De novo assembly and characterization of a highly degenerated ZW sex chromosome in the fish Megaleporinus macrocephalus

BACKGROUND

Megaleporinus macrocephalus (piauçu) is a Neotropical fish within Characoidei that presents a well-established heteromorphic ZZ/ZW sex determination system and thus constitutes a good model for studying W and Z chromosomes in fishes. We used PacBio reads and Hi-C to assemble a chromosome-level reference genome for M. macrocephalus. We generated family segregation information to construct a genetic map, pool sequencing of males and females to characterize its sex system, and RNA sequencing to highlight candidate genes of M. macrocephalus sex determination.

RESULTS

The reference genome of M. macrocephalus is 1,282,030,339 bp in length and has a contig and scaffold N50 of 5.0 Mb and 45.03 Mb, respectively. In the sex chromosome, based on patterns of recombination suppression, coverage, FST, and sex-specific SNPs, we distinguished a putative W-specific region that is highly differentiated, a region where Z and W still share some similarities and is undergoing degeneration, and the PAR. The sex chromosome gene repertoire includes genes from the TGF-β family (amhr2, bmp7) and the Wnt/β-catenin pathway (wnt4, wnt7a), some of which are differentially expressed.

CONCLUSIONS

The chromosome-level genome of piauçu exhibits high quality, establishing a valuable resource for advancing research within the group. Our discoveries offer insights into the evolutionary dynamics of Z and W sex chromosomes in fish, emphasizing ongoing degenerative processes and indicating complex interactions between Z and W sequences in specific genomic regions. Notably, amhr2 and bmp7 are potential candidate genes for sex determination in M. macrocephalus.

Transcriptome profiling of gonad-stimulating factors in thoracic ganglia and a potential role of Indian hedgehog gene in vitellogenesis of banana shrimp Fenneropenaeus merguiensis

Shrimp reproduction is controlled by several factors. Central nervous tissues, especially thoracic ganglia and brain, are known sources of gonad stimulating factors (GSFs) in crustaceans, but the GSFs in shrimp have not yet been clarified. Hence, we aimed to characterize and study putative GSFs from thoracic ganglia of adult female Fenneropenaeus merguiensis. An analysis of thoracic ganglia transcriptome revealed 3224 putative GSFs of a total 77,681 unigenes. Only 376 putative GSFs were differentially expressed during ovarian developmental stages. Eight candidate GSFs were validated for their expression patterns in thoracic ganglia, including the Indian hedgehog gene. F. merguiensis Indian hedgehog (FmIHH) was then investigated for its role in vitellogenesis. The obtained full-length cDNA of FmIHH was similar to other crustacean IHHs rather than Sonic and Desert HHs. The FmIHH was dominantly expressed in thoracic ganglia, and its expression was significantly increased in the vitellogenic stages before being downregulated at the mature stage of ovarian development. Injection of the recombinant FmIHH (His-TF-IHH) protein stimulated vitellogenin expression in ovaries on day 3 and 7, and also increased the gonadosomatic index. In addition, crustacean hyperglycemic hormone expression and total sugar were significantly decreased in eyestalks and hemolymph, respectively, after injection of His-TF-IHH, while lactic acid was increased. Both total sugar and lactic acid were unchanged in ovaries of His-TF-IHH injected shrimp. These results suggested that FmIHH plays a crucial role in vitellogenesis and regulate sugar uptake during ovarian development.

Comparative transcriptome analysis of gonads in male and female Pseudobagrus ussuriensis (Bagridae, Siluriformes)

As an important aquaculture fish in the Heilongjiang River Basin, Pseudobagrus ussuriensis has high economic value, and all-male culture is beneficial to the economic development of this fish. In this study, the transcriptomes of gonads in males and females were analyzed, and some genes related to gonad development were found. A total of 82,931 unigenes were found (average length 1504 bp, N50 1829 bp). In addition, 4689 differentially expressed genes (DEGs; including 1424 genes upregulated and 3265 genes downregulated in males) were identified. Some genes associated with testis development (such as Dmrt1 and Ropn1l) were significantly upregulated in males, while genes related to ovary development (such as Wnt2, PLC, Cyp19a, ZP3) were significantly downregulated in males, demonstrating that these genes have a crucial influence on gonad development in P. ussuriensis. Some signaling pathways related to gonad development were found, such as the Wnt pathway and oocyte meiosis. The results of RNA-seq obtained in this study provide theoretical data for elucidating the potential mechanism of gonad development of P. ussuriensis and reliable genomic data for the establishment of mono-sex breeding of P. ussuriensis.

Protein-protein networks analysis of differentially expressed genes unveils the key phenomenon of biological process with respect to reproduction in endangered catfish, C. Magur

Clarias magur (magur) is an important freshwater catfish with high potential in the aquaculture sector in its geographical ranges of distribution. One of the impediments to realise its full aquaculture potential is the lack of understanding key genes involved in its reproduction pathways. Nonetheless, very limited information is available on brain and gonads, with respect to reproduction related issues of magur at molecular level. The present study was aimed at understanding the interaction of the brain-gonad system by analysing differentially expressed genes (DEG) in brains and gonads of male and female magur using a protein-protein network interaction study. In brief, 641, 541, 225 and 245 DEGs, respectively, in ovary, testis and female brain and male-brain of magur were used as input in String database 11.0 and Cytoscape v 3.8.0 plug-in Network Analyzer for PPI network construction followed by network superimposition, network merging and analysis. A total of 13 key genes in female brain & ovary and 12 key genes in male brain & testis were obtained based on the network topological parameter betweenness centrality and nodes degree. Among them, cyp19a1b and amh genes in male brain-testis and Tp53 and exo1 genes in female brain-ovary were identified as hub genes having a high level of interaction and expression with other key genes in the network. Further, functional annotation study of these genes revealed their active involvement in important pathways related to reproduction. This is the first report exploring the interaction of brain and gonads in the regulation of magur reproduction through a protein-protein interaction network. The 25 key genes identified in the combined network are involved in various pathways, like neuropeptide signalling pathway, oxytocin receptor-mediated signalling pathway, corticotrophin-releasing factor receptor signalling pathway and reproduction process, which could lead to a better understanding of the magur reproductive system.

