High temperature increases the gsdf expression in masculinization of genetically female Japanese flounder (Paralichthys olivaceus)

Molecular biological, physiological, cytological, and epigenetic mechanisms of environmental sex differentiation in teleosts: A systematic review

Human activities have been exerting widespread stress and environmental risks in aquatic ecosystems. Environmental stress, including temperature rise, acidification, hypoxia, light pollution, and crowding, had a considerable negative impact on the life histology of aquatic animals, especially on sex differentiation (SDi) and the resulting sex ratios. Understanding how the sex of fish responds to stressful environments is of great importance for understanding the origin and maintenance of sex, the dynamics of the natural population in the changing world, and the precise application of sex control in aquaculture. This review conducted an exhaustive search of the available literature on the influence of environmental stress (ES) on SDi. Evidence has shown that all types of ES can affect SDi and universally result in an increase in males or masculinization, which has been reported in 100 fish species and 121 cases. Then, this comprehensive review aimed to summarize the molecular biology, physiology, cytology, and epigenetic mechanisms through which ES contributes to male development or masculinization. The relationship between ES and fish SDi from multiple aspects was analyzed, and it was found that environmental sex differentiation (ESDi) is the result of the combined effects of genetic and epigenetic factors, self-physiological regulation, and response to environmental signals, which involves a sophisticated network of various hormones and numerous genes at multiple levels and multiple gradations in bipotential gonads. In both normal male differentiation and ES-induced masculinization, the stress pathway and epigenetic regulation play important roles; however, how they co-regulate SDi is unclear. Evidence suggests that the universal emergence or increase in males in aquatic animals is an adaptation to moderate ES. ES-induced sex reversal should be fully investigated in more fish species and extensively in the wild. The potential aquaculture applications and difficulties associated with ESDi have also been addressed. Finally, the knowledge gaps in the ESDi are presented, which will guide the priorities of future research.

Transcriptomic Networks Reveal the Tissue-Specific Cold Shock Responses in Japanese Flounder (Paralichthys olivaceus)

Low temperature is among the important factors affecting the distribution, survival, growth, and physiology of aquatic animals. In this study, coordinated transcriptomic responses to 10 °C acute cold stress were investigated in the gills, hearts, livers, and spleens of Japanese flounder (Paralichthys olivaceus), an important aquaculture species in east Asia. Histological examination suggested different levels of injury among P. olivaceus tissues after cold shock, mainly in the gills and livers. Based on transcriptome and weighted gene coexpression network analysis, 10 tissue-specific cold responsive modules (CRMs) were identified, revealing a cascade of cellular responses to cold stress. Specifically, five upregulated CRMs were enriched with induced differentially expressed genes (DEGs), mainly corresponding to the functions of "extracellular matrix", "cytoskeleton", and "oxidoreductase activity", indicating the induced cellular response to cold shock. The "cell cycle/division" and "DNA complex" functions were enriched in the downregulated CRMs for all four tissues, which comprised inhibited DEGs, suggesting that even with tissue-specific responses, cold shock may induce severely disrupted cellular functions in all tissues, reducing aquaculture productivity. Therefore, our results revealed the tissue-specific regulation of the cellular response to low-temperature stress, which warrants further investigation and provides more comprehensive insights for the conservation and cultivation of P. olivaceus in cold water.

Gonadal Soma-Derived Factor Expression is a Potential Biomarker for Predicting the Effects of Endocrine-Disrupting Chemicals on Gonadal Differentiation in Japanese Medaka (Oryzias Latipes)

Chemicals with androgenic or estrogenic activity induce the sex reversal and/or intersex condition in various teleost fish species. Previously, we reported that exposure to 17α-methyltestosterone, bisphenol A, or 4-nonylphenol induces changes in expression of the gonadal soma-derived factor (gsdf) gene accompanied by disruption of gonadal differentiation in Japanese medaka (Oryzias latipes). These findings suggest that gsdf expression might be a useful biomarker for predicting the potential effect of chemicals on gonadal differentiation. We examined the gsdf expression in Japanese medaka exposed to chemicals with estrogenic or androgenic activity. Exposure to the androgenic steroid 17β-trenbolone at 0.5-22.1 μg/L induced the development of ovotestis (presence of ovarian tissue with testicular tissue) and female-to-male sex reversal in XX embryos, and exposure at 6.32 and 22.1 μg/L significantly increased gsdf expression in XX embryos compared with controls at developmental stage 38 (1 day before hatching). In the present study, no statistically significant difference in gsdf mRNA expression was observed after exposure to 17β-estradiol, 17α-ethinylestradiol, and 4-t-octylphenol, which have estrogenic activity. In addition, antiandrogenic chemicals or chemicals without endocrine-disrupting activity did not induce changes in gsdf expression in XX or XY embryos. Thus, an increase in gsdf expression after androgen exposure was observed in XX embryos. Together, these findings indicate that gsdf expression might be useful for predicting the adverse effect of chemicals on gonadal differentiation. Environ Toxicol Chem 2022;41:1875-1884. © 2022 SETAC.

