Two Figla homologues have disparate functions during sex differentiation in half-smooth tongue sole (Cynoglossus semilaevis)

Molecular Characterization, Expression Pattern, DNA Methylation and Gene Disruption of Figla in Blotched Snakehead (Channa maculata)

Figla is one of the earliest expressed genes in the oocyte during ovarian development. In this study, Figla was characterized in C. maculata, one of the main aquaculture species in China, and designated as CmFigla. The length of CmFigla cDNA was 1303 bp, encoding 197 amino acids that contained a conserved bHLH domain. CmFigla revealed a female-biased expression patterns in the gonads of adult fish, and CmFigla expression was far higher in ovaries than that in testes at all gonadal development stages, especially at 60~180 days post-fertilization (dpf). Furthermore, a noteworthy inverse relationship was observed between CmFigla expression and the methylation of its promoter in the adult gonads. Gonads at 90 dpf were used for in situ hybridization (ISH), and CmFigla transcripts were mainly concentrated in oogonia and the primary oocytes in ovaries, but undetectable in the testes. These results indicated that Figla would play vital roles in the ovarian development in C. maculata. Additionally, the frame-shift mutations of CmFigla were successfully constructed through the CRISPR/Cas9 system, which established a positive foundation for further investigation on the role of Figla in the ovarian development of C. maculata. Our study provides valuable clues for exploring the regulatory mechanism of Figla in the fish ovarian development and maintenance, which would be useful for the sex control and reproduction of fish in aquaculture.

Master-Key Regulators of Sex Determination in Fish and Other Vertebrates-A Review

In vertebrates, mainly single genes with an allele ratio of 1:1 trigger sex-determination (SD), leading to initial equal sex-ratios. Such genes are designated master-key regulators (MKRs) and are frequently associated with DNA structural variations, such as copy-number variation and null-alleles. Most MKR knowledge comes from fish, especially cichlids, which serve as a genetic model for SD. We list 14 MKRs, of which dmrt1 has been identified in taxonomically distant species such as birds and fish. The identification of MKRs with known involvement in SD, such as amh and fshr, indicates that a common network drives SD. We illustrate a network that affects estrogen/androgen equilibrium, suggesting that structural variation may exert over-expression of the gene and thus form an MKR. However, the reason why certain factors constitute MKRs, whereas others do not is unclear. The limited number of conserved MKRs suggests that their heterologous sequences could be used as targets in future searches for MKRs of additional species. Sex-specific mortality, sex reversal, the role of temperature in SD, and multigenic SD are examined, claiming that these phenomena are often consequences of artificial hybridization. We discuss the essentiality of taxonomic authentication of species to validate purebred origin before MKR searches.

Identification and Expression Pattern of cyp26b1 Gene in Gonad of the Chinese Tongue Sole (Cynoglossus semilaevis)

Simple Summary

In fish, it is obvious that the asynchronous development of the gonads and sexual dimorphism limit the development of aquaculture, so the research into sex-differentiation and gonadal growth is very important. Due to the sexual reversal phenomenon (genetic females becoming phenotypic males), the Chinese tongue sole (Cynoglossus semilaevis) is a great model for investigating sex-differentiation. Herein, we report one gene involved in sex-differentiation and gonadal growth of the Chinese Tongue Sole. The gene cyp26b1 (cytochrome P450 family 26 subfamily b member 1) is a metabolizing Retinoic Acid (RA) enzyme. Since it regulates RA to control sex determination and differentiation, cyp26b1 is considered a critical part of mammals’ ovary-antagonizing and testis-determining downstream passageway of Sry (sex-determining region Y) and Sox9 (sry-box transcription factor 9). In fish, the related research is reported only on the Japanese flounder (Paralichthys olivaceus) and zebrafish (Danio rerio). In the current investigation, the identification and expression pattern of the cyp26b1 gene in the Chinese tongue sole suggested that cyp26b1 might impact sex-differentiation and gonadal development.

Abstract

As an RA-metabolizing enzyme, cyp26b1 has a substantial impact on RA-signaling pathways. The cyp26b1 gene from the Chinese tongue sole was cloned and identified in this investigation. The cyp26b1 ORF was 1536 bp in length and encoded a 512 amino acid protein. A quantitative real-time PCR (qPCR) indicated that the cyp26b1 expression is no significant sexual dimorphism in the gonads at the 80 days post-hatching (dph) stages. After 4 months post-hatching (mph), the expression of cyp26b1 showed sexual dimorphism and lower level of expression in the ovaries than in the testes. An in situ hybridization demonstrated that cyp26b1 mRNA was primarily located in the testis. Interestingly, the cyp26b1 mRNA probe was also detected in the ovaries. These results suggested that cyp26b1 participates in the sex-differentiation and gonadal development of the Chinese tongue sole.

