Establishment and characterization of an ovarian cell line from half-smooth tongue sole Cynoglossus semilaevis

Identification and Functional Analysis of E3 Ubiquitin Ligase g2e3 in Chinese Tongue Sole, Cynoglossus semilaevis

Gametogenesis, the intricate developmental process responsible for the generation of germ cells (gametes), serves as a fundamental prerequisite for the perpetuation of the reproductive cycle across diverse organisms. The g2e3 enzyme is a putative ubiquitin E3 ligase implicated in the intricate regulatory mechanisms underlying cellular proliferation and division processes. The present study delves into the function of G2/M phase-specific E3 ubiquitin protein ligase (Cs-g2e3) in gametogenesis in Chinese Tongue Sole (Cynoglossus semilaevis). Sequence analysis shows that the Cs-g2e3 mRNA spans 6479 bp, encoding a 733 amino acid protein characterized by three conserved structural domains: PHD, RING, and HECT-typical of HECT E3 ubiquitin ligases. The predominant expression of Cs-g2e3 in the gonad tissues is further verified by qPCR. The expression profile of Cs-g2e3 in the gonads of the Chinese Tongue Sole is analyzed at different ages, and the results show that its expression peaks at 8 months of age and then begins to decline and stabilize. It is noteworthy that the expression level remains significantly elevated compared to that observed during the juvenile period. In situ hybridization shows that the mRNA of Cs-g2e3 is mainly localized in the germ cells of the ovary and the testis. RNA interference experiments show that the knockdown of Cs-g2e3 in ovarian and testicular germ cell lines significantly downregulates the expression of key genes involved in oogenesis (e.g., sox9 and cyp19a) and spermatogenesis (e.g., tesk1 and piwil2), respectively. Furthermore, the analysis of mutations in the transcription factor binding sites reveals that mutations within the Myogenin, YY1, and JunB binding sites significantly impact the transcriptional activity of the Cs-g2e3 gene, with the mutation in the YY1 binding site exhibiting the most pronounced effect (p < 0.001). This study contributes to a deeper understanding of the tissue-specific expression patterns of Cs-g2e3 across various tissues in Cynoglossus semilaevis, as well as the potential regulatory influences of transcription factors on its promoter activity. These findings may facilitate future research endeavors aimed at elucidating the expression and functional roles of the Cs-g2e3 gene.

Unveiling Gene Expression Dynamics during Early Embryogenesis in Cynoglossus semilaevis: A Transcriptomic Perspective

Commencing with sperm-egg fusion, the early stages of metazoan development include the cleavage and formation of blastula and gastrula. These early embryonic events play a crucial role in ontogeny and are accompanied by a dramatic remodeling of the gene network, particularly encompassing the maternal-to-zygotic transition. Nonetheless, the gene expression dynamics governing early embryogenesis remain unclear in most metazoan lineages. We conducted transcriptomic profiling on two types of gametes (oocytes and sperms) and early embryos (ranging from the four-cell to the gastrula stage) of an economically valuable flatfish-the Chinese tongue sole Cynoglossus semilaevis (Pleuronectiformes: Cynoglossidae). Comparative transcriptome analysis revealed that large-scale zygotic genome activation (ZGA) occurs in the blastula stage, aligning with previous findings in zebrafish. Through the comparison of the most abundant transcripts identified in each sample and the functional analysis of co-expression modules, we unveiled distinct functional enrichments across different gametes/developmental stages: actin- and immune-related functions in sperms; mitosis, transcription inhibition, and mitochondrial function in oocytes and in pre-ZGA embryos (four- to 1000-cell stage); and organ development in post-ZGA embryos (blastula and gastrula). These results provide insights into the intricate transcriptional regulation of early embryonic development in Cynoglossidae fish and expand our knowledge of developmental constraints in vertebrates.