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.

New insights on folliculogenesis and follicular placentation in marine viviparous fish black rockfish (Sebastes schlegelii)

In viviparous fish, a considerable degree of variation in placental structures have been described. However, no distinct structures are reported in Scorpaenidae. In this study, we demonstrate a new type of folliculogenesis and follicular placentation in Sebastes schlegelii. Before copulation, the germinal epithelium gradually surrounds the oocytes and develops into individually follicles with a stalk-like structure hanging on the ovigerous lamella, which ensures each follicle have access to spermatozoa after copulation. From V to early gestation stage, the cyp17-I highly expressesaccompanied by cyp19a1a signals disappearance, and 11-ketotestosterone level keeps rising and peaks at blastula stage, while 17β-estradiol declines to the bottom. Meanwhile, the theca cells rapidly proliferate and invade outwards forming a highly hypertrophied and folded microvillous placenta. This unbalance of hormone might be an important factor driving the theca cells proliferation and invasion. Additionally, some conserved genes related to mammalian placentation are significantly high expression in follicular placenta suggesting the high convergence in vertebrate placenta evolution.

Molecular Mechanism Based on Histopathology, Antioxidant System and Transcriptomic Profiles in Heat Stress Response in the Gills of Japanese Flounder

As an economically important flatfish in Asia, Japanese flounder is threatened by continuously rising temperatures due to global warming. To understand the molecular responses of this species to temperature stress, adult Japanese flounder individuals were treated with two kinds of heat stress—a gradual temperature rise (GTR) and an abrupt temperature rise (ATR)—in aquaria under experimental conditions. Changes in histopathology, programmed cell death levels and the oxidative stress status of gills were investigated. Histopathology showed that the damage caused by ATR stress was more serious. TUNEL signals confirmed this result, showing more programmed cell death in the ATR group. In addition, reactive oxygen species (ROS) levels and the 8-O-hDG contents of both the GTR and ATR groups increased significantly, and the total superoxide dismutase (T-SOD) activities and total antioxidant capacity (T-AOC) levels decreased in the two stressed groups, which showed damage to antioxidant systems. Meanwhile, RNA-seq was utilized to illustrate the molecular mechanisms underyling gill damage. Compared to the control group of 18 °C, 507 differentially expressed genes (DEGs) were screened in the GTR group; 341 were up-regulated and 166 were down-regulated, and pathway enrichment analysis indicated that they were involved in regulation and adaptation, including chaperone and folding catalyst pathways, the mitogen-activated protein kinase signaling (MAPK) pathway and DNA replication protein pathways. After ATR stress, 1070 DEGs were identified, 627 were up-regulated and 423 were down-regulated, and most DEGs were involved in chaperone and folding catalyst and DNA-related pathways, such as DNA replication proteins and nucleotide excision repair. The annotation of DEGs showed the great importance of heat shock proteins (HSPs) in protecting Japanese flounder from heat stress injury; 12 hsp genes were found after GTR, while 5 hsp genes were found after ATR. In summary, our study records gill dysfunction after heat stress, with different response patterns observed in the two experimental designs; chaperones were activated to defend heat stress after GTR, while replication was almost abandoned due to the severe damage consequent on ATR stress.

Dynamic changes in mitochondrial DNA, morphology, and fission during oogenesis of a seasonal-breeding teleost, Pampus argenteus

Mitochondria play crucial roles during oocyte development. In this study, we have investigated mitochondrial morphology, mtDNA, Ca²⁺-ATP enzyme activity, and mitochondrial fission factor (mff) expression levels during oogenesis of the silver pomfret Pampus argenteus. The mtDNA increased with oocyte development, and mitochondrial morphology and distribution were stage-specific. In the perinucleolar oocytes, oval mitochondria were dispersed in the cytoplasm. In previtellogenic oocytes, mitochondria massively increased and aggregated, forming mitochondrial clouds. At the same time, two morphologically different types of mitochondria had been distinguished, one of which was elongated with well-developed cristae, and the other was round with distorted and fused cristae. During vitellogenesis, the increases in mitochondria with well-developed cristae and in Ca²⁺-ATPase enzymatic activity were accompanied by an accumulation of yolk substance, suggesting the possible participation of mitochondria in the formation of vitellogenesis. Furthermore, we examined the cDNA of mff its transcript levels in relation to oocyte development. The transcript levels of mff were high in the perinucleolar stage, increasing to the highest level at the previtellogenic stage. Immunocytochemistry showed that MFF was detected in the cytoplasm of previtellogenic and midvitellogenic oocytes. We speculated that the mff-mediated mitochondrial fission may play a crucial role in oocyte development, especially in vitellogenesis.

