Antagonistic roles of Dmrt1 and Foxl2 in sex differentiation via estrogen production in tilapia as demonstrated by TALENs

Gsdf is not indispensable for male differentiation in the medaka species Oryzias hubbsi

Sex determination mechanisms differ widely among vertebrates, particularly in fish species, where diverse sex chromosomes and sex-determining genes have evolved. However, the sex-differentiation pathways activated by these sex-determining genes appear to be conserved. Gonadal soma-derived growth factor (Gsdf) is one of the genes conserved across teleost fish, especially in medaka fishes of the genus Oryzias, and is implicated in testis differentiation and germ cell proliferation. However, its role in sex differentiation remains unclear. In this study, we investigated Gsdf function in Oryzias hubbsi, a species with a ZW sex-determination system. We confirmed its male-dominant expression, as in other species. However, histological analyses revealed no male-to-female sex reversal in Gsdf-knockout fish, contrary to findings in other medaka species. Genetic sex determination remained intact without Gsdf function, indicating a Gsdf-independent sex-differentiation pathway in O. hubbsi. Instead, Gsdf loss led to germ cell overproliferation in both sexes and accelerated onset of meiosis in testes, suggesting a role in germ cell proliferation. Notably, the feminizing effect of germ cells observed in O. latipes was absent, suggesting diverse germ cell-somatic cell relationships in Oryzias gonad development. Our study highlights species-specific variations in the molecular pathways governing sex determination and differentiation, emphasizing the need for further exploration to elucidate the complexities of sexual development.

Decoding the fish genome opens a new era in important trait research and molecular breeding in China

Aquaculture represents the fastest-growing global food production sector, as it has become an essential component of the global food supply. China has the world's largest aquaculture industry in terms of production volume. However, the sustainable development of fish culture is hindered by several concerns, including germplasm degradation and disease outbreaks. The practice of genomic breeding, which relies heavily on genome information and genotypephenotype relationships, has significant potential for increasing the efficiency of aquaculture production. In 2014, the completion of the genome sequencing and annotation of the Chinese tongue sole signified the beginning of the fish genomics era in China. Since then, domestic researchers have made dramatic progress in functional genomic studies. To date, the genomes of more than 60 species of fish in China have been assembled and annotated. Based on these reference genomes, evolutionary, comparative, and functional genomic studies have revolutionized our understanding of a wide range of biologically and economically important traits of fishes, including growth and development, sex determination, disease resistance, metamorphosis, and pigmentation. Furthermore, genomic tools and breeding techniques such as SNP arrays, genomic selection, and genome editing have greatly accelerated genetic improvement through the incorporation of functional genomic information into breeding activities. This review aims to summarize the current status, advances, and perspectives of the genome resources, genomic study of important traits, and genomic breeding techniques of fish in China. The review will provide aquaculture researchers, fish breeders, and farmers with updated information concerning fish genomic research and breeding technology. The summary will help to promote the genetic improvement of production traits and thus will support the sustainable development of fish aquaculture.

The potential regulatory role of the non-coding RNAs in regulating the exogenous estrogen-induced feminization in Takifugu rubripes gonad

Estrogen plays a pivotal role in the early stage of sex differentiation in teleost. However, the underlying mechanisms of estrogen-induced feminization process are still needed for further clarification. Here, the comparative analysis of whole-transcriptome RNA sequencing was conducted between 17beta-Estradiol induced feminized XY (E-XY) gonads and control gonads (C) in Takifugu rubripes. A total of 57 miRNAs, 65 lncRNAs, and 4 circRNAs were found to be expressed at lower levels in control-XY (C-XY) than that in control-XX (C-XX), and were up-regulated in XY during E2-induced feminization process. The expression levels of 24 miRNAs, and 55 lncRNAs were higher in C-XY than that in C-XX, and were down-regulated in E2-treated XY. Furthermore, a correlation analysis was performed between miRNA-seq and mRNA-seq data. In C-XX/C-XY, 114 differential expression (DE) miRNAs were predicted to target to 904 differential expression genes (DEGs), while in C-XY/E-XY, 226 DEmiRNAs were predicted to target to 2,048 DEGs. In C-XX/C-XY, and C-XY/E-XY, KEGG pathway enrichment analysis showed that those targeted genes were mainly enriched in MAPK signaling, calcium signaling, steroid hormone biosynthesis and ovarian steroidogenesis pathway. Additionally, the competitive endogenous RNA (ceRNA) regulatory network was constructed by 24 miRNAs, 21 lncRNAs, 4 circRNAs and 5 key sex-related genes. These findings suggested that the expression of critical genes in sex differentiation were altered in E2-treated XY T. rubripes may via the lncRNA-miRNA-mRNA regulation network to facilitate the differentiation and maintenance of ovaries. Our results provide a new insight into the comprehensive understanding of the effects of estrogen signaling pathways on sex differentiation in teleost gonads.

A computational analysis of the molecular mechanisms underlying the effects of ibuprofen and dibutyl phthalate on gene expression in fish

The impact of emerging pollutants such as ibuprofen and dibutyl phthalate on aquatic species is a growing concern and the need for proper assessment and evaluation of these toxicants is imperative. The objective of this study was to examine the toxicogenomic impacts of ibuprofen and dibutyl phthalate on Clarias gariepinus, a widely distributed African catfish species. Results showed that exposure to the test compounds caused significant changes in gene expression, including upregulation of growth hormone, interleukin, melatonin receptors, 17β-Hydroxysteroid Dehydrogenase, heat shock protein, doublesex, and mab-3 related transcription factor. On the other hand, expression of forkhead Box Protein L2 and cytochrome P450 was downregulated, revealing a potential to induce female to male sex reversal. The binding affinities and hydrophobic interactions of the test compounds with the reference genes were also studied, showing that ibuprofen had the lowest binding energy and the highest affinity for the docked genes. Both compounds revealed a mutual molecular interaction with amino acids residues within the catalytic cavity of the docked genes. These results provide new insights into the toxic effects of ibuprofen and dibutyl phthalate on Clarias gariepinus, contributing to a better understanding of the environmental impact of these pollutants.

Foxp2 deficiency impairs reproduction by modulating the hypothalamic-pituitary-gonadal axis in zebrafish†

FOXP2 was initially characterized as a transcription factor linked to speech and language disorders. Single-cell RNA sequencing reveals that Foxp2 is enriched in the gonadotrope cluster of the pituitary gland and colocalized with the hormones LHB and FSHB in chickens and mice, implying that FOXP2 might be associated with reproduction in vertebrates. Herein, we investigated the roles of foxp2 in reproduction in a Foxp2-deficient zebrafish model. The results indicated that the loss of Foxp2 inhibits courtship behavior in adult male zebrafish. Notably, Foxp2 deficiency disrupts gonad development, leading to retardation of follicle development and a decrease in oocytes in females at the full-growth stage, among other phenotypes. The transcriptome analysis (RNA-seq) also revealed that differentially expressed genes clustered into the estrogen signaling and ovarian steroidogenesis-related signaling pathways. In addition, we found that Foxp2 deficiency could modulate the hypothalamic-pituitary-gonadal axis, especially the regulation of lhb and fshb expression, in zebrafish. In contrast, the injection of human chorionic gonadotropin, a specific LH agonist, partially rescues Foxp2-impaired reproduction in zebrafish, suggesting that Foxp2 plays an important role in the regulation of reproduction via the hypothalamic-pituitary-gonadal axis in zebrafish. Thus, our findings reveal a new role for Foxp2 in the regulation of reproduction in vertebrates.

Dmrt1 is the only male pathway gene tested indispensable for sex determination and functional testis development in tilapia

Sex is determined by multiple factors derived from somatic and germ cells in vertebrates. We have identified amhy, dmrt1, gsdf as male and foxl2, foxl3, cyp19a1a as female sex determination pathway genes in Nile tilapia. However, the relationship among these genes is largely unclear. Here, we found that the gonads of dmrt1;cyp19a1a double mutants developed as ovaries or underdeveloped testes with no germ cells irrespective of their genetic sex. In addition, the gonads of dmrt1;cyp19a1a;cyp19a1b triple mutants still developed as ovaries. The gonads of foxl3;cyp19a1a double mutants developed as testes, while the gonads of dmrt1;cyp19a1a;foxl3 triple mutants eventually developed as ovaries. In contrast, the gonads of amhy;cyp19a1a, gsdf;cyp19a1a, amhy;foxl2, gsdf;foxl2 double and amhy;cyp19a1a;cyp19a1b, gsdf;cyp19a1a;cyp19a1b triple mutants developed as testes with spermatogenesis via up-regulation of dmrt1 in both somatic and germ cells. The gonads of amhy;foxl3 and gsdf;foxl3 double mutants developed as ovaries but with germ cells in spermatogenesis due to up-regulation of dmrt1. Taking the respective ovary and underdeveloped testis of dmrt1;foxl3 and dmrt1;foxl2 double mutants reported previously into consideration, we demonstrated that once dmrt1 mutated, the gonad could not be rescued to functional testis by mutating any female pathway gene. The sex reversal caused by mutation of male pathway genes other than dmrt1, including its upstream amhy and downstream gsdf, could be rescued by mutating female pathway gene. Overall, our data suggested that dmrt1 is the only male pathway gene tested indispensable for sex determination and functional testis development in tilapia.

