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

The expression profiles of cyp19a1, sf-1, esrs and gths in the brain-pituitary during gonadal sex differentiation in juvenile Japanese eels

Eels are gonochoristic species whose gonadal differentiation initiates at the yellow eel stage and is influenced by environmental factors. We revealed some sex-related genes were sex dimorphically expressed in gonads during gonadal sex differentiation of Japanese eel (Anguilla japonica); however, the expression of sex-related genes in the brain-pituitary during gonadal sex differentiation in eels is still unclear. This study aimed to investigate the sex-related gene expressions in the brain-pituitary and tried to clarify their roles in the brain and gonads during gonadal sex differentiation. Based on our previous histological study, the control eels developed as males, and estradiol-17β (E2) was used for feminization. Our results showed that during testicular differentiation, the brain cyp19a1 transcripts and aromatase proteins were increased significantly; moreover, the cyp19a1, sf-1, foxl2s, and esrs (except gperb) transcripts in the midbrain/pituitary also were increased significantly. Forebrain gnrh1 transcripts increased slightly during gonadal differentiation of both sexes, but the gnrhr1b and gnrhr2 transcripts in the midbrain/pituitary were stable during gonadal differentiation. The expression levels of gths and gh in the midbrain/pituitary were significantly increased during testicular differentiation and were much higher in males than in E2-feminized females. These results implied that endogenous estrogens might play essential roles in the brain/pituitary during testicular differentiation, sf-1, foxl2s, and esrs may have roles in cyp19a1 regulation in the midbrain/pituitary of Japanese eels. For the GnRH-GTH axis, gths, especially fshb, may be regulated by esrs and involved in regulating testicular differentiation and development in Japanese eels.

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.

Tilapia, a good model for studying reproductive endocrinology

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

Estrodiol-17β and aromatase inhibitor treatment induced alternations of genome-wide DNA methylation pattern in Takifugu rubripes gonads

Estradiol-17β (E2) and aromatase inhibitor (AI) exposure can change the phenotypic sex of fish gonads. To investigated whether alterations in DNA methylation is involved in this process, the level of genome-wide DNA methylation in Takifugu rubripes gonads was quantitatively analyzed during the E2-induced feminization and AI-induced masculinization processes in this study. The methylation levels of the total cytosine (C) in control-XX(C-XX), control-XY (C-XY), E2-treated-XY (E-XY) and AI-treated-XX (AI-XX) were 9.11%, 9.19%, 8.63% and 9.23%, respectively. In the C-XX vs C-XY comparison, 4,196 differentially methylated regions (DMRs) overlapped with the gene body of 2,497 genes and 608 DMRs overlapped with the promoter of 575 genes. In the E-XY vs C-XY comparison, 6,539 DMRs overlapped with the gene body of 3,416 genes and 856 DMRs overlapped with the promoter of 776 genes. In the AI-XX vs C-XX comparison, 2,843 DMRs overlapped with the gene body of 1,831 genes and 461 DMRs overlapped with the promoter of 421 genes. Gonadal genomic methylation mainly occurred at CG sites and the genes that overlapped with DMRs on CG context were most enriched in the signaling pathways related to gonad differentiation, such as the Wnt, TGF-β, MAPK, CAM and GnRH pathways. The DNA methylation levels of steroid synthesis genes and estrogen receptor genes promoter or gene body were negative correlated with their expression. After bisulfite sequencing verification, the DNA methylation level of the amhr2 promoter in XY was increased after E2 treatment, which consistent with the data from the genome-wide DNA methylation sequencing. In C-XY group, the expression of amhr2 was significantly higher than that in E-XY (p < 0.05). Additionally, dnmt1, which is responsible for methylation maintenance, expressed at similar level in four groups (p > 0.05). dnmt3, tet2, and setd1b, which were responsible for methylation modification, expressed at significantly higher levels in E-XY compared to the C-XY (p < 0.05). Dnmt3 and tet2 were expressed at significantly higher levels in AI-XX than that in C-XX (p < 0.05). These results indicated that E2 and AI treatment lead to the aberrant genome-wide DNA methylation level and expression level of dnmt3, tet2, and setd1b in T. rubripes gonad.

Whole-genome assembly and annotation for the little yellow croaker (Larimichthys polyactis) provide insights into the evolution of hermaphroditism and gonochorism

The evolutionary direction of gonochorism and hermaphroditism is an intriguing mystery to be solved. The special transient hermaphroditic stage makes the little yellow croaker (Larimichthys polyactis) an appealing model for studying hermaphrodite formation. However, the origin and evolutionary relationship between of L. polyactis and Larimichthys crocea, the most famous commercial fish species in East Asia, remain unclear. Here, we report the sequence of the L. polyactis genome, which we found is ~706 Mb long (contig N50 = 1.21 Mb and scaffold N50 = 4.52 Mb) and contains 25,233 protein-coding genes. Phylogenomic analysis suggested that L. polyactis diverged from the common ancestor, L. crocea, approximately 25.4 million years ago. Our high-quality genome assembly enabled comparative genomic analysis, which revealed several within-chromosome rearrangements and translocations, without major chromosome fission or fusion events between the two species. The dmrt1 gene was identified as the male-specific gene in L. polyactis. Transcriptome analysis showed that the expression of dmrt1 and its upstream regulatory gene (rnf183) were both sexually dimorphic. Rnf183, unlike its two paralogues rnf223 and rnf225, is only present in Larimichthys and Lates but not in other teleost species, suggesting that it originated from lineage-specific duplication or was lost in other teleosts. Phylogenetic analysis shows that the hermaphrodite stage in male L. polyactis may be explained by the sequence evolution of dmrt1. Decoding the L. polyactis genome not only provides insight into the genetic underpinnings of hermaphrodite evolution, but also provides valuable information for enhancing fish aquaculture.

Development of Ovaries and Sex Change in Fish: Bringing Potential into Action

BACKGROUND

Encompassing about half of the 60,000 species of vertebrates, fish display the greatest diversity of sex determination mechanisms among metazoans. As such that phylum offers a unique playground to study the impressive variety of gonadal morphogenetic strategies, ranging from gonochorism, with either genetic or environmental sex determination, to unisexuality, with either simultaneous or consecutive hermaphroditism.

SUMMARY

From the two main types of gonads, the ovaries embrace the important role to produce the larger and non-motile gametes, which is the basis for the development of a future organism. The production of the egg cells is complex and involves the formation of follicular cells, which are necessary for the maturation of the oocytes and the production of feminine hormones. In this vein, our review focuses on the development of ovaries in fish with special emphasis on the germ cells, including those that transition from one sex to the other as part of their life cycle and those that are capable of transitioning to the opposite sex depending on environmental cues.