Chromosome level reference of Atlantic halibut Hippoglossus hippoglossus provides insight into the evolution of sexual determination systems

Changes in the genetic mechanisms that control sexual determination have occurred independently across the tree of life, and with exceptional frequency in teleost fishes. To investigate the genomic changes underlying the evolution of sexual determination, we sequenced a chromosome-level genome, multitissue transcriptomes, and reduced representation population data for the Atlantic halibut (Hippoglossus hippoglossus), which has an XY/XX sex determination mechanism and has recently diverged (0.9-3.8 Ma) from the Pacific halibut (Hippoglossus stenolepis), which has a ZZ/ZW system. We used frequency and coverage-based population approaches to identify a putative sex-determining factor, GSDF. We characterized regions with elevated heterozygosity and linkage disequilibrium indicating suppression of recombination across a nascent sex chromosome. We detected testis-specific expression of GSDF, the sequence of which is highly conserved across flatfishes. Based on evidence from genome-wide association, coverage, linkage disequilibrium, testis and brain transcriptomes, and sequence conservation with other flatfishes, we propose a mechanism for the recent evolution of an XY sex-determination mechanism in Atlantic halibut. Changes to the ancestral sex-determining gene DMRT1 in regulating the downstream gene GSDF probably coincided with GSDF, or a proximal regulatory element of it, becoming the primary sex-determining factor. Our results suggest changes to a small number of elements can have drastic repercussions for the genomic substrate available to sex-specific evolutionary forces, providing insight into how certain elements repeatedly evolve to control sex across taxa. Our chromosome-level assembly, multitissue transcriptomes, and population genomic data provide a valuable resource and understanding of the evolution of sexual systems in fishes.

R-spondin1 in loach (Misgurnus anguillicaudatus): Identification, characterization, and analysis of its expression patterns and DNA methylation in response to high-temperature stress

With a well-understood function in mammals, R-spondin1 (Rspo1) is an important regulator of ovarian development via the Wnt/β-catenin pathway. Rspo1 deficiency causes retardation of ovarian development in XX fish, and increases Rspo1 function induces femininity and sex reversal in XY fish. In this study, Rspo1 was successfully cloned from loach (Misgurnus anguillicaudatus), and its expression profile was analyzed. The full-length cDNA of Misgurnus anguillicaudatus Rspo1 (MaRspo1) comprised 1322 bp and included an open reading frame (ORF) of 795 bp, which encoded a predicted polypeptide measuring 264 amino acids in length. Phylogenetic and gene structure analyses showed a highly conserved sequence of MaRspo1 (identical to the Rspo1 genes of other species), consisting of an N-terminal signal peptide (SP), two furin-like cysteine-rich domains (FU1 and FU2), a thrombospondin type 1 repeat (TSP1) and a C-terminal region. Real-time PCR revealed the female-biased expression profile of MaRspo1, with the highest expression level among tested tissues detected in ovary. Investigation of MaRspo1 expression levels throughout the early development stage (10-60 days post hatching) under three temperature treatments (25 °C, 28 °C, and 31 °C) revealed significantly differential expression of MaRspo1 among the three temperature groups, with decreased MaRspo1 expression in the high-temperature (31 °C) group. The results of DNA methylation analysis indicated that exposure to high temperature during early development can increase the average promoter methylation level of MaRspo1 in both females and males. Taken together, the results of this study provide the basis for the further investigation of the molecular mechanism of Rspo1 in response to temperature.

Exposure to 4-nonylphenol induces a shift in the gene expression of gsdf and testis-ova formation and sex reversal in Japanese medaka (Oryzias latipes)