Comparative gonad transcriptome analysis in cobia (Rachycentron canadum)

Background: Cobia (Rachycentron canadum) is a species of fish with high commercial potential particularly due to fast growth rates. The evidence of sexual size dimorphism favoring females indicate potential benefits in having a monosex culture. However, the involvement of genetic factors responsible for sexual development and gonadal maintenance that produces phenotypic sex in cobia is largely unknown. Methods: In the present study, we performed transcriptome sequencing of cobia to identify sex-biased significantly differentially expressed genes (DEGs) in testes and ovaries. The reliability of the gonad transcriptome data was validated by qPCR analysis of eight selected significantly differential expressed sex-related candidate genes. Results: This comparative gonad transcriptomic analysis revealed that 7,120 and 4,628 DEGs are up-regulated in testes or ovaries, respectively. Further functional annotation analyses identified 76 important candidate genes involved in sex determination cascades or sex differentiation, including 42 known testis-biased DEGs (dmrt1, amh and sox9 etc.), and 34 known ovary-biased DEGs (foxl2, sox3 and cyp19a etc.). Moreover, eleven significantly enriched pathways functionally related to sex determination and sex differentiation were identified, including Wnt signaling pathway, oocyte meiosis, the TGF-beta signaling pathway and MAPK signaling pathway. Conclusion: This work represents the first comparative gonad transcriptome study in cobia. The putative sex-associated DEGs and pathways provide an important molecular basis for further investigation of cobia's sex determination, gonadal development as well as potential control breeding of monosex female populations for a possible aquaculture setting.

Effects of Secretoneurin and Gonadotropin-Releasing Hormone Agonist on the Spawning of Captive Greater Amberjack (Seriola dumerili)

The greater amberjack (Seriola dumerili), a pelagic marine species with a global distribution, has considerable worldwide potential as an aquaculture species. However, difficulties have been encountered in inducing spontaneous spawning in cultured fish stocks. In this study, we analysed the key regulatory factors, secretoneurin (SN) and gonadotropin-releasing hormone (GnRH), in greater amberjack. Active peptides of SN and GnRH, SdSNa, and SdGnRH, respectively, were obtained by comparative analysis of homologous proteins from different species. Amino acid substitutions of the SdGnRH decapeptide at position 6 with a dextrorotatory (D) amino acid and at position 10 with an ethylamide group yielded a super-active agonist (SdGnRHa). The injection of SdSNa and SdGnRHa elevated luteinizing hormone, thyroid-stimulating hormone, and oxytocin levels in the sera of sexually mature fish, whereas it reduced the level of follicle-stimulating hormone. Furthermore, in response to the SdSNa and SdGnRHa injections, we detected an increase in the expression of genes associated with oocyte development and spermatogenesis. We established that the greater amberjack cultured along the southern coast of China reached sexual maturity at three years of age, and its reproductive season extended from February to April. Spawning of the cultured greater amberjack was successfully induced with a single injection of SdGnRHa/SdSN/DOM/HCG. Our findings indicate that similar to GnRHa, SNa is a potential stimulator of reproduction that can be used to artificially induce spawning in marine fish.

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 (<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 <2-year-old fish. However, precocious femaleness (oocytes entering the vitellogenic oocyte stage) is observed in testis-removed fish in <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.

Sex-Specific Genomic Region Identification and Molecular Sex Marker Development of Rock Bream (Oplegnathus fasciatus)

Rock bream (Oplegnathus fasciatus) is a valuable commercial marine teleost species, which exhibits sexual dimorphism in growth performance. However, the absence of a rapid and cost-effective sex identification method based on sex-specific genetic marker has impeded study on sex determination mechanisms and breeding applications. In the present study, we firstly developed the PCR method for identifying potential sex-specific sequences in Oplegnathus fasciatus with the next-generation sequencing. Sex-specific genomic regions/loci for sex determination were discovered on Chr2 and Chr6 by genome-wide association analysis, sequencing depth, and heterozygosity comparison between females and males. Candidate sex-determining genes (CCDC63, ITR, WNT4) were furtherly detected in transcriptome data of testes and ovaries. Taken together, a male-specific 34-bp deletion on the Chr2 was identified and developed into molecular marker of sex for O. fasciatus. After validation in individuals with known phenotypic sexes, the accuracy was 100%. This study gives an insight into the mechanism of sex determination in O. fasciatus, and the gender marker is crucial both for future genomic research and for development of efficient and sustainable aquaculture practice.

Mode of uterine milk secretion in the white shark

Examination of the uterus of a dead female white shark (Carcharodon carcharias), which contained the earliest known white shark embryos, revealed that the uterine wall produces lipid-rich secretion (histotroph or "uterine milk") for embryonic nutrition. Uterine tissue was processed for light and electron microscopy, and immunohistochemical techniques to identify its secretory mechanism. Our results indicate that the white shark uterus secretes lipids via holocrine secretion. This type of secretion is characterized by the release of large lipid droplets accumulated in the epithelial cells into the uterine lumen through cell disintegration. The secretory epithelium of the uterus is stratified, and new surface epithelial cells are continuously supplied from deeper epithelial layers to replace the dead secretory cells at the surface. This vertical replacement possibly facilitates the active renewal of the surface epithelium, which is necessary for maintaining holocrine secretory mechanisms. These secretory mechanisms are different from those of myliobatiform stingrays, another elasmobranch taxon that exhibits lipid histotrophy. This may reflect the different origins of lipid histotrophy between these taxa. This article is protected by copyright. All rights reserved.

The significant sex-biased expression pattern of Sp-Wnt4 provides novel insights into the ovarian development of mud crab (Scylla Paramamosain)

The wingless-type MMTV integration site family member-4 (Wnt4), a member of the wingless-related integration site (Wnt) family, is widely accepted as a key regulator of ovarian development in mammals. In this study, a full-length cDNA of Wnt4 (designated as Sp-Wnt4) was cloned, characterized, and functionally studied in mud crab (Scylla paramamosain). The full-length cDNA of Sp-Wnt4 consists of 2659 bp with an open reading frame (ORF) encoding 359 amino acids, a 907 bp 5'-UTR and a 672 bp 3'-UTR. Sp-Wnt4 contains 25 cysteine (Cys) residues and three potential N-glycosylation sites. Sp-Wnt4 protein shared the highest identity (98.9%) to the Wnt4 protein of Portunus trituberculatus. The phylogenetic tree showed that Sp-Wnt4 and Wnt4 protein of Malacostracan crustaceans clustered together, indicating that they had a close genetic distance. Sp-Wnt4 was expressed at a higher level in the ovary compared to other tissues, with the highest expression level at the third stage (O-III) of the ovarian development (P < 0.05). A downward trend was observed in the expression level of Sp-Wnt4 from the embryo stage to crablet stages (P < 0.05). After unilateral eyestalk ablation, the expression level of Sp-Wnt4 significantly increased in testis (14-fold) and downregulated (3.1-fold) in the gill (P < 0.05) of females. In situ hybridization (ISH) assay revealed that Sp-Wnt4 transcripts were mainly localized in the cytoplasm of oocyte cells. These findings showed that Sp-Wnt4 play crucial roles in the ovarian development of S. paramamosain. In conclusion, our study provides novel insights into the evolution and roles of the Wnt4 gene.

