Cloning of brain aromatase gene and expression of brain and ovarian aromatase genes during sexual differentiation in genetic male and female Nile tilapia Oreochromis niloticus

Triclosan, an antimicrobial drug, induced reproductive impairment in the freshwater fish, Anabas testudineus (Bloch, 1792)

Triclosan (TCS), an antimicrobial drug, is known to occupy different compartments in aquatic ecosystems. The present study focused to evaluate the reproductive toxicity of triclosan, at environmentally relevant (0.009 and 9 μg L-1) and sublethal (176.7 μg L-1) concentrations for 90 days in the pre-spawning phase of the fish, Anabas testudineus. The reproductive biomarkers, namely, gonadal steroidogenic enzymes, expression of aromatic genes, levels of serum gonadotropins, sex hormones, and histology of gonads were analyzed. The weight of the animal, brain weights along with gonadosomatic index decreased while mucus deposition increased significantly at all concentrations of triclosan as the primary defensive mechanism to prevent the entry of toxicants. Triclosan disrupted gonadal steroidogenesis as evidenced by a reduction in the activities of gonadal steroidogenic enzymes. The expressions of cyp19a1a and cyp19a1b genes were up-regulated in the brain of both sexes and testis, while down-regulated in the ovary indicating estrogenic effects of the compound. The endocrine-disrupting effects of triclosan were confirmed. The current results suggest that chronic exposure to triclosan altered reproductive endpoints thereby impairing normal reproductive functions in fish.

Cyp19a1a Promotes Ovarian Maturation through Regulating E2 Synthesis with Estrogen Receptor 2a in Pampus argenteus (Euphrasen, 1788)

In the artificial breeding of Pampus argenteus (Euphrasen, 1788), female fish spawn before male release sperm, which indicates rapid ovarian development. In fish, aromatase is responsible for converting androgens into estrogens and estrogen plays a crucial role in ovarian development. In this study, we aimed to investigate the potential role of brain-type and ovarian-type aromatase to study the rapid ovarian development mechanism. The results showed that cyp19a1a was mainly expressed in the ovary and could be classified as the ovarian type, whereas cyp19a1b could be considered as the brain type for its expression was mainly in the brain. During ovarian development, the expression of cyp19a1a in the ovary significantly increased from stage IV to stage V and Cyp19a1a signals were present in the follicle cells, while cyp19a1b expression in the pituitary gland decreased from stage IV to stage V. To further investigate the function of Cyp19a1a, recombinant Cyp19a1a (rCyp19a1a) was produced and specific anti-Cyp19a1a antiserum was obtained. The expressions of cyp19a1a, estrogen receptors 2 alpha (esr2a), and androgen receptor alpha (arα) were significantly upregulated in the presence of rCyp19a1a. Meanwhile, cyp19a1a was expressed significantly after E2 treatment in both ovarian and testicular tissue culture. Taken together, we found two forms of aromatase in silver pomfret. The ovarian-type aromatase might play an important role in ovarian differentiation and maturation, and participate in E2 synthesis through co-regulation with esr2a. The brain-type aromatase cyp19a1b might be involved in the regulation of both brain and gonadal development.

Cloning, computational analysis and expression profiling of steroid 5 alpha-reductase 1 (SRD5A1) gene during reproductive phases and ovatide stimulation in endangered catfish, Clarias magur

The cloning and characterization of the complete coding sequence of the Clarias magur SRD5A1 (CmSRD5A1) gene, which encodes an enzyme responsible for regulating steroid levels by converting testosterone into 5α-dihydrotestosterone (DHT), have been successfully achieved. DHT plays a vital role in enabling the complete expression of testosterone's actions in neuroendocrine tissues. The ORF of the full-length cDNA sequence of SRD5A1 was 795 bp, translating into 265 amino acids, with a total length of 836 bp including UTRs. Like other vertebrates, the signal peptide analysis revealed that SRD5A1 is a non-secretory protein, and hydropathy profiles indicated that it is hydrophobic in nature. The 3D structure of CmSRD5A1 sequence generated above was predicted using highly accurate AlphaFold 2 in Google Colab online platform. CmSRD5A1 contains seven transmembrane helices connected by six loops, with the N-termini located on the periplasmic side and C-termini on the cytosolic side. Structural superimposition with known bacterial and human SRD5As showed very high structural similarity. The electrostatic potential calculation and surface analysis of CmSRD5A1 revealed the presence of a large cavity with two openings one highly electropositive towards the cytosolic side and another relatively neutral towards the transmembrane region. The structural comparison revealed that the electropositive side of the cavity should bind to NADPH and the steroid hormone in the hydrophobic environment. Polar residues binding to NADPH are highly conserved and the same as known strictures. The conserved residues involved in hydrogen bonding with the ketone group at C-3 in the steroids hence fevering Δ4 double-bond reduction are identified as E66 and Y101. Our findings showed that SRD5A1 expression was lower during the spawning phase than the preparatory phase in female fish, while the administration of Ovatide (a GnRH analogue) resulted in up-regulation of expression after 6 h of injection in the ovary. In males, the lowest expression was observed during the preparatory phase and peaked at 16 h post- Ovatide injection in the testis. The expression of SRD5A1 in the brain of female fish was slightly higher during the Ovatide stimulation phase than the spawning phase. This study represents the first report on the cloning and characterization of the full-length cDNA of SRD5A1 in Indian catfish.

CYP19A1 May Influence Lambing Traits in Goats by Regulating the Biological Function of Granulosa Cells

Simple Summary

Aromatase (CYP19A1), a member of the cytochrome family, is widely expressed in ovarian and granulosa cells and is primarily responsible for the conversion of androgens to estrogens. Increased expression of CYP19A1 in follicular granulosa cells has implications for cell proliferation, steroid hormone secretion, and the expression of related functional indicator genes. We hypothesize that CYP19A1 may indirectly influence lambing numbers in goats by regulating follicular cell growth and development, as well as ovarian ovulation.