Profiling the Physiological Roles in Fish Primary Cell Culture

Fish primary cell culture has emerged as a valuable tool for investigating the physiological roles and responses of various cell types found in fish species. This review aims to provide an overview of the advancements and applications of fish primary cell culture techniques, focusing on the profiling of physiological roles exhibited by fish cells in vitro. Fish primary cell culture involves the isolation and cultivation of cells directly derived from fish tissues, maintaining their functional characteristics and enabling researchers to study their behavior and responses under controlled conditions. Over the years, significant progress has been made in optimizing the culture conditions, establishing standardized protocols, and improving the characterization techniques for fish primary cell cultures. The review highlights the diverse cell types that have been successfully cultured from different fish species, including gonad cells, pituitary cells, muscle cells, hepatocytes, kidney and immune cells, adipocyte cells and myeloid cells, brain cells, primary fin cells, gill cells, and other cells. Each cell type exhibits distinct physiological functions, contributing to vital processes such as metabolism, tissue regeneration, immune response, and toxin metabolism. Furthermore, this paper explores the pivotal role of fish primary cell culture in elucidating the mechanisms underlying various physiological processes. Researchers have utilized fish primary cell cultures to study the effects of environmental factors, toxins, pathogens, and pharmaceutical compounds on cellular functions, providing valuable insights into fish health, disease pathogenesis, and drug development. The paper also discusses the application of fish primary cell cultures in aquaculture research, particularly in investigating fish growth, nutrition, reproduction, and stress responses. By mimicking the in vivo conditions in vitro, primary cell culture has proven instrumental in identifying key factors influencing fish health and performance, thereby contributing to the development of sustainable aquaculture practices.

Establishment and identification of a skin cell line from Chinese tongue sole (Cynoglossus semilaevis) and analysis of the changes in its transcriptome upon LPS stimulation

The Chinese tongue sole (Cynoglossus semilaevis) holds significant economic importance within the fishing industry along the eastern coasts of China. In recent years, the frequent outbreaks of bacterial diseases have become a common concern as the aquaculture scale expands. The majority of the diseased fish exhibit symptoms such as skin congestion, damage and skin ulceration. As the skin serves as the first line of defense against bacterial infections, establishing a skin cell line for immunological research on Chinese tongue sole's response to bacterial infection is of utmost importance. In this study, a cell line named CSS (derived from the skin of the Chinese tongue sole) was successfully established. The cells have demonstrated stability during passages and exhibit a multipolar fibroblast-like morphology. They were cultured in L-15 medium with 20% serum and have been successfully passed through 60 passages over a period of 20 months. The identification of the mitochondrial CO1 gene confirmed that the cell originated from Chinese tongue sole. The karyotype detection revealed that the cell had a chromosome number of 2n = 42. After being stored in liquid nitrogen for 15 months, the cells can maintain more than 75% viability upon recovery. After transfecting with cy3-labeled scramble siRNA and pEGFP-N3 plasmid, clear fluorescence was observed in the transfected cells. We observed that lipopolysaccharide (LPS) from Escherichia coli significantly upregulate the gene expression of various immune-related pathways at 2 h in the CSS cell line. Additionally, the differentially expressed genes showed a higher enrichment in immune-related pathways at 2 and 6 h after stimulation compared to the 24 h point. Moreover, we identified 347 genes that exhibited a gradual increase in expression during the 0-24 h stimulation period. These genes were primarily enriched in pathways related to Autophagy, GABAergic synapse, Apelin signaling and Ferroptosis. In general, the CSS cell line established in this study exhibits stable growth and can serve as a valuable tool for in vitro studies of immunology and other basic biologies of Chinese tongue sole.

Establishment and characterization of the ovary cell line derived from two-spot puffer Takifugu bimaculatus and its application for gene editing and marine toxicology

Takifugu bimaculatus is a marine fish with high nutritional value. Its ovary contains tetrodotoxin (TTX) which is a severe neurotoxin that limits its edible value of it. To understand the mechanism of oogenesis and production of TTX in T. bimaculatus, an ovarian cell line named TBO from an adolescent ovary was established. TBO was composed of fibroblast-like cells that expressed the ovarian follicle cells marker gene Foxl2 and highly expressed TTX binding protein 2 (PSTBP2) but did not express the germ cells marker gene Vasa. Therefore, TBO seems to be mainly composed of follicle cells and possibly a small percentage of oocytes. Electroporation was used to successfully transfect the pEGFP-N1 and pNanog-N1 vectors into the TBO cell line with a high transfection efficiency. The morphological changes and survival rates of the exposed cells proved that this cell line was effective for exposure to conotoxins (CTXs), another group of toxins related to food safety. Furthermore, PSTBP2 was knocked out in TBO using CRISPR/Cas9 technology, showing that sgRNA2 could mutate PSTBP2. The results suggested that TBO will be more convenient, efficient, and rapid for reproduction and toxicology investigation, and gene editing. This study laid the groundwork for future research into the fish gonadal cell culture and food-related marine toxins. In conclusion, a cell line has been generated from T. bimaculatus, which might represent a valuable model for fish studies in the fields of toxicology and gene editing.