Genetic improvement in edible fish: status, constraints, and prospects on CRISPR-based genome engineering

Conventional selective breeding in aquaculture has been effective in genetically enhancing economic traits like growth and disease resistance. However, its advances are restricted by heritability, the extended period required to produce a strain with desirable traits, and the necessity to target multiple characteristics simultaneously in the breeding programs. Genome editing tools like zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) are promising for faster genetic improvement in fishes. CRISPR/Cas9 technology is the least expensive, most precise, and well compatible with multiplexing of all genome editing approaches, making it a productive and highly targeted approach for developing customized fish strains with specified characteristics. As a result, the use of CRISPR/Cas9 technology in aquaculture is rapidly growing, with the main traits researched being reproduction and development, growth, pigmentation, disease resistance, trans-GFP utilization, and omega-3 metabolism. However, technological obstacles, such as off-target effects, ancestral genome duplication, and mosaicism in founder population, need to be addressed to achieve sustainable fish production. Furthermore, present regulatory and risk assessment frameworks are inadequate to address the technical hurdles of CRISPR/Cas9, even though public and regulatory approval is critical to commercializing novel technology products. In this review, we examine the potential of CRISPR/Cas9 technology for the genetic improvement of edible fish, the technical, ethical, and socio-economic challenges to using it in fish species, and its future scope for sustainable fish production.

Comparative genomic survey and functional analysis of DKKL1 during spermatogenesis in the Chinese soft-shelled turtle (Pelodiscus sinensis)

A feature of the Chinese soft-shelled turtle (Pelodiscus sinensis) is seasonal spermatogenesis; however, the underlying molecular mechanism is not well clarified. Here, we firstly cloned and characterized P. sinensis DKKL1, and then performed comparative genomic studies, expression analysis, and functional validation. P. sinensis DKKL1 had 2 putative N-glycosylation sites and 16 phosphorylation sites. DKKL1 also had classic transmembrane structures that were extracellularly localized. DKKL1's genetic distance was close to turtles, followed by amphibians and mammals, but its genetic distance was far from fishes. DKKL1 genes from different species shared distinct genomic characteristics. Meanwhile, they were also relatively conserved among themselves, at least from the perspective of classes. Notably, the transcription factors associated with spermatogenesis were also identified, containing CTCF, EWSR1, and FOXL2. DKKL1 exhibited sexually dimorphic expression only in adult gonads, which was significantly higher than that in other somatic tissues (P < 0.001), and was barely expressed in embryonic gonads. DKKL1 transcripts showed a strong signal in sperm, while faint signals were detected in other male germ cells. DKKL1 in adult testes progressively increased per month (P < 0.05), displaying a seasonal expression trait. DKKL1 was significantly downregulated in testes cells after the sex hormones (17β-estradiol and 17α-methyltestosterone) and Wnt/β-catenin inhibitor treatment (P < 0.05). Likewise, the Wnt/β-catenin inhibitor treatment dramatically repressed CTCF, EWSR1, and FOXL2 expression. Conversely, they were markedly upregulated after the 17β-estradiol and 17α-methyltestosterone treatment, suggesting that the three transcription factors might bind to different promoter regions, thereby negatively regulating DKKL1 transcription in response to the changes in the estrogen and androgen pathways, and positively controlling DKKL1 transcription in answer to the alterations in the Wnt/β-catenin pathway. Knockdown of DKKL1 significantly reduced the relative expression of HMGB2 and SPATS1 (P < 0.01), suggesting that it may be involved in seasonal spermatogenesis of P. sinensis through a positive regulatory interaction with these two genes. Overall, our findings provide novel insights into the genome evolution and potential functions of seasonal spermatogenesis of P. sinensis DKKL1.

Tilapia, a good model for studying reproductive endocrinology

The Nile tilapia (Oreochromis niloticus), with a system of XX/XY sex determination, is a worldwide farmed fish with a shorter sexual maturation time than that of most cultured fish. Tilapia show a spawning cycle of approximately 14 days and can be artificially propagated in the laboratory all year round to obtain genetically all female (XX) and all male (XY) fry. Its genome sequence has been opened, and a perfect gene editing platform has been established. With a moderate body size, it is convenient for taking enough blood to measure hormone level. In recent years, using tilapia as animal model, we have confirmed that estrogen is crucial for female development because 1) mutation of star2, cyp17a1 or cyp19a1a (encoding aromatase, the key enzyme for estrogen synthesis) results in sex reversal (SR) due to estrogen deficiency in XX tilapia, while mutation of star1, cyp11a1, cyp17a2, cyp19a1b or cyp11c1 affects fertility due to abnormal androgen, cortisol and DHP levels in XY tilapia; 2) when the estrogen receptors (esr2a/esr2b) are mutated, the sex is reversed from female to male, while when the androgen receptors are mutated, the sex cannot be reversed; 3) the differentiated ovary can be transdifferentiated into functional testis by inhibition of estrogen synthesis, and the differentiated testis can be transdifferentiated into ovary by simultaneous addition of exogenous estrogen and androgen synthase inhibitor; 4) loss of male pathway genes amhy, dmrt1, gsdf causes SR with upregulation of cyp19a1a in XY tilapia. Disruption of estrogen synthesis rescues the male to female SR of amhy and gsdf but not dmrt1 mutants; 5) mutation of female pathway genes foxl2 and sf-1 causes SR with downregulation of cyp19a1a in XX tilapia; 6) the germ cell SR of foxl3 mutants fails to be rescued by estrogen treatment, indicating that estrogen determines female germ cell fate through foxl3. This review also summarized the effects of deficiency of other steroid hormones, such as androgen, DHP and cortisol, on fish reproduction. Overall, these studies demonstrate that tilapia is an excellent animal model for studying reproductive endocrinology of fish.

Simultaneous determination of methyltestosterone and its metabolite in fish by gas chromatography-mass spectrometry

Methyltestosterone is one of the banned drugs in aquaculture, and it should be monitored in food-producing animals. 17α-Methyl-5β-androstane-3α,17β-diol, as the main metabolite of methyltestosterone in vertebrates, could be used as another marker for controlling the administration of methyltestosterone, due to its high residual concentration and slow elimination rate. In this study, an analytical method based on gas chromatography-mass spectrometry (GC-MS) was developed and validated for the simultaneous determination of methyltestosterone and its main metabolite in fish. After pretreatment by liquid-liquid extraction with n-hexane and solid phase extraction with C18 and NH2 columns, the target analytes in the muscle tissues were extracted and concentrated, and the influence of the sample matrix was eliminated. Then, the prepared samples were separated and detected with GC-MS in the selected ion monitoring (SIM) mode. Methyltestosterone-D3 was chosen as the internal standard for quantitation. After optimization, the limits of detection for methyltestosterone and 17α-methyl-5β-androstane-3α,17β-diol were 20 μg kg-1 and 15 μg kg-1, respectively. The limits of quantitation were both 50 μg kg-1. The calibration curves showed good linearity in the concentration range from 50.0 ng mL-1 to 500.0 ng mL-1. The correlation coefficients of methyltestosterone and 17α-methyl-5β-androstane-3α,17β-diol were more than 0.9990. The recoveries of the analytes in real samples were in the range of 99.7-116.6% with the relative standard deviation of 5.2-8.3%. The established method could meet the demand for simultaneous detection of methyltestosterone and its major metabolite, and it could be used to provide more information on the abuse of methyltestosterone in food-producing animals.