Absence of Figla-like Gene Is Concordant with Femaleness in Cichlids Harboring the LG1 Sex-Determination System

Oreochromis niloticus has been used as a reference genome for studies of tilapia sex determination (SD) revealing segregating genetic loci on linkage groups (LGs) 1, 3, and 23. The master key regulator genes (MKR) underlying the SD regions on LGs 3 and 23 have been already found. To identify the MKR in fish that segregate for the LG1 XX/XY SD-system, we applied short variant discovery within the sequence reads of the genomic libraries of the Amherst hybrid stock, Coptodon zillii and Sarotherodon galilaeus, which were aligned to a 3-Mbp-region of the O. aureus genome. We obtained 66,372 variants of which six were concordant with the XX/XY model of SD and were conserved across these species, disclosing the male specific figla-like gene. We further validated this observation in O. mossambicus and in the Chitralada hybrid stock. Genome alignment of the 1252-bp transcript showed that the figla-like gene’s size was 2664 bp, and that its three exons were capable of encoding 99 amino acids including a 45-amino-acid basic helix–loop–helix domain that is typical of the ovary development regulator—factor-in-the-germline-alpha (FIGLA). In Amherst gonads, the figla-like gene was exclusively expressed in testes. Thus, the figla-like genomic presence determines male fate by interrupting the female developmental program. This indicates that the figla-like gene is the long-sought SD MKR on LG1.

A Potential Role of the Translation Elongation Factor eef1a1 in Gonadal High-Temperature Perception in Chinese Tongue Sole (Cynoglossus semilaevis)

Simple Summary

The eukaryotic elongation factor 1 alpha (eef1a) gene is vital for protein translation by delivering aminoacylated tRNAs to the A/P site of the ribosome via the GTP-dependent reaction. Here, the Chinese tongue sole (Cynoglossus semilaevis) eef1a1 gene was identified, and its potential role in gonadal high-temperature perception was assessed. The full-length sequence of eef1a1 cDNA was 1809 base pair (bp) encoding a putative protein of 461 amino acids. The expression levels of eef1a1 in the ovary were significantly higher than that in the testis from 6 mpf to 3 ypf. Under high-temperature induction during sex differentiation, eef1a1 was significantly down-regulated in males, while the difference was not detected in females. Furthermore, the rapid response of eef1a1 to environmental high temperature was assessed in vitro. Our findings suggest that C. semilaevis eef1a1 might be essential for the molecular response regulatory network of external temperature affecting internal sex differentiation.

Abstract

The eukaryotic translation elongation factor 1 alpha (eef1a) gene has a well-defined role in protein synthesis. However, its role in external temperature perception and internal sex differentiation and development is still unclear. In this study, eef1a1 was identified and functionally analyzed in Chinese tongue sole (Cynoglossus semilaevis). The eef1a1 cDNA, 1809 bp in length, had a 1386 bp open reading frame (ORF) that encoded a 461 amino acid polypeptide containing one EF-1_alpha domain. eef1a1 expression levels were investigated across different tissues and during gonadal development. In the gonad, eef1a1 showed a sexually dimorphic expression pattern with a statistically higher expression level in the ovary than in the testis from 6 months postfertilization to 3 years postfertilization. Under high temperature (28 °C) treatment during C. semilaevis sex differentiation (from 30 days postfertilization to 3 months postfertilization), eef1a1 was statistically down-regulated in males, while the difference was not detected in females. In addition, the dual-luciferase assay exhibited that eef1a1 can respond to high temperature rapidly. Based on these results, C. semilaevis eef1a1 might have a dual role in the perception of external temperature changes and sex differentiation regulation.

pitpβ_w Encoding Phosphatidylinositol Transfer Protein Is Involved in Female Differentiation of Chinese Tongue Sole, Cynoglossus semilaevis

