Aromatase plays a key role during normal and temperature-induced sex differentiation of tilapia Oreochromis niloticus

Potential Implications of Acid-Sensing Ion Channels ASIC2 and ASIC4 in Gonadal Differentiation of Dicentrarchus labrax Subjected to Water Temperature Increase during Gonadal Development

In this study, the expression and implication of acid-sensing ion channels 2 and 4 (ASIC2 and ASIC4) in the gonadal sex differentiation of Dicentrarchus labrax (D. labrax), subjected to increasing water temperatures during gonadal development, were evaluated. Two groups were selected: a control group (CG), in which the average water temperature was maintained at 15 °C and increased to 20 °C in 20 days until weaning; and an experimental group (EG), in which the water temperature was retained at 15 °C for 60 days; thereafter, the temperature was increased daily by 0.5 °C until it reached 20 °C up to the weaning time. Ten fish from the CG and 13 fish from the EG were sampled randomly on the 335th day after hatching (dph). A higher percentage of gonad differentiation in ovaries rather than in testes was observed in the EG compared to the CG (p = 0.01). ASIC2 and ASIC4 were detected for the first time in D. labrax ovaries by indirect immunofluorescence. Both ASIC2 and ASIC4 were expressed in previtellogenic oocytes of ovaries and in scattered cells within some testes, and were most likely intratesticular previtellogenic oocytes in both the CG and EG groups. The CG group showed a higher expression of ASIC4 than the EG cohort (p < 0.05). The results gathered in this study revealed the capacity of water temperature to influence both gonadal differentiation and growth in this gonochoristic fish species, and suggests the possible role of ASIC2 and ASIC4 in gonad differentiation and gamete development in D. labrax.

Effect of high temperatures on sex ratio and differential expression analysis (RNA-seq) of sex-determining genes in Oreochromis niloticus from different river basins in Benin

The high temperature sex reversal process leading to functional phenotypic masculinization during development has been widely described in Nile tilapia (Oreochromis n iloticus) under laboratory or aquaculture conditions and in the wild. In this study, we selected five wild populations of O. niloticus from different river basins in Benin and produced twenty full-sib families of mixed-sex (XY and XX) by natural reproduction. Progenies were exposed to room temperature or high (36.5°C) temperatures between 10 and 30 days post-fertilization (dpf). In control groups, we observed sex ratios from 40% to 60% males as expected, except for 3 families from the Gobé region which showed a bias towards males. High temperature treatment significantly increased male rates in each family up to 88%. Transcriptome analysis was performed by RNA-sequencing (RNA-seq) on brains and gonads from control and treated batches of six families at 15 dpf and 40 dpf. Analysis of differentially expressed genes, differentially spliced genes, and correlations with sex reversal was performed. In 40 dpf gonads, genes involved in sex determination such as dmrt1, cyp11c1, amh, cyp19a1b, ara, and dax1 were upregulated. In 15 dpf brains, a negative correlation was found between the expression of cyp19a1b and the reversal rate, while at 40 dpf a negative correlation was found between the expression of foxl2, cyp11c1, and sf1 and positive correlation was found between dmrt1 expression and reversal rate. Ontology analysis of the genes affected by high temperatures revealed that male sex differentiation processes, primary male sexual characteristics, autophagy, and cilium organization were affected. Based on these results, we conclude that sex reversal by high temperature treatment leads to similar modifications of the transcriptomes in the gonads and brains in offspring of different natural populations of Nile tilapia, which thus may activate a common cascade of reactions inducing sex reversal in progenies.

Molecular biological, physiological, cytological, and epigenetic mechanisms of environmental sex differentiation in teleosts: A systematic review

Human activities have been exerting widespread stress and environmental risks in aquatic ecosystems. Environmental stress, including temperature rise, acidification, hypoxia, light pollution, and crowding, had a considerable negative impact on the life histology of aquatic animals, especially on sex differentiation (SDi) and the resulting sex ratios. Understanding how the sex of fish responds to stressful environments is of great importance for understanding the origin and maintenance of sex, the dynamics of the natural population in the changing world, and the precise application of sex control in aquaculture. This review conducted an exhaustive search of the available literature on the influence of environmental stress (ES) on SDi. Evidence has shown that all types of ES can affect SDi and universally result in an increase in males or masculinization, which has been reported in 100 fish species and 121 cases. Then, this comprehensive review aimed to summarize the molecular biology, physiology, cytology, and epigenetic mechanisms through which ES contributes to male development or masculinization. The relationship between ES and fish SDi from multiple aspects was analyzed, and it was found that environmental sex differentiation (ESDi) is the result of the combined effects of genetic and epigenetic factors, self-physiological regulation, and response to environmental signals, which involves a sophisticated network of various hormones and numerous genes at multiple levels and multiple gradations in bipotential gonads. In both normal male differentiation and ES-induced masculinization, the stress pathway and epigenetic regulation play important roles; however, how they co-regulate SDi is unclear. Evidence suggests that the universal emergence or increase in males in aquatic animals is an adaptation to moderate ES. ES-induced sex reversal should be fully investigated in more fish species and extensively in the wild. The potential aquaculture applications and difficulties associated with ESDi have also been addressed. Finally, the knowledge gaps in the ESDi are presented, which will guide the priorities of future research.