KEY MESSAGES

Clearly, establishing an individual as either a female or a male is not accomplished by the sole development of two types of gonads. In most cases, that dichotomy, be it final or transient, is accompanied by coordinated transformations across the entire organism, leading to changes in the physiological sex as a whole. These coordinated transformations require both molecular and neuroendocrine networks, but also anatomical and behavioural adjustments. Remarkably, fish managed to tame the ins and outs of sex reversal mechanisms to take the most advantages of changing sex as adaptive strategies in some situations.

The Ovarian Transcriptome at the Early Stage of Testis Removal-Induced Male-To-Female Sex Change in the Protandrous Black Porgy Acanthopagrus schlegelii

Unlike gonochoristic fishes, sex is fixed after gonadal differentiation (primary sex determination), and sex can be altered in adults (secondary sex determination) of hermaphroditic fish species. The secondary sex determination of hermaphroditic fish has focused on the differences between testicular tissue and ovarian tissue during the sex change process. However, comprehensive studies analyzing ovarian tissue or testicular tissue independently have not been performed. Hermaphroditic black porgy shows a digonic gonad (ovarian tissue with testicular tissue separated by connective tissue). Protandrous black porgy has stable maleness during the first two reproductive cycles (&lt;2 years old), and approximately 50% enter femaleness (natural sex change) during the third reproductive cycle. Precocious femaleness is rarely observed in the estradiol-17β (E2)-induced female phase (oocytes maintained at the primary oocyte stage), and a reversible female-to-male sex change is found after E2 is withdrawn in &lt;2-year-old fish. However, precocious femaleness (oocytes entering the vitellogenic oocyte stage) is observed in testis-removed fish in &lt;2-year-old fish. We used this characteristic to study secondary sex determination (femaleness) in ovarian tissue via transcriptomic analysis. Cell proliferation analysis showed that BrdU (5-bromo-2′-deoxyuridine)-incorporated germline cells were significantly increased in the testis-removed fish (female) compared to the control (sham) fish (male) during the nonspawning season (2 months after surgery). qPCR analysis showed that there were no differences in pituitary-releasing hormones (lhb and gtha) in pituitary and ovarian steroidogenesis-related factors (star, cyp11a1, hsd3b1, and cyp19a1a) or female-related genes (wnt4a, bmp15, gdf9, figla, and foxl2) in ovarian tissues between intact and testis-removed fish (2 months after surgery). Low expression of pituitary fshb and ovarian cyp17a1 was found after 2 months of surgery. However, we did find small numbers of genes (289 genes) showing sexual fate dimorphic expression in both groups by transcriptomic analysis (1 month after surgery). The expression profiles of these differentially expressed genes were further examined by qPCR. Our present work identified several candidate genes in ovarian tissue that may be involved in the early period of secondary sex determination (femaleness) in black porgy. The data confirmed our previous suggestion that testicular tissue plays an important role in secondary sex determination in protandrous black porgy.

Evolutionary dynamics and conserved function of the Tudor domain-containing (TDRD) proteins in teleost fish

Tudor domain-containing (TDRD) proteins, the germline enriched protein family, play essential roles in the process of gametogenesis and genome stability through their interaction with the PIWI-interacting RNA (piRNA) pathway. Several studies have suggested the rapid evolution of the piRNA pathway in teleost lineages with striking reproductive diversity. However, there is still limited information about the function and evolution of Tdrd genes in teleost species. In this study, through genome wide screening, 13 Tdrd family genes were identified in economically important aquaculture fish, including spotted sea bass (Lateolabrax maculatus), Asian sea bass (Lates calcarifer), and tongue sole (Cynoglossus semilaevis). With copy number, structure, phylogeny, and synteny analysis, duplication of Tdrd6 and Tdrd7, as well as loss of Stk31 and Tdrd10, were characterized in teleost lineages. Codon based molecular evolution analysis indicated faster evolution of teleost Tdrd genes than that in mammals, potentially associated with the accelerated evolution of the piRNA pathway in teleost lineages. The evolutionary diversity of Tdrd genes was also detected between different teleost lineages. RNA-seq analysis showed that most teleost Tdrd genes were dominantly expressed in gonads, particularly highly expressed in testis, such as Tdrd6, Tdrd7a, Tdrd9, Ecat8, and Tdrd15. The varied expression and evolutionary pattern between the duplicated Tdrd6 and Tdrd7 in teleosts may indicate their functional diversification. All these results suggest a conserved function of teleost Tdrd family in gametogenesis and the piRNA pathway, which could lay a foundation for the evolution of Tdrd genes and be helpful for further deciphering of Tdrd functions in teleosts.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-021-00118-7.

Comparative Proteomics of the Acanthopagrus schlegelii Gonad in Different Sex Reversal

A substantial proportion of Acanthopagrus schlegelii individuals change sex from male to female during their lifetime. However, the mechanisms underlying sex change are unknown. In this research, iTRAQ analyses of proteins obtained from A.schlegelii gonads in four different stages of development were compared. In total, 4692 proteins were identified, including common sex-specific proteins, such as sperm-associated antigen 6 and cilia- and flagella-associated proteins in males, and zona pellucida sperm-binding proteins in females. Furthermore, proteins involved in the integrin signaling pathway, inflammation mediated by the chemokine and cytokine signaling pathways, pyruvate metabolism, CCKR signaling map, de novo purine biosynthesis and the ubiquitin proteasome pathway were upregulated in female gonads, whereas proteins implicated in DNA replication, the heterotrimeric G-protein signaling pathway, Gi alpha- and Gs alpha-mediated pathways, wnt signaling pathway, and hedgehog signaling pathway were upregulated in male gonads. Interestingly, cathepsins were only identified in ovaries, indicating their potential involvement in rapid ovarian development. Apoptosis-related proteins expressed in ovaries (such as MAPK and Cdc42) may protect them from cancer. This is the first report on the gonad proteome from A.schlegelii in different stages of sex reversal, and the results provide important fundamental data for studying the molecular mechanisms of sex reversal.

