The role of Amh signaling in teleost fish--Multiple functions not restricted to the gonads

Comparative transcriptome analysis identified genes involved in ovarian development in Takifugu rubripes

Ovarian development is a complex process involving multiple genes, but the molecular mechanisms underlying this process in Takifugu rubripes remain poorly understood. This study aimed to identify genes associated with ovarian development in T. rubripes and to investigate the regulatory mechanisms of oocyte maturation. Transcriptome data were compared across four different developmental stages (stage II to V) to identify differentially expressed genes (DEGs) and perform GO and KEGG enrichment analysis. The expression patterns of randomly selected genes were then validated by qPCR. The results yielded a total of 1,289,401,820 raw data from all libraries, with 16,929 DEGs identified across all comparison groups. The DEGs were predominantly enriched in ovarian steroidogenesis, estrogen-mediated signaling, and TGF-beta signaling pathways. The qPCR analysis showed that cyp17a1 was identified as being expressed at similar levels in stage II and III. Thereafter, cyp17a1 was observed to undergo a continuous increase in expression from stage III to V. cyp19a1, nanos1, foxl2 and ar were identified as being expressed at similar levels at stage II and III, then increase in expression from stage III to IV and subsequent downregulation from stage IV to V. hsd17b1 was identified as being expressed at similar levels at stage II and IV. This study represents a transcriptomic study of ovarian development in female T. rubripes. Several essential ovarian-related genes and sex-related biological pathways were identified. The results will improve our understanding of the molecular mechanisms underlying ovarian development in this species.

In vivo treatment with a non-aromatizable androgen rapidly alters the ovarian transcriptome of previtellogenic secondary growth coho salmon (Onchorhynchus kisutch)

Recent evidence suggests that androgens are a potent driver of growth during late the primary stage of ovarian follicle development in teleosts. We have previously shown that the non-aromatizable androgen, 11-ketotestosterone (11-KT), both advances ovarian follicle growth in vivo and dramatically alters the primary growth ovarian transcriptome in coho salmon. Many of the transcriptomic changes pointed towards 11-KT driving process associated with the transition to a secondary growth phenotype. In the current study, we implanted previtellogenic early secondary growth coho salmon with cholesterol pellets containing 11-KT and performed RNA-Seq on ovarian tissue after 3 days in order to identify alterations to the ovarian transcriptome in early secondary growth. We identified 8,707 contiguous sequences (contigs) that were differentially expressed (DE) between control and 11-KT implanted fish and were able to collapse those to 3,853 gene-level IDs, more than a 3-fold more DE contigs than at the primary growth stage we reported previously. These contigs included genes encoding proteins involved in steroidogenesis, vitellogenin and lipid uptake, follicle stimulating hormone signaling, growth factor signaling, and structural proteins, suggesting androgens continue to promote previtellogenic secondary growth.

Hypogonadotropic hypogonadism in male tilapia lacking a functional rln3b gene

Relaxin 3 is a neuropeptide that plays a crucial role in reproductive functions of mammals. Previous studies have confirmed that rln3a plays an important role in the male reproduction of tilapia. To further understand the significance of its paralogous gene rln3b in male fertility, we generated a homozygous mutant line of rln3b in Nile tilapia. Our findings indicated that rln3b mutation delayed spermatogenesis and led to abnormal testes structure. Knocking out rln3b gene resulted in a decrease in sperm count, sperm motility and male fish fertility. TUNEL detection revealed a small amount of apoptosis in the testes of rln3b-/- male fish at 390 days after hatching (dah). RT-qPCR analysis demonstrated that mutation of rln3b gene caused a significant downregulation of steroid synthesis-related genes such as cyp17a1, cyp11b2, germ cell marker gene, Vasa, and gonadal somatic cell marker genes of amh and amhr2. Furthermore, we found a significant down-regulation of hypothalamic-pituitary-gonadal (HPG) axis-related genes, while a significantly up-regulation of the dopamine synthetase gene in the rln3b-/- male fish. Taken together, our data strongly suggested that Rln3b played a crucial role in the fertility of XY tilapia by regulating HPG axis genes.

Endocrine and Transcriptome Changes Associated with Testicular Growth and Differentiation in Atlantic Salmon (Salmo salar L.)