Combined effects of rearing temperature regime (thermocycle vs. constant temperature) during early development and thermal treatment on Nile tilapia (Oreochromis niloticus) sex differentiation

In nature, water temperature experiences daily variations known as thermocycles. Temperature is the main environmental factor that influences sex determination in most teleost fish. The purpose of this study was to examine the effects of rearing temperature (thermocycle (TC) vs. constant (CTE)) on development and a posterior thermal shock throughout the period of sex differentiation of Nile tilapia (Oreochromis niloticus). Embryos and larvae were kept under two temperature regimes: TC of 31 °C:25 °C day:night vs. CTE of 28 °C from 0 to 11 dpf. After this period, the larvae from each group were subjected to either heat treatment (HT, 36 °C for 12 days) or kept under the same rearing temperatures until 23 dpf (Control, C). Then all the groups remained at constant temperature until 270 dpf, when blood and gonads were collected. Larval samples were used to examine the expression of genes related to male (amh, ara, sox9a, dmrt1a) and female (cyp19a1a, foxl2, era) sexual differentiation. In juveniles, sex was characterized by histology, the gonadal expression of the genes involved in the sex steroid synthesis was analyzed by qPCR, and plasma testosterone (T) and estradiol (E₂) levels were analyzed by ELISA. In larvae, daily TCs increased the survival rate against HT and up-regulated the expression of ovarian differentiation genes. In juveniles, TC + C induced a higher proportion of females and higher cyp19a1a expression compared to CTE + C. HT induced changes in the CTE group by up-regulating testicular differentiation genes and down-regulating female promoting genes, which did not occur in the TC group. Juveniles from TC + C group presented a higher proportion of females with higher E₂ and cyp19a1a than CTE + HT. Fish from the CTE + HT group showed a higher percentage of males with highest T and amh. These findings indicate that daily TCs during larval development promote ovarian differentiation and diminish the masculinizing effects of HT.

AOP Report: Adverse Outcome Pathways for Aromatase Inhibition or Androgen Receptor Agonism Leading to Male-Biased Sex Ratio and Population Decline in Fish

Screening and testing of potential endocrine-disrupting chemicals for ecological effects are examples of risk assessment/regulatory activities that can employ adverse outcome pathways (AOPs) to establish linkages between readily measured alterations in endocrine function and whole organism- and population-level responses. Of particular concern are processes controlled by the hypothalamic-pituitary-gonadal/thyroidal (HPG/T) axes. However, the availability of AOPs suitable to meet this need is currently limited in terms of species and life-stage representation relative to the diversity of endpoints influenced by HPG/T function. In our report we describe two novel AOPs that comprise a simple AOP network focused on the effects of chemicals on sex differentiation during early development in fish. The first AOP (346) documents events starting with inhibition of cytochrome P450 aromatase (CYP19), resulting in decreased availability of 17β-estradiol during gonad differentiation, which increases the occurrence of testis formation, resulting in a male-biased sex ratio and consequent population-level declines. The second AOP (376) is initiated by activation of the androgen receptor (AR), also during sexual differentiation, again resulting in a male-biased sex ratio and population-level effects. Both AOPs are strongly supported by existing physiological and toxicological evidence, including numerous fish studies with model CYP19 inhibitors and AR agonists. Accordingly, AOPs 346 and 376 provide a basis for more focused screening and testing of chemicals with the potential to affect HPG function in fish during early development. Environ Toxicol Chem 2023;42:747-756. Published 2023. This article is a U.S. Government work and is in the public domain in the USA.

Screening and Validation of p38 MAPK Involved in Ovarian Development of Brachymystax lenok

Brachymystax lenok (lenok) is a rare cold-water fish native to China that is of high meat quality. Its wild population has declined sharply in recent years, and therefore, exploring the molecular mechanisms underlying the development and reproduction of lenoks for the purposes of artificial breeding and genetic improvement is necessary. The lenok comparative transcriptome was analyzed by combining single molecule, real-time, and next generation sequencing (NGS) technology. Differentially expressed genes (DEGs) were identified in five tissues (head kidney, spleen, liver, muscle, and gonad) between immature [300 days post-hatching (dph)] and mature [three years post-hatching (ph)] lenoks. In total, 234,124 and 229,008 full-length non-chimeric reads were obtained from the immature and mature sequencing data, respectively. After NGS correction, 61,405 and 59,372 non-redundant transcripts were obtained for the expression level and pathway enrichment analyses, respectively. Compared with the mature group, 719 genes with significantly increased expression and 1,727 genes with significantly decreased expression in all five tissues were found in the immature group. Furthermore, DEGs and pathways involved in the endocrine system and gonadal development were identified, and p38 mitogen-activated protein kinases (MAPKs) were identified as potentially regulating gonadal development in lenok. Inhibiting the activity of p38 MAPKs resulted in abnormal levels of gonadotropin-releasing hormone, follicle-stimulating hormone, and estradiol, and affected follicular development. The full-length transcriptome data obtained in this study may provide a valuable reference for the study of gene function, gene expression, and evolutionary relationships in B. lenok and may illustrate the basic regulatory mechanism of ovarian development in teleosts.

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.

Epigenetic Regulation and Environmental Sex Determination in Cichlid Fishes

Studying environmental sex determination (ESD) in cichlids provides a phylogenetic and comparative approach to understand the evolution of the underlying mechanisms, their impact on the evolution of the overlying systems, and the neuroethology of life history strategies. Natural selection normally favors parents who invest equally in the development of male and female offspring, but evolution may favor deviations from this 50:50 ratio when environmental conditions produce an advantage for doing so. Many species of cichlids demonstrate ESD in response to water chemistry (temperature, pH, and oxygen concentration). The relative strengths of and the exact interactions between these factors vary between congeners, demonstrating genetic variation in sensitivity. The presence of sizable proportions of the less common sex towards the environmental extremes in most species strongly suggests the presence of some genetic sex-determining loci acting in parallel with the ESD factors. Sex determination and differentiation in these species does not seem to result in the organization of a final and irreversible sexual fate, so much as a life-long ongoing battle between competing male- and female-determining genetic and hormonal networks governed by epigenetic factors. We discuss what is and is not known about the epigenetic mechanism behind the differentiation of both gonads and sex differences in the brain. Beyond the well-studied tilapia species, the 2 best-studied dwarf cichlid systems showing ESD are the South American genus Apistogramma and the West African genus Pelvicachromis. Both species demonstrate male morphs with alternative reproductive tactics. We discuss the further neuroethology opportunities such systems provide to the study of epigenetics of alternative life history strategies and other behavioral variation.