Roles of anti-Müllerian hormone and its duplicates in sex determination and germ cell proliferation of Nile tilapia

Duplicates of amh are crucial for fish sex determination and differentiation. In Nile tilapia, unlike in other teleosts, amh is located on X chromosome. The Y chromosome amh (amhΔ-y) is mutated with 5 bp insertion and 233 bp deletion in the coding sequence, and tandem duplicate of amh on Y chromosome (amhy) has been identified as the sex determiner. However, the expression of amh, amhΔ-y, and amhy, their roles in germ cell proliferation and the molecular mechanism of how amhy determines sex is still unclear. In this study, expression and functions of each duplicate were analyzed. Sex reversal occurred only when amhy was mutated as revealed by single, double, and triple mutation of the 3 duplicates in XY fish. Homozygous mutation of amhy in YY fish also resulted in sex reversal. Earlier and higher expression of amhy/Amhy was observed in XY gonads compared with amh/Amh during sex determination. Amhy could inhibit the transcription of cyp19a1a through Amhr2/Smads signaling. Loss of cyp19a1a rescued the sex reversal phenotype in XY fish with amhy mutation. Interestingly, mutation of both amh and amhy in XY fish or homozygous mutation of amhy in YY fish resulted in infertile females with significantly increased germ cell proliferation. Taken together, these results indicated that up-regulation of amhy during the critical period of sex determination makes it the sex-determining gene, and it functions through repressing cyp19a1a expression via Amhr2/Smads signaling pathway. Amh retained its function in controlling germ cell proliferation as reported in other teleosts, while amhΔ-y was nonfunctionalized.

Involvement of Transforming Growth Factor Beta Family Genes in Gonadal Differentiation in Japanese Eel, Anguilla japonica, According to Sex-Related Gene Expressions

The gonochoristic feature with environmental sex determination that occurs during the yellow stage in the eel provides an interesting model to investigate the mechanisms of gonadal development. We previously studied various sex-related genes during gonadal sex differentiation in Japanese eels. In the present study, the members of transforming growth factor beta (TGF-β) superfamily were investigated. Transcript levels of anti-Müllerian hormone, its receptor, gonadal soma-derived factor (amh, amhr2, and gsdf, respectively) measured by real-time polymerase chain reaction (qPCR) showed a strong sexual dimorphism. Transcripts were dominantly expressed in the testis, and their levels significantly increased with testicular differentiation. In contrast, the expressions of amh, amhr2, and gsdf transcripts were low in the ovary of E2-feminized female eels. In situ hybridization detected gsdf (but not amh) transcript signals in undifferentiated gonads. amh and gsdf signals were localized to Sertoli cells and had increased significantly with testicular differentiation. Weak gsdf and no amh signals were detected in early ovaries of E2-feminized female eels. Transcript levels of amh and gsdf (not amhr2) decreased during human chorionic gonadotropin (HCG)-induced spermatogenesis in males. This study suggests that amh, amhr2, and especially gsdf might be involved in the gene pathway regulating testicular differentiation of Japanese eels.

Might Gene Duplication and Neofunctionalization Contribute to the Sexual Lability Observed in Fish?

Sex determination and differentiation varies widely across vertebrates, but is most dramatically diverse in fishes. Among fishes sex reversal and sex change are observed in 41 teleost families spanning 7 orders. These sex-changing fish perhaps highlight better than any other system that sex determination is not the narrow and fixed construct we once thought, but a plastic trait that is better viewed as a reaction norm. However, while this stunning transformation is increasingly understood, a fundamental question arises, which is why some fish species have retained this inherent plasticity in sexual fate, while others have not? Here, we explore our current understanding of sex change in fish, some of the factors that permit and constrain sex reversal, and posit that gene duplication and neofunctionalization contribute to the sexual lability observed in fish.

Environmental Cues and Mechanisms Underpinning Sex Change in Fish

Fishes are the only vertebrates that undergo sex change during their lifetime, but even within this group, a unique reproductive strategy is displayed by only 1.5% of the teleosts. This lability in alternating sexual fate is the result of the simultaneous suppression and activation of opposing male and female networks. Here, we provide a brief review summarizing recent advances in our understanding of the environmental cues that trigger sex change and their perception, integration, and translation into molecular cascades that convert the sex of an individual. We particularly focus on molecular events underpinning the complex behavioral and morphological transformation involved in sex change, dissecting the main molecular players and regulatory networks that shape the transformation of one sex into the opposite. We show that histological changes and molecular pathways governing gonadal reorganization are better described than the neuroendocrine basis of sex change and that, despite important advances, information is lacking for the majority of hermaphrodite species. We highlight significant gaps in our knowledge of how sex change takes place and suggest future research directions.

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

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

Molecular and cellular regulation on sex change in hermaphroditic fish, with a special focus on protandrous black porgy, Acanthopagrus schlegelii

In teleost fish, sex can be determined by genetic factors, environmental factors, or both. Unlike in gonochoristic fish, in which sex is fixed in adults, sex can change in adults of hermaphroditic fish species. Thus, sex is generated during the initial gonadal differentiation stage (primary sex differentiation) and later during sexual fate alternation (secondary sex differentiation) in hermaphroditic fish species. Depending on the species, sex phase alternation can be induced by endogenous cues (such as individual age and body size) or by social cues (such as sex ratio or relative body size within the population). In general, the fluctuation in plasma estradiol-17β (E2) levels is correlated with the sexual fate alternation in hermaphroditic fish. Hormonal treatments can artificially induce sexual phase alternation in sequential hermaphroditic fishes, but in a transient and reversible manner. This is the case for the E2-induced female phase in protandrous black porgy and the methyltestosterone (MT)- or aromatase inhibitor (AI)-induced male phase in protogynous grouper. Recent reviews have focused on the different forms of sex change in fish who undergo sequential sex change, especially in terms of gene expression and the role of hormones. In this review, we use the protandrous black porgy, a nonsocial cue-influenced hermaphroditic species, with digonic gonads (ovarian and testis separated by a connective tissue), as a model to describe our findings and discuss the molecular and cellular regulation of sexual fate determination in hermaphroditic fish.

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

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

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.

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

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

The Genetics and Epigenetics of Sex Change in Fish

Fish show extraordinary sexual plasticity, changing sex naturally as part of their life cycle or reversing sex because of environmental stressors. This plasticity shows that sexual fate is not an irreversible process but the result of an ongoing tug-of-war for supremacy between male and female signaling networks. The behavioral, gonadal, and morphological changes involved in this process are well described, yet the molecular events that underpin those changes remain poorly understood. Epigenetic modifications emerge as a critical link between environmental stimuli, the onset of sex change, and subsequent maintenance of sexual phenotype. Here we synthesize current knowledge of sex change, focusing on the genetic and epigenetic processes that are likely involved in the initiation and regulation of sex change. We anticipate that better understanding of sex change in fish will shed new light on sex determination and development in vertebrates and on how environmental perturbations affect sexual fate.

