Characterization and gonadal expression of FOXL2 relative to Cyp19a genes in spotted scat Scatophagus argus

Cyp19a1a Promotes Ovarian Maturation through Regulating E2 Synthesis with Estrogen Receptor 2a in Pampus argenteus (Euphrasen, 1788)

In the artificial breeding of Pampus argenteus (Euphrasen, 1788), female fish spawn before male release sperm, which indicates rapid ovarian development. In fish, aromatase is responsible for converting androgens into estrogens and estrogen plays a crucial role in ovarian development. In this study, we aimed to investigate the potential role of brain-type and ovarian-type aromatase to study the rapid ovarian development mechanism. The results showed that cyp19a1a was mainly expressed in the ovary and could be classified as the ovarian type, whereas cyp19a1b could be considered as the brain type for its expression was mainly in the brain. During ovarian development, the expression of cyp19a1a in the ovary significantly increased from stage IV to stage V and Cyp19a1a signals were present in the follicle cells, while cyp19a1b expression in the pituitary gland decreased from stage IV to stage V. To further investigate the function of Cyp19a1a, recombinant Cyp19a1a (rCyp19a1a) was produced and specific anti-Cyp19a1a antiserum was obtained. The expressions of cyp19a1a, estrogen receptors 2 alpha (esr2a), and androgen receptor alpha (arα) were significantly upregulated in the presence of rCyp19a1a. Meanwhile, cyp19a1a was expressed significantly after E2 treatment in both ovarian and testicular tissue culture. Taken together, we found two forms of aromatase in silver pomfret. The ovarian-type aromatase might play an important role in ovarian differentiation and maturation, and participate in E2 synthesis through co-regulation with esr2a. The brain-type aromatase cyp19a1b might be involved in the regulation of both brain and gonadal development.

Cloning, Identification, and Functional Analysis of the Foxl2 Gene in Procambarus clarkii

Procambarus clarkii is the most widely distributed freshwater shrimp in China, with important economic value and great potential for development. The forkheadboxL2 (Foxl2) gene has been found to be involved in the reproductive development of many crustaceans. To understand the role of the Foxl2 gene in the gonad development of P. clarkii, we designed CDS-specific primers for the P. clarkii Foxl2 (PcFoxl2) gene and cloned its CDS sequence using RT-PCR. The nucleotide and protein sequence information was then analyzed through bioinformatics analysis. The expression and subcellular localization of PcFoxl2 in various tissues were detected using qRT-PCR and in situ hybridization. The effects of PcFoxl2 knockdown on gonad development were investigated using RNA interference. The results showed that the CDS length of the PcFoxl2 gene was 1614 bp and encoded 537 amino acids. Protein sequence comparison and phylogenetic analysis showed that PcFoxl2 was the closest relative to Crayfish. qRT-PCR analysis indicated that the expression level of PcFoxl2 in the testis was significantly higher (>40 fold) than that in the ovary (p < 0.01). The in situ hybridization results showed that PcFoxl2 was expressed in both the cytoplasm and the nucleus of egg cells, and that the expression was strongest in egg cells at the early stage of yolk synthesis, while weak in the secondary oocytes. The positive signal was strongest in the spermatocyte nucleolus, while only a trace signal was observed in the cytoplasm. After interfering with the PcFoxl2 gene using dsRNA, the expression of PcFoxl2 in the RNA interference group was significantly lower than that in the control group, and this interference effect lasted for one week. Moreover, the gonad index of the experimental group was significantly lower than that of the control group (p < 0.05) after 10 days of P. clarkii cultivation following PcFoxl2 knockdown. The expression levels of the nanos and S3a genes, which are related to gonad development, decreased significantly after PcFoxl2 gene interference. The results suggest that the Foxl2 gene is involved in the growth and development of gonads, particularly in the development of testis, and is related to the early development of oocytes. This study provides a theoretical basis for the artificial breeding of P. clarkii.