Phosphatidylinositol transfer protein (pitp) plays an important role in phospholipid transfer in animals. A pitp variant (pitpβ_w) in Chinese tongue sole was identified by transcriptomic analysis for its female-biased expression. The coding sequence of pitpβ_w was 816 bp, encoding a 371-amino-acid protein. pitpβ_w showed female-biased expression and was relatively high in brain, muscle, and ovary tissues. In different developmental stages of the ovary, pitpβ_w could be detected from 40 days until 3 years post hatching, and the highest expression was observed at 90 days. In situ hybridization revealed that pitpβ_w was predominantly localized in early-stage oocytes (I-III stages). After siRNA-mediated knockdown of pitpβ_w in an ovarian cell line, the expression of sox9a was reduced, while that of figla_tv1 and sox9b was significantly increased. Our findings suggest that pitpβ_w might be involved in female differentiation and early oogenesis.

Sex Determination and Differentiation in Teleost: Roles of Genetics, Environment, and Brain

The fish reproductive system is a complex biological system. Nonetheless, reproductive organ development is conserved, which starts with sex determination and then sex differentiation. The sex of a teleost is determined and differentiated from bipotential primordium by genetics, environmental factors, or both. These two processes are species-specific. There are several prominent genes and environmental factors involved during sex determination and differentiation. At the cellular level, most of the sex-determining genes suppress the female pathway. For environmental factors, there are temperature, density, hypoxia, pH, and social interaction. Once the sexual fate is determined, sex differentiation takes over the gonadal developmental process. Environmental factors involve activation and suppression of various male and female pathways depending on the sexual fate. Alongside these factors, the role of the brain during sex determination and differentiation remains elusive. Nonetheless, GnRH III knockout has promoted a male sex-biased population, which shows brain involvement during sex determination. During sex differentiation, LH and FSH might not affect the gonadal differentiation, but are required for regulating sex differentiation. This review discusses the role of prominent genes, environmental factors, and the brain in sex determination and differentiation across a few teleost species.

A simple and rapid method for fish sex identification based on recombinase-aided amplification and its use in Cynoglossus semilaevis

Fish sex identification is a basic technique of great importance for both fish genetic studies and fisheries. Due to the sexual reversal phenomenon in many fish species, a simple and rapid molecular identification method for fish genetic sex is urgently needed to suit versatile detection scenarios, such as point-of-need applications. In this study, we took Cynoglossus semilaevis as an example, established a recombinase-aided amplification (RAA)-based method for sex identification, and combined the RAA-detection with two result visualization approaches with distinct features, capillary electrophoresis (CE) and lateral flow dipstick (LFD). Specific primers and probe were designed to specifically detect the sex chromosome W of C. semilaevis in order to distinguish the genetic sex between males, pseudo-males and females. To evaluate the performance of our methods, the genetic sex for twenty-eight males, sixty-eight pseudo-males and fifty-four females were examined with the RAA-based method and classical PCR-based genotyping method, demonstrating the consistent results of sex identification between both methods. The RAA-LFD method is operationally simple, rapid (~ 30 min) and holds great potential for point-of-need applications of fish sex identification, including fishery fields. The method presented here could be effective for identifying fish gender with the ZW karyotype.

lncRNA DMRT2-AS acts as a transcriptional regulator of dmrt2 involving in sex differentiation in the Chinese tongue sole (Cynoglossus semilaevis)

Long non-coding RNAs (lncRNAs) contribute to various biological processes, including sexual development. As a member of the DMRT family, dmrt2 plays a very important role in sex determination and differentiation. In this study, we cloned and characterized the lncRNA DMRT2-AS (referred to as dmrt2 antisense) associated with dmrt2 from the gonads of the Chinese tongue sole (Cynoglossus semilaevis). The full-length cDNA of DMRT2-AS was 537 bp. Based on a sequence alignment, DMRT2-AS overlapped with dmrt2 in reverse on exon 4 and intron 3, with a region of overlap of 221 bp on exon 4. RT-qPCR showed that DMRT2-AS was highly expressed in the testis of Chinese tongue sole. In addition, the expression of DMRT2-AS increased continuously during male gonadal development. In vitro experiments and bioinformatics predictions showed that DMRT2-AS promoted the expression of dmrt2 at the transcriptional level. These results suggest that DMRT2-AS acts as a transcriptional regulator of dmrt2 and plays an important role in the gonadal differentiation of male.