Activation of the brain-pituitary-gonadotropic axis in the black porgy Acanthopagrus schlegelii during gonadal differentiation and testis development and effect of estradiol treatment

Previous studies revealed an estradiol (E2)-dependent peak in brain activity, including neurosteroidogenesis and neurogenesis in the black porgy during the gonadal differentiation period. The brain-pituitary-gonadotropic axis is a key regulator of reproduction and may also be involved in gonadal differentiation, but its activity and potential role in black porgy during the gonadal differentiation period is still unknown. The present study analyzed the expression of regulatory factors involved in the gonadotropic axis at the time of gonadal differentiation (90, 120, 150 days after hatching [dah]) and subsequent testicular development (180, 210, 300 dah). In agreement with previous studies, expression of brain aromatase cyp19a1b peaked at 120 dah, and this was followed by a gradual increase during testicular development. The expression of gonadotropin subunits increased slightly but not significantly during gonadal differentiation and then increased significantly at 300 dah. In contrast, the expression of brain gnrh1 and pituitary gnrh receptor 1 (gnrhr1) exhibited a pattern with two peaks, the first at 120 dah, during the period of gonadal differentiation, and the second peak during testicular development. Gonad fshr and lhcgr increased during gonadal differentiation period with highest transcript level in prespawning season during testicular development. This suggests that the early activation of brain gnrh1, pituitary gnrhr1 and gths, and gonad gthrs might be involved in the control of gonadal differentiation. E2 treatment increased brain cyp19a1b expression at each sampling time, in agreement with previous studies in black porgy and other teleosts. E2 also significantly stimulated the expression of pituitary gonadotropin subunits at all sampling times, indicating potential E2-mediated steroid feedback. In contrast, no significant effect of E2 was observed on gnrh1. Moreover, treatment of AI or E2 had no statistically significant effect on brain gnrh1 transcription levels during gonadal differentiation. This indicated that the early peak of gnrh1 expression during the gonadal differentiation period is E2-independent and therefore not directly related to the E2-dependent peak in brain neurosteroidogenesis and neurogenesis also occurring during this period in black porgy. Both E2-independent and E2-dependent mechanisms are thus involved in the peak expression of various genes in the brain of black porgy at the time of gonadal differentiation.

Socially controlled male-to-female sex reversal in the protogynous orange-spotted grouper, Epinephelus coioides

Socially controlled sex change in teleosts is a dramatic example of adaptive reproductive plasticity. In many cases, the occurrence of sex change is triggered by a change in the social context, such as the disappearance of the dominant individual. The orange-spotted grouper Epinephelus coioides is a typical protogynous hermaphrodite fish that changes sex from female to male and remains male throughout its life span. In this study, male-to-female sex reversal in male Epinephelus coioides was successfully induced by social isolation. The body length and mass, gonadal change, serum sex steroid hormone levels and sex-related gene expression patterns during the process of socially controlled male-to-female sex reversal in E. coioides were systematically examined. This report investigates the physiological mechanisms of the socially controlled male-to-female sex reversal process in a protogynous hermaphrodite grouper species. The results enable us to study the physiological control of sex change, not only from female to male, but also from male to female.

Natural sex change in fish

Sexual fate can no longer be considered an irreversible deterministic process that once established during early embryonic development, plays out unchanged across an organism's life. Rather, it appears to be a dynamic process, with sexual phenotype determined through an ongoing battle for supremacy between antagonistic male and female developmental pathways. That sexual fate is not final and is actively regulated via the suppression or activation of opposing genetic networks creates the potential for flexibility in sexual phenotype in adulthood. Such flexibility is seen in many fish, where sex change is a usual and adaptive part of the life cycle. Many fish are sequential hermaphrodites, beginning life as one sex and changing sometime later to the other. Sequential hermaphrodites include species capable of female-to-male (protogynous), male-to-female (protandrous), or bidirectional (serial) sex change. These natural forms of sex change involve coordinated transformations across multiple biological systems, including behavioral, anatomical, neuroendocrine and molecular axes. Here we review the biological processes underlying this amazing transformation, focusing particularly on the molecular aspects, where new genomic technologies are beginning to help us understand how sex change is initiated and regulated at the molecular level.

The Gene Toolkit Implicated in Functional Sex in Sparidae Hermaphrodites: Inferences From Comparative Transcriptomics

Sex-biased gene expression is the mode through which sex dimorphism arises from a nearly identical genome, especially in organisms without genetic sex determination. Teleost fishes show great variations in the way the sex phenotype forms. Among them, Sparidae, that might be considered as a model family displays a remarkable diversity of reproductive modes. In this study, we sequenced and analyzed the sex-biased transcriptome in gonads and brain (the tissues with the most profound role in sexual development and reproduction) of two sparids with different reproductive modes: the gonochoristic common dentex, Dentex dentex, and the protandrous hermaphrodite gilthead seabream, Sparus aurata. Through comparative analysis with other protogynous and rudimentary protandrous sparid transcriptomes already available, we put forward common male and female-specific genes and pathways that are probably implicated in sex-maintenance in this fish family. Our results contribute to the understanding of the complex processes behind the establishment of the functional sex, especially in hermaphrodite species and set the groundwork for future experiments by providing a gene toolkit that can improve efforts to control phenotypic sex in finfish in the ever-increasingly important field of aquaculture.

Initiation of sex change and gonadal gene expression in black sea bass (Centropristis striata) exposed to exemestane, an aromatase inhibitor

Many teleost fishes exhibit sequential hermaphroditism, where male or female gonads develop first and later undergo sex change. Model sex change species are characterized by social hierarchies and coloration changes, which enable experimental manipulations to better understand these processes. However, other species such as the protogynous black sea bass (Centropristis striata) do not exhibit these characteristics and instead receive research attention due to their importance in fisheries or aquaculture. Black sea bass social structure is unknown, which makes sex change sampling difficult, and few molecular resources are available. The purpose of the present study was to induce sex change using exemestane, an aromatase inhibitor, and assess gonadal gene expression using sex markers (amh, zpc2) and genes involved in steroidogenesis (cyp19a1a, cyp11b), estrogen signaling (esr1, esr2b), and apoptosis or atresia (aen, casp9, fabp11, parg, pdcd4, rif1). Overall, dietary exemestane treatment was effective, and most exposed females exhibited early histological signs of sex change and significantly higher rates of ovarian atresia relative to control females. Genes associated with atresia did not reflect this, however, as expression patterns in sex changing gonads were overall similar to those of ovaries, likely due to a whole ovary dilution effect of the RNA. Still, small but insignificant expression decreases during early sex change were detected for ovary-related genes (aen, casp9, fabp11, zpc2) and anti-apoptotic factors (parg, rif1). Exemestane treatment did not impact spermatogenesis or testicular gene expression, but testes were generally characterized by elevated steroidogenic enzyme and estrogen receptor mRNAs. Further research will be needed to understand these processes in black sea bass, using isolated ovarian follicles and multiple stages of sex change.