Sex determination, gonadal sex differentiation, and plasticity in vertebrate species

A diverse array of sex determination (SD) mechanisms, encompassing environmental to genetic, have been found to exist among vertebrates, covering a spectrum from fixed SD mechanisms (mammals) to functional sex change in fishes (sequential hermaphroditic fishes). A major landmark in vertebrate SD was the discovery of the SRY gene in 1990. Since that time, many attempts to clone an SRY ortholog from nonmammalian vertebrates remained unsuccessful, until 2002, when DMY/dmrt1by was discovered as the SD gene of a small fish, medaka. Surprisingly, however, DMY/dmrt1by was found in only 2 species among more than 20 species of medaka, suggesting a large diversity of SD genes among vertebrates. Considerable progress has been made over the last 3 decades, such that it is now possible to formulate reasonable paradigms of how SD and gonadal sex differentiation may work in some model vertebrate species. This review outlines our current understanding of vertebrate SD and gonadal sex differentiation, with a focus on the molecular and cellular mechanisms involved. An impressive number of genes and factors have been discovered that play important roles in testicular and ovarian differentiation. An antagonism between the male and female pathway genes exists in gonads during both sex differentiation and, surprisingly, even as adults, suggesting that, in addition to sex-changing fishes, gonochoristic vertebrates including mice maintain some degree of gonadal sexual plasticity into adulthood. Importantly, a review of various SD mechanisms among vertebrates suggests that this is the ideal biological event that can make us understand the evolutionary conundrums underlying speciation and species diversity.

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.

Controlled release of sex steroids through osmotic pump alters brain GnRH1 and catecholaminergic system dimorphically in the catfish, Clarias gariepinus

The present study aimed to evaluate osmotic pump-mediated controlled release of estrogen in males and androgen in females to analyze the impact on gonadotropin-releasing hormone (GnRH1), catecholamines (CAs) and other associated genes in the catfish, Clarias gariepinus. During pre-spawning phase, catfish were separately implanted osmotic pumps loaded with 17β-estradiol (E₂) in males and 17α-methyltestosterone (MT) in females at a dose of 10 μg/100 μl or saline (100 μl) controls into both sexes to release for 21 days and all fishes were maintained as per the duration. Further, GnRH1 expression levels were analysed in the discrete regions of brain after E₂ and MT treatments in male and female catfish, respectively using qPCR which revealed that GnRH1 expression was significantly higher in E₂ treated male as compared to the control. On the other hand, GnRH1 expression was lower in MT treated female when compared to the control in the discrete regions of brain. In addition, certain brain and monoaminergic system related genes showed a differential response. Catfish GnRH1 could be localized in preoptic area-hypothalamus (POA-HYP) that correlated with the expression profile in the discrete regions of catfish brain. Serum levels of sex steroids in the treated male fish indicated that the treatment of E₂ could maintain and impart feminization effect even in the presence of endogenous androgen during gonadal recrudescence while such an effect was not seen in females with androgen treatment. Measurement of CAs, L-3,4-dihydroxyphenylalanine, dopamine and norepinephrine levels in the male and female brain after the controlled release of E₂ and MT, respectively confirmed the modulation of neurotransmitters in the E₂treated male than MT treated female fish. These results collectively suggest the severity of estrogenic over androgenic compounds to alter reproductive status even at a minimal dose by targeting CAs and GnRH1 at the level of brain of catfish. This study provides insights into the reproductive toxicity of sex steroid analogues at the level of brain GnRH1 and CA-ergic system in addition to serum T, 11-KT and E₂ levels during gonadal recrudescence, which is a crucial period of gametogenesis preceding spawning.

Inhibition of ovarian development and instances of sex reversal in genotypic female sablefish (Anoplopoma fimbria) exposed to elevated water temperature

This study determined high temperature effects on ovarian development in a marine groundfish species, sablefish (Anoplopoma fimbria), with potential application in sex reversal or sterilization for aquaculture. Monosex female (XX-genotype) sablefish larvae (∼30 mm) were randomly divided into three groups and exposed to control (15.6 °C ± 0.8 °C), moderate (20.4 °C ± 0.5 °C), or high (21.7 °C ± 0.5 °C) temperatures for 19 weeks. Treated fish were then tagged and transferred to ambient seawater (11.2 °C ± 2.3 °C) for one year to determine whether temperature effects on reproductive development were maintained post-treatment. Fish were periodically sampled for gonadal histology, gene expression and plasma 17β-estradiol (E2) analyses to assess gonadal development. Short-term (4-week) exposure to elevated temperatures had only minor effects, whereas longer exposure (12-19 weeks) markedly inhibited ovarian development. Fish from the moderate and high treatment groups had significantly less developed ovaries relative to controls, and mRNA levels for germ cell (vasa, zpc) and apoptosis-associated genes (p53, casp8) generally indicated gonadal degeneration. The high treatment group also had significantly reduced plasma E2 levels and elevated gonadal amh gene expression. After one year at ambient temperatures, however, ovaries of moderate and high treatment fish exhibited compensatory recovery and were indistinguishable from controls. Two genotypic females possessing immature testes (neomales) were observed in the high treatment group, indicating sex reversal had occurred (6% rate). These results demonstrate that extreme elevated temperatures may inhibit ovarian development or trigger sex reversal. High temperature treatment is likely not an effective sterilization method but may be preferable for sablefish neomale broodstock production.