Alternative splicing of histone demethylase Kdm6bb mediates temperature-induced sex reversal in the Nile tilapia

Sex determination in many fish species is remarkably plastic and temperature sensitive. Nile tilapia display a genetic sex-determination system (XX/XY). However, high-temperature treatment during critical thermosensitive periods can induce XX females into XXm pseudo-males, and this phenomenon is termed temperature-induced sex reversal (TISR). To investigate the molecular mechanism of TISR in Nile tilapia, we performed Iso-seq analysis and found a dramatic effect of high temperature on gene alternative splicing (AS). Kdm6bb histone demethylase showed a novel AS at intron 5 that generates Kdm6bb_tv1 transcripts without intron 5 and Kdm6bb_tv2 with intron 5. Kdm6bb_tv1 encodes a full-length protein while Kdm6bb_tv2 encodes a truncated protein. Expression analysis revealed that intron 5 splicing of Kdm6bb is male and gonad biased at larval stage, and only gonad biased at adult stage. High-temperature treatment induced intron 5 splicing in the gonads of XX and XY fish, resulting in increased Kdm6bb_tv1 expression. To directly test the role of Kdm6bb_tv1 in Nile tilapia TISR, we knocked out expression of Kdm6bb_tv1. However, Kdm6bb_tv1-/- homozygous mutants showed embryonic lethality. Overexpression of Kdm6bb_tv1, but not Kdm6bb_tv2, induced sex reversal of XX females into pseudo-males. Overexpression of Kdm6bb_tv1, as with high-temperature treatment, modified the promotor region of Gsdf and Dmrt1 by demethylating the trimethylated lysine 27 of histone 3 (H3K27me3), thereby increasing expression. Collectively, these studies demonstrate that AS of Kdm6bb intron 5 increases the expression of Kdm6bb_tv1, which acts as a direct link between high temperature and activation of Gsdf and Dmrt1 expression, leading to male sex determination.

Genome editing in East African cichlids and tilapias: state-of-the-art and future directions

African cichlid fishes of the Cichlidae family are a group of teleosts important for aquaculture and research. A thriving research community is particularly interested in the cichlid radiations of the East African Great Lakes. One key goal is to pinpoint genetic variation underlying phenotypic diversification, but the lack of genetic tools has precluded thorough dissection of the genetic basis of relevant traits in cichlids. Genome editing technologies are well established in teleost models like zebrafish and medaka. However, this is not the case for emerging model organisms, such as East African cichlids, where these technologies remain inaccessible to most laboratories, due in part to limited exchange of knowledge and expertise. The Cichlid Science 2022 meeting (Cambridge, UK) hosted for the first time a Genome Editing Workshop, where the community discussed recent advances in genome editing, with an emphasis on CRISPR/Cas9 technologies. Based on the workshop findings and discussions, in this review we define the state-of-the-art of cichlid genome editing, share resources and protocols, and propose new possible avenues to further expand the cichlid genome editing toolkit.

Estrodiol-17β and aromatase inhibitor treatment induced alternations of genome-wide DNA methylation pattern in Takifugu rubripes gonads

Estradiol-17β (E2) and aromatase inhibitor (AI) exposure can change the phenotypic sex of fish gonads. To investigated whether alterations in DNA methylation is involved in this process, the level of genome-wide DNA methylation in Takifugu rubripes gonads was quantitatively analyzed during the E2-induced feminization and AI-induced masculinization processes in this study. The methylation levels of the total cytosine (C) in control-XX(C-XX), control-XY (C-XY), E2-treated-XY (E-XY) and AI-treated-XX (AI-XX) were 9.11%, 9.19%, 8.63% and 9.23%, respectively. In the C-XX vs C-XY comparison, 4,196 differentially methylated regions (DMRs) overlapped with the gene body of 2,497 genes and 608 DMRs overlapped with the promoter of 575 genes. In the E-XY vs C-XY comparison, 6,539 DMRs overlapped with the gene body of 3,416 genes and 856 DMRs overlapped with the promoter of 776 genes. In the AI-XX vs C-XX comparison, 2,843 DMRs overlapped with the gene body of 1,831 genes and 461 DMRs overlapped with the promoter of 421 genes. Gonadal genomic methylation mainly occurred at CG sites and the genes that overlapped with DMRs on CG context were most enriched in the signaling pathways related to gonad differentiation, such as the Wnt, TGF-β, MAPK, CAM and GnRH pathways. The DNA methylation levels of steroid synthesis genes and estrogen receptor genes promoter or gene body were negative correlated with their expression. After bisulfite sequencing verification, the DNA methylation level of the amhr2 promoter in XY was increased after E2 treatment, which consistent with the data from the genome-wide DNA methylation sequencing. In C-XY group, the expression of amhr2 was significantly higher than that in E-XY (p < 0.05). Additionally, dnmt1, which is responsible for methylation maintenance, expressed at similar level in four groups (p > 0.05). dnmt3, tet2, and setd1b, which were responsible for methylation modification, expressed at significantly higher levels in E-XY compared to the C-XY (p < 0.05). Dnmt3 and tet2 were expressed at significantly higher levels in AI-XX than that in C-XX (p < 0.05). These results indicated that E2 and AI treatment lead to the aberrant genome-wide DNA methylation level and expression level of dnmt3, tet2, and setd1b in T. rubripes gonad.

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

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

Inhibition of the NF-κB signaling pathway affects gonadal differentiation and leads to male bias in Paramisgurnus dabryanus

In recent years, sex-controlled breeding has emerged as an effective strategy to enhance the yields of economic animals with different growth characteristics, while increasing the economic benefits of aquaculture. It is known that the NF-κB pathway participates in gonadal differentiation and reproduction. Therefore, we used the large-scale loach as a research model for the present study and selected an effective inhibitor of the NF-κB signaling pathway (QNZ). This, to investigates the impacts of the NF-κB signaling pathway on gonadal differentiation during a critical period of gonad development and after maturation. Simultaneously, the sex ratio bias and the reproductive capacities of adult fish were analyzed. Our results indicated that the inhibition of the NF-κB signaling pathway influenced the expression of genes related to gonad development, regulated the gene expression related to the brain-gonad-liver axis of juvenile loaches, and finally impacted the gonadal differentiation of the large-scale loach and promoted a male-biased sex ratio. Meanwhile, high QNZ concentrations affected the reproductive abilities of adult loaches and inhibited the growth performance of offspring. Thus, our results deepened the exploration of sex control in fish and provided a certain research basis for the sustainable development of the aquaculture industry.

Gonadal transcriptomes reveal sex-biased expression genes associated with sex determination and differentiation in red-tail catfish (Hemibagrus wyckioides)

BACKGROUND

Red-tail catfish (Hemibagrus wyckioides) is an important commercially farmed catfish in southern China. Males of red-tail catfish grow faster than females, suggesting that all-male catfish will produce more significant economic benefits in aquaculture practice. However, little research has been reported on sex determination and gonadal development in red-tail catfish.

RESULTS

In this study, we performed the first transcriptomic analysis of male and female gonads at four developmental stages at 10, 18, 30, and 48 days post hatching (dph) using RNA-seq technology. A total of 23,588 genes were screened in 24 sequenced samples, of which 28, 213, 636, and 1381 differentially expressed genes (DEGs) were detected at four developmental stages, respectively. Seven candidate genes of sex determination and differentiation were further identified. Real-time quantitative PCR (RT-qPCR) further confirmed that anti-Mullerian hormone (amh), growth differentiation factor 6a (gdf6a), testis-specific gene antigen 10 (tsga10), and cytochrome P450 family 17 subfamily A (cyp17a) were highly expressed mainly in the male, while cytochrome P450 family 19 subfamily A polypeptide 1b (cyp19a1b), forkhead box L2 (foxl2), and hydroxysteroid 17-beta dehydrogenase 1 (hsd17b1) were highly expressed in the female. The KEGG pathway enrichment data showed that these identified DEGs were mainly involved in steroid hormone biosynthesis and TGF-β signaling pathways.

CONCLUSIONS

Based on RNA-seq data of gonads at the early developmental stages, seven DEGs shared by the four developmental stages were identified, among which amh and gdf6a may be the male-biased expression genes, while foxl2, cyp19a1b and hsd17b1 may be the female-biased expression genes in red-tail catfish. Our study will provide crucial genetic information for the research on sex control in red-tail catfish, as well as for exploring the evolutionary processes of sex determination mechanisms in fish.