MT-Feeding-Induced Impermanent Sex Reversal in the Orange-Spotted Grouper during Sex Differentiation

In this study, we systematically investigated the process of sex reversal induced by 17-methyltestosterone (MT) feeding and MT-feeding withdrawal at the ovary differentiation stage in orange-spotted groupers, Epinephelus coioides. Gonadal histology showed that MT feeding induced a precocious sex reversal from immature ovaries to testes, bypassing the formation of an ovarian cavity, and MT-feeding withdrawal led to an ovarian fate. In both the MT feeding and MT-feeding withdrawal phases, cytochrome P450 family 11 subfamily B (cyp11b) gene expression and serum 11-KT levels were not significantly changed, suggesting that the MT-treated fish did not generate endogenous steroids, even though active spermatogenesis occurred. Finally, by tracing doublesex-expressing and Mab-3-related transcription factor 1 (dmrt1)-expressing cells and TUNEL (terminal deoxynucleotidyl transferase 2-deoxyuridine, 5-triphosphate nick end labeling) assays, we found that the efferent duct formed first, and then, the germ cells and somatic cells of the testicular tissue were generated around the efferent duct during MT-feeding-induced precocious sex reversal. Collectively, our findings provide insights into the molecular and cellular mechanisms underlying sex reversal induced by exogenous hormones during sex differentiation in the protogynous orange-spotted grouper.

Experimental evidence of masculinization by continuous illumination in a temperature sex determination teleost (Atherinopsidae) model: is oxidative stress involved?

The present study evaluates the influence of continuous light on phenotypic sex ratios in Chirostoma estor, a temperature sex determination animal model. Relative gene expression levels of 5 day old larvae were performed on two early gonad differentiation genes (sox9 and foxl2), two stress axis activation genes (gcr1 and crf) and four reactive oxygen species (ROS) antagonist effector genes (sod2, ucp2, gsr and cat). Two light treatments were applied from fertilization; control (12L:12D) simulated natural photoperiod and a continuous illumination photoperiod. By the end of the trial (12 weeks after hatching), differentiated and normal gonads were clearly identifiable in both treatments by histological observations. Regarding sex ratio, 73% of phenotypic males were found in continuous illumination compared with 40% in controls. Consistently, the sox9 gene (involved in early testis differentiation) showed an over expression in 64% of the individual larvae analysed compared with foxl2 (ovarian differentiation) suggesting a masculinization tendency in continuous illumination. On the other hand, only 36% of individuals showed the same tendency in the control treatment consistent with phenotypic sex ratios found under normal culture conditions. Relative gene expression results did not show significant difference in sod2, ucp2 and gcr1 levels, but cat, gsr and crf showed significantly higher expression levels in the continuous illumination treatment suggesting that both, the stress axis and ROS response mechanisms were activated at this time. This study suggests, a link between continuous light, oxidative stress and environmental sex determination in vertebrates. However, further research is necessary to describe this possible upstream mechanism that may drive some aspects of sexual plasticity in vertebrates.

Age and growth of black seabream Acanthopagrus schlegelii(Sparidae) in Hong Kong and adjacent waters of the northern South China Sea

Age and growth of the black seabream Acanthopagrus schlegelii (family Sparidae) from the northern South China Sea (NSCS) were studied by reading growth rings in sectioned sagittal otoliths. Ring formation frequency was determined to be annual by using marginal increment analysis. The von Bertalanffy growth function parameters were estimated as: L_∞  = 43.7 cm L_S ; K =0.22 year; t₀  = -1.59 years. Functional males are significantly younger than functional females, with sexually transitional individuals between the modal ages of males and females supporting protandry in this species. Males become sexually mature within 1 year and 50% age at sex change is at 2 years. The maximum age recorded for both males and females sampled was 9 years which is lower than for conspecifics elsewhere and may reflect high fishing pressure in the study area when compared with conspecifics in other areas or could reflect latitudinal effects. Otolith mass was significantly and positively related to age, providing a cheap and quick alternative method for approximating age. Acanthopagrus schlegelii is a relatively fast-growing and rapidly maturing species attaining a similar asymptotic length to conspecifics. A need for fishery management is indicated to protect both young juveniles and older adults, especially females, to increase reproductive output and safeguard fishery production.

Primary males guide the femaleness through the regulation of testicular Dmrt1 and ovarian Cyp19a1a in protandrous black porgy

Controlling the development of the sexes is critically important for the broodstock management in aquaculture. Sex steroids are widely used for sex control of fish. However, hermaphroditic fish have a plastic sex, and a stable sex is difficult to maintain with sex steroids. We used the black porgy (Acanthopagrus schlegelii) as a model to understand the possible mechanism of sexual fate decision. Low exogenous estradiol (E2) induced male development. In contrast, high exogenous E2 induced the regression of the testis and the development of the ovary and resulted in an unstable expression of femaleness (passive femaleness, with ovaries containing only the primary oocytes). The removal of testicular tissue by surgery resulted in the early development of vitellogenic oocytes and active femaleness. Our data also demonstrated that the male-to-female sex change is blocked by the maintenance of male function with gonadotropin-induced dmrt1 expression in the testis. Furthermore, our data also indicated that ovarian cyp19a1a expression is regulated by the testis through epigenetic modifications. Therefore, the primary male guides the femaleness in the protandrous black porgy and the transition of sexual fate from male to female is determined by the status of the testicular tissue.

Action of the Metalloproteinases in Gonadal Remodeling during Sex Reversal in the Sequential Hermaphroditism of the Teleostei Fish Synbranchus marmoratus (Synbranchiformes: Synbranchidae)

Teleostei present great plasticity regarding sex change. During sex reversal, the whole gonad including the germinal epithelium undergoes significant changes, remodeling, and neoformation. However, there is no information on the changes that occur within the interstitial compartment. Considering the lack of information, especially on the role played by metalloproteinases (MMPs) in fish gonadal remodeling, the aim of this study was to evaluate the action of MMPs on gonads of sex reversed females of Synbranchus marmoratus, a fresh water protogynic diandric fish. Gonads were processed for light microscopy and blood samples were used for the determination of plasma sex steroid levels. During sex reversal, degeneration of the ovaries occurred and were gradually replaced by the germinal tissue of the male. The action of the MMPs induces significant changes in the interstitial compartment, allowing the reorganization of germinal epithelium. Leydig cells also showed an important role in female to male reversion. The gonadal transition coincides with changes in circulating sex steroid levels throughout sex reversion. The action of the MMPs, in the gonadal remodeling, especially on the basement membrane, is essential for the establishment of a new functional germinal epithelium.