Effects of short-term water velocity stimulation on the biochemical and transcriptional responses of grass carp (Ctenopharyngodon idellus)

Since 2011, ecological operation trials of the Three Gorges Reservoir (TGR) have been continuously conducted to improve the spawning quantity of the four major Chinese carp species below the Gezhouba Dam. In particular, exploring the effects of short-term water velocity stimulation on ovarian development in grass carp (Ctenopharyngodon idellus) is essential to understand the response of natural reproduction to ecological flows. We performed ovary histology analysis and biochemical assays among individuals with or without stimulation by running water. Although there were no obvious effects on the ovarian development characteristics of grass carp under short-term water velocity stimulation, estradiol, progesterone, follicle-stimulating hormone (FSH), and triiodothyronine (T3) concentrations were elevated. Then, we further explored the ovarian development of grass carp under short-term water velocity stimulation by RNA sequencing of ovarian tissues. In total, 221 and 741 genes were up- or downregulated under short-term water velocity stimulation, respectively, compared to the control group. The majority of differentially expressed genes (DEGs) were enriched in pathways including ABC transporters, cytokine-cytokine receptor interaction, ECM-receptor interaction, and steroid hormone biosynthesis. Important genes including gpr4, vtg1, C-type lectin, hsd17b1, cyp19a1a, cyp17a1, and rdh12 that are involved in ovarian development were regulated. Our results provide new insights and reveal potential regulatory genes and pathways involved in the ovarian development of grass carp under short-term water velocity stimulation, which may be beneficial when devising further ecological regulation strategies.

Molecular cloning and expression patterns of a sex-biased transcriptional factor Foxl2 in the giant freshwater prawn (Macrobrachium rosenbergii)

BACKGROUD

Macrobrachium rosenbergii is an economically important species that is widely cultivated in some Asian nations. Foxl2 is a transcriptional regulator of ovarian differentiation and development. The aim of this study was to study the bioinformatics features and expression patterns of M. rosenbergii Foxl2 (MrFoxl2).

METHODS

In this study, all experimental animals were mature M. rosenbergii (9-12 cm) individuals. The foxl2 gene was identified and characterized in the genome of M. rosenbergii using molecular cloning, bioinformatic analysis, in situ hybridization, and quantitative analysis.

RESULTS

The identified cDNA encoded a putative 489-amino-acid MrFoxl2 protein. Bioinformatics analysis revealed a low identity of MrFoxl2 to other crustacean orthologues. The closest phylogenetic relationship was to Foxl2 of Eriocheir sinensis. The result of in situ hybridization demonstrated that transcripts of MrFoxl2 in M. rosenbergii were identified in spermatocytes, oocytes, and secretory epithelial cells of the vas deferens. The result of q-PCR suggested that a high expression of MrFoxl2 was identified in the testis, vas deferens, and ovaries. During ovarian development, MrFoxl2 expression was the highest in stage I.

CONCLUSION

Our findings suggest that MrFoxl2 may play a role in gonadal development in both female and male M. rosenbergii.

Feminization of channel catfish with 17β-oestradiol involves methylation and expression of a specific set of genes independent of the sex determination region

Exogenous oestrogen 17β-oestradiol (E2) has been shown to effectively induce feminization in teleosts. However, the molecular mechanisms underlying the process remain unclear. Here, we determined global DNA methylation and gene expression profiles of channel catfish (Ictalurus punctatus) during early sex differentiation after E2 treatment. Overall, the levels of global DNA methylation after E2 treatment were not significantly different from those of controls. However, a specific set of genes were differentially methylated, which included many sex differentiation-related pathways, such as MARK signalling, adrenergic signalling, Wnt signalling, GnRH signalling, ErbB signalling, and ECM-receptor interactions. Many genes involved in these pathways were also differentially expressed after E2 treatment. Specifically, E2 treatments resulted in upregulation of female-related genes and downregulation of male-related genes in genetic males during sex reversal. However, E2-induced sex reversal did not cause sex-specific changes in methylation profiles or gene expression within the sex determination region (SDR) on chromosome 4, suggesting that E2-induced sex reversal was a downstream process independent of the sex determination process that was regulated by sex-specific methylation within the SDR.

Characterization, expression, and regulatory effects of nr0b1a and nr0b1b in spotted scat (Scatophagus argus)