Sexual maturation of Atlantic salmon males is marked by dramatic endocrine changes and rapid growth of the testes, resulting in an increase in the gonad somatic index (GSI). We examined the association of gonadal growth with serum sex steroids, as well as pituitary and testicular gene expression levels, which were assessed with a DNA oligonucleotide microarray. The testes transcriptome was stable in males with a GSI < 0.08% despite the large difference between the smallest and the largest gonads. Fish with a GSI ≥ 0.23% had 7-17 times higher serum levels of five male steroids and a 2-fold increase in progesterone, without a change in cortisol and related steroids. The pituitary transcriptome showed an upregulation of the hormone-coding genes that control reproduction and behavior, and structural rearrangement was indicated by the genes involved in synaptic transmission and the differentiation of neurons. The observed changes in the abundance of testicular transcripts were caused by the regulation of transcription and/or disproportional growth, with a greater increase in the germinative compartment. As these factors could not be separated, the transcriptome results are presented as higher or lower specific activities (HSA and LSA). LSA was observed in 4268 genes, including many genes involved in various immune responses and developmental processes. LSA also included genes with roles in female reproduction, germinal cell maintenance and gonad development, responses to endocrine and neural regulation, and the biosynthesis of sex steroids. Two functional groups prevailed among HSA: structure and activity of the cilia (95 genes) and meiosis (34 genes). The puberty of A. salmon testis is marked by the predominance of spermatogenesis, which displaces other processes; masculinization; and the weakening of external regulation. Results confirmed the known roles of many genes involved in reproduction and pointed to uncharacterized genes that deserve attention as possible regulators of sexual maturation.

Molecular and Physiological Effects of 17α-methyltestosterone on Sex Differentiation of Black Rockfish, Sebastes schlegelii

It is widely known that all-female fish production holds economic value for aquaculture. Sebastes schlegelii, a preeminent economic species, exhibits a sex dimorphism, with females surpassing males in growth. In this regard, achieving all-female black rockfish production could significantly enhance breeding profitability. In this study, we utilized the widely used male sex-regulating hormone, 17α-methyltestosterone (MT) at three different concentrations (20, 40, and 60 ppm), to produce pseudomales of S. schlegelii for subsequent all-female offspring breeding. Long-term MT administration severely inhibits the growth of S. schlegelii, while short term had no significant impact. Histological analysis confirmed sex reversal at all MT concentrations; however, both medium and higher MT concentrations impaired testis development. MT also influenced sex steroid hormone levels in pseudomales, suppressing E2 while increasing T and 11-KT levels. In addition, a transcriptome analysis revealed that MT down-regulated ovarian-related genes (cyp19a1a and foxl2) while up-regulating male-related genes (amh) in pseudomales. Furthermore, MT modulated the TGF-β signaling and steroid hormone biosynthesis pathways, indicating its crucial role in S. schlegelii sex differentiation. Therefore, the current study provides a method for achieving sexual reversal using MT in S. schlegelii and offers an initial insight into the underlying mechanism of sexual reversal in this species.

Interaction of 17α-ethinylestradiol and methyltestosterone in western mosquitofish (Gambusia affinis) across two generations

The interactions between estrogen and androgen in aquatic animals remain largely unknown. In this study, two generations (F0 and F1) of western mosquitofish (Gambusia affinis) were continuously exposed to 17α-ethinylestradiol (EE2, 10 ng/L), methyltestosterone (MT, 10 ng/L (MTL); 50 ng/L (MTH)), and mixtures (EE2+MTL and EE2+MTH). Various endpoints, including sex ratio (phenotypic and genetic), secondary sex characteristics, gonadal histology, and transcriptional profile of genes, were examined. The results showed that G. affinis exposed to MTH and EE2+MTH had a > 89.7 % of phenotypic males in F1 generation, with 34.5 and 50.0 % of these males originated from genetic females, respectively. Moreover, females from F0 and F1 generations exposed to MTH and EE2+MTH exhibited masculinized anal fins and skeletons. The combined effect of MT and EE2 on most endpoints was dependent on MT. Furthermore, significant transcriptional alterations in certain target genes were observed in both the F0 and F1 generations by EE2 and MT alone and by mixtures, showing some degree of interactions. These findings that the effects of EE2+MTH were primarily on the phenotypic sex of G. affinis in offspring generation suggest that G. affinis under chronic exposure to the binary mixture contaminated water could have sex-biased populations.