Primordial Germ Cell Migration and Histological and Molecular Characterization of Gonadal Differentiation in Pachón Cavefish Astyanax mexicanus

The genetic regulatory network governing vertebrate gonadal differentiation appears less conserved than previously thought. Here, we investigated the gonadal development of Astyanax mexicanus Pachón cavefish by looking at primordial germ cells (PGCs) migration and proliferation, gonad histology, and gene expression patterns. We showed that PGCs are first detected at the 80% epiboly stage and then reach the gonadal primordium at 1 day post-fertilization (dpf). However, in contrast to the generally described absence of PGCs proliferation during their migration phase, PGCs number in cavefish doubles between early neurula and 8-9 somites stages. Combining both gonadal histology and vasa (germ cell marker) expression patterns, we observed that ovarian and testicular differentiation occurs around 65 dpf in females and 90 dpf in males, respectively, with an important inter-individual variability. The expression patterns of dmrt1, gsdf, and amh revealed a conserved predominant male expression during cavefish gonadal development, but none of the ovarian differentiation genes, i. e., foxl2a, cyp19a1a, and wnt4b displayed an early sexually dimorphic expression, and surprisingly all these genes exhibited predominant expression in adult testes. Altogether, our results lay the foundation for further research on sex determination and differentiation in A. mexicanus and contribute to the emerging picture that the vertebrate sex differentiation downstream regulatory network is less conserved than previously thought, at least in teleost fishes.

Interactive role of Wnt signaling and Zn in regulating testicular function of the common carp, Cyprinus carpio

Wnt signaling is conserved among all species and plays a significant role in various cellular processes including reproduction. The present study identified significant involvement of wnt4a, wnt5b, and wnt8a signaling in the testicular growth of common carp,Cyprinus carpio. Predominant expression of wnt4a, wnt5b, and wnt8a was found in the gonads and Wnt4a was localized in spermatocytes and interstitial cells. Ontogeny and testicular phase-wise analysis signified the importance of wnt isofoms analyzed in this study. Specific pathway activation of Wnt signaling revealed that Wnt4a and Wnt5b act through non-canonical while Wnt8a prefers the canonical pathway. The Wnt signaling regulates several steroidogenic enzyme and testis-related genes which was confirmed by the Wnt blockade experiments. Incidentally, zinc (Zn) is an essential trace element involved in the progression of spermatogenesis in teleosts. In adult male carp, a single administration of Zn at different doses elevated the expression of Wnt and Zn transporter genes and a single-dose (30 μg/g body weight [BW]) of Zn treatment elevated steroidogenic enzyme and testis-related genes which coincided with elevated androgens. Conversely, single-dose administration of Zn chelator to the Zn administered (30 μg/g BW) fish reversed the effects emphasizing a prominent role of Zn in the testicular function perhaps through Wnt signaling. Similar effects were observed in the in vitro experiments using the Zn chelator. Bioaccumulation of Zn and histological analysis revealed the importance of Zn in progression of spermatogenesis and sperm motility. Various assays related to cell viability and proliferation exhibited the role of Zn in promoting spermatogenic cell progression. Flow cytometric analysis confirmed Zn-induced elevation of Wnt and Zn transporter genes in germ and supporting cells. Furthermore, the effects of Zn are dose-related in carp. Taken together, it seems wnt4a, wnt5b, and wnt8a play an important role in testis and exposure of Wnt inhibitor, canonical as well as non-canonical activators, and Zn confirmed that Zn regulates Wnt signaling vis-à-vis promoting spermatogenesis in the common carp.

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.

Gonadal Transcriptome Analysis of Sex-Related Genes in the Protandrous Yellowfin Seabream (Acanthopagrus latus)

Yellowfin seabream (Acanthopagrus latus), a protandrous hermaphroditic fish, is a good model for studying the mechanism of sex reversal. However, limited knowledge is known about the genetic information related to reproduction and sex differentiation in this species. Here, we performed de novo transcriptome sequencing analysis of the testis, ovotestis, and ovary to identify sex-related genes in yellowfin seabream. The results assembled 71,765 unigenes in which 16,126 and 17,560 unigenes were differentially expressed in the ovotestis and ovary compared to the testis, respectively. The most differentially expressed gene (DEG)-enriched Kyoto Encyclopedia of Genes and Genomes and GO pathways were closely associated with the synthesis of sex steroid hormones. Functional analyses identified 55 important sex-related DEGs, including 32 testis-biased DEGs (dmrt1, amh, and sox9, etc.), 20 ovary-biased DEGs (cyp19a, foxl2, and wnt4, etc.), and 3 ovotestis-biased DEGs (lhb, dmrt2, and foxh1). Furthermore, the testis-specific expression of dmrt1 and the brain-pituitary-ovary axis expression of foxl2 were characterized, suggesting that they might play important roles in sex differentiation in yellowfin seabream. Our present work provided an important molecular basis for elucidating the mechanisms underlying sexual transition and reproductional regulation in yellowfin seabream.

Transcriptome of tambaqui Colossoma macropomum during gonad differentiation: Different molecular signals leading to sex identity

Tambaqui (Colossoma macropomum) is the major native species in Brazilian aquaculture, and we have shown that females exhibit a higher growth compared to males, opening up the possibility for the production of all-female population. To date, there is no information on the sex determination and differentiation molecular mechanisms of tambaqui. In the present study, transcriptome sequencing of juvenile trunks was performed to understand the molecular network involved in the gonadal sex differentiation. The results showed that before differentiation, components of the Wnt/β-catenin pathway, fox and fst genes imprint female sex development, whereas antagonistic pathways (gsk3b, wt1 and fgfr2), sox9 and genes for androgen synthesis indicate male differentiation. Hence, in undifferentiated tambaqui, the Wnt/β-catenin exerts a role on sex differentiation, either upregulated in female-like individuals, or antagonized in male-like individuals.