Nuclear receptor subfamily 0 group B member 1 (Nr0b1) belongs to the nuclear receptor (NR) superfamily. It plays critical roles in sex determination, sex differentiation, and gonadal development in mammals. In this study, the duplicated genes nr0b1a and nr0b1b were identified in spotted scat (Scatophagus argus). Phylogenetic and synteny analyses revealed that, unlike nr0b1a, nr0b1b was retained in several species of teleosts after an nr0b1 gene duplication event but was secondarily lost in other fish species, amphibians, reptiles, birds, and mammals. In a sequence analysis, only 1.5 LXXLL-related repeat motifs were identified in spotted scat Nr0b1a, Nr0b1b, and non-mammalian Nr0b1a/Nr0b1, different from the 3.5 repeat motifs in mammalian Nr0b1. By qPCR, nr0b1a and nr0b1b were highly expressed in testes from stages IV to V and in ovaries from stages II to IV, respectively. Male-to-female sex reversal was induced in XY spotted scat by the administration of exogenous E2. A qPCR analysis showed that nr0b1b mRNA expression was higher in sex-reversed XY fish than in control XY fish, with no difference in nr0b1a. A luciferase assay showed that spotted scat Nr0b1a and Nr0b1b did not individually activate cyp19a1a gene transcription. As in mammals, spotted scat Nr0b1a suppressed Nr5a1-mediated cyp19a1a expression, despite containing only 1.5 LXXLL-related repeat motifs in its N-terminal region, while Nr0b1b stimulated Nr5a1-mediated cyp19a1a transcription. These results demonstrated that nr0b1a and nr0b1b in spotted scat have distinct expression patterns and regulatory effects and further indicate that nr0b1b might be involved in ovarian development by regulating Nr5a1-mediated cyp19a1a expression.

Identification and functional analysis of a sex-biased transcriptional factor Foxl2 in the bay scallop Argopecten irradians irradians

Transcription factor Foxl2 is an evolutionarily conserved gene playing pivotal roles in regulation of early ovarian differentiation and maintenance in animals. However, the Foxl2 gene has not been thoroughly studied in hermaphroditic scallops. In this study, we cloned and characterized a Foxl2 (designated as AiFoxl2) from the bay scallop Argopecten irradians irradians. The open reading frame of AiFoxl2 was 1122 bp encoding 373 amino acids residues and contained a conserved forkhead box domain. Quantitative real-time PCR showed that AiFoxl2 was mainly expressed in the ovary. Moreover, the highest expression of AiFoxl2 in the ovary was detected at proliferative stage and growing stage, while the lowest level was found at resting stage. During the embryonic and larval development, expression of AiFoxl2 was found first in fertilized eggs, increased significantly at the blastula stage, and reached peak value at the D-larvae stage. When AiFoxl2 was knocked down, testis development-related genes (Dmrt1, Sox7 and Sox9) were up-regulated significantly while the ovary development-related genes (Vg, HSD14, and GATA-1) were down-regulated manifestly. These findings suggested that AiFoxl2 was a female-related gene in A. i. irradians and may be involved in regulation of ovarian development and differentiation.

ddRADseq-assisted construction of a high-density SNP genetic map and QTL fine mapping for growth-related traits in the spotted scat (Scatophagus argus)

BACKGROUND

Scatophagus argus is a popular farmed fish in several countries of Southeast Asia, including China. Although S. argus has a highly promising economic value, a significant lag of breeding research severely obstructs the sustainable development of aquaculture industry. As one of the most important economic traits, growth traits are controlled by multiple gene loci called quantitative trait loci (QTLs). It is urgently needed to launch a marker assisted selection (MAS) breeding program to improve growth and other pivotal traits. Thus a high-density genetic linkage map is necessary for the fine mapping of QTLs associated with target traits.

RESULTS

Using restriction site-associated DNA sequencing, 6196 single nucleotide polymorphism (SNP) markers were developed from a full-sib mapping population for genetic map construction. A total of 6193 SNPs were grouped into 24 linkage groups (LGs), and the total length reached 2191.65 cM with an average marker interval of 0.35 cM. Comparative genome mapping revealed 23 one-to-one and 1 one-to-two syntenic relationships between S. argus LGs and Larimichthys crocea chromosomes. Based on the high-quality linkage map, a total of 44 QTLs associated with growth-related traits were identified on 11 LGs. Of which, 19 significant QTLs for body weight were detected on 9 LGs, explaining 8.8-19.6% of phenotypic variances. Within genomic regions flanking the SNP markers in QTL intervals, we predicted 15 candidate genes showing potential relationships with growth, such as Hbp1, Vgll4 and Pim3, which merit further functional exploration.

CONCLUSIONS

The first SNP genetic map with a fine resolution of 0.35 cM for S. argus has been developed, which shows a high level of syntenic relationship with L. crocea genomes. This map can provide valuable information for future genetic, genomic and evolutionary studies. The QTLs and SNP markers significantly associated with growth-related traits will act as useful tools in gene mapping, map-based cloning and MAS breeding to speed up the genetic improvement in important traits of S. argus. The interesting candidate genes are promising for further investigations and have the potential to provide deeper insights into growth regulation in the future.