Gestodene causes masculinization of the western mosquitofish (Gambusia affinis): Insights from ovary metabolomics

Gestodene (GES) is a common synthetic progesterone frequently detected in aquatic environments. Chronic exposure to GES can cause masculinization of a variety of fish; however, whether metabolism is closely related to the masculinization has yet to be explored. Hence, the ovary metabolome of adult female western mosquitofish (Gambusia affinis) after exposing to GES (0.0, 5.0, 50.0, and 500.0 ng/L) for 40 days was analyzed by using high-performance liquid chromatography ionization with quadrupole time-of-flight tandem mass spectrometry (HPLC-QTOF-MS). The results showed that GES increased the levels of cysteine, taurine, ophthalmic acid and cAMP while decreased methionine, these metabolites changes may owing to the oxidative stress of the ovaries; while taurcholic acid and uric acid were decreased along with induced oocyte apopotosis. Steroids hormone metabolism was also significantly affected, with progesterone and cortisol being the most affected. Enzyme-linked immunoassay results showed that estradiol levels were decreased while testosterone levels were increased with GES exposure. In addition, correlation analysis showed that the differential metabolites of some amino acids (e.g. leucine) were strongly correlated with the levels of steroids hormones secreted by the pituitary gland. The results of this study suggest that GES affects ovarian metabolism via the hypothalamus-pituitary-gonad and hypothalamic-pituitary-adrenal axes, impair antioxidant capacity, induce apoptosis in the ovary of G. affinis, and finally caused masculinization.

An evaluation of DNA sample source and molecular markers to determine gender in the short-beaked echidna (Tachyglossus aculeatus)

The short-beaked echidna is sexually monomorphic such that gender identification without veterinary intervention is challenging. The aim of this study was to evaluate and compare the most optimal noninvasive genetic source by extracting echidna genomic DNA (gDNA) from fecal scats, plucked hair, and quills to perform genetic sex testing using a range of molecular markers. Sex determination of 14 captive short-beaked echidnas was determined by amplifying isolated DNA from noninvasive samples, targeting two Y chromosome (male-specific) genes (mediator complex subunit 26 Y-gametologue [CRSPY] and anti-Müllerian hormone Y-gametologue [AMHY]), in addition to four confirmed sex-specific RADseq markers. Results of noninvasive samples were compared with blood samples and clinical records. Receiver operating characteristic curves were used to assess accuracy of sex determination of markers for each sample type. The gender of the echidnas was successfully identified on 75% of occasions using fecal samples, 90.6% occasions using hair, and 84.6% occasions with quills. Overall, the male-specific RADseq markers accurately identified the sex of echidnas with all sample types for 90% of animals; compared with 81.5% using CRSPY, and 82.0% using AMHY to identify sex. Collection of hair, quills, and feces provides a useful alternative to invasively collected samples, however, the accuracy of results depends on sample type and genetic marker selected. We found gender determination in the short-beaked echidna was most accurate using four male-specific RADseq markers on gDNA isolated from blood and hair. The noninvasive genetic sexing techniques documented here will inform and facilitate husbandry and genetic management of captive echidna populations.

Genome-wide identification, evolution and expression of TGF-β signaling pathway members in mandarin fish (Siniperca chuatsi)

Members of the transforming growth factor β (TGF-β) signaling pathway regulate diverse cellular biological processes in embryonic and tissue development. We took mandarin fish (Siniperca chuatsi) as the research object to identify all members of the TGF-β signaling pathway, measure their expression pattern in the key period post hatching, and further explore their possible role in the process of sex regulation. Herein, we identified eighty-three TGF-β signaling pathway members and located them on chromosomes based on the genome of mandarin fish. TGF-β signaling pathway members were highly conserved since each TGF-β subfamily clustered with orthologs from other species. Transcriptome analysis, qRT-PCR and in situ hybridization demonstrated that most mandarin fish TGF-β signaling pathway members presented stage-specific and/or sex-dimorphic expression during gonadal development, and different members of the TGF-β signaling pathway participated in different stages of gonadal development. Taken together, our results provide new insight into the role of TGF-β signaling pathway members in the sex regulation of mandarin fish.

Developmental regulation of the male urogenital papilla in the male marine teleost black rockfish, Sebastes schlegelii (Hilgendorf, 1880)†

The male external genitalia of the black rockfish (Sebastes schlegelii Hilgendorf, 1880) is a fleshy protrusion known as the urogenital papilla (UGP), which functions to deliver sperm into the female reproductive tract for internal fertilization. It is not known which genes regulate the development of the UGP. The aim of this study was to identify key genes that regulate the development of the UGP in black rockfish and to determine the distribution of androgen receptor gene (ar) in the UGP. A total of 26 adult males and 560 juvenile fish were used in the experiment, in which we divided all normally developing juveniles into normal development and androgen groups. We added methyltestosterone solution (100 μg/l) to the androgen group-treated fish tank, soaked for 2 h per day for 38 days, and sampled 5~10 samples each time every 5 days during the culture process. Gene expression changes related to UGP were analyzed with tissue specificity between control and androgen groups during sex differentiation, adult male maturation, and the copulation stage (September to December) using real-time quantitative polymerase chain reaction. The expression of ar was also localized by two-color in situ hybridization in the UGP region of juvenile fish. Androgen treatment enhanced ar expression levels and the ar signal was stronger in the UGP region of both adult breeding fish and androgen-treated juvenile fish. This study provides insights into the regulation of the external genitalia of black rockfish and presents vital information for the artificial breeding of viviparous fish.