Abstract

Abnormal expression of CYP19A1, a gene related to steroid hormone synthesis, causes steroid hormone disruption and leads to abnormal ovulation in granulosa cells. However, the exact mechanism of CYP19A1 regulation is unclear. In this study, we confirmed the localization of CYP19A1 in goat ovarian tissues using immunohistochemistry. Subsequently, we investigated the effects of CYP19A1 on granulosa cell proliferation, steroid hormone secretion, and expression of candidate genes for multiparous traits by overexpressing and silencing CYP19A1 in goat granulosa cells (GCs). The immunohistochemistry results showed that CYP19A1 was expressed in all types of follicular, luteal, and granulosa cells, with subcellular localization results revealing that CYP19A1 protein was mainly localized in the cytoplasm and nucleus. Overexpression of CYP19A1 significantly increased the mRNA levels of CYP19A1, FSHR, and INHBA, which are candidate genes for multiple birth traits in goats. It also promoted cell proliferation, PCNA and Cyclin E mRNA levels in granulosa cells, and secretion of estrogen and progesterone. However, it inhibited the mRNA levels of STAR, CYP11A1, and 3βSHD, which are genes related to steroid synthesis. Silencing CYP19A1 expression significantly reduced CYP19A1, FSHR, and INHBA mRNA levels in granulosa cells and inhibited granulosa cell proliferation and PCNA and Cyclin E mRNA levels. It also reduced estrogen and progesterone secretion but enhanced the mRNA levels of STAR, CYP11A1, and 3βSHD. CYP19A1 potentially influenced the lambing traits in goats by affecting granulosa cell proliferation, hormone secretion, and expression of candidate genes associated with traits for multiple births.

Developmental changes in gene expression and gonad morphology during sex differentiation in Atlantic salmon (Salmo salar)

The Atlantic salmon (Salmo salar) is a globally important species for its value in fisheries and aquaculture, and as a research model. In order to characterise aspects of sex differentiation at the morphological and mRNA level in this species, the present study examined developmental changes in gonad morphology and gene expression in males and females between 0 and 79 days post hatch (dph). Morphological differentiation of the ovary (indicated by the formation of germ cell cysts) became apparent from 52 dph. By 79 dph, ovarian phenotype was evident in 100% of genotypic females. Testes remained in an undifferentiated-like state throughout the experiment, containing germ cells dispersed singularly within the gonadal region distal to the mesentery. There were no significant sex-related differences in gonad cross-section size, germ cell number or germ cell diameter during the experiment. The expression of genes involved in teleost sex differentiation (anti-müllerian hormone (amh), cytochrome P450, family 19, subfamily A, polypeptide 1a (cyp19a1a), forkhead box L2a (foxl2a), gonadal soma-derived factor (gsdf), r-spondin 1 (rspo1), sexually dimorphic on the Y chromosome (sdY)), retinoic acid-signalling (aldehyde dehydrogenase 1a2 (aldh1a2), cytochrome P450 family 26 a1 (cyp26a1), cytochrome P450 family 26 b1 (cyp26b1), t-box transcription factor 1 (tbx1a)) and neuroestrogen production (cytochrome P450, family 19, subfamily A, polypeptide 1b (cyp19a1b)) was investigated. Significant sex-related differences were observed only for the expression of amh, cyp19a1a, gsdf and sdY. In males, amh, gsdf and sdY were upregulated from 34, 59 and 44 dph respectively. In females, cyp19a1a was upregulated from 66 dph. Independent of sex, foxl2a expression was highest at 0 dph and had reduced ∼ 47-fold by the time of morphological sex differentiation at 52 dph. This study provides new insights into the timing and sequence of some physiological changes associated with sex differentiation in Atlantic salmon. These findings also reveal that some aspects of the mRNA sex differentiation pathways in Atlantic salmon are unique compared to other teleost fishes, including other salmonids.

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.

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.

Epigenetic regulation of gonadal and brain aromatase expression in a cichlid fish with environmental sex determination

A fit animal must develop testes or ovaries, with brain and physiology to match. In species with alternative male morphs this coordination of development across tissues operates within sexes as well as between. For Pelvicachromis pulcher, an African cichlid in which early pH exposure influences both sex and alternative male morph, we sequence both copies of aromatase (cyp19a1), a key gene for sex determination. We analyze gene expression and epigenetic state, comparing gonad and brain tissue from females, alternative male morphs, and fry. Relative to brain, we find elevated expression of the A-copy in the ovaries but not testes. Methylation analysis suggests strong epigenetic regulation, with one region specifying sex and another specifying tissue. We find elevated brain expression of the B-copy with no sex or male morph differences. B-copy methylation follows that of the A-copy rather than corresponding to B-copy expression. In 30-day old fry, we see elevated B-copy expression in the head, but we do not see the expected elevated A-copy expression in the trunk that would reflect ovarian development. Interestingly, the A-copy epialleles that distinguish ovaries from testes are among the most explanatory patterns for variation among fry, suggesting epigenetic marking of sex prior to differentiation and thus laying the groundwork for mechanistic studies of epigenetic regulation of sex and morph differentiation.

Pseudobranch mimics gill in expressing Na+K+-ATPase 1 α-subunit and carbonic anhydrase in concert with H+-ATPase in adult hilsa (Tenualosa ilisha) during river migration

In hilsa (Tenualosa ilisha), pseudobranch comprises a row of parallel filaments bear numerous leaf-like lamellae arranged on both sides throughout its length. The purpose of this study was to elucidate involvement of pseudobranchial Na⁺, K⁺-ATPase (NKA) 1 α-subunit, and carbonic anhydrase (CA) in concert with H⁺-ATPase (HAT) compared to their branchial counterparts in freshwater acclimation of hilsa during spawning migration from off-shore of the Bay of Bengal to the Bhagirathi-Hooghly zones of the Ganga river system in India. Adult hilsa fish were collected from seawater (SW), freshwater 1 (FW1), and freshwater 2 (FW2) locations, where the salinity level was 26-28‰, 1-5‰, and 0-0.04‰, respectively. Hilsa migrating through freshwater showed a consistent decrease in the plasma osmolality, sodium (Na⁺) and chloride (Cl^-) ion levels indicates unstable ionic homeostasis. The mRNA expression and activity of NKA 1 α-subunit in pseudobranch as well as in true gills declined with the migration to upstream locations. The pseudobranchial CA activity almost mirrors its branchial counterpart most notably while hilsa entered the freshwater zone, in the upstream river suggesting its diverse role in hypo-osmotic regulatory acclimation. Nevertheless, the H⁺-ATPase activity of both the tissues increased with the freshwater entry and remained similar during up-river movement into the freshwater environment. The results confirm that the pseudobranchial NKA 1 α-subunit mRNA expression and activity mimic its branchial counterpart in the process of ionoregulatory acclimation during migration through salt barriers. Also, the increase in the activities of pseudobranchial and branchial CA in concert with H⁺-ATPase (HAT) during freshwater acclimation of hilsa suggests their critical involvement in ion uptake.