The transcriptomic signature of different sexes in two protogynous hermaphrodites: Insights into the molecular network underlying sex phenotype in fish

Sex differentiation is a puzzling problem in fish due to the variety of reproductive systems and the flexibility of their sex determination mechanisms. The Sparidae, a teleost family, reflects this remarkable diversity of sexual mechanisms found in fish. Our aim was to capture the transcriptomic signature of different sexes in two protogynous hermaphrodite sparids, the common pandora Pagellus erythrinus and the red porgy Pagrus pagrus in order to shed light on the molecular network contributing to either the female or the male phenotype in these organisms. Through RNA sequencing, we investigated sex-specific differences in gene expression in both species' brains and gonads. The analysis revealed common male and female specific genes/pathways between these protogynous fish. Whereas limited sex differences found in the brain indicate a sexually plastic tissue, in contrast, the great amount of sex-biased genes observed in gonads reflects the functional divergence of the transformed tissue to either its male or female character. Α common "crew" of well-known molecular players is acting to preserve either sex identity of the gonad in these fish. Lastly, this study lays the ground for a deeper understanding of the complex process of sex differentiation in two species with an evolutionary significant reproductive system.

Reproductive biology of black seabream Acanthopagrus schlegelii, threadfin porgy Evynnis cardinalis and red pargo Pagrus major in the northern South China Sea with consideration of fishery status and management needs

The reproductive biology of three commercially significant seabream species (family: Sparidae) Acanthopagrus schlegelii, Evynnis cardinalis and Pagrus major, taken from Hong Kong and adjacent northern South China Sea (SCS) waters, were investigated for their sexual patterns, spawning seasons, length at maturity and exploitation in relation to their conservation and management status. Histological analysis showed E. cardinalis and P. major to be functionally gonochoristic, the latter having a bisexual juvenile stage and being a rudimentary hermaphrodite. Acanthopagrus schlegelii is a protandric hermaphrodite. Standard length (L_S ) at 50% sex change for A. schlegelii is 291 mm. L_S at 50% female maturity for E. cardinalis and P. major are 117 and 332 mm, respectively. For all three species, the spawning period falls between November and March. The study highlights geographical differences in reproductive biology among the species and a paucity of fishery or other population-related data. While heavy fishing pressure, life-history characteristics and absence of effective management throughout the geographic ranges of these species make them susceptible to overfishing, they nonetheless appear to be generally more resilient than many other taxa that comprise the multi-species fisheries of the region, possibly due to their relatively rapid sexual maturation and spatial movement patterns. Overall, however, little is known of the biology, fishing history and current fishery status of sparids in general in the northern SCS and the current study is one of the first to examine such aspects of this family in the region and to consider appropriate management options.

Bending Genders: The Biology of Natural Sex Change in Fish

Sexual fate is no longer seen as an irreversible deterministic switch set during early embryonic development but as an ongoing battle for primacy between male and female developmental trajectories. That sexual fate is not final and must be actively maintained via continuous suppression of the opposing sexual network creates the potential for flexibility into adulthood. In many fishes, sexuality is not only extremely plastic, but sex change is a usual and adaptive part of the life cycle. Sequential hermaphrodites begin life as one sex, changing sometime later to the other, and include species capable of protandrous (male-to-female), protogynous (female-to-male), or serial (bidirectional) sex change. Natural sex change involves coordinated transformations across multiple biological systems, including behavioural, anatomical, neuroendocrine, and molecular axes. We here review the biological processes underlying this amazing transformation, focussing particularly on its molecular basis, which remains poorly understood, but where new genomic technologies are significantly advancing our understanding of how sex change is initiated and progressed at the molecular level. Knowledge of how a usually committed developmental process remains plastic in sequentially hermaphroditic fishes is relevant to understanding the evolution and functioning of sexual developmental systems in vertebrates generally, as well as pathologies of sexual development in humans.

Sex Change in Clownfish: Molecular Insights from Transcriptome Analysis

Sequential hermaphroditism is a unique reproductive strategy among teleosts that is displayed mainly in fish species living in the coral reef environment. The reproductive biology of hermaphrodites has long been intriguing; however, very little is known about the molecular pathways underlying their sex change. Here, we provide the first de novo transcriptome analyses of a hermaphrodite teleost´s undergoing sex change in its natural environment. Our study has examined relative gene expression across multiple groups-rather than just two contrasting conditions- and has allowed us to explore the differential expression patterns throughout the whole process. Our analysis has highlighted the rapid and complex genomic response of the brain associated with sex change, which is subsequently transmitted to the gonads, identifying a large number of candidate genes, some well-known and some novel, involved in the process. The present study provides strong evidence of the importance of the sex steroidogenic machinery during sex change in clownfish, with the aromatase gene playing a central role, both in the brain and the gonad. This work constitutes the first genome-wide study in a social sex-changing species and provides insights into the genetic mechanism governing social sex change and gonadal restructuring in protandrous hermaphrodites.

Sexual plasticity: A fishy tale

Teleost fish exhibit remarkably diverse and plastic patterns of sexual development. One of the most fascinating modes of plasticity is functional sex change, which is widespread in marine fish including species of commercial importance; however, the regulatory mechanisms remain elusive. In this review, we explore such sexual plasticity in fish, using the bluehead wrasse (Thalassoma bifasciatum) as the primary model. Synthesizing current knowledge, we propose that cortisol and key neurochemicals modulate gonadotropin releasing hormone and luteinizing hormone signaling to promote socially controlled sex change in protogynous fish. Future large-scale genomic analyses and systematic comparisons among species, combined with manipulation studies, will likely uncover the common and unique pathways governing this astonishing transformation. Revealing the molecular and neuroendocrine mechanisms underlying sex change in fish will greatly enhance our understanding of vertebrate sex determination and differentiation as well as phenotypic plasticity in response to environmental influences. Mol. Reprod. Dev. 84: 171-194, 2017. © 2016 Wiley Periodicals, Inc.