Potential Involvement of ewsr1-w Gene in Ovarian Development of Chinese Tongue Sole, Cynoglossus semilaevis

Simple Summary

Sexual dimorphism is a phenomenon commonly existing in animals. Chinese tongue sole Cynoglossus semilaevis is an economical marine fish with obvious female-biased size dimorphism. So, it is important to explore the molecular mechanism beyond gonadal development for sex control in aquaculture industry. RNA-binding protein Ewing Sarcoma protein-like (ewsr1) gene is important for mouse gonadal development and reproduction, however there are limited studies on this gene in teleost. In this study, two ewsr1 genes were cloned and characterized from C. semilaevis. The ewsr1-w gene, located in W chromosomes, showed female-biased expression during C. semilaevis gonadal development. In addition, knock-down effect and transcriptional regulation of Cs-ewsr1-w further suggested its essential role in ovarian development. This study broadened our understanding on ewsr1 function in teleost, and provided genetic resources for the further development of sex control breeding techniques in C. semilaevis aquaculture.

Abstract

Ewsr1 encodes a protein that acts as a multifunctional molecule in a variety of cellular processes. The full-length of Cs-ewsr1-w and Cs-ewsr1-z were cloned in Chinese tongue sole (Cynoglossus semilaevis). The open reading frame (ORF) of Cs-ewsr1-w was 1,767 bp that encoded 589 amino acids, while Cs-ewsr1-z was 1,794 bp that encoded 598 amino acids. Real-time PCR assays showed that Cs-ewsr1-w exhibited significant female-biased expression and could be hardly detected in male. It has the most abundant expression in ovaries among eight healthy tissues. Its expression in ovary increased gradually from 90 d to 3 y with C. semilaevis ovarian development and reached the peak at 3 y. After Cs-ewsr1-w knockdown with siRNA interference, several genes related to gonadal development including foxl2, sox9b and pou5f1 were down-regulated in ovarian cell line, suggesting the possible participation of Cs-ewsr1-w in C. semilaevis ovarian development. The dual-luciferase reporter assay revealed that the -733/-154 bp Cs-ewsr1-w promoter fragment exhibited strong transcription activity human embryonic kidney (HEK) 293T cell line. The mutation of a MAF BZIP Transcription Factor K (Mafk) binding site located in this fragment suggested that transcription factor Mafk might play an important role in Cs-ewsr1-w basal transcription. Our results will provide clues on the gene expression level, transcriptional regulation and knock-down effect of ewsr1 gene during ovarian development in teleost.

Function of Foxl2 and Dmrt1 proteins during gonadal differentiation in the olive flounder Paralichthys olivaceus

Study on fish sex differentiation is important both from academic and practical aspects. Foxl2 and Dmrt1 are important transcription factors that should be involved in fish gonadal differentiation, but there is still no direct evidence to clarify their protein functions. Olive flounder Paralichthys olivaceus, an important mariculture fish in China, Japan, and Korea, shows sex-dimorphic growth. In this study, the Foxl2 and Dmrt1 proteins were detected in granulosa cells of the ovary and Sertoli cells of the testis, respectively, showing significant sex-dimorphic expression patterns. Then, bioactive high-purity Foxl2 and Dmrt1 recombinant proteins were obtained in vitro. Furthermore, effects of the recombinant Foxl2 and Dmrt1 during gonadal differentiation period were evaluated by intraperitoneal injection in juvenile fish. Compared with the control group, the male rate in the Dmrt1 group increased from 0 % to 82 %, showing for the first time in fish that the recombinant Dmrt1 could alter the sex phenotype. In addition, transcription levels of cyp19a and its transcription factors also changed after the recombinant Foxl2 and Dmrt1 injection. These findings reveal that Foxl2 and Dmrt1 are vital regulators for fish gonadal differentiation by regulating cyp19a expression, and also provide a new approach for sex control in fish aquaculture.

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.

A Z-Linked E3 Ubiquitin Ligase Cs-rchy1 Is Involved in Gametogenesis in Chinese Tongue Sole, Cynoglossus semilaevis

Simple Summary

The sexual growth dimorphism prevails in animals and this phenomenon is even more obvious in marine fish, so understanding the mechanism of gonadal development and gametogenesis is of great importance for sex control, thus increased productivity in aquaculture. In mammal, ubiquitin ligase plays a versatile role in gonadal development and spermatogenesis, whereas its function in fish is little reported. Using Cynoglossus semilaevis (one-year-old female individual usually grows 2–4 times bigger than male) as the fish model, a Z-chromosome linked ubiquitin ligase neurl3 was previously identified and characterized, which suggested its involvement in spermatogenesis. However, in this study, characterization of another Z-chromosome linked ubiquitin ligase Cs-rchy1 suggested it might function both in spermatogenesis and oogenesis, as well as the potential role in growth. These data may provide the genetic resource for gene editing or marker exploration in future.