Development of Ovaries and Sex Change in Fish: Bringing Potential into Action

BACKGROUND

Encompassing about half of the 60,000 species of vertebrates, fish display the greatest diversity of sex determination mechanisms among metazoans. As such that phylum offers a unique playground to study the impressive variety of gonadal morphogenetic strategies, ranging from gonochorism, with either genetic or environmental sex determination, to unisexuality, with either simultaneous or consecutive hermaphroditism.

SUMMARY

From the two main types of gonads, the ovaries embrace the important role to produce the larger and non-motile gametes, which is the basis for the development of a future organism. The production of the egg cells is complex and involves the formation of follicular cells, which are necessary for the maturation of the oocytes and the production of feminine hormones. In this vein, our review focuses on the development of ovaries in fish with special emphasis on the germ cells, including those that transition from one sex to the other as part of their life cycle and those that are capable of transitioning to the opposite sex depending on environmental cues.

KEY MESSAGES

Clearly, establishing an individual as either a female or a male is not accomplished by the sole development of two types of gonads. In most cases, that dichotomy, be it final or transient, is accompanied by coordinated transformations across the entire organism, leading to changes in the physiological sex as a whole. These coordinated transformations require both molecular and neuroendocrine networks, but also anatomical and behavioural adjustments. Remarkably, fish managed to tame the ins and outs of sex reversal mechanisms to take the most advantages of changing sex as adaptive strategies in some situations.

dmrtb1 is involved in the testicular development in Larimichthys crocea

In brief

dmrtb1 performs critical functions in sex determination/differentiation and gonadal development in many organisms, but its role in teleost is rarely studied. Through gene cloning, in situ hybridization, and RNA interference technology, the function of dmrtb1 in testicular development of large yellow croaker (Larimichthys crocea) was studied; our study will be helpful in understanding further the molecular regulation mechanism of Lcdmrtb1/Lcdmrt6 in testicular development in L. crocea, and our results enrich the theory of fish dmrts involved in reproductive regulation and provide a new idea for sex control breeding of L. crocea by manipulating reproductive pathway.

Abstract

Doublesex- and mab-3-related transcription factor B1 (dmrtb1/dmrt6) belongs to one of the members of DMRT family, which performs critical functions in sex determination and differentiation, gonadal development, and functional maintenance. However, knowledge of its exact mechanism remains unclear in teleost. Very little is known about the role of dmrtb1 in the gonad development of Larimichthys crocea. In this study, a dmrtb1 homolog in L. crocea named as Lcdmrtb1 with the full-length cDNA was isolated and characterized. Except for the conserved DM domain, the other regions had low homology. Of the tissues sampled, Lcdmrtb1 was only found to be highly expressed in the testis. In situ hybridization of testis revealed Lcdmrtb1 in both spermatogonia and spermatocytes. After Lcdmrtb1 interference in the testis cells (LYCT) of L. crocea, the expression levels of Lcdmrtb1 and Lcdmrt1 were significantly decreased; subsequently, testicular cell morphology changed from fibrous to round and their growth rate slowed. Similarly, the expression levels of Lcdmrtb1, Lcdmrt1, sox9a/b, and amh were significantly decreased after RNAi in the testis. Furthermore, it was discovered that the spermatogonia had disappeared, and the Sertoli cells had been reduced. The results of immunohistochemistry showed that the expression of Sox9 protein in the testis was not detected after dmrtb1 was knocked down. These results indicated that the absence of Lcdmrtb1 not only greatly inhibited cell growth and destroyed the morphology of testis cells but also down-regulated Lcdmrt1 expression in the testis. This study will be helpful in understanding further the molecular regulation mechanism of Lcdmrtb1/Lcdmrt6 in testicular development in L. crocea.

Genome-wide identification, evolution and expression analysis of bone morphogenetic protein (BMP) gene family in chinese soft-shell turtle (Pelodiscus sinensis)

Introduction: Bone morphogenetic proteins (BMPs) play a crucial role in bone formation and differentiation. Recent RNA-Seq results suggest that BMPs may be involved in the sex differentiation of P. sinensis, yet more relevant studies about BMPs in P. sinensis are lacking. Methods: Herein, we identified BMP gene family members, analyzed the phylogeny, collinear relationship, scaffold localization, gene structures, protein structures, transcription factors and dimorphic expression by using bioinformatic methods based on genomic and transcriptomic data of P. sinensis. Meanwhile, qRT-PCR was used to verify the RNA-Seq results and initially explore the function of the BMPs in the sex differentiation of P. sinensis. Results: A total of 11 BMP genes were identified, 10 of which were localized to their respective genomic scaffolds. Phylogenetic analysis revealed that BMP genes were divided into eight subfamilies and shared similar motifs ("WII", "FPL", "TNHA", "CCVP", and "CGC") and domain (TGF-β superfamily). The results of the sexually dimorphic expression profile and qRT-PCR showed that Bmp2, Bmp3, Bmp15l, Bmp5, Bmp6 and Bmp8a were significantly upregulated in ovaries, while Bmp2lb, Bmp7, Bmp2bl and Bmp10 were remarkable upregulated in testes, suggesting that these genes may play a role in sex differentiation of P. sinensis. Discussion: Collectively, our comprehensive results enrich the basic date for studying the evolution and functions of BMP genes in P. sinensis.

Sexually dimorphic expression of foxl2 in the sea urchin (Mesocentrotus nudus)

Sea urchin (Mesocentrotus nudus) is an important economically mariculture species in several Asian countries, and gonads are the sole edible parts for people. In addition to commercial value, it is an excellent model for studying gonadal development, sex determination and sex differentiation. Identify sex-related genes is an effective way to reveal the molecular mechanism of gonadal development. In the present study, the foxl2 homologous gene was identified in M. nudus. Foxl2 is not a maternal factor, and is detected for the first time in two-arm stages. Additionally, the expression of foxl2 in the testis is higher than in the ovaries at the same developmental stages. The foxl2 transcripts were specifically enriched in the cytoplasm of germ cellsboth in the ovary and testis, but their proteins were more concentrated in the area near the oocyte nucleus. Overall, this study contributes to our understanding of the dynamic and sexually dimorphic expression pattern of foxl2 and provide a useful germ cell marker during gametogenesis in sea urchin.

Identification and Expression Analysis of Dsx and Its Positive Transcriptional Regulation of IAG in Black Tiger Shrimp (Penaeus monodon)

Doublesex (Dsx) is a polymorphic transcription factor of the DMRTs family, which is involved in male sex trait development and controls sexual dimorphism at different developmental stages in arthropods. However, the transcriptional regulation of the Dsx gene is largely unknown in decapods. In this study, we reported the cDNA sequence of PmDsx in Penaeus monodon, which encodes a 257 amino acid polypeptide. It shared many similarities with Dsx homologs and has a close relationship in the phylogeny of different species. We demonstrated that the expression of the male sex differentiation gene Dsx was predominantly expressed in the P. monodon testis, and that PmDsx dsRNA injection significantly decreased the expression of the insulin-like androgenic gland hormone (IAG) and male sex-determining gene while increasing the expression of the female sex-determining gene. We also identified a 5′-flanking region of PmIAG that had two potential cis-regulatory elements (CREs) for the PmDsx transcription. Further, the dual-luciferase reporter analysis and truncated mutagenesis revealed that PmDsx overexpression significantly promoted the transcriptional activity of the PmIAG promoter via a specific CRE. These results suggest that PmDsx is engaged in male reproductive development and positively regulates the transcription of the PmIAG by specifically binding upstream of the promoter of the PmIAG. It provides a theoretical basis for exploring the sexual regulation pathway and evolutionary dynamics of Dmrt family genes in P. monodon.

MiR-34b/c play a role in early sex differentiation of Amur sturgeon, Acipenser schrenckii

Background

Sex differentiation can be viewed as a controlled regulatory balance between sex differentiation-related mRNAs and post-transcriptional mechanisms mediated by non-coding RNAs. In mammals, increasing evidence has been reported regarding the importance of gonad-specific microRNAs (miRNAs) in sex differentiation. Although many fishes express a large number of gonadal miRNAs, the effects of these sex-biased miRNAs on sex differentiation in teleost fish remain unknown. Previous studies have shown the exclusive and sexually dimorphic expression of miR-34b/c in the gonads of the Amur sturgeon (Acipenser schrenckii), suggesting its potential role in the sex differentiation process.