Two distinct vitellogenin genes are similar in function and expression in the bigfin reef squid Sepioteuthis lessoniana

Unlike vitellogenin, which is the sole major precursor of yolk protein in all oviparous vertebrates, a variety of major precursor of yolk proteins are found among oviparous invertebrates. Sea urchins have a transferrin-like yolk protein, while all other major precursors of yolk proteins in oviparous invertebrates belong to the superfamily of large lipid transfer proteins (LLTPs). However, a comprehensive understanding of vitellogenesis is absent in cephalopods. To understand control of vitellogenesis by the LLTPs gene, two vitellogenins (VTG1 and VTG2), two apolipophorins (APOLP2A and APOLP2B), and a cytosolic large subunit of microsomal triglyceride transfer protein (MTTP) found in the bigfin reef squid. Only the two VTGs showed high levels of expression in mature females compared to males. We further analyzed the expression profile and localization of both VTGs/VTGs during ovarian development. Our data showed that VTGs/VTGs expressions were correlated to the female reproductive cycle. Ovarian VTG1 and VTG2 were localized in the follicle cells but not in oocytes. In addition, VTG1 and VTG2 were represented in follicle cells and oocytes. Thus, our results showed that both VTGs were synthesized by follicle cells and are then delivered to oocytes. In addition, we demonstrated that VTGs were the major precursor of yolk protein in bigfin reef squid. We also found differential proteolytic cleavage processes of VTG1 and VTG2 during VTGs accumulation in oocytes. Therefore, our data shed light on the molecular mechanism of the yolk accumulation pathway in cephalopods.

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.

Cloning and characterization of wnt4a gene in a natural triploid teleost, Qi river crucian carp (Carassius auratus)

WNT4 (wingless-type MMTV integration site family, member 4) plays a key role in the ovarian differentiation and development in mammals. However, the possible roles of Wnt4 during gonadal differentiation and development need further clarification in teleosts. In this study, we cloned and characterized the full-length cDNA of Qi river crucian carp (Carassius auratus) wnt4a gene (CA-wnt4a). The cDNA of CA-wnt4a is 2337 bp, including the ORF of 1059 bp, encoding a putative protein with a transmembrane domain and a WNT family domain. Sequence and phylogenetic analyses revealed that the CA-Wnt4a identified is a genuine Wnt4a. Tissue distribution analysis showed that CA-wnt4a is expressed in all the tissues examined, including ovary. CA-wnt4a undergoes a stepwise increase in the embryonic stages, suggesting that CA-wnt4a might be involved in the early developmental stage. Ontogenic analysis demonstrated that CA-wnt4a expression is upregulated in the ovaries at 30-50 days after hatching (dah), the critical period of sex determination/differentiation in Qi river crucian carp. From 90 dah, the expression of CA-wnt4a was gradually downregulated in the developing ovaries. Immunohistochemistry demonstrated that CA-Wnt4a was expressed in the somatic and germ cells of the ovary by 30 dah, thereafter, positive signals of Wnt4a were detected in the somatic cells, oogonia and primary growth oocytes from 60 dah. In the sex-reversed testis induced by letrozole treatment, the expression level of CA-wnt4a was significantly downregulated. When CA-wnt4a expression was inhibited by injection of FH535 (an inhibitor of canonical Wnt/β-catenin signal pathway) in the ovaries, levels of cyp19a1a, foxl2 mRNA were significantly downregulated, while sox9b and cyp11c1 were upregulated, which suggested that together with Foxl2-leading estrogen pathway, CA-wnt4a signaling pathway might be involved in ovarian differentiation and repression of the male pathway gene expression in Qi river crucian carp.

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.

Comparative transcriptome analysis of diploid and triploid hybrid groupers (Epinephelus coioides♀ × E. lanceolatus♂) reveals the mechanism of abnormal gonadal development in triploid hybrids

In our previous studies, diploid and triploid hybrids have been detected from the hybridization of Epinephelus coioides♀ × E. lanceolatus♂. The triploid groupers have been found to be delayed in gonadal development, but the mechanism remains poorly understood. In this study, we examined the gonadal development, assayed the serum steroid hormone levels, and compared the BP (brain and pituitary) and G (gonad) transcriptomes of 18-month-old diploid and triploid hybrids. The results showed that levels of serum estradiol-17β and testosterone were significantly higher in triploid groupers. The RNA-seq data revealed that 1518 and 14,963 differentially expressed genes were identified in the BP and G transcriptome, respectively. Further analysis revealed that the expression levels of genes involved in the sexual differentiation pathway and sex steroid synthesis pathway are significantly higher in triploid hybrids. Our findings provided a comprehensive insight into a better understanding of the regulatory mechanisms of sterility in triploid hybrid fish.

Identification of critical sex-biased genes in Andrias davidianus by de novo transcriptome

The Chinese giant salamander Andrias davidianus is a protected amphibian with high nutritional and economic value. Understanding its sex determination mechanism is important for improving culture techniques and sex control in breeding. However, little information on the characterization of critical genes involved in sex is available. Herein, sequencing of ovary and test produced 40,783,222 and 46,128,902 raw reads, respectively, which were jointly assembled into 80,497 unigenes. Of these, 36,609 unigenes were annotated, of which 8907 were female-biased and 10,385 were male-biased. Several sex-related pathways were observed, including the Wnt signaling pathway. After elevated temperature and estrogen exposure, neomale and neofemale specimens were identified by a female-specific marker for the first time. RT-qPCR analysis showed the expression profile of ten selected sex-biased genes to be exhibited consistently in male and neomale and in female and neofemale, with the exception of the Amh and TfIIIa genes. Results suggested that these genes may play important roles in A. davidianus sex determination and gonad development. This provides a basis for further investigation of the molecular mechanisms of sex determination in amphibians.

Expression patterns of sex differentiation-related genes during gonadal sex change in the protogynous wrasse, Halichoeres trimaculatus

The three-spot wrasse, Halichoeres trimaculatus, can change sex from female to male (i.e. protogyny) due to sharp decrease in endogenous estrogen. During the sex change, ovarian tissue degenerates and testicular tissue arises newly. Finally, ovarian tissue disappears completely and replaces into mature testis. In order to predict the molecular mechanisms controlling the processes of sex change, we investigated the expression patterns of four genes (rspo1, figla, sox9b and amh), which have been thought to be associated with ovarian/testicular differentiation in vertebrates. Expression levels of rspo1 and figla, which play important roles for ovarian differentiation in vertebrates, were stable until the middle stage of the sex change, and subsequently down-regulated. Therefore, it was indicated that decrease in rspo1 and figla could result from ovarian degeneration. On the other hand, basis on the expression pattern, it was indicated that sox9b and amh, which are involved in testicular differentiation in vertebrates, were implicated in testicular formation and spermatogenesis during the sex change as well. The present results could be fundamental information for investigating the relationship between these factors and E2 depletion, which is crucial trigger for sex change.