Temperature preference of Nile tilapia (Oreochromis niloticus) juveniles induces spontaneous sex reversal

Nile tilapia (Oreochromis niloticus) is an African freshwater fish that displays a genetic sex determination system (XX|XY) where high temperatures (above 32°C to 36.5°C) induce masculinization. In Nile tilapia, the thermosensitive period was reported from 10 to 30 days post fertilization. In their natural environment, juveniles may encounter high temperatures that are above the optimal temperature for growth (27-30°C). The relevance of the thermal sex reversal mechanism in a natural context remains unclear. The main objective of our study is to determine whether sexually undifferentiated juveniles spontaneously prefer higher, unfavorable temperatures and whether this choice skews the sex ratio toward males. Five full-sib progenies (from 100% XX crosses) were subjected to (1) a horizontal three-compartment thermal step gradient (thermal continuum 28°C- 32°C- 36.5°C) during the thermosensitive period, (2) a control continuum (28°C- 28°C- 28°C) and (3) a thermal control tank (36.5°C). During the first days of the treatment, up to an average of 20% of the population preferred the masculinizing compartment of the thermal continuum (36.5°C) compared to the control continuum. During the second part of the treatment, juveniles preferred the lower, nonmasculinizing 32°C temperature. This short exposure to higher temperatures was sufficient to significantly skew the sex ratio toward males, compared to congeners raised at 28°C (from 5.0 ± 6.7% to 15.6 ± 16.5% of males). The proportion of males was significantly different in the thermal continuum, thermal control tank and control continuum, and it was positively correlated among populations. Our study shows for the first time that Nile tilapia juveniles can choose a masculinizing temperature during a short period of time. This preference is sufficient to induce sex reversal to males within a population. For the first time, behavior is reported as a potential player in the sex determination mechanism of this species.

Time matters! Developmental shift in gene expression between the head and the trunk region of the cichlid fish Astatotilapia burtoni

BACKGROUND

Differential gene expression can be translated into differing phenotypic traits. Especially during embryogenesis, specific gene expression networks regulate the development of different body structures. Cichlid fishes, with their impressive phenotypic diversity and propensity to radiate, are an emerging model system in the genomics era. Here we set out to investigate gene expression throughout development in the well-studied cichlid fish Astatotilapia burtoni, native to Lake Tanganyika and its affluent rivers.

RESULTS

Combining RNA-sequencing from different developmental time points as well as integrating adult gene expression data, we constructed a new genome annotation for A. burtoni comprising 103,253 transcripts (stemming from 52,584 genomic loci) as well as a new reference transcriptome set. We compared our transcriptome to the available reference genome, redefining transcripts and adding new annotations. We show that about half of these transcripts have coding potential. We also characterize transcripts that are not present in the genome assembly. Next, using our newly constructed comprehensive reference transcriptome, we characterized differential gene expression through time and showed that gene expression is shifted between different body parts. We constructed a gene expression network that identified connected genes responsible for particular phenotypes and made use of it to focus on genes under potential positive selection in A. burtoni, which were implicated in fin development and vision.

CONCLUSIONS

We provide new genomic resources for the cichlid fish Astatotilapia burtoni, which will contribute to its further establishment as a model system. Tracing gene expression through time, we identified gene networks underlying particular functions, which will help to understand the genetic basis of phenotypic diversity in cichlids.

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.

Differences in aromatase expression and steroid hormone concentrations in the reproductive tissues of male domestic turkeys (Meleagris gallopavo) with white and yellow semen

1. To show hormonal differences between male turkeys with yellow semen syndrome (YSS) and white, normal semen (WNS), the expression of aromatase, oestrogen receptor α (ERα), and oestrogen receptor β (ERβ) as well as testosterone and oestradiol concentrations in YSS and WNS testes, epididymis, and ductus deferens were examined. 2. To measure gene expression levels of aromatase and oestrogen receptors (ERs), three complementary techniques (real-time PCR, Western blot, and immunohistochemistry) were used, whereas steroid hormone levels were determined radio-immunologically. 3. Upregulation of aromatase and ERα mRNAs in YSS testes (P < 0.05; P < 0.01), epididymis (P < 0.001; P < 0.001), and ductus deferens (P < 0.05; P < 0.01) compared to those of WNS tissues was detected. Significant increases in the levels of aromatase and ERα proteins were detected in YSS testes (P < 0.001; P < 0.05), epididymis (P < 0.001; P < 0.001), and ductus deferens (P < 0.001; P < 0.05). The expression of ERβ mRNA and protein level was upregulated in the testes (P < 0.05; P < 0.01) and epididymis (P < 0.001; P < 0.01) but not in ductus deferens where it was downregulated (P < 0.01; P < 0.01). Increased intensity of immunoreactive proteins in YSS versus WNS reproductive tissues corroborated gene expression results. 4. Testosterone concentration diminished in YSS epididymis (P < 0.05) and ductus deferens (P < 0.05), but not in the testes, remaining at high level (P < 0.05) compared to WNS values. Concomitantly, increased oestradiol concentration was found in YSS testes (P < 0.05) and epididymis (P < 0.05) but decreased in the ductus deferens (P < 0.05). 5. From the published literature, this study is the first to demonstrate the ability for androgen aromatisation in the turkey reproductive tissues and to show the cellular targets for locally produced oestrogens. The data suggested that the androgen/oestrogen ratio is a mechanistic basis for amplification of differences between turkeys with white and yellow semen and that these results can have a relevance in applied sciences to widen the knowledge on domestic bird reproduction.