Exposure to estrone disrupts the endocrine system of western mosquitofish (Gambusia affinis)

Estrone (E1) is one of the predominant natural estrogens detected in aquatic environments, yet little is known about its effects on the endocrine system in fish. In this study, the sex ratio, secondary sexual characteristics, gonadal histology, and transcriptional levels of genes closely related to sex differentiation and hypothalamic-pituitary-gonadal-liver (HPGL) axis were assessed in western mosquitofish (Gambusia affinis) after a full life-cycle exposure to E1 (0, 25.4, 143, 740, and 4300 ng/L) for 119 days. The results showed that exposure to 4300 ng/L of E1 resulted in 100% female and inhibited the growth of females. Exposure to environmentally relevant concentrations of E1 (143 and 740 ng/L) led to obvious feminization of skeletons and anal fins in males. Exposure to 740 and 4300 ng/L of E1 increased the proportion of mature spermatocytes in females, and exposure to 143 and 740 ng/L decreased the proportion of mature spermatocytes in males. Moreover, the transcripts of genes related to sex differentiation and HPGL axis were changed in the E1-exposed adult fish and embryos inside females. This study has provided valuable data on the endocrine disruption effects of E1 at environmentally relevant concentrations in G. affinis.

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.

Inactivation of the Anti-Müllerian Hormone Receptor Type 2 (amhrII) Gene in Northern Pike (Esox lucius) Results in Male-To-Female Sex Reversal

BACKGROUND

The anti-müllerian hormone (Amh) pathway is crucial for sexual development in teleosts. A male-specific duplicate of anti-müllerian hormone (amhby) was previously identified as the northern pike (Esox lucius) master sex determination gene. However, the role of its putative cognate receptor, i.e., the anti-müllerian hormone receptor type 2 (amhrII) was unclear in this species.

OBJECTIVE

Here, we investigated the role of amhrII during sexual development of northern pike.

METHOD

We generated stable mutants with deletions in exon 9 of amhrII, inactivating the AmhrII protein using a CRISPR-Cas9-mediated gene knockout strategy.

RESULT

The inactivation of amhrII in northern pike results in a high level of male-to-female sex reversal.

CONCLUSION

This result demonstrates that amhrII is necessary for male sexual development in northern pike and supports the idea that AmhrII is a conserved regulator of the teleosts sex differentiation network.

Development of epigenetic biomarkers for the identification of sex and thermal stress in fish using DNA methylation analysis and machine learning procedures

The sex ratio is a key ecological demographic parameter crucial for population viability. However, the epigenetic mechanisms operating during gonadal development regulating gene expression and the sex ratio remain poorly understood. Moreover, there is interest in the development of epigenetic markers associated with a particular phenotype or as sentinels of environmental effects. Here, we profiled DNA methylation and gene expression of 10 key genes related to sex development and stress, including steroidogenic enzymes, and growth and transcription factors. We provide novel information on the sex-related differences and on the influence of elevated temperature on these genes in zebrafish, a species with mixed genetic and environmental influences on sex ratios. We identified both positive (e.g., amh, cyp11c and hsd11b2) and negative (e.g., cyp11a1 and dmrt1) correlations in unexposed males, and negative correlation (amh) in exposed females between DNA methylation and gene expression levels. Further, we combined DNA methylation analysis with machine learning procedures and found a series of informative CpGs capable not only of correctly identifying sex (based on cyp19a1a DNA methylation levels) but also of identifying whether males and females had been exposed to abnormally elevated temperature when young (based on amh and foxl2a DNA methylation levels, respectively). This was achieved in the absence of conspicuous morphological alterations of the gonads. These DNA methylation-based epigenetic biomarkers represent molecular resources that can correctly recapitulate past thermal history and pave the way for similar findings in other species to assess potential ecological effects of environmental disturbances in the context of climate change.