Expression pattern and functional analysis of R-spondin1 in tongue sole Cynoglossus semilaevis

R-spondin 1 (Rspo1) is a potential female-determining gene in mammals that could regulate the Wnt/β-catenin signaling pathway. The deletion of Rspo1 causes sex reversal in females. To investigate sexual determination and differentiation, we cloned and analyzed the Rspo1 gene in Cynoglossus semilaevis. Phylogenetic and gene structure analyses revealed that Rspo1 gene exhibited high sequence conservation and contained an N-terminal signal peptide, two furin-like cysteine-rich domains (FU1 and FU2), a thrombospondin type 1 repeat, and a C-terminal region enriched with basic charged amino acids. qRT-PCR revealed that Rspo1 expressed sexual dimorphism in gonad, with higher expression levels in the ovary than in the testis, thus, suggesting the involvement of Rspo1 in gonad differentiation. In situ hybridization results demonstrated that Rspo1 was expressed in premature germ cells, including spermatogonia and spermatocytes in the testis and stage II and stage III oocytes in the ovary. The methylation levels in two CpG sites of Rspo1 promoter significantly differed among females, males, and pseudomales. After 30days of exposure to high temperature, the expression of Rspo1 significantly decreased in female individuals, some of which were prone to males. However, no difference of Rspo1 gene expression was observed between the control group and high-temperature group in males. These preliminary findings suggested that Rspo1 played a crucial role in sex determination and development. This study laid the groundwork for further sex control breeding techniques in C. semilaevis.

Robust gdf9 and bmp15 expression in the oocytes of ovotestes through the Figla-independent pathway in the hermaphroditic black porgy, Acanthopagrus schlegelii

More than 1,500 fish species are hermaphroditic, but no hermaphroditic lineage appears to be evolutionarily ancient in fishes. Thus, whether more than one sex at a time was present during the evolutionary shift from gonochorism to hermaphroditism in fishes is an intriguing question. Ectopic oocytes were created in the ovotestes of protandrous black porgy via the withdrawal of estradiol (E2) administration. These ectopic oocytes reprogrammed the surrounding cells, which changed from Sertoli cells to follicle-like cells. We observed that gdf9 and bmp15 expression was localized in the primary oocytes and gradually decreased after oocytes entered a secondary oocyte stage. Robust expression of gdf9 and bmp15 in ectopic oocytes was associated with the surrounding Sertoli cells. However, blocking Cyp19a1a activity and increasing androgen levels did not stimulate the expression of gdf9 and bmp15. Thus, the robust gdf9 and bmp15 expression was not related to the inappropriate male microenvironment. Furthermore, in vitro data demonstrated that gdf9 and bmp15 were not downstream genes of Figla signaling. Therefore, our results suggest that there are two independent mechanisms, a Figla-dependent pathway and a Figla-independent pathway, by which oocyte-surrounding cells are altered from a male somatic fate to a female somatic fate. This functional switch might clarify how oocytes created an appropriate microenvironment during the transition from the ancient gonochorism to the present hermaphroditism.

In vivo induction of human chorionic gonadotropin by osmotic pump advances sexual maturation during pre-spawning phase in adult catfish

Gonadal maturation is a critical event wherein gonads, under the influence of several hormones and factors, undergo cyclic morphological and physiological changes to produce functional gametes during the spawning phase. However, artificial induction can be effectively used to advance the maturation of gonad vis-à-vis spawning like behavior in seasonal breeders during the off-breeding season. In the present study, osmotic pumps loaded with 5000IU of human chorionic gonadotropin (hCG) or saline as control were implanted intraperitoneally for 21days during the pre-spawning phase (May-June) in catfish Clarias batrachus and C. gariepinus. Significant increase in gonado-somatic index and sperm motility, and in the levels of certain sex steroids were observed in the hCG treated catfish when compared to control while estradiol-17β (E₂) was low. Histological analysis in hCG treated testis revealed densely packed sperm and/or spermatids inside the lumen wherein the control testis displayed normal characteristics of the pre-spawning phase. In females, histological analysis showed a significant increase in post-vitellogenic full-grown immature follicles as seen in the spawning phase. In accordance with this, the steroid hormone profile correlated well with steroidogenic shift from E₂ to 17α,20β-DP indicating oocyte maturation. However, in the control ovaries of C. batrachus, perinucleolar and pre-vitellogenic oocytes were seen to be predominant. In addition, when compared with the control, the hCG treated group displayed a significant increase in the transcripts of several genes associated with gonadal growth. Taken together, artificial induction by slow release of hCG is an effective strategy to advance sexual maturation in catfish in a programmed manner.

Identification and analysis of the β-catenin1 gene in half-smooth tongue sole (Cynoglossus semilaevis)

β-catenin is a key signalling molecule in the canonical Wnt pathway, which plays a role in cell adhesion, embryogenesis and sex determination. However, little is known about its function in teleosts. We cloned and characterized the full-length β-catenin1 gene from half-smooth tongue sole (Cynoglossus semilaevis), which was designated CS-β-catenin1. The CS-β-catenin1 cDNA consists of 2,346 nucleotides and encodes a protein with 782 amino acids. Although CS-β-catenin1 was transcribed in the gonads of both sexes, the level was significantly higher in ovaries compared to testes. Furthermore, the mRNA level of CS-β-catenin1 was significantly upregulated at 160 days and constantly increased until 2 years of age. In situ hybridization revealed that CS-β-catenin1 mRNA was mainly localized in oocyte cells, especially in stage I, II and III oocytes. When CS-β-catenin1 expression was inhibited by injection of quercetin in the ovaries, levels of CS-Figla and CS-foxl2 mRNA were significantly down-regulated, and CS-dmrt1 was up-regulated, which suggested that CS-β-catenin1 is a potential upstream gene of CS-Figla and is involved in the development of the ovaries, i.e., folliculogenesis.