Abstract

Ubiquitin ligase (E3) plays a versatile role in gonadal development and spermatogenesis in mammals, while its function in fish is little reported. In this study, a Z-chromosome linked ubiquitin ligase rchy1 in C. semilaevis (Cs-rchy1) was cloned and characterized. The full-length cDNA was composed of 1962 bp, including 551 bp 5′UTR, 736 bp 3′UTR, and 675 bp ORF encoding a 224-amino-acid (aa) protein. Cs-rchy1 was examined among seven different tissues and found to be predominantly expressed in gonads. In testis, Cs-rchy1 could be detected from 40 days post hatching (dph) until 3 years post hatching (yph), but there was a significant increase at 6 months post hatching (mph). In comparison, the expression levels in ovary were rather stable among different developmental stages. In situ hybridization showed that Cs-rchy1 was mainly localized in germ cells, that is, spermatid and spermatozoa in testis and stage I, II and III oocytes in ovary. In vitro RNA interference found that Cs-rchy1 knockdown resulted in the decline of sox9 and igf1 in ovarian cell line and down-regulation of cyp19a in the testicular cell line. These data suggested that Cs-rchy1 might participate in gonadal differentiation and gametogenesis, via regulating steroid hormone synthesis.

A combination of genome-wide association study screening and SNaPshot for detecting sex-related SNPs and genes in Cynoglossus semilaevis

Chinese tongue sole (Cynoglossus semilaevis) males and females exhibit great differences in growth rate and appearance. The species is heterogametic (ZW/ZZ) and has sex-reversed "pseudomales" that are genetically female and physiologically male. In this study, we identified eight sex-specific single nucleotide polymorphism (SNP) markers for the sex identification of C. semilaevis by using a combination of genome-wide association study (GWAS) screening and SnaPshot validation. Candidate SNPs were screened using genotyping by sequencing to perform GWAS of the differential SNPs between the sexes of C. semilaevis. The SNP loci were amplified using a multiplex PCR system and detected via SNaPshot, which enables multiplexing of up to 30-40 SNPs in a single assay and ensures high accuracy of the results. The molecular markers detected in our study were used to successfully identify normal males and pseudomales from 45 caught and 40 cultured C. semilaevis specimens. Linkage disequilibrium analysis showed that the eight SNP loci were related to each other, with a strong linkage. Moreover, we investigated the expression of prdm6 mRNA containing a missense SNP and confirmed that the gene is differentially expressed in the gonads of the different sexes of C. semilaevis; the expression of prdm6 mRNA was significantly higher in the males than in the females and pseudomales. This means prdm6 may be related to sex differentiation in C. semilaevis.

Using ovarian and brain cell culture from the Mullet, Liza klunzingeri, to assess the inhibitory effects of benzo[a]pyrene on aromatase activity

We assessed the toxic effects of benzo[a]pyrene (BaP) on cell viability, aromatase (Aro) activity and steroid production using ovarian and brain cell cultures obtained from Mullet, Liza klunzingeri. The brain and ovary were minced and digested, and the cells were suspended in Leibovitz's L-15 medium supplemented with 15% and 20% fetal bovine serum. The cell suspensions were seeded on 25-cm² cell-culture flasks at 1 × 10⁶ cells/mL and incubated at 25 °C for 2 weeks. A BaP concentration of 10^-5 mol/L was accepted as the half-maximal inhibitory concentration. Ovarian and brain cells were exposed to different concentrations of BaP [0 (control), 10^-6 , 2 × 10^-6 , 3 × 10^-6 mol/L] and incubated at 30 °C. At different sampling times (0, 12, 24 and 48 h) 40 ng/10⁵ cells of 1,4,6-androstatriene-3,17-dione (ATD) was added to each well. Aro activity, 17β-estradiol (E2) and ATD production were determined. The sensitivity of the cultivated ovarian and brain cells to BaP increased dose dependently. BaP was a potent inhibitor of Aro activity at 2 × 10^-6 and 3 × 10^-6 mol/L, both in the cultivated brain and ovarian cells at different sampling times, with 10^-6 mol/L BaP found to be the least potent Aro inhibitor. E2 production decreased from cultivated ovarian and brain cells treated by different concentrations of BaP. In conclusion, BaP is able to change the activity of Aro and disrupt the biosynthesis of estrogens, and thus affects reproduction in fish.