Cloning and functional analysis of c/ebpα as negative regulator of dmrt1 in Chinese tongue sole (Cynoglossus semilaevis)

c/ebpα is a member of the C/EBP family of transcription factors, which are involved in cell growth and differentiation and have a conserved basic leucine zipper (bZIP) domain. However, little is known about its function in sex determination and differentiation. In the present study, c/ebpα was cloned from the gonads of Chinese tongue sole (Cynoglossus semilaevis). The full-length cDNA of c/ebpα was 1583 bp, with a 198-bp 5' UTR, a 446-bp 3' UTR, and a 939-bp open reading frame encoding a 312-amino acid peptide. qRT-PCR revealed that c/ebpα was predominantly expressed in undifferentiated gonads of male C. semilaevis at 30 dpf and 60 dpf and peaked at 60 dpf. Expression levels of c/ebpα in the testis were constantly higher than those in ovaries at all developmental stages. Moreover, a dual-luciferase assay revealed that c/ebpα could negatively regulate the male-determining gene dmrt1 in vitro. These results provide fundamental information indicating that C. semilaevis c/ebpa might be involved in early gonadal differentiation and functions as a negative regulator of dmrt1 by repressing its transcription.

Transcriptional activity of FIGLA, NEUROG2, and EGR1 transcription factors associated with polymorphisms in the proximal regulatory region of GPR54 gene in cattle

Activity of transcription factors affect synthesis of G-protein coupled receptor 54 (GPR54), an important factor in regulation of initiation of puberty. Expression of the GPR54 gene in cattle is associated with polymorphisms in the proximal regulatory region (PRR) of the GPR54 gene. Transcription resulting in production of GPR54 mRNA transcript occurs as a result of transcription factor (TF) interactions in the PRR. Polymorphisms in the PRR may be associated with extent of activity of these TFs. Folliculogenesis-specific BHLH TF (FIGLA), neurogenin 2 (NEUROG2), and early growth response 1 (EGR1) are important in modulation of ovarian follicle development and neurons synthesizing GnRH, thus, regulating biosynthesis of luteinizing hormone. The aim of this study, therefore, was to assess the transcription-activating potential of binding sites for FIGLA, NEUROG2, and EGR1 TFs in the GPR54 promoter of cattle. Two luciferase-based promoters, ATC and CCT, which contain three single nucleotide polymorphisms (SNPs), A/C-794, T/C-663, and C/T-601, in the GPR54 PRR, were analyzed to evaluate gene expression and activation of different promoters by FIGLA, NEUROG2, and EGR1. The FIGLA induced GPR54 transcription through the CCT, whereas NEUROG2 and EGR1 induced GPR54 transcription through the ATC promoter-binding site. The CCT-activating effects of FIGLA were greater (2.56-fold) than the ATC-activating effects (P < 0.05). The ATC-activating effects of NEUROG2 and EGR1 were markedly greater (12.91- and 8.41-fold; P < 0.01) than CCT-activating effects. The polymorphisms, CCT and ATC, of the cattle GPR54 affect the activity of transcription factors, therefore, have an important effect on production of GPR54 mRNA transcript.

Expression analysis and characterization of dmrt2 in Chinese tongue sole (Cynoglossus semilaevis)

Dmrt2 is a member of the dmrt gene family with a conserved zinc finger-like DNA-binding motif (DM domain). In the present study, CS-dmrt2 was cloned from the gonads of Chinese tongue sole (Cynoglossus semilaevis). The full-length cDNA of CS-dmrt2 is 2834 bp in length, with a 251 bp 5'-untranslated region (UTR), a 1086 bp 3'-UTR and a 1503 bp open reading frame (ORF) that encodes a 501-amino-acid peptide. qPCR revealed that CS-dmrt2 was mainly expressed in C. semilaevis testes. In situ hybridization (ISH) showed CS-dmrt2 expression throughout early gonadal development (36 days after hatching (dah) and 86 dah), but the expression was higher in male gonads than in female gonads. CS-dmrt2 mRNA was highly expressed in male germ cells. Comparison of methylation levels between females and males demonstrated hypo-methylated levels of the CS-dmrt2 promoter in the male gonads, which is consistent with the high mRNA expression. These results suggest that the CS-dmrt2 gene may play a functional role in gonadal differentiation/development and germ cell maturation in the testis.