Expression of wnt4/5 during reproductive cycle of catfish and wnt5 promoter analysis

Signaling molecules, Wnt4 and Wnt5, are essential for ovarian growth during developmental stages in mammals. Although these molecules were identified in several teleosts, their precise expression and role in reproductive processes have not yet been explored in any lower vertebrates. In view of this, using catfish, Clarias batrachus as an animal model, cloning and expression analysis of wnt4 and wnt5 were analyzed in different tissues, at various developmental stages, during ovarian reproductive cycle and after gonadotropin induction. These studies indicate a plausible influence of Wnts in ovarian development and recrudescence. Transcript and protein localization revealed their presence in peri-nucleolar, pre-vitellogenic, vitellogenic and follicular layer of post-vitellogenic oocytes. Synchronous expression of pax2 and wnt5 during the ovarian development and recrudescence of catfish led us to analyze the importance of putative binding element of Pax2 in the 5'-promoter motif of wnt5 Promoter activity of wnt5 was analyzed by luciferase assays after transfecting progressive deletion constructs in pGL3 basic vector into the mammalian cell lines (HEK 293 and CHO). The constructs having putative Pax2 motif showed high promoter activity compared with controls. Likewise, the constructs with site-directed mutagenesis showed increased activity after supplementing recombinant Pax2 indicating the prominence of this motif in wnt5 promoter, in vitro Electrophoretic gel mobility shift, supershift and chromatin immunoprecipitation assays confirmed the binding of Pax2 to its corresponding cis-acting element in the upstream of wnt5 This study is the first of its kind to report the critical transcriptional interaction of Pax2 on wnt5 vis-à-vis ovarian development in teleosts.

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.

Molecular cloning and sexually dimorphic expression of wnt4 in olive flounder (Paralichthys olivaceus)

WNT4 (wingless-type MMTV integration site family, member 4) is regarded as a key regulator of gonad differentiation in mammalians. However, the potential role of wnt4 in teleosts during gonad differentiation and development is still unclear. The full-length cDNA sequence of wnt4 in olive flounder (Paralichthys olivaceus) was obtained using RACE (rapid amplification of cDNA ends) technique. The wnt4 ORF contains 1059 nucleotides, encoding a protein with a signal peptide domain and a wnt family domain. Expression in tissues of adult flounders was analyzed by real-time RT-PCR. The results showed that wnt4 was widely expressed in multiple tissues of flounders, and the expression level was significantly higher in ovary than in testis. Then wnt4 expression pattern was investigated during gonadal differentiation period and at gonadal development stages (I-V). The results showed the expression levels were significantly higher in testis than in ovary during gonadal differentiation. Notably, wnt4 expression had a very significant increase before testis differentiation. At gonad different developmental stages, there was no expression signal at stage I or stage II, and the expression of wnt4 was much stronger in ovary than in testis at stage III and stage IV, followed by a faint expression in stage V in both sexes. Our results imply that cloned wnt4 could be wnt4a. It is a sex-related gene and its expression pattern in gonadal differentiation period of flounder is different from that in mammalians or other teleosts. Flounder wnt4 might play more important role in testis than in ovary during gonadal differentiation.

Synergistic role of β-catenin1 and 2 in ovarian differentiation and maintenance of female pathway in Nile tilapia

Two β-catenin (β-cat) genes exist in teleosts but little is known about their expression and function in ovarian development. We identified β-cat1 and β-cat2 from the Nile tilapia. β-cat1 and β-cat2 displayed a similar expression pattern in the ovary during development, and were mainly expressed in the oogonia and oocytes. In luciferase assays, β-cat1 activated the TOPFlash reporter dose-dependently, whereas β-cat2 failed to do so. Cotransfection of β-cat1 and β-cat2 synergistically enhanced the expression of the reporter. A specific interaction between β-cat1 and β-cat2 was also observed in a mammalian two-hybrid assay. Furthermore, tilapia recombinant Dkk1, an inhibitor of the β-cat pathway, decreased β-cat1 and β-cat2, while increased sox9, dmrt1, cyp11b2 and foxl2 expression in the in vitro cultured tilapia ovary, which could be abolished by simultaneous treatment with Bio, an agonist of β-cat. Consistently, β-cat1 or β-cat2 knockdown in XX fish by TALENs caused the retardation of ovarian differentiation and masculinization, as reflected by the upregulation of dmrt1, cyp11b2, sox9, and serum 11-KT level. On the contrary, serum E2 level was unchanged even though foxl2 transcription was upregulated. These data suggestes that both β-cat1 and β-cat2 are important members and play synergistic roles in the canonical Wnt signal pathway in fish. Independent of Foxl2-leading estrogen pathway, they might be involved in ovarian differentiation and repression of the male pathway gene expression in tilapia.

R-spondin1 signaling pathway is required for both the ovarian and testicular development in a teleosts, Nile tilapia (Oreochromis niloticus)

The furin-domain-containing peptide R-spondin 1 (RSPO1) has recently emerged as an important regulator of ovarian development, upregulating the WNT/β-catenin pathway to oppose testis formation in mammals. However, little information has been reported on the Rspo1 signaling pathway in teleosts. In this study, Rspo1 was isolated from the gonads of the Nile tilapia, Oreochromis niloticus. An in situ hybridization analysis demonstrated that Rspo1 is expressed in the germ cells of the ovary and the testis. An ontogenic analysis demonstrated that Rspo1 expression is upregulated just before meiotic initiation in both the ovary and testis during the early developmental stages of the tilapia. The expression pattern is sexually dimorphic from 20days after hatching, with higher expression in the ovary. The reduction of Rspo1 expression by transcription activator-like (TAL) effector nuclease (TALEN) caused retarded ovarian development, the ectopic expression of male-dominant genes, and increased serum 11-ketotestosterone. Intriguingly, a deficiency of Rspo1 in XY fish caused a delay in spermatogenesis, the inhibition of igf3 and amh expression and a reduction in serum 11-ketotestosterone. Furthermore, incubation with FH535, an inhibitor of the Rspo1/Wnt pathway, decreased β-catenin, while increased cyp11c1 and dmrt1 expression in the in vitro cultured ovaries; decreased cyp11c1, amh and igf3 expression in the in vitro cultured testes. Taken together, our data suggest that the Rspo1 signaling pathway might be involved in both ovarian and testicular development in the tilapia.