An estradiol-17β/miRNA-26a/cyp19a1a regulatory feedback loop in the protogynous hermaphroditic fish, Epinephelus coioides

Cyp19a1a is a key gene responsible for the production of estradiol-17β (E₂), the main functional estrogen and a major downstream regulator of reproduction in teleost fish. It is widely known that CYP19 gene expression, aromatase activity, and E₂ production can influence gonadal differentiation and sex reversal in teleost fish, but the feedback mechanisms whereby E₂ regulates cyp19a1a remain poorly understood, especially regarding the potential roles of endogenous small RNA molecules (miRNAs). Here, we identified miR-26a-5p as a regulatory factor of its predicted target gene (cyp19a1a). In vitro and in vivo studies showed that miR-26a-5p can decrease cyp19a1a expression. Furthermore, high doses of E₂ act as a repressor of miR-26a-5p. This study proposes a regulatory feedback loop whereby E₂ regulates cyp19a1a through miR-26a-5p, and suggests that this positive feedback is an important aspect of the control of E₂ production.

Novel Sex Chromosomes in 3 Cichlid Fishes from Lake Tanganyika

African cichlids are well known for their adaptive radiations, but it is now apparent that they also harbor an extraordinary diversity of sex chromosome systems. In this study, we sequenced pools of males and females from species in 3 different genera of cichlids from Lake Tanganyika. We then searched for regions that were differentiated following the patterns expected for sex chromosomes. We report 2 novel sex chromosomes systems, an XY system on LG19 in Tropheus sp. "black" and a ZW system on LG7 in Hemibates stenosoma. We also identify a ZW system on LG5 in Cyprichromis leptosoma that may be convergent with a system previously described in Lake Malawi cichlids. Our data also identify candidate single nucleotide polymorphisms for the blue/yellow tail color polymorphism observed among male C. leptosoma.

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.

New Insights Into the Role of Follicle-Stimulating Hormone in Sex Differentiation of the Protogynous Orange-Spotted Grouper, Epinephelus coioides

Follicle-stimulating hormone (FSH) signaling is considered to be essential for early gametogenesis in teleosts, but its functional roles during sex differentiation are largely unknown. In this study, we investigated the effects of long-term and short-term FSH injection on sex differentiation in the protogynous orange-spotted grouper (Epinephelus coioides). Long-term FSH treatment initially promoted the formation of ovaries but subsequently induced a male fate. The expression of female pathway genes was initially increased but then decreased, whereas the expression of male pathway genes was up-regulated only during long-term FSH treatment. The genes related to the synthesis of sex steroid hormones, as well as serum 11-ketotestosterone and estradiol, were also up-regulated during long-term FSH treatment. Short-term FSH treatment activated genes in the female pathway (especially cyp19a1a) at low doses but caused inhibition at high doses. Genes in the male pathway were up-regulated by high concentrations of FSH over the short term. Finally, we found that low, but not high, concentrations of FSH treatment activated cyp19a1a promoter activities in human embryonic kidney (HEK) 293 cells. Overall, our data suggested that FSH may induce ovarian differentiation or a change to a male sex fate in the protogynous orange-spotted grouper, and that these processes occurred in an FSH concentration-dependent manner.

Social factors and aromatase gene expression during adult male-to-female sex change in captive leopard grouper Mycteroperca rosacea

Social factors and aromatase gene expression in the leopard grouper Mycteroperca rosacea was studied when captive fish were separated by sex during the reproductive (April-June) and post-reproductive (July-September) seasons. Monosex females, monosex males, and mixed-sex, held in social sextet units were analyzed for sex steroids throughout confinement. At the end of the experiment, the gonad-sex was defined by histology, and gonad and brain aromatase gene expressions were quantified. Only males held in the monosex social units changed sex. Histology showed one male remained unchanged, six were found in a transitional sexual stage, in which two had intersex-predominantly-testes, and four had a more defined intersex ovo-testes pattern, and 11 were immature de novo females (neofemales). Neofemales and most intersex fish did not survive. In spring, 11-ketosterone showed a specific male profile, which suggests that male-to-female sex change was not triggered during the reproductive season. The low steroid levels in summer made it impossible to associate the sex change to a gonad hormonal shift; in September, gonad aromatase gene expression was not significantly different among groups. However, brain aromatase expression in intersex fish was significantly higher than monosex females, mixed-sex females, and neofemale groups. These results suggest that in the absence of female hormonal compounds, and at a time when male gonad steroidogenesis was diminished, the brain mediated male-to-male social-behavioral interactions, including stress, by increasing aromatization, resulting in derived intersex-male, which triggered more aromatization, followed by a sex change.

Rapid sex steroid effects on reproductive responses in male goldfish: Sensory and motor mechanisms

Contribution to Special Issue on Fast effects of steroids. Although we have learned a great deal about the molecular mechanisms through which sex steroids rapidly affect cellular physiology, we still know little about the links between those mechanisms and behavioral output, nor about their functional consequences in natural contexts. In this review, we first briefly discuss the contexts associated with rapid effects of sex steroids on reproductive behaviors and their likely functional outcomes, as well the sensory, motor, and motivational mechanisms associated with those effects. We then discuss our recent studies on the rapid effects of testosterone in goldfish. Those studies indicate that testosterone, through its aromatization and the subsequent activation of estrogen receptors, rapidly stimulates physiological processes related to the release of milt/sperm through likely influences on motor pathways, as well as behavioral responses to female visual stimuli that may reflect, in part, influences on early stages of sensory processing. Such motor and sensory mechanism are likely important for sperm competition and mate detection / tracking, respectively, in competitive mating contexts. We also present preliminary data on rapid effects of testosterone on responses to pheromones that may not involve estrogen receptors, suggesting a dissociation in the receptor mechanisms that mediate behavioral responses in different sensory modalities. Lastly, we briefly discuss the implications of our work on unresolved questions about rapid sex steroid neuromodulation in fish.

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.

Gene Expression of Aromatases, Steroid Receptor, GnRH and GTHs in the Brain during the Formation of Ovarian Cavity in Red Spotted Grouper, Epinephelus akaara

Red spotted grouper, Epinephelus akaara, is a popular aquaculture species and a protogynous hermaphrodite. Induction of artificial sex change at the time of primary sex differentiation is of interest but has not been successful due to the lack of necessary basic information. To find out the potential neuroendocrine influence on the primary sex differentiation, the expression of key genes in the brain was investigated during the formation of ovarian cavity. Expression of cyp19a1b, esr1, gnrhr1, fsh, lh and cga in the brain was positively associated with the formation of ovarian cavity, showing gradual increase as the formation proceeds. However, the expression of gnrh1 was suppressed during the early part of the ovarian cavity formation, signifying potential hypothalamic influence on the primary sex differentiation in this species.