Molecular mechanism of endocrine system impairment by 17α-methyltestosterone in gynogenic Pengze crucian carp offspring

The effects of synthetic androgen 17α-methyltestosterone (MT) on endocrine impairment were examined in crucian carp. Immature 7-month old mono-female Pengze crucian carp (Pcc) F2 offspring were exposed to 50 and 100 μg/L of MT (week 2, 4, and 8). Gonadosomatic index, hepatosomatic index and intestine weight altered considerably and oocyte development was repressed. In the treatment groups, ovarian 11-ketotestosterone decreased, whereas 17β-estradiol and testosterone increased, and ovarian aromatase activities increased at week 4. However, in the brain tissue, those values significantly decreased. Quantitative RT-PCR analysis demonstrated changes in steroid receptor genes and upregulation of steroidogenic genes (Pcc-3bhsd, Pcc-11bhsd2 Pcc-cyp11a1), while the other three steroidogenic genes (Pcc-cyp17a1, Pcc-cyp19a1a and Pcc-star) decreased from week 4 to week 8. Ovarian, hepatic Pcc-vtg B and vitellogenin concentration increased in both 50 and 100 μg/L of MT exposure groups. This study adds further information regarding the effects of androgens on the development of previtellogenic oocytes, which suggests that MT could directly target estrogen signaling pathway, or indirectly affect steroidogenesis and vitellogenesis.

The effects of temperature on ovarian aromatase (cyp19a1a) expression and sex differentiation in summer flounder (Paralichthys dentatus)

Female summer flounder grow considerably faster and larger than males, and a tremendous increase in performance can therefore be realized through production of monosex female populations. Rearing temperature has been shown to affect sex differentiation in other teleost species by influencing expression of genes encoding transcription factors or enzymes involved in endocrine function. Cyp19a1a is a well-studied gene that had been shown to play a role in ovarian development, and exhibits sexually dimorphic expression in other species. In the present study, summer flounder (37 days post-hatch; DPH) were raised at 13, 16 or 19 °C. Fish from all three treatments were sampled throughout development and analyzed in qPCR to determine cyp19a1a gene expression levels. Sex ratios of additional fish grown to ≥150 mm at each temperature treatment were determined. Low female production was achieved overall (26.9, 17.6 and 0% at 13, 16 and 19 °C, respectively). Cyp19a1a expression was significantly lower at 52 DPH (~15 mm total length) at the male-producing temperature (19 °C) and increased to similar levels as other treatments at 66 DPH. Expression levels later in juvenile development (66-191 DPH) largely decreased with fish size. The period of sex differentiation in summer flounder remains unknown, but cyp19a1a expression patterns suggest that it may occur earlier in development than that of congenerics. Further research is necessary to understand the sex-determining mechanisms in this species before sexually dimorphic growth can be used to achieve economic advantages in commercial production.

Cloning and transcriptional expression of a novel gene during sex inversion of the rice field eel (Monopterus albus)

We performed annealing control primer (ACP)-based differential-display reverse transcription-polymerase chain reaction (DDRT-PCR) to isolate differentially expressed genes (DEGs) from the stage IV ovary and ovotestis of the rice field eel, Monopterus albus. Using 20 arbitrary ACP primers, 14 DEG expressed-sequence tags were identified and sequenced. The transcriptional expression of one DEG, G2, was significantly greater in the ovotestis than the stage IV ovary. To understand the role of G2 in sex inversion, G2 cDNA was cloned and semi-RT-PCR, real time PCR were performed during gonad development. The full-length G2 cDNA was 650 base pairs (bp) and it comprised a 5′-untranslated region (UTR) of 82 bp, a 3′-UTR of 121 bp and an open reading frame of 444 bp that encoded a 148-amino acid protein. The expression of G2 was weak during early ovarian development until the stage IV ovary, but expression increased significantly with gonad development. We speculate that G2 may play an important function during sex inversion and testis development in the rice field eel, but the full details of the function of this gene requires further research.

Large-scale transcriptome sequencing reveals novel expression patterns for key sex-related genes in a sex-changing fish

Background

Teleost fishes exhibit remarkably diverse and plastic sexual developmental patterns. One of the most astonishing is the rapid socially controlled female-to-male (protogynous) sex change observed in bluehead wrasses (Thalassoma bifasciatum). Such functional sex change is widespread in marine fishes, including species of commercial importance, yet its underlying molecular basis remains poorly explored.

Methods

RNA sequencing was performed to characterize the transcriptomic profiles and identify genes exhibiting sex-biased expression in the brain (forebrain and midbrain) and gonads of bluehead wrasses. Functional annotation and enrichment analysis were carried out for the sex-biased genes in the gonad to detect global differences in gene products and genetic pathways between males and females.

Results

Here we report the first transcriptomic analysis for a protogynous fish. Expression comparison between males and females reveals a large set of genes with sex-biased expression in the gonad, but relatively few such sex-biased genes in the brain. Functional annotation and enrichment analysis suggested that ovaries are mainly enriched for metabolic processes and testes for signal transduction, particularly receptors of neurotransmitters and steroid hormones. When compared to other species, many genes previously implicated in male sex determination and differentiation pathways showed conservation in their gonadal expression patterns in bluehead wrasses. However, some critical female-pathway genes (e.g., rspo1 and wnt4b) exhibited unanticipated expression patterns. In the brain, gene expression patterns suggest that local neurosteroid production and signaling likely contribute to the sex differences observed.

Conclusions

Expression patterns of key sex-related genes suggest that sex-changing fish predominantly use an evolutionarily conserved genetic toolkit, but that subtle variability in the standard sex-determination regulatory network likely contributes to sexual plasticity in these fish. This study not only provides the first molecular data on a system ideally suited to explore the molecular basis of sexual plasticity and tissue re-engineering, but also sheds some light on the evolution of diverse sex determination and differentiation systems.

Electronic supplementary material

The online version of this article (doi:10.1186/s13293-015-0044-8) contains supplementary material, which is available to authorized users.