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.

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.

Neuroendocrine disruption of organizational and activational hormone programming in poikilothermic vertebrates

In vertebrates, sexual differentiation of the reproductive system and brain is tightly orchestrated by organizational and activational effects of endogenous hormones. In mammals and birds, the organizational period is typified by a surge of sex hormones during differentiation of specific neural circuits; whereas activational effects are dependent upon later increases in these same hormones at sexual maturation. Depending on the reproductive organ or brain region, initial programming events may be modulated by androgens or require conversion of androgens to estrogens. The prevailing notion based upon findings in mammalian models is that male brain is sculpted to undergo masculinization and defeminization. In absence of these responses, the female brain develops. While timing of organizational and activational events vary across taxa, there are shared features. Further, exposure of different animal models to environmental chemicals such as xenoestrogens such as bisphenol A-BPA and ethinylestradiol-EE2, gestagens, and thyroid hormone disruptors, broadly classified as neuroendocrine disrupting chemicals (NED), during these critical periods may result in similar alterations in brain structure, function, and consequently, behaviors. Organizational effects of neuroendocrine systems in mammals and birds appear to be permanent, whereas teleost fish neuroendocrine systems exhibit plasticity. While there are fewer NED studies in amphibians and reptiles, data suggest that NED disrupt normal organizational-activational effects of endogenous hormones, although it remains to be determined if these disturbances are reversible. The aim of this review is to examine how various environmental chemicals may interrupt normal organizational and activational events in poikilothermic vertebrates. By altering such processes, these chemicals may affect reproductive health of an animal and result in compromised populations and ecosystem-level effects.

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.

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.

Global DNA Methylation Changes in Nile Tilapia Gonads during High Temperature-Induced Masculinization

In some fish species, high or low temperature can switch the sex determination mechanisms and induce fish sex reversal when the gonads are undifferentiated. During this high or low temperature-induced sex reversal, the expressions of many genes are altered. However, genome-wide DNA methylation changes in fish gonads after high or low temperature treatment are unclear. Herein, we compared the global DNA methylation changes in the gonads from control females (CF), control males (CM), high temperature-treated females (TF), and high temperature-induced males (IM) from the F8 family of Nile tilapia (Oreochromis niloticus) using methylated DNA immunoprecipitation sequencing. The DNA methylation level in CF was higher than that in CM for various chromosomes. Both females and males showed an increase in methylation levels on various chromosomes after high-temperature induction. We identified 64,438 (CF/CM), 63,437 (TF/IM), 98,675 (TF/CF), 235,270 (IM/CM) and 119,958 (IM/CF) differentially methylated regions (DMRs) in Nile tilapia gonads, representing approximately 0.70% (CF/CM), 0.69% (TF/IM), 1.07% (TF/CF), 2.56% (IM/CM), and 1.30% (IM/CF)of the length of the genome. A total of 89 and 65 genes that exhibited DMRs in their gene bodies and promoters were mapped to the Nile tilapia genome. Furthermore, more than half of the genes with DMRs in the gene body in CF/CM were also included in the IM/CM, TF/CF, TF/IM, and IM/CF groups. Additionally, many important pathways, including neuroactive ligand-receptor interaction, extracellular matrix-receptor interaction, and biosynthesis of unsaturated fatty acids were identified. This study provided an important foundation to investigate the molecular mechanism of high temperature-induced sex reversal in fish species.

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

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

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.

Coordinated microRNA and messenger RNA expression profiles for understanding sexual dimorphism of gonads and the potential roles of microRNA in the steroidogenesis pathway in Nile tilapia (Oreochromis niloticus)

Sexual dimorphism is a widespread phenomenon in animals. However, the potential role of microRNAs (miRNAs) in regulating this dimorphism is not fully understood. In our study, we used an integrated approach to identify functional targets of miRNA by combining the paired expression profiles of miRNAs and messenger RNAs (mRNAs) in ovaries and testes of young Nile tilapia, Oreochromis niloticus. The results revealed that 67 upregulated and nine downregulated miRNAs and 2299 upregulated and 3260 downregulated genes were identified in the ovary compared with those in the testis (P < 0.01). The target genes of differentially expressed miRNAs were predicted and overlapped with the differentially expressed mRNAs. Furthermore, Kyoto Encyclopedia of Genes and Genomes pathway analyses were conducted in these coincident genes. By correlating miRNA-mRNA and predicting computational target, two types of negatively regulatory miRNA-mRNA correlations (upregulated or downregulated miRNA and downregulated or upregulated mRNA) were obtained. Seven functional miRNA-target gene pairs, miR-17-5p/DMRT1, miR-20a/DMRT1, miR-138/CYP17A2, miR-338/CYP17A2, miR-200a/CYP17A2, miR-456/AMH, and miR-138/AMH, were predicted at the sequence level and further detected by real-time polymerase chain reaction on the basis of the significantly negative relationships. Our results suggest that the integrated analysis of miRNA and mRNA expression profiling can provide novel insights into the molecular mechanism of sexual dimorphism.

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.