Expression Patterns of CREBs in Oocyte Growth and Maturation of Fish

In fish, oocyte meiotic maturation is regulated by 17α, 20β-dihydroxy-progesterone through cAMP. To study the role of cAMP response element binding protein (CREB) in meiotic maturation, we cloned and characterized the expression pattern of CREBs from two fish models, the Nile tilapia and catfish. In the Nile tilapia three different CREBs were identified where in CREB1 was found in many tissues including gonads with abundant expression in testis. CREB2, few amino acids shorter than CREB1, was expressed in several tissues with abundant expression in ovary. In addition, a 3’UTR variant form, CREB3 was exclusively found in ovary. During natural 14-day ovarian cycle of the Nile tilapia, CREB1 expression was stable throughout vitellogenesis with a sharp decrease on the day of spawning. In contrast, CREB2 remain unchanged throughout the ovarian cycle, however elevated in 11-day full-grown immature ovarian follicle and after hCG-induction. Interestingly, CREB3 expression was induced three folds on the day of spawning as well as during hCG-induced oocyte maturation. Based on the synergistic expression pattern, CREB1 is likely to control oocyte growth, whereas CREB 2 and 3 contribute to oocyte maturation in tilapia and the latter seems to be critical. In catfish, a single form of CREB showed a maximum expression during spawning phase and hCG-induced maturation both in vivo and in vitro augmented CREB expression. These results suggest that spatial and temporal expression of CREBs seems to be important for final oocyte maturation and may also regulate oocyte growth in fish.

Molecular cloning and characterization of brain and ovarian cytochrome P450 aromatase genes in the catfish Heteropneustes fossilis: Sex, tissue and seasonal variation in, and effects of gonadotropin on gene expression

Cytochrome P450 aromatase (Cyp19arom) is the rate-limiting enzyme controlling estrogen biosynthesis, coded by Cyp19a1 in most gnathostomes. Most teleosts have two forms expressed differentially in ovary (cyp19a1a) and neural tissue (cyp19a1b). In this study, full length cDNAs of 2006 bp and 1913 bp with ORFs of 1575 bp and 1488 bp were isolated from the brain and ovary, respectively, of the catfish Heteropneustes fossilis, an air-breathing species with high aquaculture potential. The ORFs encode predicted proteins of 495 and 524 amino acid residues, respectively. The proteins show 62% identity with each other and cluster in two distinct clades (the brain type and ovary type) in the teleost taxon, separated from the tetrapod type. In the in situ localization study, both cyp19a1a and cyp19a1b transcripts were localized in the brain but the signal intensity was higher for the brain type paralog. The transcript signals were observed in the radial glial cells and in neuronal populations of the dorso-lateral region of the telencephalon, pre-tectum, hypothalamus and medulla oblongata. In the ovary, both paralogs were expressed in the follicular layer with a high signal intensity of the ovarian type (cyp19a1a). The differential expression of the gene paralogs was evident from qPCR analysis. Cyp19a1b has relatively a high abundance in the female brain, followed by other peripheral tissues (gonads, liver, gill, kidney and muscle). On the other hand, cyp19a1a has relatively a high transcript abundance in the ovary and female brain, followed by the testis and male brain, and female liver and muscle. The expression was low in male liver and muscle, and the lowest in the gill and kidney. The expression of the two paralogs exhibit brain regional differences; both types have relatively a high transcript abundance in telencephalon-preoptic area with the cyp19a1b expression higher in females than males. In hypothalamus, the expression of both types is higher in males than females. In medulla, the expression of the cyp19a1b is higher than cyp19a1a, and the transcript abundance of the ovarian type is higher in females than males. The expression of the gene paralogs elicits significant seasonal variations in the ovary and brain. In both tissues, the expression increases from the resting to preparatory phases, and decreases through the prespawning phase to low levels in spawning phase. In vivo and/or in vitro treatments with human chorionic gonadotropin (hCG) stimulated the expression of the gene paralogs in the brain and ovary, time-dependently. In conclusion, both paralogs have an overlapping distribution at different levels of the brain-pituitary-gonad axis and may function as a single functional unit as far as the estrogen synthesis is concerned.

Localization of steroidogenic enzymes and Foxl2a in the gonads of mature zebrafish (Danio rerio)

In zebrafish, the identification of the cells expressing steroidogenic enzymes and their regulators is far from completely fulfilled though it could provide crucial information on the elucidation of the role of these enzymes. The aim of this study was to better characterize the expression pattern of steroidogenic enzymes involved in estrogen and androgen production (Cyp17-I, Cyp11c1, Cyp19a1a and Cyp19a1b) and one of their regulators (Foxl2a) in zebrafish gonads. By using immunohistochemistry, we localized the steroid-producing cells in mature zebrafish gonads and determined different expression patterns between males and females. All these steroidogenic enzymes and Foxl2a were detected both in the testis and ovary. In the testis, they were all localized both in Leydig and germ cells except Cyp19a1b which was only detected in germ cells. In the ovary, Cyp17-I, Cyp19a1a and Foxl2a were immunolocalized in both somatic and germ cells while Cyp19a1b was only detected in germ cells and Cyp11c1 in somatic cells. Moreover, Cyp19a1a and Foxl2a did not display exactly the same patterns of spatial localization but their expressions were correlated suggesting a possible regulation of cyp19a1a gene by Foxl2a in zebrafish. Comparative analysis revealed a dimorphic expression of Cyp11c1, Cyp19a1a, Cyp19a1b and Foxl2a between males and females. Overall, our study provides a detailed description of the expression of proteins involved in the biosynthesis of steroidal hormones at the cellular scale within gonads, which is critical to further elucidating the intimate roles of the enzymes and the use of the zebrafish as a model in the field of endocrinology.

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.

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.

The regulation of aromatase and androgen receptor expression during gonad development in male and female European eel

This research investigated the regulation of aromatase and androgen receptor gene expression in the brain-pituitary-gonad (BPG) axis of male and female European eels (Anguilla anguilla) during induced sexual maturation. Complete A. anguilla aromatase (aa-cyp19a1) and partial androgen receptor α and β (aa-ara and aa-arb) sequences were isolated, and qPCR assays were validated and used for quantification of transcript levels for these three genes. Expression levels of the genes varied with sex, tissue and stage of maturation. aa-arb was expressed at higher levels than aa-ara in the pituitary and gonad in both sexes, suggesting aa-arb is the physiologically most important androgen receptor in these tissues. In the female brain, a decrease in aa-ara and an increase in aa-cyp19a1 were observed at the vitellogenic stage. In contrast, a progressive increase in all three genes was observed in the pituitary and ovaries throughout gonadal development, with aa-arb and aa-cyp19a1 reaching significantly higher levels at the vitellogenic stage. In the male pituitary, a decrease in aa-arb and an increase in aa-cyp19a1 were observed at the beginning of spermatogenesis, and thereafter remained low and high, respectively. In the testis, the transcript levels of androgen receptors and aa-cyp19a1 were higher during the early stages of spermatogenesis and decreased thereafter. These sex-dependent differences in the regulation of the expression of aa-ara, aa-arb and cyp19a1 are discussed in relation to the role of androgens and their potential aromatization in the European eel during gonadal maturation.