Transcriptome analysis reveals differentially expressed genes associated with germ cell and gonad development in the Southern bluefin tuna (Thunnus maccoyii)

BACKGROUND

Controlling and managing the breeding of bluefin tuna (Thunnus spp.) in captivity is an imperative step towards obtaining a sustainable supply of these fish in aquaculture production systems. Germ cell transplantation (GCT) is an innovative technology for the production of inter-species surrogates, by transplanting undifferentiated germ cells derived from a donor species into larvae of a host species. The transplanted surrogates will then grow and mature to produce donor-derived seed, thus providing a simpler alternative to maintaining large-bodied broodstock such as the bluefin tuna. Implementation of GCT for new species requires the development of molecular tools to follow the fate of the transplanted germ cells. These tools are based on key reproductive and germ cell-specific genes. RNA-Sequencing (RNA-Seq) provides a rapid, cost-effective method for high throughput gene identification in non-model species. This study utilized RNA-Seq to identify key genes expressed in the gonads of Southern bluefin tuna (Thunnus maccoyii, SBT) and their specific expression patterns in male and female gonad cells.

RESULTS

Key genes involved in the reproductive molecular pathway and specifically, germ cell development in gonads, were identified using analysis of RNA-Seq transcriptomes of male and female SBT gonad cells. Expression profiles of transcripts from ovary and testis cells were compared, as well as testis germ cell-enriched fraction prepared with Percoll gradient, as used in GCT studies. Ovary cells demonstrated over-expression of genes related to stem cell maintenance, while in testis cells, transcripts encoding for reproduction-associated receptors, sex steroids and hormone synthesis and signaling genes were over-expressed. Within the testis cells, the Percoll-enriched fraction showed over-expression of genes that are related to post-meiosis germ cell populations.

CONCLUSIONS

Gonad development and germ cell related genes were identified from SBT gonads and their expression patterns in ovary and testis cells were determined. These expression patterns correlate with the reproductive developmental stage of the sampled fish. The majority of the genes described in this study were sequenced for the first time in T. maccoyii. The wealth of SBT gonadal and germ cell-related gene sequences made publicly available by this study provides an extensive resource for further GCT and reproductive molecular biology studies of this commercially valuable fish.

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.

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.

Comparative transcriptomics in East African cichlids reveals sex- and species-specific expression and new candidates for sex differentiation in fishes

Males and females of the same species differ largely in gene expression, which accounts for most of the morphological and physiological differences and sex-specific phenotypes. Here, we analyzed sex-specific gene expression in the brain and the gonads of cichlid fishes from Lake Tanganyika belonging to four different lineages, so-called tribes (Eretmodini, Ectodini, Haplochromini, and Lamprologini), using the outgroup Nile tilapia (Oreochromis niloticus) as reference. The comparison between male and female brains revealed few differences between the sexes, consistent in all investigated species. The gonads, on the other hand, showed a large fraction of differentially expressed transcripts with the majority of them showing the same direction of expression in all four species. All here-studied cichlids, especially the three investigated mouth-breeding species, showed a trend toward more male- than female biased transcripts. Transcripts, which were female-biased in expression in all four species, were overrepresented on linkage group (LG)1 in the reference genome and common male-biased transcripts showed accumulation on LG23, the presumable sex chromosomes of the Nile tilapia. Sex-specific transcripts contained candidate genes for sex determination and differentiation in fishes,especially members of the transforming growth factor-b-superfamily and the Wnt-pathway and also prominent members of the sox-, dm-domain-, and high mobility group-box families. We further confirmed our previous finding on species/lineage-specific gene expression shifts in the sex steroid pathway, including synthesizing enzymes as the aromatase cyp19a1 and estrogen and androgen receptors.

Cloning and characterization of wnt4a gene and evidence for positive selection in half-smooth tongue sole (Cynoglossus semilaevis)

Wnt4 gene plays a role in developmental processes in mammals. However, little is known regarding its function in teleosts. We cloned and characterized the full-length half-smooth tongue sole (Cynoglossus semilaevis) wnt4a gene (CS-wnt4a). CS-wnt4a cDNA was 1746 bp in length encoding 353aa. CS-wnt4a expression level was highest in the testis, and gradually increased in the developing gonads until 1 year of age. In situ hybridization revealed that CS-wnt4a expression level was highest in stage II oocytes and sperm in the adult ovary and testis, respectively. CS-wnt4a expression level was significantly up-regulated in the gonads after exposure to high temperature. The level of methylation of the CS-wnt4a first exon was negatively correlated with the expression of CS-wnt4a. The branch-site model suggested that vertebrate wnt4a differed significantly from that of wnt4b, and that the selective pressures differed between ancestral aquatic and terrestrial organisms. Two positively selected sites were found in the ancestral lineages of teleost fish, but none in the ancestral lineages of mammals. One positively selected site was located on the α-helices of the 3D structure, the other on the random coil. Our results are of value for further study of the function of wnt4 and the mechanism of selection.

Conserved action of β-catenin during female fate determination in the red-eared slider turtle

In reptiles such as the red-eared slider turtle Trachemys scripta, development of an ovary from the bipotential gonad requires a coordinated expansion of the cortical domain and regression of the medulla. While estrogen, which is necessary and sufficient for ovarian development in non-mammalian vertebrates, is thought to feminize both compartments, it remains unclear whether there is a signaling relationship between the two domains that coordinates their fates. We show that aromatase, the estrogen-synthesizing enzyme, is localized to the medulla of the differentiating turtle ovary and that differentiation of the medulla precedes and is independent of cortical expansion. Coordinated feminization of the two domains may therefore rely on an estrogenic signal from the differentiating medulla. In eutherian mammals, where estrogen is dispensable for early ovary development, the canonical Wnt signaling pathway is critical for female fate determination. Whether this function is conserved among vertebrates and how it is potentially integrated with estrogen signaling are uncertain. Using a novel in vitro turtle gonad culture system, we demonstrate that ectopic activation of the canonical Wnt signaling pathway in presumptive male gonads induced a partial sex-reversal of the medulla, but inhibition of the pathway was not sufficient to sex-reverse differentiating ovaries. These patterns are similar to those previously observed in mice. Wnt signaling appears to function downstream of estrogen, as ectopic activation of the pathway rescued female development when estrogen synthesis was inhibited. Our findings therefore suggest that the ovary-promoting effects of the Wnt signaling pathway may be functionally conserved between mammals and reptiles.