High temperature-induced sterility in the female Nile tilapia, Oreochromis niloticus

High temperature treatments induce germ cell loss in gonads of vertebrate animals, including fish. It could be a reliable source for induction of sterility if the treatments led to a permanent loss of germ cells. Here we report that heat treatment at 37 °C for 45-60 days caused a complete loss of germ cells in female Nile tilapia, Oreochromis niloticus, and that sterility was achieved in fish at all stages of their life cycle. Unlike previous observations, germ cells did not repopulate even after returning them to the water at control conditions suggesting permanent depletion of germ cells. Gonadal somatic cells immunopositive for 3β-hydroxysteroid dehydrogenase (3β-HSD) were clustered at one end of the germ cell depleted gonads close to the blood vessel. Serum level of testosterone, 11-ketotestosterone, and 17β-estradiol was significantly decreased in sterile fish compared to control. Body weight of sterile fish was higher than control fish at the end of experiment. Our observations of increased growth and permanent sterilization in the high temperature-treated fish suggest that this method could be an appropriate and eco-friendly tool for inducing sterility in fish with a higher thermal tolerance.

A syntenic region conserved from fish to Mammalian x chromosome

Sex chromosomes bearing the sex-determining gene initiate development along the male or female pathway, no matter which sex is determined by XY male or ZW female heterogamety. Sex chromosomes originate from ancient autosomes but evolved rapidly after the acquisition of sex-determining factors which are highly divergent between species. In the heterogametic male system (XY system), the X chromosome is relatively evolutionary silent and maintains most of its ancestral genes, in contrast to its Y counterpart that has evolved rapidly and degenerated. Sex in a teleost fish, the Nile tilapia (Oreochromis niloticus), is determined genetically via an XY system, in which an unpaired region is present in the largest chromosome pair. We defined the differences in DNA contents present in this chromosome with a two-color comparative genomic hybridization (CGH) and the random amplified polymorphic DNA (RAPD) approach in XY males. We further identified a syntenic segment within this region that is well conserved in several teleosts. Through comparative genome analysis, this syntenic segment was also shown to be present in mammalian X chromosomes, suggesting a common ancestral origin of vertebrate sex chromosomes.

Genetics and timing of sex determination in the East African cichlid fish Astatotilapia burtoni

Background

The factors determining sex are diverse in vertebrates and especially so in teleost fishes. Only a handful of master sex-determining genes have been identified, however great efforts have been undertaken to characterize the subsequent genetic network of sex differentiation in various organisms. East African cichlids offer an ideal model system to study the complexity of sexual development, since many different sex-determining mechanisms occur in closely related species of this fish family. Here, we investigated the sex-determining system and gene expression profiles during male development of Astatotilapia burtoni, a member of the rapidly radiating and exceptionally species-rich haplochromine lineage.

Results

Crossing experiments with hormonally sex-reversed fish provided evidence for an XX-XY sex determination system in A. burtoni. Resultant all-male broods were used to assess gene expression patterns throughout development of a set of candidate genes, previously characterized in adult cichlids only.

Conclusions

We could identify the onset of gonad sexual differentiation at 11–12 dpf. The expression profiles identified wnt4B and wt1A as the earliest gonad markers in A. burtoni. Furthermore we identified late testis genes (cyp19a1A, gsdf, dmrt1 and gata4), and brain markers (ctnnb1A, ctnnb1B, dax1A, foxl2, foxl3, nanos1A, nanos1B, rspo1, sf-1, sox9A and sox9B).

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-014-0140-5) contains supplementary material, which is available to authorized users.

Characterization and differential expression patterns of conserved microRNAs and mRNAs in three genders of the rice field eel (Monopterus albus)

MicroRNAs (miRNAs) are endogenous small RNAs that can regulate target mRNAs by binding to their sequences in the 3' untranslated region. The expression of miRNAs and their biogenetic pathway are involved in sexual differentiation and in the regulation of the development of germ cells and gonadal somatic cells. The rice field eel (Monopterus albus) undergoes a natural sexual transformation from female to male via an intersex stage during its life cycle. To investigate the molecular mechanisms of this sexual transformation, miRNAs present in the different sexual stages of the rice field eel were identified by high-throughput sequencing technology. A significantly differential expression among the 3 genders (p < 0.001) was observed for 48 unique miRNAs and 3 miRNAs*. Only 9 unique miRNAs showed a more than 8-fold change in their expression among the 3 genders, including mal-miR-430a and mal-miR-430c which were higher in females than in males. However, mal-miR-430b was only detected in males. Several potential miRNA target genes (cyp19a, cyp19b, nr5a1b, foxl2 amh, and vasa) were also investigated. Real-time RT-PCR demonstrated highly specific expression patterns of these genes in the 3 genders of the rice field eel. Many of these genes are targets of mal-miR-430b according to the TargetScan and miRTarBase. These results suggest that the miR-430 family may be involved in the sexual transformation of the rice field eel.

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.

Identification of male-specific amh duplication, sexually differentially expressed genes and microRNAs at early embryonic development of Nile tilapia (Oreochromis niloticus)

Background

The probable influence of genes and the environment on sex determination in Nile tilapia suggests that it should be regarded as a complex trait. Detection of sex determination genes in tilapia has both scientific and commercial importance. The main objective was to detect genes and microRNAs that were differentially expressed by gender in early embryonic development.