Temperature induced-masculinisation in the Nile tilapia causes rapid up-regulation of both dmrt1 and amh expressions

Nile tilapia has primarily a XX/XY sex determining system but minor genetic factors as well as temperature can override the major factors. Female XX progenies can be sex-reversed into functional males by rearing at high temperatures (>34°C) from 10dpf onwards. Temperature effects on the molecular pathways during sex differentiation in teleosts need to be deciphered. The temporal expression profiles of cyp19a1a and foxl2, two ovarian-developmental genes and dmrt1 and amh, two testes-developmental genes were analysed during key stages of the sex differentiation of genetic all-females, all-males and temperature-masculinised XX females (TM) tilapia. Overall QPCR analysis was similar between gonads and trunks. Both amh and dmrt1 expressions were up-regulated simultaneously in TM already at 13-15dpf. Dmrt1 expression became markedly elevated ∼3-fold higher than XY male levels at 20-26dpf whereas amh had similar levels to XY males. Foxl2 and cyp19a1a expression profiles were similar. Both were up-regulated at early stages in TM but repressed after 17-19dpf, whilest levels continued to increase in XX-females. Our results show that temperature action on tilapia testis development induces the rapid increase of both dmrt1 and amh expressions followed by the down-regulation of foxl2 and cyp19a1a. This suggests that dmrt1 and/or amh may be the modulator(s) of the down-regulation of foxl2 and/or cyp19a1a.

Zebrafish embryos/larvae for rapid determination of effects on hypothalamic-pituitary-thyroid (HPT) and hypothalamic-pituitary-interrenal (HPI) axis: mRNA expression

To identify and prioritize chemicals that may affect thyroid and adrenal/interregnal endocrine system and to reduce cost and animal use by conventional toxicity assay, an in vivo screening assay was developed using zebrafish embryos/larvae based on measurement of expression of genes that were suggested to play important roles in hypothalamic-pituitary-thyroid (HPT) and hypothalamic-pituitary-interrenal (HPI) axis. Model chemicals that could modulate HPT and HPI axis in adult fish were selected in assay validation, including anti-thyroid agent 6-Propyl-2-thiouracil (PTU) and cytochrome P450 11B (Cyp11b) enzyme inhibitor metyrapone (MET). Zebrafish embryos were exposed to different concentrations of model chemical from 4h post-fertilization (hpf) to 5d post-fertilization (dpf). Exposure to PTU increased mRNA expression of sodium iodide symporter (nis) and thyroglobulin (tg) involved in HPT axis, and MET treatment up-regulated all the mRNA expression tested involved in HPI axis by a compensatory mechanism. These results suggested that HPT and HPI axis were active upon chemical exposure at least at 5 dpf zebrafish. Furthermore, we studied the effects of PTU or MET on the cross-talk between HPT and HPI axis. The results demonstrated that PTU and MET could affect cross-talk responses in zebrafish embryos/larvae.

Estrogen oversees the maintenance of the female genetic program in terminally differentiated gonochorists

The sexual plasticity of the gonads is not retained after the completion of sex differentiation in vertebrates, except in some hermaphroditic species. Here, we report that the depletion of estradiol-17β (E2) by aromatase inhibitors (AI) for up to six months resulted in a functional female-to-male sex reversal in sexually-mature adults of two gonochoristic fish species, Nile tilapia and medaka. The sex-reversed fish showed a typical male pattern of E2 and androgen levels, secondary sexual characteristics, and male-like sex behavior, producing fertile sperm. Conversely, co-treatment of E2 inhibited AI-induced sex reversal. In situ hybridization of medaka gonads during AI-induced sex reversal indicated that cysts on the dorsal side of the adult ovaries are the origin of germ cells and Sertoli cells in the newly formed testicular tissue. Gonochoristic fish maintain their sexual plasticity until adulthood and E2 plays a critical role in maintaining the female phenotype.

Creating females? Developmental effects of 17α-ethynylestradiol on the mangrove rivulus' ovotestis

Interest in the occurrence and fate of trace organic contaminants in the aquatic environment and their potential effects on all organisms has increased over the past two decades. Researches on contaminants have included both natural and synthetic estrogenic contaminants, neuroactive pharmaceuticals, and other endocrine disrupting chemicals that are mediated by the androgen and progesterone receptors. Exposure to very low concentrations (ng/L or parts per trillion) of compounds such as 17α-ethynylestradiol (EE(2)), a synthetic estrogen, can affect gonadal development, viability and production of eggs, fertilization rate, and sexual differentiation in fishes. Researchers and aquaculturists have used exposures to relatively higher concentrations of androgens and estrogens, for example 17α-methyltestosterone and EE(2), respectively, to direct sexual differentiation in a number of fishes. Rivulus is an androdioecious teleost that in nature exists mostly as selfing, simultaneous hermaphrodites as well as a small number of males that outcross with hermaphrodites. No one has either collected females in the wild or created functional females in the laboratory. This study had two goals: (1) to develop a reliable protocol to produce female rivulus to enable downstream technologies such as embryo injections and (2) to investigate developmental effects of EE(2) on the sexual outcome, reproductive health, and relevant gene expression in rivulus. With these goals in mind, we exposed newly hatched rivulus to nominal concentrations of 0.1, 0.5, or 1.0 parts per million (ppm) EE(2) for 4 weeks, grew them to maturity in control water, and then compared egg production; production and viability of embryos; age of reproductive maturity; and gene expression in the brain, gonad, and liver. Expression levels of seven genes with known relevance to gonadal development and function (cyp19a1b, cyp19a1a, dmrt1, figα, ERα, ERβ, and vtg) were measured using quantitative polymerase chain reaction (PCR). There was a significant decrease in cyp19a1a gene expression in the brain, corresponding to increased exposure to EE(2). Gonadal gene expression for cyp19a1a, ERα, and dmrt1 also decreased in response to EE(2). Vtg expression in the liver was unaffected. Our hypothesis that exposure to EE(2) during gonadal differentiation would direct female development was not supported by the data. However, treated fish exhibited impaired reproductive health that included reduced expression of relevant genes and, importantly, decreased fertility, increased sterility, and delay of age of reproductive maturity. The results of this study suggest that the development and maintenance of a simultaneous hermphrodite ovotestis may be particularly sensitive to its hormonal milieu.