Comparative transcriptome analysis of male and female gonads reveals sex-biased genes in spotted scat (Scatophagus argus)

Scatophagus argus is a new emerging aquaculture fish in East and Southeast Asia. To date, research on reproductive development and regulation in S. argus is lacking. Additionally, genetic and genomic information about reproduction, such as gonadal transcriptome data, is also lacking. Herein, we report the first gonadal transcriptomes of S. argus and identify genes potentially involved in reproduction and gonadal development. A total of 136,561 unigenes were obtained by sequencing of testes (n = 3) and ovaries (n = 3) at stage III. Genes upregulated in males and females known to be involved in gonadal development and gametogenesis were identified, including male-biased dmrt1, amh, gsdf, wt1a, sox9b, and nanos2, and female-biased foxl2, gdf9, bmp15, sox3, zar1, and figla. Serum estradiol-17β and 11-ketotestosterone levels were biased in female and male fish, respectively. Sexual dimorphism of serum steroid hormone levels were interpreted after expression analysis of 20 steroidogenesis-related genes, including cyp19a1a and cyp11b2. This gonadal transcript dataset will help investigate functional genes related to reproduction in S. argus.

FoxL2 combined with Cyp19a1a regulate the spawning upstream migration in Coilia nasus

FoxL2 is a member of the forkhead/HNF-3-related family of transcription factors which provides tissue specific gene regulation. It is known to regulate ovarian aromatase, which plays a crucial role in ovarian development and mature. To understand the role of FoxL2/ovarian aromatase encoded gene Cyp19a1a during ovarian development and recrudescence, we identified cDNA characteristics of FoxL2 and Cyp19a1a, analyzed its temporal expression both at transcript and protein levels in the anadromous fish, Coilia nasus. Tissue distribution pattern revealed that FoxL2 mRNA expression level was highest in ovary, while Cyp19a1a mRNA was highest in brain. During the upstream migration cycle, in ovary, the FoxL2 mRNA temporal expression peaked at the multiplication stage (stage III in May), the Cyp19a1a mRNA expression peaked at the onset stage (stage I in March). It was found that their mRNA transcripts were maintained at high level during the migration stage (from stage I in March to stage VI in July). Additionally, the strongest immunolabeling positive signals of Cyp19a1a and FoxL2 proteins were mainly found in the cytoplasm of olfactory bulb cell, stratum granulare and neurogliocyte cells and development stage oocytes. Data indicated that FoxL2 and Cyp19a1a were inducible and functional in the C. nasus ovary development and migration process. Therefore, the present results can be regarded as evidence for indispensable roles of FoxL2 and Cyp19a1a in the ovary development and migratory behavior at gene expression patterns and encoded protein distribution level.

Phoenixin: Expression at different ovarian development stages and effects on genes ralated to reproduction in spotted scat, Scatophagus argus

Phoenixin (Pnx), a recently discovered neuropeptide, has been implicated in reproduction. Pnx mainly exists in two active isoforms, phoenixin-14 (Pnx-14) and phoenixin-20 (Pnx-20). However, little is known about the functions of Pnx in teleosts. To determine the roles of Pnx in the regulation of reproduction in Scatophagus argus, the physiological characterization of the Pnx was analyzed. During ovary development, the expression of pnx in phase IV was higher than in phase II and III in the hypothalamus. In the pituitary, pnx expression was highest in phase IV, moderate in phase III, and lowest in phase II. When hypothalamus and pituitary fragments were cultured in vitro with Pnx-14 and Pnx-20 (10 nM and 100 nM) for 6 h, the expression of GnRHR (gonadotropin releasing hormone receptor), lh (luteinizing hormone) and fsh (follicular stimulating hormone) in the pituitary increased significantly, except GnRH (gonadotropin releasing hormone) in the hypothalamus. Similarly, the expression of GnRHR, lh and fsh in the pituitary increased significantly after injecting S. argus with Pnx-14 and Pnx-20 (10 ng/g and 100 ng/g body weight (bw)), except GnRHR and fsh treated with 10 ng/gbw Pnx-20 in the pituitary and GnRHs in the hypothalamus. These results indicate that Pnx may not only stimulate the reproduction of the S. argus through the hypothalamic-pituitary-gonadal (HPG) axis, but also directly through the pituitary.