The branched isomer mixture 4-nonylphenol (4-NP) has been used worldwide as a surfactant, and can have endocrine-disrupting effects on aquatic organisms. For instance, 4-NP induces the formation of testis-ova (i.e., testicular and ovarian tissue in the same gonad) or male to female sex reversal of various teleost fishes. Recently, our group revealed that altered gsdf gene expression is associated with disruption of gonadal differentiation in Japanese medaka (Oryzias latipes) embryos exposed to methyltestosterone or bisphenol A, suggesting that gsdf might be useful as a biomarker for predicting the impact of endocrine-disrupting chemicals (EDCs) on gonadal differentiation. Here, we used 4-NP to examine further whether gsdf expression at the embryo stage is useful for predicting EDC impact on gonadal sex differentiation. When fertilized medaka eggs were exposed to 32 or 100 μg/L 4-NP, testis-ova in genetic males and sex reversal from genetic male to phenotypic female were observed. At stage 38 (just before hatching), 4-NP exposure at 1-100 μg/L did not affect gsdf expression in XX embryos compared with the nontreated control; however, in XY embryos, the gsdf expression in the 100 μg/L-exposed group was significantly lower than that in the controls. The 4-NP concentration at which gsdf expression was suppressed was equal to that at which testis-ova and sex reversal were induced. These results indicate that expression of the gsdf gene at the embryonic stage in medaka is a useful biomarker for predicting the impact of EDCs on sexual differentiation.

Transcriptome Profiling and Analysis of Genes Associated with High Temperature-Induced Masculinization in Sex-Undifferentiated Nile Tilapia Gonad

Artificially high temperatures during critical thermosensitive periods (TSPs) can induce the sex reversal of Nile tilapia (Oreochromis niloticus) females into pseudomales; Nile tilapia is a GSD + TE (genotypic plus temperature effects) fish species. Previous studies have shown that water temperature affects the expression levels of many genes in the gonad or brain in various teleost species. However, few studies on the effect of temperature at the whole-gonad transcriptomic level in the early stage of sex differentiation have been reported in fish species exhibiting GSD + TE. In this study, RNA-Seq was performed to characterize the transcriptomic profile and identify genes exhibiting temperature- and sex-biased expressions in the Nile tilapia gonad at 21 dpf. A total of 42 genes were found to be associated with both high-temperature treatment and sex development, as the expression levels of these genes differed in both FC (female control) vs MC (male control) and FC vs FT (high temperature-treated females in the TSP). Among these genes, the transcriptional alterations of many male sex determination and differentiation genes, such as Dmrt1, Gsdf, and the DNA damage-inducible protein GADD45 alpha, suggested that the male pathway is initiated after high-temperature treatment and that its initiation may play a role in high temperature-induced masculinization in Nile tilapia. The qRT-PCR validation results for thirteen differentially expressed genes showed that the Pearson's correlation of the log₁₀ fold change values between the qPCR and RNA-Seq results was 0.70 (p < 0.001), indicating the accuracy and reliability of the RNA-Seq results. Our study provides insights into how high-temperature treatment induces the sex reversal of Nile tilapia females.

Transcriptomic analysis of dead end knockout testis reveals germ cell and gonadal somatic factors in Atlantic salmon

Background

Sustainability challenges are currently hampering an increase in salmon production. Using sterile salmon can solve problems with precocious puberty and genetic introgression from farmed escapees to wild populations. Recently sterile salmon was produced by knocking out the germ cell-specific dead end (dnd). Several approaches may be applied to inhibit Dnd function, including gene knockout, knockdown or immunization. Since it is challenging to develop a successful treatment against a gene product already existing in the body, alternative targets are being explored. Germ cells are surrounded by, and dependent on, gonadal somatic cells. Targeting genes essential for the survival of gonadal somatic cells may be good alternative targets for sterility treatments. Our aim was to identify and characterize novel germ cell and gonadal somatic factors in Atlantic salmon.

Results

We have for the first time analysed RNA-sequencing data from germ cell-free (GCF)/dnd knockout and wild type (WT) salmon testis and searched for genes preferentially expressed in either germ cells or gonadal somatic cells. To exclude genes with extra-gonadal expression, our dataset was merged with available multi-tissue transcriptome data. We identified 389 gonad specific genes, of which 194 were preferentially expressed within germ cells, and 11 were confined to gonadal somatic cells. Interestingly, 5 of the 11 gonadal somatic transcripts represented genes encoding secreted TGF-β factors; gsdf, inha, nodal and two bmp6-like genes, all representative vaccine targets. Of these, gsdf and inha had the highest transcript levels. Expression of gsdf and inha was further confirmed to be gonad specific, and their spatial expression was restricted to granulosa and Sertoli cells of the ovary and testis, respectively. Finally, we show that inha expression increases with puberty in both ovary and testis tissue, while gsdf expression does not change or decreases during puberty in ovary and testis tissue, respectively.

Conclusions

This study contributes with transcriptome data on salmon testis tissue with and without germ cells. We provide a list of novel and known germ cell- and gonad somatic specific transcripts, and show that the expression of two highly active gonadal somatic secreted TGF-β factors, gsdf and inha, are located within granulosa and Sertoli cells.