Sexually dimorphic and ontogenetic expression of dmrt1, cyp19a1a and cyp19a1b in Gobiocypris rarus

Fish have diverse sex determination and differentiation. DMRT1 and aromatase are conserved in the phyla and play pivotal roles in sex development. Gobiocypris rarus is a small fish used as a model in aquatic toxicology in China and has been used to study the effects of environmental endocrine disruptors on gene expression, but its sexual development remains elusive. Here, we report the full-length cDNA of G. rarus dmrt1 and its expression along with the expression of cyp19a1a and cyp19a1b, two genes encoding gonad and brain type aromatases, in adults and during ontogenesis. Both cyp19a1a and dmrt1 are expressed in the ovary and testis but show sexual dimorphism. Expression of cyp19a1a in the ovary is higher than in testes and dmrt1 follows the opposite pattern. Juvenile gonad histology changes at 15 days after hatching. The dimorphic expression of dmrt1 and cyp19a1a appears from 5 days after hatching, which is earlier than histological change. cyp19a1b is expressed coordinately with cyp19a1a until 15 days after hatching. These results show that dmrt1 and cyp19a1a play important roles in sex determination and sex differentiation in G. rarus.

The brain of teleost fish, a source, and a target of sexual steroids

Neurosteroids are defined as steroids de novo synthesized in the central nervous system. While the production of neurosteroids is well documented in mammals and amphibians, there is less information about teleosts, the largest group of fish. Teleosts have long been known for their high brain aromatase and 5α-reductase activities, but recent data now document the capacity of the fish brain to produce a large variety of sex steroids. This article aims at reviewing the available information regarding expression and/or activity of the main steroidogenic enzymes in the brain of fish. In addition, the distribution of estrogen, androgen, and progesterone nuclear receptors is documented in relation with the potential sites of production of neurosteroids. Interestingly, radial glial cells acting as neuronal progenitors, appear to be a potential source of neurosteroids, but also a target for centrally and/or peripherally produced steroids.

Cloning and differential expression of FOXL2 during ovarian development and recrudescence of the catfish, Clarias gariepinus

FOXL2 is a member of the forkhead/HNF-3-related family of transcription factors which provides tissue-specific gene regulation. It is known to regulate ovarian aromatase, (cyp19a1a) which plays a crucial role in ovarian differentiation. To understand the role of FOXL2 in gonads and brain during ovarian development and recrudescence, we cloned the full-length cDNA of FOXL2 and analyzed its spatio-temporal expression both at transcript and protein levels in the air-breathing catfish, Clarias gariepinus. Based on its deduced amino acid sequence, an antigenic peptide conjugated with a carrier protein was synthesized which was then used for raising antibody that reacted specifically with FOXL2. Tissue distribution pattern of FOXL2 revealed its presence prominently in ovary and female brain with sexual dimorphism. Highest expression of FOXL2 was observed in ovary and brain during prespawning phase indicating an important role for this correlate in ovarian recrudescence. Human chorionic gonadotropin (hCG) treatment, in vitro and in vivo, induced FOXL2 expression in the ovary during preparatory and prespawning phases. Similar type of enhanced expression was evident in brain after hCG-induction during the prespawning phase. The ontogeny of FOXL2 showed sexual dimorphic expression pattern both in gonads and brain. Based on our previous studies, the expression pattern of FOXL2 was found to be synchronous not only with that of ovarian cyp19a1a but also with brain cyp19a1b. Present study substantiates the role of FOXL2 in the regulation of aromatase in teleosts and also designates FOXL2 as a potential ovary and brain marker during female sex development in catfish.

FTZ-F1 and FOXL2 up-regulate catfish brain aromatase gene transcription by specific binding to the promoter motifs

Cytochrome P450 aromatase (cyp19) catalyzes the conversion of androgens into estrogens. Teleosts have distinct, ovarian specific (cyp19a1a) and brain specific (cyp19a1b) cyp19 genes. Previous studies in teleosts demonstrated regulation of cyp19a1a expression by the NR5A nuclear receptor subfamily as well as a fork head transcription factor, FOXL2. In the present study, we investigated the involvement of fushi tarazu factor 1, FTZ-F1, a NR5A subfamily member, and FOXL2 in the regulation of cyp19a1b expression in brain of the air-breathing catfish, Clarias gariepinus. Based on the synchronous expression pattern of cyp19a1b, FTZ-F1 and FOXL2 in the brain, we isolated the 5' upstream region of cyp19a1b to analyse regulatory motifs. Promoter motif analysis revealed FTZ-F1/NR5A1 and FOXL2 binding nucleotide sequences. Transient transfection studies showed that FTZ-F1 and FOXL2 together enhanced the transcriptional activity of cyp19a1b gene in mammalian cell lines. Mutation in either of their putative binding sites within the cyp19a1b promoter abolished this effect. Electrophoretic gel mobility shift experiments indicated that FTZ-F1 and FOXL2 proteins bind to the synthesized radio-labelled oligomers used as probes and mobility shifted upon addition of their respective antibodies. Chromatin immunoprecipitation assay confirmed the binding of both these transcription factors to their corresponding cis-acting elements in the upstream region of cyp19a1b. To our knowledge, this study is the first report on the transcriptional regulation of cyp19a1b by FTZ-F1 and FOXL2 in a teleost fish.

A review of studies on androgen and estrogen exposure in fish early life stages: effects on gene and hormonal control of sexual differentiation

Teleost fish are unique among vertebrates in that phenotypic sex or onset of sex inversion can be easily manipulated by hormonal treatments. In recent years, researchers have begun reporting concentrations of synthetic and natural hormones in the environment. Although concentrations are very low (in the parts per trillion to low parts per billion), they are still of concern because of the high potency of synthetic hormones and the enhanced susceptibility of teleost fishes, especially early life stages, to hormonal exposures. In this review, we will focus on sex differentiation in teleost fishes and how these processes in fish early life stages may be impacted by environmental hormones which are known to contaminate aquatic environments. We will start by reviewing information on sources and concentrations of hormones in the environment and continue by summarizing the state of knowledge of sex differentiation in teleost gonochoristic fishes, including information on genes involved (e.g. cyp19, dmrt1, sox9 and foxl2). We will end our review with a summary of studies that have examined the effects of androgens and estrogens on fish sex differentiation after exposure of fish embryos and larvae and with ideas for future research.