Results

Using quantitative real-time PCR (qPCR), we observed that miR-34b/c showed consistent spatiotemporal expression patterns; the expression levels significantly increased during early sex differentiation. Using in situ hybridization, miR-34c was found to be located in the germ cells. In primary germ cells in vitro, the group subjected to overexpression and inhibition of miR-34c showed significantly higher proliferation ability and lower apoptosis, respectively, compared to the corresponding control group. Luciferase reporter assays using the ar-3′UTR-psiCHECK-2 luciferase vector suggested a targeted regulatory interaction between miR-34b/c and the 3′UTR of the androgen receptor (ar) mRNA. Furthermore, miR-34b/c and ar showed negative expression patterns during early sex differentiation. Additionally, a negative feedback regulation pattern was observed between foxl2 expression in the ovaries and amh and sox9 expression in the testes during early sex differentiation.

Conclusions

This study sheds new light on the roles of miR-34b/c in gonad development of Amur sturgeon, and provides the first comprehensive evidence that the gonad-predominant microRNAs may have a major role in sex differentiation in teleost fish.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-022-00469-6.

Becoming female: Ovarian differentiation from an evolutionary perspective

Differentiation of the bipotential gonadal primordium into ovaries and testes is a common process among vertebrate species. While vertebrate ovaries eventually share the same functions of producing oocytes and estrogens, ovarian differentiation relies on different morphogenetic, cellular, and molecular cues depending on species. The aim of this review is to highlight the conserved and divergent features of ovarian differentiation through an evolutionary perspective. From teleosts to mammals, each clade or species has a different story to tell. For this purpose, this review focuses on three specific aspects of ovarian differentiation: ovarian morphogenesis, the evolution of the role of estrogens on ovarian differentiation and the molecular pathways involved in granulosa cell determination and maintenance.

Transcriptome analysis revealed gene expression feminization of testis after exogenous tetrodotoxin administration in pufferfish Takifugu flavidus

Tetrodotoxin (TTX) is a deadly neurotoxin and usually accumulates in large amounts in the ovaries but is non-toxic or low toxic in the testis of pufferfish. The molecular mechanism underlying sexual dimorphism accumulation of TTX in ovary and testis, and the relationship between TTX accumulation with sex related genes expression remain largely unknown. The present study investigated the effects of exogenous TTX treatment on Takifugu flavidus. The results demonstrated that exogenous TTX administration significantly incresed level of TTX concentration in kidney, cholecyst, skin, liver, heart, muscle, ovary and testis of the treatment group (TG) than that of the control group (CG). Transcriptome sequencing and analysis were performed to study differential expression profiles of mRNA and piRNA after TTX administration of the ovary and testis. The results showed that compared with female control group (FCG) and male control group (MCG), TTX administration resulted in 80 and 23 piRNAs, 126 and 223 genes up and down regulated expression in female TTX-treated group (FTG), meanwhile, 286 and 223 piRNAs, 2 and 443 genes up and down regulated expression in male TTX-treated group (MTG). The female dominant genes cyp19a1, gdf9 and foxl2 were found to be up-regulated in MTG. The cyp19a1, whose corresponding target piRNA uniq_554482 was identified as down-regulated in the MTG, indicating the gene expression feminization in testis after exogenous TTX administration. The KEGG enrichment analysis revealed that differentially expressed genes (DEGs) and piRNAs (DEpiRNAs) in MTG vs MCG group were more enriched in metabolism pathways, indicating that the testis produced more metabolic pathways in response to exogenous TTX, which might be a reason for the sexual dimorphism of TTX distribution in gonads. In addition, TdT-mediated dUTP-biotin nick end labeling staining showed that significant apoptosis was detected in the MTG testis, and the role of the cell apoptotic pathways was further confirmed. Overall, our research revealed that the response of the ovary and testis to TTX administration was largely different, the ovary is more tolerant whereas the testis is more sensitive to TTX. These data will deepen our understanding on the accumulation of TTX sexual dimorphism in Takifugu.

Foxl2 is required for the initiation of the female pathway in a temperature-dependent sex determination system in Trachemys scripta

KDM6B-mediated epigenetic modification of the testicular regulator Dmrt1 has previously been identified as the primary switch of the male pathway in a temperature-dependent sex-determination (TSD) system; however, the molecular network of the female pathway has not yet been established. Here, we have functionally characterized for the first time an upstream regulator of the female pathway, the forkhead transcription factor FOXL2, in Trachemys scripta, a turtle species with a TSD system. FOXL2 exhibited temperature-dependent female-specific expression patterns before the onset of gonadal differentiation and was preferentially localized in ovarian somatic cells. Foxl2 responded rapidly to temperature shifts and estrogen. Importantly, forced expression of Foxl2 at the male-producing temperature led to male-to-female sex reversal, as evidenced by the formation of an ovary-like structure, and upregulation of the ovarian regulators Cyp19a1 and R-spondin1. Additionally, knockdown of Foxl2 caused masculinization at the female-producing temperature, which was confirmed by loss of the female phenotype, development of seminiferous tubules, and elevated expression of Dmrt1 and Sox9. Collectively, we demonstrate that Foxl2 expression is necessary and sufficient to drive ovarian determination in T. scripta, suggesting a crucial role of Foxl2 in female sex determination in the TSD system.

Comprehensive Transcriptome Analysis of Gonadal and Somatic Tissues for Identification of Sex-Related Genes in the Largemouth Bass Micropterus salmoides

Largemouth bass (Micropterus salmoides) is an economically important fish. It can spawn many times during a breeding season, and there are no obvious morphological characteristics to distinguish male and female juvenile fish. So far, little is known about the genes regulating their sexual development in this species. Here, we performed RNA sequencing (RNA-Seq) analysis of the testis, ovary, and somatic tissue to identify sex-related genes in the largemouth bass. A total of 51,672 unigenes were obtained via the transcriptome analysis, and 5900 differential expression genes (DEGs), including 3028 up-regulated and 2872 down-regulated DEGs, were obtained in the somatic tissue, testis, and ovary. DEGs were retrieved by making comparisons: somatic tissue vs testis (1733-up and 1382-down), testis vs ovary (841-up and 807-down), and ovary vs somatic tissue (454-up and 683-down). Finally, functional annotation identified 22 key sex-related DEGs, including 13 testis-biased DEGs (dmrt1, cyp11b1, sox9, spata4, spata22, spata17, fshr, fem-1a, wt1, daz1, amh, vasa, and piwi1) and 9 ovary-biased DEGs (foxl2, gdf9, zp3, sox3, cyp19a, bmp15, fem-1b, fig. la, and piwi2). This result was further confirmed by the tissue expression detection via RT-PCR and RT-qPCR. Protein-protein interacting (PPI) network analysis revealed that the testis-specific dmrt1 interacts directly with the testis-biased DEGs (cyp11b1 and spata4) and the ovary-biased DEGs (foxl2, gdf9, zp3, sox3, cyp19a, and bmp15), suggesting that the dmrt1 as a sex-determining gene can play a dual role through inducing the testis-biased DEGs and inhibiting the ovary-biased DEGs during the testicular development. Our present results provide useful molecular data for a better understanding of sexual development in the largemouth bass.

TALEN-Mediated Gene Editing of slc24a5 (Solute Carrier Family 24, Member 5) in Kawakawa, Euthynnus affinis

Transcription activator-like effector (TALE) nucleases (TALENs) mediated gene editing methods are becoming popular and have revealed the staggering complexity of genome control during development. Here, we present a simple and efficient gene knockout using TALENs in kawakawa, Euthynnus affinis, using slc24a5. We examined slc24a5 gene expression and functional differences between two TALENs that hold the TALE scaffolds, +153/+47 and +136/+63 and target slc24a5. Developmental changes in slc24a5 transcripts were seen in early-stage embryos by real-time PCR; slc24a5 expression was first detected 48 h post fertilization (hpf), which increased dramatically at 72 hpf. Four TALENs, 47- and 63-type of two different target loci (A and B), respectively, were constructed using Platinum TALEN and evaluated in vitro by a single-strand annealing (SSA) assay. TALEN activities were further evaluated in vivo by injecting TALEN mRNAs in the two-cell stage of the zygote. Most of the TALEN-induced mutants showed mosaic patterns in the retinal pigment epithelium (RPE) and fewer melanin pigments on the body at 72 hpf and later when compared to the control, implying the gene’s association with melanin pigment formation. A heteroduplex mobility assay (HMA) and the genome sequence further confirmed the TALEN-induced mutations of substitution, insertion, and deletion at an endogenous locus.