R-spondin1, WNT4, and the CTNNB1 signaling pathway: strict control over ovarian differentiation

Sex differentiation is a unique developmental process. Starting from a bipotential gonad, it gives rise to the ovary and the testis, two highly specialized organs that differ morphologically and physiologically despite sharing common reproductive and endocrine functions. This highlights the specific plasticity of the gonadal precursors and the existence of complex antagonistic genetic regulation. Mammalian sex determination is controlled by paternal transmission of the Y-linked gene, sex-determining region Y (SRY). Using mouse models, it has been shown that the main role of Sry is to activate the expression of the transcription factor Sox9; either one of these two genes is necessary and sufficient to allow testicular development through Sertoli cell differentiation. Thus, defects in SRY/Sry and/or SOX9/Sox9 expression result in male-to-female sex reversal of XY individuals. Molecular mechanisms governing ovarian differentiation remained unknown for a long time, until the discovery of the roles of R-spondin1 (RSPO1) and WNT4. In XX individuals, activation of the β-catenin signaling pathway by the secreted proteins RSPO1 and WNT4 is required to allow granulosa cell differentiation and, in turn, ovarian differentiation. Thus, mutations in RSPO1 result in female-to-male sex reversal of XX patients, and mouse models have allowed the identification of genetic cascades activated by RSPO1 and WNT4 to regulate ovarian development. In this review, we will discuss the respective roles of RSPO1, WNT4, and the β-catenin signaling pathway during ovarian differentiation in mice.

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.

Gonad differentiation in zebrafish is regulated by the canonical Wnt signaling pathway

Zebrafish males undergo a "juvenile ovary-to-testis" gonadal transformation process. Several genes, including nuclear receptor subfamily 5, group A (nr5a) and anti-Müllerian hormone (amh), and pathways such as Tp53-mediated germ-cell apoptosis have been implicated in zebrafish testis formation. However, our knowledge of the regulation of this complex process is incomplete, and much remains to be investigated about the molecular pathways and network of genes that control it. Using a microarray-based analysis of transforming zebrafish male gonads, we demonstrated that their transcriptomes undergo transition from an ovary-like pattern to an ovotestis to a testis-like profile. Microarray results also validated the previous histological and immunohistochemical observation that there is high variation in the duration and extent of commitment to the juvenile ovary phase among individuals. Interestingly, global gene expression profiling of diverging zebrafish juvenile ovaries and transforming ovotestes revealed that some members of the canonical Wnt/beta-catenin signaling pathway were differentially expressed between these two phases. To investigate whether Wnt/beta-catenin signaling plays a role in zebrafish gonad differentiation, we used the Tg (hsp70l:dkk1b-GFP)w32 line to inhibit Wnt/beta-catenin signaling during gonad differentiation. Activation of dkk1b-GFP expression by heat shock resulted in an increased proportion of males and corresponding decrease in gonadal aromatase gene (cyp19a1a) expression. The Wnt target gene, lymphocyte enhancer binding factor 1 (lef1), was also down-regulated in the process. Together, these results provide the first functional evidence that, similarly to mammals, Wnt/beta-catenin signaling is a "pro-female" pathway that regulates gonad differentiation in zebrafish.

A sex-associated sequence identified by RAPD screening in gynogenetic individuals of turbot (Scophthalmus maximus)

Understanding the genetic basis of sex determination mechanisms is essential for improving the productivity of farmed aquaculture fish species like turbot (Scophthalmus maximus). In culture conditions turbot males grow slower than females starting from eight months post-hatch, and this differential growth rate is maintained until sexual maturation is reached, being mature females almost twice as big as males of the same age. The goal of this study was to identify sex-specific DNA markers in turbot using comparative random amplified polymorphism DNA (RAPD) profiles in males and females to get new insights of the genetic architecture related to sex determination. In order to do this, we analyzed 540 commercial 10-mer RAPD primers in male and female pools of a gynogenetic family because of its higher inbreeding, which facilitates the detection of associations across the genome. Two sex-linked RAPD markers were identified in the female pool and one in the male pool. After the analysis of the three markers on individual samples of each pool and also in unrelated individuals, only one RAPD showed significant association with females. This marker was isolated, cloned and sequenced, containing two sequences, a microsatellite (SEX01) and a minisatellite (SEX02), which were mapped in the turbot reference map. From this map position, through a comparative mapping approach, we identified Foxl2, a relevant gene related to initial steps of sex differentiation, and Wnt4, a gene related with ovarian development, close to the microsatellite and minisatellite markers, respectively. The position of Foxl2 and Wnt4 was confirmed by linkage mapping in the reference turbot map.

Small-scale transcriptomics reveals differences among gonadal stages in Asian seabass (Lates calcarifer)

BACKGROUND

The Asian seabass (Lates calcarifer) is a protandrous hermaphrodite that typically matures as a male at approximately 2-4 years of age and then changes sex in subsequent years. Although several sexual maturation stages have been described histologically for both testis and ovary, the underlying gene expression profiles remain lacking. The development of a gene expression platform is therefore necessary to improve our understanding of the gonad development of this cultured teleost species.

METHODS

Thirty Asian seabass gonads were collected from farms in Singapore, examined histologically and staged according to their sex and gonadal maturation status. Partial coding sequences of 24 sex-related genes were cloned using degenerate primers and were sequenced. Additional 13 cDNA sequences were obtained through next-generation sequencing. A real-time qPCR was then performed using the microfluidic-based Fluidigm 48.48 Dynamic arrays.

RESULTS

We obtained 17 ovaries and 13 testes at various stages of sexual maturation. Of the 37 genes that were tested, 32 (86%) showed sexually dimorphic expression. These genes included sex-related genes, sox9, wt1, amh, nr5a2, dmrt1 and nr0b1, which showed testis-enhanced expression similar to other vertebrate species. Known male- and female-enhanced germ cells markers, which were established from studies in other species, similarly showed testis- and ovary-enhanced expression, respectively, in the Asian seabass. Three pro-Wnt signaling genes were also upregulated in the ovary, consistent with existing studies that suggested the role of Wnt signaling in ovarian differentiation in teleosts and mammals. The expression patterns of genes involved in steroidogenesis, retinoic acid metabolism, apoptosis and NF-κB signaling were also described. We were able to classify gonads according to sex and gonadal maturation stages, based on their small-scale transcriptomic profiles, and to uncover a wide variation in expression profiles among individuals of the same sex.

CONCLUSIONS

The analysis of a selected set of genes related to reproduction and in sufficient number of individuals using a qPCR array can elucidate new insights into the molecular mechanisms involved in Asian seabass gonad development. Given the conservation of gene expression patterns found in this study, these insights may also help us draw parallels with other teleosts.