Results

Artificial fertilization of Oreochromis niloticus XX females with either sex-reversed ΔXX males or genetically-modified YY ‘supermales’ resulted in all-female and all-male embryos, respectively. RNA of pools of all-female and all-male embryos at 2, 5 and 9 dpf were used as template for a custom Agilent eArray hybridization and next generation sequencing. Fifty-nine genes differentially expressed between genders were identified by a false discovery rate of p < 0.05. The most overexpressed genes were amh and tspan8 in males, and cr/20β-hsd, gpa33, rtn4ipl and zp3 in females (p < 1 × 10⁻⁹). Validation of gene expression using qPCR in embryos and gonads indicated copy number variation in tspan8, gpa33, cr/20β-hsd and amh. Sequencing of amh identified a male-specific duplication of this gene, denoted amhy, differing from the sequence of amh by a 233 bp deletion on exonVII, hence lacking the capability to encode the protein motif that binds to the transforming growth factor beta receptor (TGF-β domain). amh and amhy segregated in the mapping family in full concordance with SD-linked marker on LG23 signifying the QTL for SD. We discovered 831 microRNAs in tilapia embryos of which nine had sexually dimorphic expression patterns by a false discovery rate of p < 0.05. An up-regulated microRNA in males, pma-mir-4585, was characterized with all six predicted target genes including cr/20β-hsd, down-regulated in males.

Conclusions

This study reports the first discovery of sexually differentially expressed genes and microRNAs at a very early stage of tilapia embryonic development, i.e. from 2 dpf. Genes with sexually differential expression patterns are enriched for copy number variation. A novel male-specific duplication of amh, denoted amhy, lacking the TGF-β domain was identified and mapped to the QTL region on LG23 for SD, thus indicating its potential role in SD.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-774) contains supplementary material, which is available to authorized users.

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.

Intersex occurrence in rainbow trout (Oncorhynchus mykiss) male fry chronically exposed to ethynylestradiol

This study aimed to investigate the male-to-female morphological and physiological transdifferentiation process in rainbow trout (Oncorhynchus mykiss) exposed to exogenous estrogens. The first objective was to elucidate whether trout develop intersex gonads under exposure to low levels of estrogen. To this end, the gonads of an all-male population of fry exposed chronically (from 60 to 136 days post fertilization – dpf) to several doses (from environmentally relevant 0.01 µg/L to supra-environmental levels: 0.1, 1 and 10 µg/L) of the potent synthetic estrogen ethynylestradiol (EE2) were examined histologically. The morphological evaluations were underpinned by the analysis of gonad steroid (testosterone, estradiol and 11-ketotestosterone) levels and of brain and gonad gene expression, including estrogen-responsive genes and genes involved in sex differentiation in (gonads: cyp19a1a, ER isoforms, vtg, dmrt1, sox9a2; sdY; cyp11b; brain: cyp19a1b, ER isoforms). Intersex gonads were observed from the first concentration used (0.01 µg EE2/L) and sexual inversion could be detected from 0.1 µg EE2/L. This was accompanied by a linear decrease in 11-KT levels, whereas no effect on E2 and T levels was observed. Q-PCR results from the gonads showed downregulation of testicular markers (dmrt1, sox9a2; sdY; cyp11b) with increasing EE2 exposure concentrations, and upregulation of the female vtg gene. No evidence was found for a direct involvement of aromatase in the sex conversion process. The results from this study provide evidence that gonads of male trout respond to estrogen exposure by intersex formation and, with increasing concentration, by morphological and physiological conversion to phenotypic ovaries. However, supra-environmental estrogen concentrations are needed to induce these changes.

Molecular players involved in temperature-dependent sex determination and sex differentiation in Teleost fish

The molecular mechanisms that underlie sex determination and differentiation are conserved and diversified. In fish species, temperature-dependent sex determination and differentiation seem to be ubiquitous and molecular players involved in these mechanisms may be conserved. Although how the ambient temperature transduces signals to the undifferentiated gonads remains to be elucidated, the genes downstream in the sex differentiation pathway are shared between sex-determining mechanisms. In this paper, we review recent advances on the molecular players that participate in the sex determination and differentiation in fish species, by putting emphasis on temperature-dependent sex determination and differentiation, which include temperature-dependent sex determination and genetic sex determination plus temperature effects. Application of temperature-dependent sex differentiation in farmed fish and the consequences of temperature-induced sex reversal are discussed.

Epigenetic control of gonadal aromatase (cyp19a1) in temperature-dependent sex determination of red-eared slider turtles

In the red-eared slider turtle (Trachemys scripta), a species with temperature-dependent sex determination (TSD), the expression of the aromatase gene during gonad development is strictly limited to the female-producing temperature. The underlying mechanism remains unknown. In this study, we identified the upstream 5'-flanking region of the aromatase gene, gonad-specific promoter, and the temperature-dependent DNA methylation signatures during gonad development in the red-eared slider turtle. The 5'-flanking region of the slider aromatase exhibited sequence similarities to the aromatase genes of the American alligator, chicken, quail, and zebra finch. A putative TATA box was located 31 bp upstream of the gonad-specific transcription start site. DNA methylation at the CpG sites between the putative binding sites of the fork head domain factor (FOX) and vertebrate steroidogenic factor 1 (SF1) and adjacent TATA box in the promoter region were significantly lower in embryonic gonads at the female-producing temperature compared the male-producing temperature. A shift from male- to female-, but not from female- to male-, producing temperature changed the level of DNA methylation in gonads. Taken together these results indicate that the temperature, particularly female-producing temperature, allows demethylation at the specific CpG sites of the promoter region which leads the temperature-specific expression of aromatase during gonad development.

Diversity and plasticity of sex determination and differentiation in fishes

Among vertebrates, fishes show an exceptional range of reproductive strategies regarding the expression of their sexuality. Fish sexualities were categorized into gonochorism, synchronous/sequential hermaphrodite, or unisexual reproduction. In gonochoristic fishes, sex is determined genetically or by environmental factors. After sex determination, the gonads are differentiated into ovary or testis, with the sex remaining fixed for the entire life cycle. In contrast, some sequential hermaphrodite fishes can change their sex from male to female (protandrous), female to male (protogynous), or serially (bi-directional sex change) in their life cycle. In many cases, sex change is cued by social factors such as the disappearance of a male or female from a group. This unique diversity in fishes provides an ideal animal model to investigate sex determination and differentiation in vertebrates. This review first discusses genetic-orientated sex determination mechanisms. Then, we address the gonadal sex differentiation process in a gonochoristic fish, using an example of the Nile tilapia. Finally, we discuss various types of sex change that occur in hermaphrodite fishes.

