected at ∼ 7 dpf in somatic cells. It appeared in germ cells at the onset of ovarian (29 dpf) and testicular (52 dpf) meiosis. In testis, Igf-1 together with steroids may regulate spermatogenesis whereas in ovary it participates in steroidogenesis regulation. Igf-1 and Igf-2 promote proliferation of follicular cells and oocyte maturation. Igf-3 expression is gonad specific and localized in the ovarian granulosa or testicular interstitial cells. In developing gonads igf-3 is up-regulated in males but down-regulated in females. In contrast, bream Gh injections increased igf-1 mRNA in male and female liver and ovaries but gonadal igf-3 was not affected. Thus, local Igf-1 and Igf-2 may play crucial roles in the formation, development and function of gonads while Igf-3 depending on the species is involved in male and female reproduction. Furthermore, precocious ethynylestradiol (EE) exposure induced lasting effects on growth, through pituitary gh inhibition, local suppression of igf-1 expression and in testis only down-regulation of igf-3 mRNA. In conclusion, SGD in tilapia may be driven through an inhibitory effect due to E2 synthesis in female and involving Igfs regulation.