Effect of In Vitro Maturation of Human Oocytes Obtained After Controlled Ovarian Hormonal Stimulation on the Expression of Development- and Zona Pellucida-Related Genes and Their Interactions

In an in vitro fertilization program, approximately 10-15% of oocytes obtained after controlled ovarian stimulation are immature, with germinal vesicles (GVs). These oocytes are usually discarded in clinical practice; however, an in vitro maturation (IVM) procedure can be applied to mature them. There are scarce data in the literature on the effect of IVM on the expression of important development- and zona pellucida (ZP)-related genes in human oocytes; therefore, we wanted to determine this. One hundred nine human oocytes were collected from patients enrolled in an intracytoplasmic sperm injection program. The expression of the BMP4, GDF9, ZP1, ZP2, ZP3, and ZP4 genes was analyzed using RT-qPCR in oocytes matured in vitro with different reproductive hormones in the IVM medium (AMH, FSH + hCG, FSH + hCG + AMH), in in vivo matured oocytes and in immature oocytes with GVs. No statistically significant differences in the expression of selected genes in oocytes were observed among groups with different reproductive hormones in IVM medium. However, several interesting significant correlations were found between BMP4 and GDF9, and ZP1 and ZP4; between GDF9 and ZP1, and ZP2 and ZP4; and between ZP1 and ZP3 and ZP4 in the in vitro matured oocytes, while no such correlations were present in other groups of oocytes. The type of reproductive hormone in the maturation medium does not affect the expression of the analyzed genes in oocytes during the maturation process. However, the in vitro maturation procedure itself generated correlations among analyzed genes that were otherwise not present in in vivo matured and immature oocytes.

Knock-down of amh transcription by antisense RNA reduces FSH and increases follicular atresia in female Oreochromis niloticus

Anti-Müllerian hormone (Amh) plays an important role in regulating gonad development in teleosts. However, little is known about the effects of Amh on follicle development. In this study, we transfected the vector containing antisense RNA fragments of the amh gene to produce Nile tilapia, Oreochromis niloticus, with knocked-down Amh function in vivo. The results confirmed that the antisense RNA effectively inhibited amh transcription and Amh protein expression in female tilapia ovarian tissue. At 180 days of age, compared with control fish, female tilapia with knocked-down Amh function showed significantly increased growth and significantly decreased ovary weight and gonadosomatic index (P < 0.05). Female fish in the control group had ruddy-colored external genitalia, eggs extruded from the abdomen when gently squeezed, and most oocytes were developmental stage V. In contrast, the external genitalia of female fish with knocked-down Amh function did not have the ruddy color, no eggs extruded from the abdomen when squeezed, most oocytes were at developmental stages II and III, and considerable follicular atresia was apparent. At 180 days of age, the transcript levels of amhrII, cyp19a1a, foxl2 and sox9b in ovarian tissue, and the titers of luteinizing hormone, follicle stimulating hormone, and estradiol in the serum, were significantly lower in fish with knocked-down Amh function than in control fish (P < 0.05). We concluded that decreased serum hormone levels and an abnormal AMH signal delayed development and caused follicular degeneration in Nile tilapia with knocked-down Amh function. These findings show that antisense RNA is a feasible approach for gene silencing in fish, and represents an accurate and effective strategy to study gene function.

Impact of germ cell ablation on the activation of the brain-pituitary-gonadal axis in precocious Atlantic salmon (Salmo salar L.) males

The germ cells are essential for sexual reproduction by giving rise to the gametes, but the importance of germ cells for gonadal somatic functions varies among vertebrates. The RNA-binding dead end (Dnd) protein is necessary for the specification and migration of primordial germ cells to the future reproductive organs. Here, we ablated the gametes in Atlantic salmon males and females by microinjecting dnd antisense gapmer oligonucleotides at the zygotic stage. Precocious maturation was induced in above 50% of both germ cell-depleted and intact fertile males, but not in females, by exposure to an off-season photoperiod regime. Sterile and fertile males showed similar body growth, but maturing fish tended to be heavier than their immature counterparts. Pituitary fshβ messenger RNA levels strongly increased in maturing sterile and fertile males concomitant with the upregulated expression of Sertoli and Leydig cell markers. Plasma concentrations of 11-ketotestosterone and testosterone in maturing sterile males were significantly higher than the basal levels in immature fish, but lower than those in maturing fertile males. The study demonstrates that germ cells are not a prerequisite for the activation of the brain-pituitary-gonad axis and sex steroidogenesis in Atlantic salmon males, but may be important for the maintenance of gonadal somatic functions.