First evidence for family-specific QTL for temperature-dependent sex reversal in Nile tilapia (Oreochromis niloticus)

This study for the first time screens microsatellite markers for associations with the temperature-dependent sex of Oreochromis niloticus. Previous studies revealed markers on linkage groups (LG) 1, 3, and 23 to be linked to the phenotypic sex of Oreochromis spp. at normal rearing temperatures. Moreover, candidate genes for sex determination and differentiation have been mapped to these linkage groups. Here, 6 families of a temperature-treated genetically all-female (XX) F(1)-population were genotyped for 21 microsatellites on the 3 LGs. No population-wide QTL (quantitative trait loci) or marker trait associations could be detected. However, family-specific QTL were found on LG 1 flanked by UNH995 and UNH104, on LG 3 at the position of GM213, and on LG 23 next to GM283. Moreover, family-specific single marker associations for UNH995 and UNH104 on LG 1, GM213 on LG 3, as well as for UNH898 and GM283 on LG 23 were detected. Yet, marker trait associations could not explain the temperature-dependent sex of all fish in the respective families. The molecular cue for the temperature-dependent sex in Nile tilapia might partially coincide with allelic variants at major and minor genetic sex determining factors. Moreover, additional QTL contributing to variable liabilities towards temperature might persist on other LGs.

Effect of rearing water temperature on protandrous sex inversion in cultured Asian Seabass (Lates calcarifer)

Asian Seabass, Lates calcarifer (Bloch, 1790), is a protandrous species cultured for Aquaculture. The cultured Asian Seabass in Australia exhibits precocious sex inversion before 2years of age. This phenomenon highly affects on maintaining a proper broodstock in a hatchery. The effect of temperature on sex inversion inducement in Asian Seabass was thus investigated at five different temperature regimes experienced in Australia. Asian Seabass (14months) grown in fresh water under natural temperature in a commercial farm in Queensland were transported to the research facility at James Cook University, Australia and held in fresh water at 28°C until acclimatized to the experimental conditions. Fish were acclimated to the experimental conditions (30ppt salinity) over the first and final week (22°C, 25°C, 28°C, 31°C and 34°C) of one month acclimatizing period. Fish were fed daily with a commercial pellet (50% protein, 18MJkg(-1)) to satiety. Blood, brain and gonad collected before transfer to the experimental temperature regime in the final week of acclimatization and at the end of the experiment were analysed. Plasma sex steroids level and aromatase activity of brain and gonad were also measured. There was an increase in plasma estradiol levels with increasing temperature from 25°C while no significant difference was observed among all treatment temperatures except at 25°C. However, fish held at 22°C showed higher estradiol level than at 25°C and 28°C. Significantly higher (p<0.05) plasma testosterone levels were detected in fish held at 31°C and 34°C while a reducing trend was observed towards lower temperature regimes. Fish held at 22°C had significantly lower plasma testosterone than all others as well those sampled at the beginning. The plasma 11-ketoTestosterone was at non-detectable levels in all experimental temperatures as shown at the beginning. The average aromatase activity in brain was highest at 28°C among all temperatures, but no significant differences (p>0.05) observed. The Average aromatase activity in gonad was highest at 31°C followed by at 34°C and 28°C. No or very low level of gonad aromatase activity recorded in fish sacrificed prior to treatment. The aromatase activity was greater in brain than in gonad suggesting that the aromatase produced in the brain yet to transfer to the gonad or brain is the first place to response for culture environmental temperature. It is concluded that plasma sex steroids levels and aromatase activity in Asian Seabass have positive response to increasing temperature in culture facilities.

DNA methylation of the gonadal aromatase (cyp19a) promoter is involved in temperature-dependent sex ratio shifts in the European sea bass

Sex ratio shifts in response to temperature are common in fish and reptiles. However, the mechanism linking temperature during early development and sex ratios has remained elusive. We show in the European sea bass (sb), a fish in which temperature effects on sex ratios are maximal before the gonads form, that juvenile males have double the DNA methylation levels of females in the promoter of gonadal aromatase (cyp19a), the enzyme that converts androgens into estrogens. Exposure to high temperature increased the cyp19a promoter methylation levels of females, indicating that induced-masculinization involves DNA methylation-mediated control of aromatase gene expression, with an observed inverse relationship between methylation levels and expression. Although different CpGs within the sb cyp19a promoter exhibited different sensitivity to temperature, we show that the increased methylation of the sb cyp19a promoter, which occurs in the gonads but not in the brain, is not a generalized effect of temperature. Importantly, these effects were also observed in sexually undifferentiated fish and were not altered by estrogen treatment. Thus, methylation of the sb cyp19a promoter is the cause of the lower expression of cyp19a in temperature-masculinized fish. In vitro, induced methylation of the sb cyp19a promoter suppressed the ability of SF-1 and Foxl2 to stimulate transcription. Finally, a CpG differentially methylated by temperature and adjacent to a Sox transcription factor binding site is conserved across species. Thus, DNA methylation of the aromatase promoter may be an essential component of the long-sought-after mechanism connecting environmental temperature and sex ratios in vertebrate species with temperature-dependent sex determination.