Cyp17a1 is Required for Female Sex Determination and Male Fertility by Regulating Sex Steroid Biosynthesis in Fish

In teleost fish, sex steroids are involved in sex determination, sex differentiation, and fertility. Cyp17a1 (Cytochrome P450 family 17 subfamily A member 1) is thought to play essential roles in fish steroidogenesis. Therefore, to further understand its roles in steroidogenesis, sex determination, and fertility in fish, we constructed a cyp17a1 gene mutant in Nile tilapia (Oreochromis niloticus). In XX fish, mutation of the cyp17a1 gene led to a female-to-male sex reversal with a significant decline in 17β-estradiol (E2) and testosterone (T) production, and ectopic expression of male-biased markers (Dmrt1 and Gsdf) in gonads from the critical window of sex determination. Sex reversal was successfully rescued via T or E2 administration, and ovarian characteristics were maintained after termination of E2 supplementation in the absence of endogenous estrogen production in cyp17a1-/- XX fish. Likewise, deficiencies in T and 11-ketotestosterone (11-KT) production in both cyp17a1-/- XX sex-reversed males and cyp17a1-/- XY mutants resulted in meiotic initiation delays, vas deferens obstruction and sterility due to excessive apoptosis and abnormal mitochondrial morphology. However, 11-KT treatment successfully rescued the dysspermia to produce normal sperm in cyp17a1-/- male fish. Significant increases in gonadotropic hormone (gth) and gth receptors in cyp17a1-/- mutants may excessively upregulate steroidogenic gene expression in Leydig cells through a feedback loop. Taken together, our findings demonstrate that Cyp17a1 is indispensable for E2 production, which is fundamental for female sex determination and differentiation in XX tilapia. Additionally, Cyp17a1 is essential for T and 11-KT production, which further promotes spermatogenesis and fertility in XY males.

Profile of gene expression changes during estrodiol-17β-induced feminization in the Takifugu rubripes brain

Background

As the critical tissue of the central nervous system, the brain has been found to be involved in gonad development. Previous studies have suggested that gonadal fate may be affected by the brain. Identifying brain-specific molecular changes that occur during estrodiol-17β (E₂) -induced feminization is crucial to our understanding of the molecular control of sex differentiation by the brains of fish.

Results

In this study, the differential transcriptomic responses of the Takifugu rubripes larvae brain were compared after E₂ treatment for 55 days. Our results showed that 514 genes were differentially expressed between E₂-treated-XX (E-XX) and Control-XX (C-XX) T. rubripes, while 362 genes were differentially expressed between E₂-treated-XY (E-XY) and Control-XY (C-XY). For example, the expression of cyp19a1b, gnrh1 and pgr was significantly up-regulated, while st, sl, tshβ, prl and pit-1, which belong to the growth hormone/prolactin family, were significantly down-regulated after E₂ treatment, in both sexes. The arntl1, bhlbe, nr1d2, per1b, per3, cry1, cipc and ciart genes, which are involved in the circadian rhythm, were also found to be altered. Differentially expressed genes (DEGs), which were identified between E-XX and C-XX, were significantly enriched in neuroactive ligand-receptor interaction, arachidonic acid metabolism, cytokine-cytokine receptor interaction and the calcium signaling pathway. The DEGs that were identified between E-XY and C-XY were significantly enriched in tyrosine metabolism, phenylalanine metabolism, arachidonic acid metabolism and linoleic acid metabolism.

Conclusion

A number of genes and pathways were identified in the brain of E₂-treated T. rubripes larvae by RNA-seq. It provided the opportunity for further study on the possible involvement of networks in the brain-pituitary-gonadal axis in sex differentiation in T. rubripes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08158-0.

Role of sex steroids in fish sex determination and differentiation as revealed by gene editing

The involvement of sex steroids in sex determination and differentiation is relatively conserved among non-mammalian vertebrates, especially in fish. Thanks to the advances in genome sequencing and genome editing, significant progresses have been made in the understanding of steroidogenic pathway and hormonal regulation of sex determination and differentiation in fish. It seems that loss of function study of single gene challenges the traditional views that estrogen is required for ovarian differentiation and androgen is needed for testicular development, but it is not so in essence. Steroidogenic enzymes can be classified into two categories based on expression and enzyme activities in fish. One type, encoded by star2, cyp17a1 and cyp19a1a, is involved in estrogen production and exclusively expressed in the gonads. Mutation of these genes results in the up-regulation of male pathway genes and sex reversal from genetic female to male. The other type, encoded by the duplicated paralogs of the above genes, including star1, cyp11a1, cyp17a2 and cyp19a1b, as well as cyp11c1 gene, is dominantly expressed both in gonads and extra-gonadal tissues. Mutation of these genes alters the steroids (androgen, DHP and cortisol) production and spermatogenesis, fertility, secondary sexual characteristics and sexual behavior, but usually does not affect the sex differentiation. For the estrogen receptors (esr1, esr2a and esr2b), single mutation failed to, but double and triple mutation leads to sex reversal from female to male, indicating that at least Esr2a and Esr2b are required to mediate the role of estrogen in sex determination proved by gene editing experiments. Taken together, results from gene editing enrich our understanding of steroid synthesis pathways and further confirm the critical role of estrogen in female sex determination by antagonizing the male pathway in fish.

Gene Expression of Takifugu rubripes Gonads During AI- or MT-induced Masculinization and E2-induced Feminization

Elucidating the global molecular changes that occur during aromatase inhibitor (AI)- or 17α-methyltestosterone (MT)-induced masculinization and estradiol-17β (E2)-induced feminization is critical to understanding the roles that endocrine and genetic factors play in regulating the process of sex differentiation in fish. Here, fugu larvae were treated with AI (letrozole), MT, or E2 from 25 to 80 days after hatching (dah), and gonadal transcriptomic analysis at 80 dah was performed. The expression of dmrt1, gsdf, foxl2, and other key genes (star, hsd3b1, cyp11c1, cyp19a1a, etc.) involved in the steroid hormone biosynthesis pathway were found be altered. The expression of dmrt1, gsdf, cyp19a1a, and foxl2 was further verified by quantitative polymerase chain reaction. In the control group, the expression of dmrt1 and gsdf was significantly higher in XY larvae than in XX larvae, while the expression of foxl2 and cyp19a1a was significantly higher in XX larvae than in XY larvae (P < .05). AI treatment suppressed the expression of foxl2 and cyp19a1a, and induced the expression of dmrt1 and gsdf in XX larvae. MT treatment suppressed the expression of foxl2, cyp19a1a, dmrt1, and gsdf in XX larvae. E2 treatment suppressed the expression of dmrt1 and gsdf, but did not restore the expression of foxl2 and cyp19a1a in XY larvae. The shared response following AI, MT, and E2 treatment suggested that these genes are essential for sex differentiation. This finding offers some insight into AI or MT-induced masculinization, and E2-induced femininization in fugu.

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.

Turning the spotlight on the C11-oxy androgens in human fetal development

Research into the biosynthesis of C11-oxy C₁₉ steroids during human fetal development, specifically fetal adrenal development and during the critical period of sex differentiation, is currently lacking. Cortisol, which possesses a C11-hydroxyl moiety has, however, been firmly established in this context. Compelling questions are whether the C11-oxy C₁₉ steroids (11β-hydroxyandrostenedione, 11β-hydroxytestosterone, 11-ketoandrostenedione and 11-ketotestosterone [11KT]) and the C11-oxy C₂₁ steroids (11β-hydroxyprogesterone and 11-ketoprogesterone) are biosynthesised during gestation, and whether these hormones circulate between the placenta and the developing fetus, and between the placenta and the mother. This review will consider the role of cortisol, 11KT and 11β-hydroxysteroid dehydrogenase type 2 (11βHSD2) in determining the sex of teleost fish, while these hormones and 11βHSD2 will also be discussed with regards to murine mammals. The focus of the review will shift to highlight the potential role of C11-oxy steroids in human fetal development based on the timely expression of steroidogenic enzymes in the adrenal, testes and ovary, as well as in the placenta; summarising reported evidence of C11-oxy steroids in neonatal life.