Effects of cortisol on female-to-male sex change in a wrasse

Sex change occurs as a usual part of the life cycle for many teleost fish and the modifications involved (behavioural, gonadal, morphological) are well studied. However, the mechanism that transduces environmental cues into the molecular cascade that underlies this transformation remains unknown. Cortisol, the main stress hormone in fish, is hypothesised to be a key factor linking environmental stimuli with sex change by initiating gene expression changes that shift steroidogenesis from oestrogens to androgens but this notion remains to be rigorously tested. Therefore, this study aimed to experimentally test the role of cortisol as an initiator of sex change in a protogynous (female-to-male) hermaphrodite, the New Zealand spotty wrasse (Notolabrus celidotus). We also sought to identify potential key regulatory factors within the head kidney that may contribute to the initiation and progression of gonadal sex change. Cortisol pellets were implanted into female spotty wrasses under inhibitory conditions (presence of a male), and outside of the optimal season for natural sex change. Histological analysis of the gonads and sex hormone analyses found no evidence of sex change after 71 days of cortisol treatment. However, expression analyses of sex and stress-associated genes in gonad and head kidney suggested that cortisol administration did have a physiological effect. In the gonad, this included upregulation of amh, a potent masculinising factor, and nr3c1, a glucocorticoid receptor. In the head kidney, hsd11b2, which converts cortisol to inactive cortisone to maintain cortisol balance, was upregulated. Overall, our results suggest cortisol administration outside of the optimal sex change window is unable to initiate gonadal restructuring. However, our expression data imply key sex and stress genes are sensitive to cortisol. This includes genes expressed in both gonad and head kidney that have been previously implicated in early sex change in several sex-changing species.

Transcriptome Profiling and Expression Localization of Key Sex-Related Genes in a Socially-Controlled Hermaphroditic Clownfish, Amphiprion clarkii

Clownfish can be an excellent research model for investigating the socially-controlled sexual development of sequential hermaphrodite teleosts. However, the molecular cascades underlying the social cues that orchestrate the sexual development process remain poorly understood. Here, we performed a comparative transcriptomic analysis of gonads from females, males, and nonbreeders of Amphiprion clarkii, which constitute a complete social group, allowing us to investigate the molecular regulatory network under social control. Our analysis highlighted that the gonads of nonbreeders and males exhibited high similarities but were far from females, both in global transcriptomic profiles and histological characteristics, and identified numerous candidate genes involved in sexual development, some well-known and some novel. Significant upregulation of cyp19a1a, foxl2, nr5a1a, wnt4a, hsd3b7, and pgr in females provides strong evidence for the importance of steroidogenesis in ovarian development and maintenance, with cyp19a1a playing a central role. Amh and sox8 are two potential key factors that may regulate testicular tissue development in early and late stages, respectively, as they are expressed at higher levels in males than in females, but with slightly different expression timings. Unlike previous descriptions in other fishes, the unique expression pattern of dmrt1 in A. clarkii implied its potential function in both male and female gonads, and we speculated that it might play promoting roles in the early development of both testicular and ovarian tissues.

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.

Cortisol safeguards oogenesis by promoting follicular cell survival

The role of glucocorticoids in oogenesis remains to be elucidated. cyp11c1 encodes the key enzyme involved in the synthesis of cortisol, the major glucocorticoid in teleosts. In our previous study, we mutated cyp11c1 in tilapia and analyzed its role in spermatogenesis. In this study, we analyzed its role in oogenesis. cyp11c1^+/- XX tilapia showed normal ovarian morphology but poor egg quality, as indicated by the mortality of embryos before 3 d post fertilization, which could be partially rescued by the supplement of exogenous cortisol to the mother fish. Transcriptome analyses revealed reduced expression of maternal genes in the eggs of the cyp11c1^+/- XX fish. The cyp11c1^-/- females showed impaired vitellogenesis and arrested oogenesis due to significantly decreased serum cortisol. Further analyses revealed decreased serum E2 level and expression of amh, an important regulator of follicular cell development, and increased follicular cell apoptosis in the ovaries of cyp11c1^-/- XX fish, which could be rescued by supplement of either exogenous cortisol or E2. Luciferase assays revealed a direct regulation of cortisol and E2 on amh transcription via GRs or ESRs. Taken together, our results demonstrate that cortisol safeguards oogenesis by promoting follicular cell survival probably via Amh signaling.