The basement membrane and the sex establishment in the juvenile hermaphroditism during gonadal differentiation of the Gymnocorymbus ternetzi (Teleostei: Characiformes: Characidae)

Although there are several studies on morphogenesis in Teleostei, until now there is no research describing the role of the basement membrane in the establishment of the germinal epithelium during gonadal differentiation in Characiformes. In attempt to study these events that result in the formation of ovarian and testicular structures, gonads of Gymnocorymbus ternetzi were prepared for light microscopy. During gonadal development in G. ternetzi, all individuals first developed ovarian tissue. The undifferentiated gonad was formed by somatic cells (SC) and primordial germ cells (PGCs). After successive mitosis, the PGCs became oogonia, which entered into meiosis originating oocytes. An interstitial tissue developed. In half of the individuals, presumptive female, prefollicle cells synthesized a basement membrane around oocyte forming a follicle. Along the ventral region of the ovary, the tissue invaginated to form the ovigerous lamellae, bordered by the germinal epithelium. Stroma developed and the follicle complexes were formed. The gonadal aromatase was detected in interstitial cells in the early steps of the gonadal differentiation in both sexes. In another half of the individuals, presumptive male, there was no synthesis of basement membrane. The interstitium was invaded by numerous granulocytes. Pre-Leydig cells proliferated. Apoptotic oocytes were observed and afterward degenerated. Spermatogonia appeared near the degenerating oocytes and associated to SCs, forming testicular tubules. Germinal epithelium developed and the basement membrane was synthesized. Concomitantly, there was decrease of the gonadal aromatase and increase in the 3β-HSD enzyme expression. Thus, the testis was organized on an ovary previously developed, constituting an indirect gonochoristic differentiation.

Activation of brain steroidogenesis and neurogenesis during the gonadal differentiation in protandrous black porgy, Acanthopagrus schlegelii

The early brain development, at the time of gonadal differentiation was investigated using a protandrous teleost, black porgy. This natural model of monosex juvenile fish avoids the potential complexity of sexual dimorphism. Brain neurogenesis was evaluated by histological analyses of the diencephalon, at the time of testicular differentiation (in fish between 90 and 150 days after hatching). Increases in the number of both Nissl-stained total brain cells, and Pcna-immunostained proliferative brain cells were observed in specific area of the diencephalon, such as ventromedialis thalami and posterior preoptic area, revealing brain cell proliferation. qPCR analyses showed significantly higher expression of the radial glial cell marker blbp and neuron marker bdnf. Strong immunohistochemical staining of Blbp and extended cellular projections were observed. A peak expression of aromatase (cyp19a1b), as well as an increase in estradiol (E2 ) content were also detected in the early brain. These data demonstrate that during gonadal differentiation, the early brain exhibits increased E2 synthesis, cell proliferation, and neurogenesis. To investigate the role of E2 in early brain, undifferentiated fish were treated with E2 or aromatase inhibitor (AI). E2 treatment upregulated brain cyp19a1b and blbp expression, and enhanced brain cell proliferation. Conversely, AI reduced brain cell proliferation. Castration experiment did not influence the brain gene expression patterns and the brain cell number. Our data clearly support E2 biosynthesis in the early brain, and that brain E2 induces neurogenesis. These peak activity patterns in the early brain occur at the time of gonad differentiation but are independent of the gonads.

Genetic basis and biotechnological manipulation of sexual dimorphism and sex determination in fish

Aquaculture has made an enormous contribution to the world food production, especially to the sustainable supply of animal proteins. The utility of diverse reproduction strategies in fish, such as the exploiting use of unisexual gynogenesis, has created a typical case of fish genetic breeding. A number of fish species show substantial sexual dimorphism that is closely linked to multiple economic traits including growth rate and body size, and the efficient development of sex-linked genetic markers and sex control biotechnologies has provided significant approaches to increase the production and value for commercial purposes. Along with the rapid development of genomics and molecular genetic techniques, the genetic basis of sexual dimorphism has been gradually deciphered, and great progress has been made in the mechanisms of fish sex determination and identification of sex-determining genes. This review summarizes the progress to provide some directive and objective thinking for further research in this field.

Genetic architecture of sex determination in fish: applications to sex ratio control in aquaculture

Controlling the sex ratio is essential in finfish farming. A balanced sex ratio is usually good for broodstock management, since it enables to develop appropriate breeding schemes. However, in some species the production of monosex populations is desirable because the existence of sexual dimorphism, primarily in growth or first time of sexual maturation, but also in color or shape, can render one sex more valuable. The knowledge of the genetic architecture of sex determination (SD) is convenient for controlling sex ratio and for the implementation of breeding programs. Unlike mammals and birds, which show highly conserved master genes that control a conserved genetic network responsible for gonad differentiation (GD), a huge diversity of SD mechanisms has been reported in fish. Despite theory predictions, more than one gene is in many cases involved in fish SD and genetic differences have been observed in the GD network. Environmental factors also play a relevant role and epigenetic mechanisms are becoming increasingly recognized for the establishment and maintenance of the GD pathways. Although major genetic factors are frequently involved in fish SD, these observations strongly suggest that SD in this group resembles a complex trait. Accordingly, the application of quantitative genetics combined with genomic tools is desirable to address its study and in fact, when applied, it has frequently demonstrated a multigene trait interacting with environmental factors in model and cultured fish species. This scenario has notable implications for aquaculture and, depending upon the species, from chromosome manipulation or environmental control techniques up to classical selection or marker assisted selection programs, are being applied. In this review, we selected four relevant species or fish groups to illustrate this diversity and hence the technologies that can be used by the industry for the control of sex ratio: turbot and European sea bass, two reference species of the European aquaculture, and salmonids and tilapia, representing the fish for which there are well established breeding programs.

The sex-specific transcriptome of the hermaphrodite sparid sharpsnout seabream (Diplodus puntazzo)

Background

Teleosts are characterized by a remarkable breadth of sexual mechanisms including various forms of hermaphroditism. Sparidae is a fish family exhibiting gonochorism or hermaphroditism even in closely related species. The sparid Diplodus puntazzo (sharpsnout seabream), exhibits rudimentary hermaphroditism characterized by intersexual immature gonads but single-sex mature ones. Apart from the intriguing reproductive biology, it is economically important with a continuously growing aquaculture in the Mediterranean Sea, but limited available genetic resources. Our aim was to characterize the expressed transcriptome of gonads and brains through RNA-Sequencing and explore the properties of genes that exhibit sex-biased expression profiles.

Results

Through RNA-Sequencing we obtained an assembled transcriptome of 82,331 loci. The expression analysis uncovered remarkable differences between male and female gonads, while male and female brains were almost identical. Focused search for known targets of sex determination and differentiation in vertebrates built the sex-specific expression profile of sharpsnout seabream. Finally, a thorough genetic marker discovery pipeline led to the retrieval of 85,189 SNPs and 29,076 microsatellites enriching the available genetic markers for this species.

Conclusions

We obtained a nearly complete source of transcriptomic sequence as well as marker information for sharpsnout seabream, laying the ground for understanding the complex process of sex differentiation of this economically valuable species. The genes involved include known candidates from other vertebrate species, suggesting a conservation of the toolkit between gonochorists and hermaphrodites.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-655) contains supplementary material, which is available to authorized users.