Dynamic epimarks in sex-related genes predict gonad phenotype in the European sea bass, a fish with mixed genetic and environmental sex determination

The integration of genomic and environmental influences into methylation patterns to bring about a phenotype is of central interest in developmental epigenetics, but many details are still unclear. The sex ratios of the species used here, the European sea bass, are determined by genetic and temperature influences. We created four families from parents known to produce offspring with different sex ratios, exposed larvae to masculinizing temperatures and examined, in juvenile gonads, the DNA methylation of seven genes related to sexual development by a targeted sequencing approach. The genes most affected by both genetics and environment were cyp19a1a and dmrt1, with contrasting sex-specific methylation and temperature responses. The relationship between cyp19a1a methylation and expression is relevant to the epigenetic regulation of vertebrate sex, and we report the evidence of such relationship only below a methylation threshold, ~ 80%, and that it was sex-specific: negatively correlated in females but positively correlated in males. From parents to offspring, the methylation in gonads was midway between oocytes and sperm, with bias towards oocytes for amh-r2, er-β2, fsh-r and cyp19a1a. In contrast, dmrt1 levels resembled those of sperm. The methylation of individual CpGs from foxl2, er-β2 and nr3c1 were conserved from parents to offspring, whereas those of cyp19a1a, dmrt1 and amh-r2 were affected by temperature. Utilizing a machine-learning procedure based on the methylation levels of a selected set of CpGs, we present the first, to our knowledge, system based on epigenetic marks capable of predicting sex in an animal with ~ 90% accuracy and discuss possible applications.

Brain Aromatase Modulates Serotonergic Neuron by Regulating Serotonin Levels in Zebrafish Embryos and Larvae

Teleost fish are known to express two isoforms of P450 aromatase, a key enzyme for estrogen synthesis. One of the isoforms, brain aromatase (AroB), cyp19a1b, is highly expressed during early development of zebrafish, thereby suggesting its role in brain development. On the other hand, early development of serotonergic neuron, one of the major monoamine neurons, is considered to play an important role in neurogenesis. Therefore, in this study, we investigated the role of AroB in development of serotonergic neuron by testing the effects of (1) estradiol (E₂) exposure and (2) morpholino (MO)-mediated AroB knockdown. When embryos were exposed to E₂, the effects were biphasic. The low dose of E₂ (0.005 µM) significantly increased serotonin (5-HT) positive area at 48 hour post-fertilization (hpf) detected by immunohistochemistry and relative mRNA levels of tryptophan hydroxylase isoforms (tph1a, tph1b, and tph2) at 96 hpf measured by semi-quantitative PCR. To test the effects on serotonin transmission, heart rate and thigmotaxis, an indicator of anxiety, were analyzed. The low dose also significantly increased heart rate at 48 hpf and decreased thigmotaxis. The high dose of E₂ (1 µM) exhibited opposite effects in all parameters. The effects of both low and high doses were reversed by addition of estrogen receptor (ER) blocker, ICI 182,780, thereby suggesting that the effects were mediated through ER. When AroB MO was injected to fertilized eggs, 5-HT-positive area was significantly decreased, while the significant decrease in relative tph mRNA levels was found only with tph2 but not with two other isoforms. AroB MO also decreased heart rate and increased thigmotaxis. All the effects were rescued by co-injection with AroB mRNA and by exposure to E₂. Taken together, this study demonstrates the role of brain aromatase in development of serotonergic neuron in zebrafish embryos and larvae, implying that brain-formed estrogen is an important factor to sustain early development of serotonergic neuron.

Transcription of CYP19A1 is directly regulated by SF-1 in the theca cells of ovary follicles in chicken

Many studies have suggested the important role of estrogen in ovarian differentiation and development of vertebrates including chicken. Cytochrome P450 aromatase, encoded by CYP19A1, is a key enzyme in estrogen synthesis, but the mechanism of CYP19A1 regulation in chicken remains unknown. Here, we found that CYP19A1 was only expressed in the theca cell layers of chicken ovary follicles. Steroidogenic factor 1 (SF-1, also named as nuclear receptor subfamily 5 group A member 1, NR5A1), a potential regulators, was expressed in both the theca cell layers and granulosa cell layers. Forkheadbox L2 (FOXL2), another potential regulator, was only expressed in the granulosa cell layers. Using luciferase assays in vitro, we found that SF-1 could activate the promoter of CYP19A1 by binding to the nuclear receptor half-site (5'-TCAAGGTCA-3') from -280 to -271 base pairs. FOXL2 did not activate the promoter of chicken CYP19A1 gene in either 293T or DF-1 cells. Overexpression of SF-1 in DF-1 cells upregulated aromatase expression, but FOXL2 could not. Taken together, our results indicated that SF-1 activates CYP19A1 mRNA expression via a conserved binding site in chicken ovary, but FOXL2 may not affect the expression of CYP19A1.