Molecular Cloning and Expression Analysis of Thyrotropin-Releasing Hormone, and Its Possible Role in Gonadal Differentiation in Rice Field eel Monopterus albus

Rice field eel (Monopterus albus), a protogynous hermaphrodite fish, is a good model for the research of sex determination and gonadal differentiation in teleosts. In this study, we cloned the full-length cDNA sequence of trh, which encoded a predicted protein with 270 amino acids. Trh mainly expressed in the brain, followed by the ovary, testis, muscle and pituitary, and had low levels in other peripheral tissues. During natural sex reversal, trh mRNA expression levels exhibited a significant increase at the late intersexual stage in the hypothalamus. In the gonad, trh mRNA expression levels showed a trend of increase followed by decrease, and only increased significantly at the middle intersexual stage. No matter static incubation or intraperitoneal (IP) injection, TRH had no significant effect on trh and thyroid-stimulating hormone βsubunit (tshβ) mRNA expression levels, and serum T3, T4 and TRH release. After static incubation of ovarian fragments by TRH, the expression of gonadal soma derived factor (gsdf) was up-regulated significantly at both the doses of 10 and 100 nM. IP injection of TRH stimulated the expression of gsdf, and inhibited the expression of ovarian aromatase gene (cyp19a1a), accompanied by the increase of serum 11-KT levels. The results indicated that TRH may play a novel role in gonadal differentiation by the regulation of gonadal differentiation-related gene expression and sex steroid hormone secretion in rice field eel.

A time-course transcriptome analysis of gonads from yellow catfish (Pelteobagrus fulvidraco) reveals genes associated with gonad development

Background

The yellow catfish, Pelteobagrus fulvidraco, is a commercially important fish species. It is widely distributed in the fresh water areas of China, including rivers, lakes, and reservoirs. Like many other aquaculture fish species, people have observed significant size dimorphism between male and female yellow catfish and it shows a growth advantage in males.

Results

Here, at the first time, the time-course transcriptome was used to explore the various expression profiles of genes in different gonad developmental stages and genders. A total of 2696 different expression genes (DEGs) were identified from different stages. Based on these DEGs, 13 gonad development related genes were identified which showed time-specific or sex biased expression patterns.

Conclusion

This study will provide the crucial information on the molecular mechanism of gonad development of female and male yellow catfish. Especially, during the different gonad development stages, these 13 gonad development related genes exhibit various expression patterns in female and male individual respectively. These results could inspire and facilitate us to understanding the various roles of these genes play in different gonad development stages and genders.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08651-0.

The miR-200 Family Targeting amh Affects the Gonadal Development of Japanese Flounder

Four members of the miR-200 family in Japanese flounder (Paralichthys olivaceus) have sex-biased expression patterns, but their target genes and how they work in the development of the gonads are rarely known. Anti-Müllerian hormone (AMH) can inhibit the development of Muller’s duct in female mammals and regulate the formation of gametes after sexual maturity. There is no Muller’s duct in teleosts, but the amh gene still exists. Knockout of amh results in sex reversal from male to female. Therefore, it is essential to explore the relationship between the miR-200 family and amh to clarify what role miR-200 plays in the development of the gonads. In Japanese flounder, the two binding sites for the miR-200 family in the 3′UTR of amh were found through bioinformatic prediction. Double luciferase and green fluorescent protein reporter experiments demonstrated amh to be directly targeted by miR-200a and miR-200b. Moreover, miR-200a and miR-200b reduced the expression of amh through site 1 rather than site 2. To explore the regulatory role of miR-200a in gonadal development, we further overexpressed miR-200a in the primary Sertoli cells of the testis. With the overexpression of miR-200a, the expression of amh decreased, while the expression of the other two male sex-related genes, dmrt1 (doublesex and mab-3 related transcription factor 1) and gsdf (diagonal soma driven factor), increased significantly. This result indicates that the miR-200 family regulates the gonadal differentiation and development by targeting amh in Japanese flounder.

Transcriptomes of testis and pituitary from male Nile tilapia (O. niloticus L.) in the context of social status