Quantitative trait loci involved in sex determination and body growth in the gilthead sea bream (Sparus aurata L.) through targeted genome scan

Among vertebrates, teleost fish exhibit a considerably wide range of sex determination patterns that may be influenced by extrinsic parameters. However even for model fish species like the zebrafish Danio rerio the precise mechanisms involved in primary sex determination have not been studied extensively. The zebrafish, a gonochoristic species, is lacking discernible sex chromosomes and the sex of juvenile fish is difficult to determine. Sequential protandrous hermaphrodite species provide distinct determination of the gender and allow studying the sex determination process by looking at the mechanism of sex reversal. This is the first attempt to understand the genetic basis of phenotypic variation for sex determination and body weight in a sequential protandrous hermaphrodite species, the gilthead sea bream (Sparus aurata). This work demonstrates a fast and efficient strategy for Quantitative Trait Loci (QTL) detection in the gilthead sea bream, a non-model but target hermaphrodite fish species. Therefore a comparative mapping approach was performed to query syntenies against two other Perciformes, the European sea bass (Dicentrarchus labrax), a gonochoristic species and the Asian sea bass (Lates calcarifer) a protandrous hermaphrodite. In this manner two significant QTLs, one QTL affecting both body weight and sex and one QTL affecting sex, were detected on the same linkage group. The co-segregation of the two QTLs provides a genomic base to the observed genetic correlation between these two traits in sea bream as well as in other teleosts. The identification of QTLs linked to sex reversal and growth, will contribute significantly to a better understanding of the complex nature of sex determination in S. aurata where most individuals reverse to the female sex at the age of two years through development and maturation of the ovarian portion of the gonad and regression of the testicular area. [Genomic sequences reported in this manuscript have been submitted to GenBank under accession numbers HQ021443-HQ021749.].

Elevated amh gene expression in the brain of male tilapia (Oreochromis niloticus) during testis differentiation

Anti-müllerian hormone (AMH) is expressed in male embryos and represses development of müllerian ducts during testis differentiation in mammals, birds and reptiles. Amh orthologues have been identified in teleosts despite them lacking müllerian ducts. Previously we found sexually dimorphic aromatase activity in tilapia brains before ovarian differentiation. This prompted us to search for further dimorphisms in tilapia brains during sex differentiation and see whether amh is expressed. We cloned the tilapia amh gene and found that it contains 7 exons but no spliced forms. The putative protein presents highest homologies with Amh proteins of pejerrey and medaka as compared to other Perciformes. We analysed amh expression in adult tissues and found elevated levels in testes, ovary and brain. Amh expression was dimorphic with higher levels in XY male brains at 10-15 dpf, when the gonads were still undifferentiated and gonadal amh was not dimorphic. Male brains had 2.7-fold higher amh expression than gonads. Thereafter, amh levels decreased in the brain while they were up-regulated in differentiating testes. Our study indicates that amh is transcribed in male brains already at 10 dpf, suggesting that sexual differentiation may be occurring earlier in tilapia brain than in gonads.

Developmental expression of steroidogenic factor-1, cyp19a1a and cyp19a1b from common carp (Cyprinus carpio)

Steroidogenic factor-1 (SF-1), cyp19a1a and cyp19a1b play pivotal roles in vertebrate steroidogenesis and reproduction. In this study, a SF-1 cDNA (EU022463) was cloned from common carp (Cyprinus carpio). The transcript contains a 1509 base pair (bp) open reading frame (ORF) encoding a 503 amino acid sequence. Comparisons of deduced amino acid sequences demonstrated that carp SF-1 is highly homologous with those of other vertebrates. Tissue specific expressions of SF-1, cyp19a1a and cyp19a1b mRNA were analyzed in 10-month-old carp. SF-1 was abundant in the hypothalamus, pituitary, gonad, spleen and liver (females only). Cyp19a1b was preferentially expressed in the brain of both sexes but also was present at much lower levels in testis, ovary and kidney (females only). Although cyp19a1a expression was preferentially expressed in ovaries, it was also present at much lower levels in brain, testis, kidney and spleen (males only). Northern blot analysis revealed that testes and brains of both sexes expressed a transcript of about 2.8 kb in size. The expression pattern of SF-1, cyp19a1a and cyp19a1b in carp gonads suggested their involvement in sexual development. In 3-month-old carp, SF-1 and cyp19a1b were expressed highly in testes but were at much lower levels in ovaries, while the opposite pattern was observed with cyp19a1a expression. In 10-month-old carp, SF-1 expression was much higher in testes than in ovaries, while the opposite pattern was observed with cyp19a1a expression. These developmental expression patterns in carp gonads suggest important roles of SF-1 and cyp19a1b in testis development and of cyp19a1a in ovary development.

Isolation of sox9 duplicates in catfish: localization, differential expression pattern during gonadal development and recrudescence, and hCG-induced up-regulation of sox9 in testicular slices

In vertebrates, sox9 is a transcription factor that plays a crucial role in testicular development and chondrogenesis. Here, we report cloning of isoforms of sox9 (sox9a and sox9b) from air-breathing catfish Clarias gariepinus, which undergoes an annual reproductive cycle. Tissue distribution pattern showed differential expression of sox9 duplicates, wherein both forms were highly expressed in brain and gonads. Furthermore, we observed a dimorphic expression pattern of sox9a and sox9b in both adult and developing gonads using RT-PCR, indicating that sox9a retained its function in testis while sox9b might have a new role to play in ovary. Changes in sox9 mRNA levels using real-time quantitative PCR (qRT-PCR) during the seasonal reproductive cycle revealed that sox9a transcript in testis was abundant during testicular recrudescence (during spermatogenesis), and its expression significantly decreased during spawning and post-spawning phases. Furthermore, treatments of human chorionic gonadotropin and 11-ketotestosterone in vitro up-regulated sox9a mRNA levels in the testicular slices at 12 and 24 h time points, suggesting that gonadotropins might stimulate sox9 expression. These results suggest that sox9 might have a plausible role in the entrainment of the testicular cycle. In contrast, during the ovarian cycle, sox9b mRNA levels gradually declined from preparatory to post-spawning phases. Immunohistochemical (IHC) data showed that, in testis, sox9 is detectable in Sertoli and spermatogonial cell types except spermatid/spermatozoa. In the ovary, it is localized in the ooplasm of primary and pre-vitellogenic oocytes. These results were further confirmed by whole-mount IHC and qRT-PCR.

Cytochrome P450 aromatases: Impact on gonadal development, recrudescence and effect of hCG in the catfish, Clarias gariepinus

Present study analyzed the importance of two forms of aromatases during ovarian development and recrudescence of north African/air-breathing catfish. We cloned both CYP19A1 (1941bp; ovarian form) and CYP19A2 (1786bp; brain form), which showed 47% homology between the two forms. Characterization of encoded proteins in non-steroidogenic COS-7 cells illustrated that both isoforms efficiently catalyzed the aromatization reaction by producing estradiol-17beta (E(2)) from testosterone. Tissue distribution pattern revealed preferential expression of CYP19A2 in brain while CYP19A1 predominated in ovary with trace amounts detected in other tissues including brain. Relative real-time PCR analysis revealed high transcript levels of both isoforms in the prespawning phase of ovarian cycle, which is in accordance with serum E(2) level. Aromatase activity in brain was comparatively lower than ovary, indicating the predominant requirement of aromatase in ovary. Ontogeny studies displayed sexual dimorphism, with early expression of CYP19A1 and CYP19A2 in ovary and brain, respectively. Phase-dependent rise of expression and enzyme activity of aromatase after hCG treatment revealed the stimulatory role of gonadotropin during preparatory and prespawning phases, preferentially to promote vitellogenesis. Lack of influence of hCG treatment during spawning phase endorses it further. A good correlation of expression, enzyme activity and serum E(2) levels suggests a crucial role of CYP19A1 during ovarian differentiation and ovarian cycle of catfish. Likewise, CYP19A2 might also be involved in these processes either indirectly or directly.