Seminal Plasma Exosomes: Promising Biomarkers for Identification of Male and Pseudo-Males in Cynoglossus semilaevis

In mammals, small RNAs enclosed in exosomes have been identified as appropriate signatures for disease diagnosis. However, there is limited information on exosomes derived from seminal plasma, and few studies have reported analyzed the composition of exosomes and enclosed small RNAs in fish. The half-smooth tongue sole (Cynoglossus semilaevis) is an economically important fish for aquaculture, and it exhibits sexual dimorphism: the female gender show higher growth rates and larger body sizes than males. Standard karyotype analysis and artificial gynogenesis tests have revealed that this species uses heterogametic sex determination (ZW/ZZ), and so-called sex-reversed pseudo-males exist. In this study, we successfully identified exosomes in the seminal plasma of C. semilaevis; to the best of our knowledge, this is the first report of exosomes in fish seminal plasma. Analysis of the nucleotide composition showed that miRNAs were dominant in the exosomes, and the miRNAs were sequenced and compared to identify signature miRNAs as sexual biomarkers. Moreover, target genes of the signature miRNAs were predicted by sequence matching and annotation. Finally, four miRNAs (dre-miR-141-3P, dre-miR-10d-5p, ssa-miR-27b-3p, and ssa-miR-23a-3p) with significant differential expression in the males and pseudo-males were selected from the signature candidate miRNAs as markers for sex identification, and their expression profiles were verified using real-time quantitative PCR. Our findings could provide an effective detection method for sex differentiation in fish.

Draft genome of the protandrous Chinese black porgy, Acanthopagrus schlegelii

Background

As one of the most popular and valuable commercial marine fishes in China and East Asian countries, the Chinese black porgy (Acanthopagrus schlegelii), also known as the blackhead seabream, has some attractive characteristics such as fast growth rate, good meat quality, resistance to diseases, and excellent adaptability to various environments. Furthermore, the black porgy is a good model for investigating sex changes in fish due to its protandrous hermaphroditism. Here, we obtained a high-quality genome assembly of this interesting teleost species and performed a genomic survey on potential genes associated with the sex-change phenomenon.

Findings

We generated 175.4 gigabases (Gb) of clean sequence reads using a whole-genome shotgun sequencing strategy. The final genome assembly is approximately 688.1 megabases (Mb), accounting for 93% of the estimated genome size (739.6 Mb). The achieved scaffold N50 is 7.6 Mb, reaching a relatively high level among sequenced fish species. We identified 19 465 protein-coding genes, which had an average transcript length of 17.3 kb. By performing a comparative genomic analysis, we found 3 types of genes potentially associated with sex change, which are useful for studying the genetic basis of the protandrous hermaphroditism.

Conclusions

We provide a draft genome assembly of the Chinese black porgy and discuss the potential genetic mechanisms of sex change. These data are also an important resource for studying the biology and for facilitating breeding of this economically important fish.

The autosomal Gsdf gene plays a role in male gonad development in Chinese tongue sole (Cynoglossus semilaevis)

Gsdf is a key gene for testicular differentiation in teleost. However, little is known about the function of Gsdf in Chinese tongue sole (Cynoglossus semilaevis). In this study, we obtained the full-length Gsdf gene (CS-Gsdf), and functional characterization revealed its potential participation during germ cell differentiation in testes. CS-Gsdf transcription was predominantly detected in gonads, while the levels in testes were significantly higher than those in ovaries. During the different developmental stages in male gonads, the mRNA level was significantly upregulated at 86 dph, and a peak appeared at 120 dph; then, the level decreased at 1 and 2 yph. In situ hybridization revealed that CS-Gsdf mRNA was mainly localized in the Sertoli cells, spermatogonia, and spermatids in mature testes. After CS-Gsdf knockdown in the male testes cell line by RNA interference, a series of sex-related genes was influenced, including several sex differentiation genes, CS-Wnt4a, CS-Cyp19a1a and CS-Star. Based on these data, we speculated that CS-Gsdf may play a positive role in germ differentiation and proliferation via influencing genes related to sex differentiation.