Establishment of a novel testicular cell line from sterlet Acipenser ruthenus and evaluation of its applications

In this study, a cell line, designated as Acipenser ruthenus testis (ART), was successfully established from testis tissues of the sterlet Acipenser ruthenus and characterized by studying and comparing the expression of specific genes between the cell line and the parent gonad tissues. The results suggested that the developed ART cell line was composed of a mixture of germ cells and somatic cells. Ploidy analysis indicated that the cell line exhibited a high degree of genetic stability and that the cells remained in a good proliferating state after being subcultured to passage 80.

Cloning, expression prolife, and immune characterization of a novel stat family member (stat5bl) in Chinese tongue sole (Cynoglossus semilaevis)

STAT plays important roles in innate immunity during JAK/STAT signaling pathway, and STAT5 is particularly focused due to the existence of duplicated forms in fish and mammal. In Chinese tongue sole, stat5bl was suggested to be a candidate related to Vibrio harveyi resistance based on previous QTL screening. In this study, the full length of stat5bl cDNA was cloned and its expression patterns were analyzed. stat5bl was predominantly expressed in immune tissues, where the highest level was observed in liver, followed by skin and gill. Time course expression patterns were examined in six tissues (liver, skin, gill, kidney, intestine, spleen) after V. harveyi infection. stat5bl could be up-regulated by V. harveyi infection in all tissues except liver, despite the timepoints of peak were different. In contrast, stat5bl was significantly downregulated in liver. To elucidate the role of stat5bl in liver, in vitro RNAi were performed using primary liver cell culture. Knockdown of stat5bl could regulate the expression of genes closely related to JAK/STAT pathway. This study would enlarge our understanding of stat5bl in fish immunity.

Establishment and characterization of a brain-cell line from kelp grouper Epinephelus moara

A new brain-cell line, EMB, was developed from kelp grouper Epinephelus moara, a cultured marine fish. The EMB cells were subcultured for more than 60 passages. The cells were cultured in Leibovitz's L-15 medium (L15) supplemented with antibiotics, foetal bovine serum (FBS), 2-mercaptoethanol (2-ME) and basic fibroblast growth factor (bFGF). The cells could grow at 18-30° C, with the maximum growth between 24 and 30° C. The optimum FBS concentration for the cells growth ranged between 15 and 20%. Chromosome analysis indicated that the modal chromosome number was 48 in the cells at passage 45. After being transfected with pEGFP-N3 plasmid, the cells could successfully express green fluorescence protein (GFP), implying that this cell line can be used for transgenic studies. A significant cytopathic effect (CPE) was observed in the cells after infection with Singapore grouper iridovirus (SGIV) or red spotted grouper nervous necrosis virus (RGNNV) and the viral replication was confirmed by quantitative real-time PCR (qrt-PCR) assay, which suggested EMB's application potential for studies of SGIV and RGNNV.

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.

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.

Expression analysis and characterization of an autosome-localized tesk1 gene in half-smooth tongue sole (Cynoglossus semilaevis)

Testis-specific protein kinase 1 (tesk1) represents a conserved gene family functioning in many cellular processes. In this study, we cloned and characterized an autosome-localized tesk1 gene (Altesk1) from Cynoglossus semilaevis. The open reading frame consists of 2088 nucleotides and encodes a 665 amino acid polypeptide. Phylogenetic analyses show that vertebrate Tesk1s are divided into two clusters based on protein length and AlTesk1 belongs to "long-type" group. Semi-quantitative PCR reveals that Altesk1 is predominantly expressed in ovary, despite of relatively low detection in some other tissues. Among different development stages, Altesk1 transcripts are only observed in ovary samples of 210-day and 1-year fish. In situ hybridization analyses have further confirmed its major localization in oocyte cells. Comparison of methylation patterns in different sexual genotypes reveals the low methylation level of Altesk1 promoter in female, which is consistent with Altesk1 high expression level in female. Taken together, this is the first time that tesk1 gene has been found to show female-biased expression and in view of this, we postulate that AlTesk1 might be involved in some cellular processes specific in ovary, e.g. oogenesis.