Involvement of FOXL2 and RSPO1 in Ovarian Determination, Development, and Maintenance in Mammals

In mammals, sex determination is a process through which the gonad is committed to differentiate into a testis or an ovary. This process relies on a delicate balance between genetic pathways that promote one fate and inhibit the other. Once the gonad is committed to the female pathway, ovarian differentiation begins and, depending on the species, is completed during gestation or shortly after birth. During this step, granulosa cell precursors, steroidogenic cells, and primordial germ cells start to express female-specific markers in a sex-dimorphic manner. The germ cells then arrest at prophase I of meiosis and, together with somatic cells, assemble into functional structures. This organization gives the ovary its definitive morphology and functionality during folliculogenesis. Until now, 2 main genetic cascades have been shown to be involved in female sex differentiation. The first is driven by FOXL2, a transcription factor that also plays a crucial role in folliculogenesis and ovarian fate maintenance in adults. The other operates through the WNT/CTNNB1 canonical pathway and is regulated primarily by R-spondin1. Here, we discuss the roles of FOXL2 and RSPO1/WNT/ CTNNB1 during ovarian development and homeostasis in different models, such as humans, goats, and rodents.

Foxl2 and Its Relatives Are Evolutionary Conserved Players in Gonadal Sex Differentiation

Foxl2 is a member of the large family of Forkhead Box (Fox) domain transcription factors. It emerged during the last 15 years as a key player in ovarian differentiation and oogenesis in vertebrates and especially mammals. This review focuses on Foxl2 genes in light of recent findings on their evolution, expression, and implication in sex differentiation in animals in general. Homologs of Foxl2 and its paralog Foxl3 are found in all metazoans, but their gene evolution is complex, with multiple gains and losses following successive whole genome duplication events in vertebrates. This review aims to decipher the evolutionary forces that drove Foxl2/3 gene specialization through sub- and neo-functionalization during evolution. Expression data in metazoans suggests that Foxl2/3 progressively acquired a role in both somatic and germ cell gonad differentiation and that a certain degree of sub-functionalization occurred after its duplication in vertebrates. This generated a scenario where Foxl2 is predominantly expressed in ovarian somatic cells and Foxl3 in male germ cells. To support this hypothesis, we provide original results showing that in the pea aphid (insects) foxl2/3 is predominantly expressed in sexual females and showing that in bovine ovaries FOXL2 is specifically expressed in granulosa cells. Overall, current results suggest that Foxl2 and Foxl3 are evolutionarily conserved players involved in somatic and germinal differentiation of gonadal sex.

Sexually dimorphic gene expression patterns during gonadal differentiation in olive flounder, Paralichthys olivaceus

The present study aims to elucidate the different expression patterns and possible roles of Doublesex and Mab-3-related transcription factor 1 (dmrt1), dmrt4, SRY-related transcription factor 9 (sox9) and cytochrome P450 aromatase 19a (cyp19a) during gonadal differentiation in olive flounder, Paralichthys olivaceus. We first analyzed the gene expression patterns in tissues using RT-PCR, which indicated dmrt1, sox9 and cyp19a were sex-related genes with sexual dimorphic expression. The quantitative expression changes of these three genes together with dmrt4 during gonadal differentiation were further examined using real-time RT-PCR. The results showed that dmrt1 was scarcely expressed in the primitive gonad and during following periods of gonadal differentiation. Its expression increased rapidly in the differentiating testis. Dmrt4 was strongly expressed in primitive gonads and much less expressed during following periods of gonadal differentiation. Its expression became strong in differentiating testes. While sox9 was highly expressed in the primitive gonad, it was expressed with fluctuations during following periods of gonadal differentiation. Cyp19a started expressing in primitive gonads, and its expression quantity fluctuated during latter periods of gonadal differentiation, but was strongly expressed in the early stage of differentiating ovaries. Results of in situ hybridization showed that dmrt4 and sox9 transcripts were both mainly localized in spermatocytes and our results suggested these four sex-related genes might be involved in gonadal differentiation through their synergistic effects in flounder.