Roles of Figla/figla in Juvenile Ovary Development and Follicle Formation During Zebrafish Gonadogenesis

Sex determination and differentiation are complex processes. As a juvenile hermaphrodite or undifferentiated gonochorist, zebrafish undergo a special juvenile ovarian phase during sex differentiation, making it an excellent model for studying early oogenesis and folliculogenesis. We provide lines of evidence at morphological, molecular, and genetic levels for roles of factor in the germline α (Figla), an oocyte-specific transcription factor, in early zebrafish gonadogenesis. As in mammals, Figla/figla was also expressed in the gonads and its expression in the ovary was also restricted to early oocytes. Disruption of figla gene by CRISPR/Cas9 led to an all-male phenotype in the mutant. Detailed analysis of early gonadal development showed that the germ cells in the mutant were clustered in cysts and underwent meiosis, forming oocytes at prefollicular chromatin nucleolar (CN) stage (stage IA). However, the subsequent transition from cystic CN oocytes to individual follicular perinucleolar oocytes (stage IB) was blocked, resulting in an all-male phenotype in the mutant. The phenotype of figla mutant could not be rescued by estrogen treatment, in contrast to cyp19a1a mutant, and introduction of tp53 mutation also had no effect, unlike in fancd1 and fancl mutants. Transcriptome analysis revealed that many biological processes and pathways related to germ cell development, especially oogenesis, were upregulated in the presence of Figla and that the regulation of figla expression may involve heat shock proteins. Our results strongly suggest important roles for Figla in juvenile ovary development, especially the formation of individual follicles from cystic oocytes.

Gonadal development and expression of sex-specific genes during sex differentiation in the Japanese eel

The process of gonadal development and mechanism involved in sex differentiation in eels are still unclear. The objectives were to investigate the gonadal development and expression pattern of sex-related genes during sex differentiation in the Japanese eel, Anguilla japonica. For control group, the elvers of 8-10cm were reared for 8months; and for feminization, estradiol-17β (E2) was orally administered to the elvers of 8-10cm for 6months. Only males were found in the control group, suggesting a possible role of environmental factors in eel sex determination. In contrast, all differentiated eels in E2-treated group were female. Gonad histology revealed that control male eels seem to differentiate through an intersexual stage, while female eels (E2-treated) would differentiate directly from an undifferentiated gonad. Tissue distribution and sex-related genes expression during gonadal development were analyzed by qPCR. The vasa, figla and sox3 transcripts in gonads were significantly increased during sex differentiation. High vasa expression occurred in males; figla and sox3 were related to ovarian differentiation. The transcripts of dmrt1 and sox9a were significantly increased in males during testicular differentiation and development. The cyp19a1 transcripts were significantly increased in differentiating and differentiated gonads, but did not show a differential expression between the control and E2-treated eels. This suggests that cyp19a1 is involved both in testicular differentiation and development in control males, and in the early stage of ovarian differentiation in E2-treated eels. Importantly, these results also reveal that cyp19a1 is not a direct target for E2 during gonad differentiation in the eel.

Molecular characterization of Pod1 during sex development in Chinese tongue sole (Cynoglossus semilaevis)

Pod1 encodes a Class II bHLH transcription factor involved in the development of a number of tissues such as gonad, spleen, lungs and heart. However, to date, little is known about its function in teleosts. In this study, we cloned and characterized Pod1 gene from Cynoglossus semilaevis. This gene contains three exons and two introns, with the full-length cDNA of 918 nucleotides that encodes a 183 amino acid protein with a conserved bHLH domain. Realtime quantitative PCR revealed that Pod1 was predominantly expressed in the testes of C. semilaevis. In different stages of testes development, Pod1 expression was undetectable up to 120 days after hatching (dah), and then increased at 210 dah and 1 year after hatching (yah). Furthermore, in situ hybridization (ISH) analysis revealed that Pod1 was mainly localized in the germ cells of testes, but was not detected in ovarian cells; which suggested its possible functions in spermatogenesis of C. semilaevis. The methylation profile analysis of Pod1 genomic sequence in the gonads showed that the differences in their putative promoter regions of Pod1 among ovary, male and pseudo-male testes were not obvious. Thus, further research might be needed to evaluate whether Pod1 expression is regulated by epigenetic level.