Homozygous mutation of foxh1 arrests oogenesis causing infertility in female Nile tilapia†

Foxh1, a member of fox gene family, was first characterized as a transcriptional partner in the formation of the Smad protein complex. Recent studies have shown foxh1 is highly expressed in the cytoplasm of oocytes in both tilapia and mouse. However, its function in oogenesis remains unexplored. In the present study, foxh1-/- tilapia was created by CRISPR/Cas9. At 180 dah (days after hatching), the foxh1-/- XX fish showed oogenesis arrest and a significantly lower GSI. The transition of oocytes from phase II to phase III and follicle cells from one to two layers was blocked, resulting in infertility of the mutant. Transcriptomic analysis revealed that expression of genes involved in estrogen synthesis and oocyte growth were altered in the foxh1-/- ovaries. Loss of foxh1 resulted in significantly decreased Cyp19a1a and increased Cyp11b2 expression, consistent with significantly lower concentrations of serum estradiol-17β (E2) and higher concentrations of 11-ketotestosterone (11-KT). Moreover, administration of E2 rescued the phenotypes of foxh1-/- XX fish, as indicated by the appearance of phase III and IV oocytes and absence of Cyp11b2 expression. Taken together, these results suggest that foxh1 functions in the oocytes to regulate oogenesis by promoting cyp19a1a expression, and therefore estrogen production. Disruption of foxh1 may block the estrogen synthesis and oocyte growth, leading to the arrest of oogenesis and thus infertility in tilapia.

The application of genome editing technology in fish

The advent and development of genome editing technology has opened up the possibility of directly targeting and modifying genomic sequences in the field of life sciences with rapid developments occurring in the last decade. As a powerful tool to decipher genome data at the molecular biology level, genome editing technology has made important contributions to elucidating many biological problems. Currently, the three most widely used genome editing technologies include: zinc finger nucleases (ZFN), transcription activator like effector nucleases (TALEN), and clustered regularly interspaced short palindromic repeats (CRISPR). Researchers are still striving to create simpler, more efficient, and accurate techniques, such as engineered base editors and new CRISPR/Cas systems, to improve editing efficiency and reduce off-target rate, as well as a near-PAMless SpCas9 variants to expand the scope of genome editing. As one of the important animal protein sources, fish has significant economic value in aquaculture. In addition, fish is indispensable for research as it serves as the evolutionary link between invertebrates and higher vertebrates. Consequently, genome editing technologies were applied extensively in various fish species for basic functional studies as well as applied research in aquaculture. In this review, we focus on the application of genome editing technologies in fish species detailing growth, gender, and pigmentation traits. In addition, we have focused on the construction of a zebrafish (Danio rerio) disease model and high-throughput screening of functional genes. Finally, we provide some of the future perspectives of this technology.

Genome Editing Using the CRISPR-Cas9 System to Generate a Solid-Red Germline of Nile Tilapia (Oreochromis niloticus)

In recent years, there has been increasing demand for red tilapia, which are commercial strains of hybrids of different tilapiine species or red variants of highly inbred Nile tilapia. However, red tilapia phenotypes are genetically unstable and affected by environmental factors, resulting in nonuniform coloration with black or dark-red color blotches that reduce their market value. Solute carrier family 45 member 2 (SLC45A2) is a membrane transporter that mediates melanin biosynthesis and is evolutionarily conserved from fish to humans. In the present study, we describe the generation of a stable and heritable red tilapia phenotype by inducing loss-of-function mutations in the slc45a2 gene. For this purpose, we identified the slc45a2 gene in Nile tilapia and designed highly specific guide RNAs (gRNA) for its genomic sequence. Multiplex microinjection of slc45a2-specific ribonucleoproteins to Nile tilapia zygotes induced up to 97-99% albinism, including loss of melanin in the eye. Next-generation sequencing of the injected zygotes demonstrated that all injected fish carried mutant alleles with variable mutagenesis efficiencies. Sanger sequencing of the genomic target region in the slc45a2 gene from fin clips, sperm, and F1 offspring of a highly mutant male identified various genomic indels and germline transmission of the sperm-identified indels. Overall, this work demonstrates the generation of somatic and germline slc45a2 mutant alleles, which leads to complete albinism in Nile tilapia.

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.

miR-133b targets tagln2 and functions in tilapia oogenesis

microRNAs (miRNAs) are important components of non-coding RNAs that participate in diverse life activities by regulating gene expression at the post transcriptional level through base complementary pairing with 3'UTRs of target mRNAs. miR-133b is a member of the miR-133 family, which play important roles in muscle differentiation and tumorigenesis. Recently, miR-133b was reported to affect estrogen synthesis by targeting foxl2 in mouse, while its role in fish reproduction remains to be elucidated. In the present study, we isolated the complete sequence of miR-133b, which was highly expressed in tilapia ovary at 30 and 90 dah (days after hatching) and subsequently decreased at 120 to 150 dah by qPCR. Interestingly, only a few oogonia were remained in the antagomir-133b treated tilapia ovary, while phase I and II oocytes were observed in the ovaries of the control group. Unexpectedly, the expression of foxl2 and cyp19a1a, as well as estradiol levels in serum were increased in the treated group. Furthermore, tagln2, an important factor for oogenesis, was predicted as the target gene of miR-133b, which was confirmed by dual luciferase reporter vector experiments. miR-133b and tagln2 were co-expressed in tilapia ovaries. Taken together, miR-133b may be involved in the early oogenesis of tilapia by regulating tagln2 expression. This study enriches the understanding of miR-133b function during oogenesis and lays a foundation for further study of the regulatory network during oogenesis.

Environmental Cues and Mechanisms Underpinning Sex Change in Fish

Fishes are the only vertebrates that undergo sex change during their lifetime, but even within this group, a unique reproductive strategy is displayed by only 1.5% of the teleosts. This lability in alternating sexual fate is the result of the simultaneous suppression and activation of opposing male and female networks. Here, we provide a brief review summarizing recent advances in our understanding of the environmental cues that trigger sex change and their perception, integration, and translation into molecular cascades that convert the sex of an individual. We particularly focus on molecular events underpinning the complex behavioral and morphological transformation involved in sex change, dissecting the main molecular players and regulatory networks that shape the transformation of one sex into the opposite. We show that histological changes and molecular pathways governing gonadal reorganization are better described than the neuroendocrine basis of sex change and that, despite important advances, information is lacking for the majority of hermaphrodite species. We highlight significant gaps in our knowledge of how sex change takes place and suggest future research directions.

Germline sexual fate is determined by the antagonistic action of dmrt1 and foxl3/foxl2 in tilapia

Germline sexual fate has long been believed to be determined by the somatic environment, but this idea is challenged by recent studies of foxl3 mutants in medaka. Here, we demonstrate that the sexual fate of tilapia germline is determined by the antagonistic interaction of dmrt1 and foxl3, which are transcriptionally repressed in male and female germ cells, respectively. Loss of dmrt1 rescued the germ cell sex reversal in foxl3Δ7/Δ7 XX fish, and loss of foxl3 partially rescued germ cell sex reversal but not somatic cell fate in dmrt1Δ5/Δ5 XY fish. Interestingly, germ cells lost sexual plasticity in dmrt1Δ5/Δ5 XY and foxl3Δ7/Δ7 XX single mutants, as aromatase inhibitor (AI) and estrogen treatment failed to rescue the respective phenotypes. However, recovery of germ cell sexual plasticity was observed in dmrt1/foxl3 double mutants. Importantly, mutation of somatic cell-specific foxl2 resulted in testicular development in foxl3Δ7/Δ7 or dmrt1Δ5/Δ5 mutants. Our findings demonstrate that sexual plasticity of germ cells relies on the presence of both dmrt1 and foxl3. The existence of dmrt1 and foxl3 allows environmental factors to influence the sex fate decision in vertebrates.

Primary sex determination in birds depends on DMRT1 dosage, but gonadal sex does not determine adult secondary sex characteristics

In birds, males are the homogametic sex (ZZ) and females the heterogametic sex (ZW). Primary sex determination is thought to depend on a sex chromosome gene dosage mechanism, and the most likely sex determinant is the Z chromosome gene Doublesex and Mab-3-Related Transcription factor 1 (DMRT1). To clarify this issue, we used a CRISPR-Cas9-based monoallelic targeting approach and sterile surrogate hosts to generate birds with targeted mutations in the DMRT1 gene. The resulting chromosomally male (ZZ) chicken with a single functional copy of DMRT1 developed ovaries in place of testes, demonstrating the avian sex-determining mechanism is based on DMRT1 dosage. These ZZ ovaries expressed typical female markers and showed clear evidence of follicular development. However, these ZZ adult birds with an ovary in place of testes were indistinguishable in appearance to wild-type adult males, supporting the concept of cell-autonomous sex identity (CASI) in birds. In experiments where estrogen synthesis was blocked in control ZW embryos, the resulting gonads developed as testes. In contrast, if estrogen synthesis was blocked in ZW embryos that lacked DMRT1, the gonads invariably adopted an ovarian fate. Our analysis shows that DMRT1 is the key sex determination switch in birds and that it is essential for testis development, but that production of estrogen is also a key factor in primary sex determination in chickens, and that this production is linked to DMRT1 expression.