Cloning and differential expression pattern of vasa in the developing and recrudescing gonads of catfish, Clarias gariepinus

Vasa gene codes for a DEAD box family protein, which plays a crucial role in primordial germ cell proliferation. In this study, we report cloning of vasa from gonads of air-breathing catfish, Clarias gariepinus, a seasonally reproducing teleost fish. We studied the expression pattern of vasa during gametogenesis using real-time PCR. We also examined the hormonal regulation on vasa in gonads of catfish. RT-PCR analysis revealed that vasa was detectable only in the gonads. Further, real-time PCR results showed that expression of vasa was seen throughout the development from embryonic stage to adult. However, the expression was more in ovary than in testis during gonadal development. In adult testis, the vasa transcripts were significantly high during spermatogenesis and it declined during spermiation. On the other hand, during ovarian recrudescence, vasa transcripts were high in immature oocytes (stages I and II oocytes) when compared to mature oocytes (stages III and IV oocytes). Human chorionic gonadotropin treatment in recrudescing ovary (in vivo) as well as in testicular slices (in vitro) resulted in up regulation of vasa mRNA in a time-dependent manner. These results together suggest that vasa gene has got an important role to play in spermatogenesis and oogenesis during recrudescence in addition to development.

Fish with thermolabile sex determination (TSD) as models to study brain sex differentiation

As fish are ectothermic animals, water temperature can affect their basic biological processes such as larval development, growth and reproduction. Similar to reptiles, the incubation temperature during early phases of development is capable to modify sex ratios in a large number of fish species. This phenomenon, known as thermolabile sex determination (TSD) was first reported in Menidia menidia, a species belonging to the family Atherinopsidae. Since then, an increasing number of fish have also been found to exhibit TSD. Traditionally, likewise in reptiles, several TSD patterns have been described in fish, however it has been recently postulated that only one, females at low temperatures and males at high temperatures, may represent the "real" or "true" TSD. Many studies regarding the influence of temperature on the final sex ratios have been focused on the expression and activity of gonadal aromatase, the enzyme involved in the conversion of androgens into estrogens and encoded by the cyp19a1a gene. In this regard, teleost fish, may be due to a whole genome duplication event, produce another aromatase enzyme, commonly named brain aromatase, encoded by the cyp19a1b gene. Contrary to what has been described in other vertebrates, fish exhibit very high levels of aromatase activity in the brain and therefore they synthesize high amounts of neuroestrogens. However, its biological significance is still not understood. In addition, the mechanism whereby temperature can induce the development of a testis or an ovary still remains elusive. In this context the present review is aimed to discuss several theories about the possible role of brain aromatase using fish as models. The relevance of brain aromatase and therefore of neuroestrogens as the possible cue for gonadal differentiation is raised. In addition, the possible role of brain aromatase as the way to keep the high levels of neurogenesis in fish is also considered. Several key examples of how teleosts and aromatase regulation can offer more insight into basic mechanisms of TSD are also reviewed.

Aromatase in the brain of teleost fish: expression, regulation and putative functions

Unlike that of mammals, the brain of teleost fish exhibits an intense aromatase activity due to the strong expression of one of two aromatase genes (aromatase A or cyp19a1a and aromatase B or cyp19a1b) that arose from a gene duplication event. In situ hybridization, immunohistochemistry and expression of GFP (green fluorescent protein) in transgenic tg(cyp19a1b-GFP) fish demonstrate that aromatase B is only expressed in radial glial cells (RGC) of adult fish. These cells persist throughout life and act as progenitors in the brain of both developing and adult fish. Although aromatase B-positive radial glial cells are most abundant in the preoptic area and the hypothalamus, they are observed throughout the entire central nervous system and spinal cord. In agreement with the fact that brain aromatase activity is correlated to sex steroid levels, the high expression of cyp19a1b is due to an auto-regulatory loop through which estrogens and aromatizable androgens up-regulate aromatase expression. This mechanism involves estrogen receptor binding on an estrogen response element located on the cyp19a1b promoter. Cell specificity is achieved by a mandatory cooperation between estrogen receptors and unidentified glial factors. Given the emerging roles of estrogens in neurogenesis, the unique feature of the adult fish brain suggests that, in addition to classical functions on brain sexual differentiation and sexual behaviour, aromatase expression in radial glial cells could be part of the mechanisms authorizing the maintenance of a high proliferative activity in the brain of fish.

Dimorphic expression of tryptophan hydroxylase in the brain of XX and XY Nile tilapia during early development

Serotonin (5-HT) is well known for modulating the release of GnRH and gonadotropin in teleosts. Reports on increased female:male ratio after the blockade of 5-HT biosynthesis proposed a role for 5-HT in brain sex differentiation. Two types of tryptophan hydroxylase (Tph), rate-limiting enzyme in the biosynthesis of 5-HT were cloned from vertebrates. In the present study, we cloned Tph from brain and evaluated its importance during early development of XX and XY Nile tilapia. Tph cloned from tilapia brain is 1888 bp in length and it encodes predicted protein of 462 amino acid residues. Tph activity of tilapia was confirmed by demonstrating the conversion of L-tryptophan to 5-hydroxy tryptophan by the recombinant protein after transient transfection of this cDNA clone in COS-7 cells. Northern blot identified single transcript around 2kb in male brain. Tissue distribution of Tph revealed high abundance in brain, kidney, liver and testis. Semi-quantitative RT-PCR revealed exclusive expression of Tph in the male brain from 5 to 20 days post hatch (dph) while in the female brain, it was from 25 dph. These results were authenticated by localization of Tph transcripts in olfactory bulb-telencephalon region of 11 dph male brain using in situ hybridization. Tph immunoreactivity (-ir) was also evident in the nucleus preopticus-periventricularis area of male brain as early as 12 dph. However, Tph-ir was observed in several regions of both male and female brain without any distinction from 30 dph. Dimorphic expression pattern of Tph during early brain development around the critical period (7-21 dph) of gonadal sex determination and differentiation may implicate a role for Tph in brain sex differentiation of tilapia.