Characterization and expression pattern of r-spondin1 in Cynoglossus semilaevis

r-spondin1 (rspo1) encodes a secreted protein that is involved in the determination and differentiation of the mammalian ovary. However, little information is yet available for teleosts. Here, we identified a homologue of rspo1 in Cynoglossus semilaevis. The full-length cDNA of rspo1 had a length of 2,703 bp with an open reading frame of 834 bp, encoding a protein with a length of 277 amino-acids. rspo1 expression was detected via qRT-PCR in various tissues, and significant sexually dimorphic expression was observed in the gonads. Furthermore, ISH located rspo1 in germ cells such as spermatogonia, spermatocytes, spermatids, spermatozoa, and oocytes, as well as in somatic cells of the gonads. Following knockdown of rspo1 in an ovarian cell line, the expressions of wnt4a, β-catenin, foxl2, and StAR were highly affected; wnt4a and β-catenin were significantly downregulated, whereas foxl2 and StAR were significantly upregulated. In summary, these data suggest that rspo1 may be involved in the regulation of ovarian development and differentiation through a conserved pathway, while the function of the gene in the testis remains elusive.

Locus Mapping, Molecular Cloning, and Expression Analysis of rps6kb2, a Novel Metamorphosis-Related Gene in Chinese Tongue Sole (Cynoglossus semilaevis)

Flatfish metamorphosis denotes the extraordinary transformation of a symmetric pelagic larva into an asymmetric benthic juvenile. This unique process involves eye migration, a 90° rotation in posture, and asymmetrical pigmentation for adaptation to a benthic lifestyle. In the present study, we used genetics to map a metamorphosis-related locus (q-10M) in the male linkage group (LG10M), a small interval of 0.9 cM corresponding to a 1.8 M-bp physical area in chromosome 9 in the Chinese tongue sole (Cynoglossus semilaevis). Combined with single-marker analysis, ribosomal protein S6 kinase 2 (rps6kb2) a member of the family of AGC kinases was identified as a novel metamorphosis-related candidate gene. Its expression pattern during metamorphosis was determined by quantitative RT-PCR and whole-mount in situ hybridization analysis. rps6kb2 gene was significantly expressed in metamorphic climax stage larvae and distributed in all the tissues transforming during metamorphosis, including tail, jaw, eye and skin of larvae. The results suggest that rps6kb2 has a general role in tissue transformations during flatfish metamorphosis including tail changes, skull remodeling, eye migration, and asymmetrical pigmentation.

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.

Identification of exosomes and its signature miRNAs of male and female Cynoglossus semilaevis

Exosomes are small membrane particles which are widely found in various cell lines and physiological fluids in mammalian. MicroRNAs (miRNAs) enclosed in exosomes have been identified as proper signatures for many diseases and response to therapies. However, the composition of exosomes and enclosed miRNAs in fishes has not been investigated. Cynoglossus semilaevis is an important commercial flatfish with ambiguous distinction between males and females before sex maturation, which leads to screening difficulty in reproduction and cultivation. An effective detection method was required for sex differentiation of C. semilaevis. In this work, we successfully identified exosomes in C. semilaevis serum. The analysis of nucleotide composition showed that miRNA dominated in exosomes. Thereafter the miRNA profiles in exosomes from males and females were sequenced and compared to identify the signature miRNAs corresponding to sex differentiation. The functions of signature miRNAs were analyzed by target matching and annotation. Furthermore, 7 miRNAs with high expression in males were selected from signature miRNAs as the markers for sex identification with their expression profiles verified by real time quantitative PCR. Exosomes were first found in fish serum in this work. Investigation of marker miRNAs supplies an effective index for the filtration of male and female C. semilaevis in cultivation.

Molecular cloning and expression analysis of the aqp1aa gene in half-smooth tongue sole (Cynoglossus semilaevis)

Aquaporin 1 (AQP1) is a member of the transmembrane water channel family of proteins with special structural features, and two AQP1 paralogous genes (aqp1aa and aqp1ab) are reported in teleosts. In the present study, the aqp1aa gene of half-smooth tongue sole (Cynoglossus semilaevis) was cloned and characterized. The full-length cDNA of aqp1aa is 1411 bp with a 786 bp open reading frame encoding a 261-amino acid putative protein with a characteristic structure consisting of 6 membrane-spanning α-helical domains and two highly conserved asparagine—proline—alanine motifs. Real-time quantitative PCR revealed that aqp1aa mRNA is expressed predominantly in the testis of males and pseudo-males, while its expression is low in the ovary and lowest in doublesex and mab-3-related transcription factor 1(DMRT1) knock out fish and triploid males. In situ hybridization indicated that aqp1aa mRNA is expressed mainly in the germ cells of males and pseudo-males, especially in spermatozoa and spermatids. These results suggest that the aqp1aa may play a role in spermatogenesis of C. semilaevis.