Effects of the Thyroid Endocrine System on Gonadal Sex Ratios and Sex-Related Gene Expression in the Pufferfish Takifugu rubripes

To examine the effect and mechanism of thyroid hormone on gonadal sex differentiation, Takifugu rubripes larvae were treated with goitrogen (methimazole, MET, 1000 g/g), and thyroxine (T4, 2nM) from 25 to 80 days after hatching (dah). Gonadal histology and sex ratios of fish were then determined at 80 dah. MET treatment induced masculinization, but T4 treatment did not induce feminization in T. rubripes larvae. Transcriptomic analysis of gonads at 80 dah was then conducted. Among the large number of differentially expressed genes between the groups, the expression of foxl2, cyp19a1a, and dmrt1 was altered. The expression of foxl2, cyp19a1a, dmrt1 and gsdf at 25, 40, 55 days after treatment (dat) was further analyzed by qPCR. MET treatment suppressed the expression of foxl2 and cyp19a1a, and induced the expression of dmrt1 in genetic females (p < 0.05). Additionally, T4 treatment induced an increase in the expression of cyp19a1a in genetic XY gonads only at 25 dat. However, the increase in cyp19a1a expression did not continue to 40 and 55 dat. This may explain why feminization of larvae was not found in the T4-treated group. Thus, the present study provides the first evidence that MET treatment causes masculinization in teleost fish. The effects of MET-induced masculinization in T. rubripes may act primarily via suppression of the expression of foxl2 and cyp19a1a, and stimulation of the expression of dmrt1. Moreover, the effects of higher concentrations of T4 or different concentrations of T3, on sex differentiation require further testing.

Regulation of spermatogenesis and reproductive capacity by Igf3 in tilapia

A novel insulin-like growth factor (igf3), which is exclusively expressed in the gonads, has been widely identified in fish species. Recent studies have indicated that Igf3 regulates spermatogonia proliferation and differentiation in zebrafish; however, detailed information on the role of this Igf needs further in vivo investigation. Here, using Nile tilapia (Oreochromis niloticus) as an animal model, we report that igf3 is required for spermatogenesis and reproduction. Knockout of igf3 by CRISPR/Cas9 severely inhibited spermatogonial proliferation and differentiation at 90 days after hatching, the time critical for meiosis initiation, and resulted in less spermatocytes in the mutants. Although spermatogenesis continued to occur later, more spermatocytes and less spermatids were observed in the igf3-/- testes when compared with wild type of testes at adults, indicating that Igf3 regulates spermatocyte to spermatid transition. Importantly, a significantly increased occurrence of apoptosis in spermatids was observed after loss of Igf3. Therefore, igf3-/- males were subfertile with drastically reduced semen volume and sperm count. Conversely, the overexpression of Igf3 in XY tilapia enhanced spermatogenesis leading to more spermatids and sperm count. Transcriptomic analysis revealed that the absence of Igf3 resulted in dysregulation of many genes involved in cell cycle, meiosis and pluripotency regulators that are critical for spermatogenesis. In addition, in vitro gonadal culture with 17α-methyltetosterone (MT) and 11-ketotestosterone (11-KT) administration and in vivo knockout of cyp11c1 demonstrated that igf3 expression is regulated by androgens, suggesting that Igf3 acts downstream of androgens in fish spermatogenesis. Notably, the igf3 knockout did not affect body growth, indicating that this Igf specifically functions in reproduction. Taken together, our data provide genetic evidence for fish igf3 in the regulation of reproductive capacity by controlling spermatogenesis.

Sex determination, gonadal sex differentiation, and plasticity in vertebrate species

A diverse array of sex determination (SD) mechanisms, encompassing environmental to genetic, have been found to exist among vertebrates, covering a spectrum from fixed SD mechanisms (mammals) to functional sex change in fishes (sequential hermaphroditic fishes). A major landmark in vertebrate SD was the discovery of the SRY gene in 1990. Since that time, many attempts to clone an SRY ortholog from nonmammalian vertebrates remained unsuccessful, until 2002, when DMY/dmrt1by was discovered as the SD gene of a small fish, medaka. Surprisingly, however, DMY/dmrt1by was found in only 2 species among more than 20 species of medaka, suggesting a large diversity of SD genes among vertebrates. Considerable progress has been made over the last 3 decades, such that it is now possible to formulate reasonable paradigms of how SD and gonadal sex differentiation may work in some model vertebrate species. This review outlines our current understanding of vertebrate SD and gonadal sex differentiation, with a focus on the molecular and cellular mechanisms involved. An impressive number of genes and factors have been discovered that play important roles in testicular and ovarian differentiation. An antagonism between the male and female pathway genes exists in gonads during both sex differentiation and, surprisingly, even as adults, suggesting that, in addition to sex-changing fishes, gonochoristic vertebrates including mice maintain some degree of gonadal sexual plasticity into adulthood. Importantly, a review of various SD mechanisms among vertebrates suggests that this is the ideal biological event that can make us understand the evolutionary conundrums underlying speciation and species diversity.

High-quality chromosome-level genomes of two tilapia species reveal their evolution of repeat sequences and sex chromosomes

Tilapias are one of the most farmed fishes that are coined as "aquatic chicken" by the food industry. Nile tilapia and blue tilapia exhibit very recent transition of sex chromosome systems since their divergence approximately five million years ago, making them a great model for elucidating the molecular and evolutionary mechanisms of sex chromosome turnovers. Studies of their sex-determining pathways are also critical for developing genetic sex control in aquaculture. We report here the newly produced genomes of Nile tilapia and blue tilapia that integrate long-read sequencing and chromatin conformation data. The two nearly complete genomes have anchored over 97% of the sequences into linkage groups (LGs), and assembled majorities of complex repetitive regions including telomeres, centromeres and rDNA clusters. In particular, we inferred two episodes of repeat expansion at LG3 respectively in the ancestor of cichlids and that of tilapias. The consequential large heterochromatic region concentrated at one end of LG3 comprises tandem arrays of mRNA and small RNA genes, among which we have identified a candidate female determining gene Paics in blue tilapia. Paics shows female-specific patterns of single-nucleotide variants, copy numbers and expression patterns in gonads during early gonadogenesis. Our work provides a very important genomic resource for functional studies of cichlids, and suggested that unequal distribution of repeat content that impacts the local recombination rate might make some chromosomes more likely to become sex chromosomes.

Natural sex change in mature protogynous orange-spotted grouper (Epinephelus coioides): gonadal restructuring, sex hormone shifts and gene profiles

Sexual patterns of teleosts are extremely diverse and include both gonochorism and hermaphroditism. As a protogynous hermaphroditic fish, all orange-spotted groupers (Epinephelus coioides) develop directly into females, and some individuals change sex to become functional males later in life. This study investigated gonadal restructuring, shifts in sex hormone levels and gene profiles of cultured mature female groupers during the first (main) breeding season of 2019 in Huizhou, China (22° 42' 02.6″ N, 114° 32' 10.1″ E). Analysis of gonadal restructuring revealed that females with pre-vitellogenic ovaries underwent vitellogenesis, spawning and regression and then returned to the pre-vitellogenic stage in the late breeding season, at which point some changed sex to become males via the intersex gonad stage. A significant decrease in the level of serum 17β-estradiol (E2) was observed during ovary regression but not during sex change, whereas serum 11-ketotestosterone (11-KT) concentrations increased significantly during sex change with the highest concentration in newly developed males. Consistent with serum hormone changes, a significant decrease in cyp19a1a expression was observed during ovary regression but not during sex change, whereas the expression of cyp11c1 and hsd11b2 increased significantly during sex change. Interestingly, hsd11b2 but not cyp11c1 was significantly upregulated from the pre-vitellogenic ovary stage to the early intersex gonad stage. These results suggest that a decrease in serum E2 concentration and downregulation of cyp19a1a expression are not necessary to trigger the female-to-male transformation, whereas increased 11-KT concentration and upregulation of hsd11b2 expression may be key events for the initiation of sex change in the orange-spotted grouper.