African cichlids are well established models for studying social hierarchies in teleosts and elucidating the effects social dominance has on gene expression. Ascension in the social hierarchy has been found to increase plasma levels of steroid hormones, follicle stimulating hormone (Fsh) and luteinizing hormone (Lh) as well as gonadosomatic index (GSI). Furthermore, the expression of genes related to gonadotropins and steroidogenesis and signaling along the brain-pituitary-gonad axis (BPG-axis) is affected by changes of an animal’s social status. In this study, we use RNA-sequencing to obtain an in-depth look at the transcriptomes of testes and pituitaries from dominant and subordinate male Nile tilapia living in long-term stable social hierarchies. This allows us to draw conclusions about factors along the brain-pituitary-gonad axis that are involved in maintaining dominance over weeks or even months. We identify a number of genes that are differentially regulated between dominant and subordinate males and show that in high-ranking fish this subset of genes is generally upregulated. Genes differentially expressed between the two social groups comprise growth factors, related binding proteins and receptors, components of Wnt-, Tgfβ- and retinoic acid-signaling pathway, gonadotropin signaling and steroidogenesis pathways. The latter is backed up by elevated levels of 11-ketotestosterone, testosterone and estradiol in dominant males. Luteinizing hormone (Lh) is found in higher concentration in the plasma of long-term dominant males than in subordinate animals. Our results both strengthen the existing models and propose new candidates for functional studies to expand our understanding of social phenomena in teleost fish.

Masculinization of Adult Gambusia holbrooki: A Case of Recapitulation of Protogyny in a Gonochorist?

17α-Methyltestosterone (MT) is a synthetic steroid that has been widely used to masculinize many fish species when administered early during larval development, however, reports on its efficacy on adults is limited. To this end, this study investigated the efficacy of MT in the masculinization of the eastern mosquitofish (G. holbrooki) at two adult stages (maiden and repeat gravid females). The treated females were fed control or respective MT incorporated feed (0-200 mg/kg diet) for 50 days. Effects of the hormone on secondary sexual characteristics, internal gonad morphology, expression of the Anti-Müllerian Hormone (amh) gene and sexual behavior of the treated females were investigated. The results showed that MT at the dose of 50 mg/kg feed stimulated secondary sexual character development, upregulated expression of amh, formation of testicular tissue and a shift in the behavior similar to those of normal males, prominently so in treated maiden gravid females. Post-treatment, long-term observations indicated that only two masculinized females reverted back to being females and gave birth to young. Induction of masculinizing effects in most individuals suggests that the sexual phenotype of this species appears to be highly plastic with potential to sex reverse at adulthood. This in combination with its small size and short reproductive cycle could provide an ideal system to explore the mechanisms of sequential hermaphroditism in fish and contribute to genetic control of this pest fish.

Intensive masculinization caused by chronic heat stress in juvenile Cynoglossus semilaevis: Growth performance, gonadal histology and gene responses

The sea temperature has been observed to chronically increase during the past decades, leaving unpredictable influences to the marine biological resources. Thus, it is of vital significance to study the biological responses of ocean inhabited organisms with the artificially stimulated heat stress environment. Cynoglossus semilaevis provides us with an ideal model to study the influence of chronic heat stress on the sexual differentiation in marine teleosts for its genetic sex determination (GSD) + environmental effected (EE) sex determination system. In this study, the comparative experiment was conducted employing heated seawater (HT group) and ambient seawater (CT group) to cultivate juvenile C. semilaevis respectively. Significant differences were exhibited in growth performance and a delayed germ cell development effect was found in pseudomales formed under chronic heat stress. Using transcriptome analysis, the transcription profile of 55 days post fertilization (dpf) and 100 dpf juveniles' gonads were studied. A total of 47 libraries were constructed with an average mapping rate of 94.63% after assembling. GO and KEGG enrichment were proceeded using DEGs screened out between (1) pseudomale gonads at 55 dpf and 100 dpf in HT and CT group (2) pseudomale and female gonads at 55 dpf and 100 dpf in HT and CT group. Terms and pathways involved in steroid stimulation, reproduction ability, germ cell proliferation et al. were shed light on. The expression pattern of 29 DEGs including amh, hsp90b1, pgr et al. were also provided to supplement the results of functional enrichment. Weighted gene co-expression networks analysis (WGCNA) was constructed and hspb8-like, histone H2A.V were exhibited to play vital roles in the heat-induced masculinization. Our findings facilitate the understanding for transcriptional variations in intensive masculinization cause by chronic heat stress of C. semilaevis and provide referable study of the influences on the teleosts in elevated sea temperature.

Recombinant anti-Müllerian hormone in the maturation medium improves the in vitro maturation of human immature (GV) oocytes after controlled ovarian hormonal stimulation

BACKGROUND

In vitro maturation (IVM) of oocytes is a laboratory method that allows the maturation of immature (GV) oocytes retrieved from patients enrolled in the in vitro fertilization (IVF) programme. However, this method is still sparsely researched and used in clinical practice, leading to suboptimal clinical results. Anti-Müllerian hormone (AMH) is an important hormone with known effects on human ovaries, especially on follicles (follicular cells) during folliculogenesis. In contrast, the effect of AMH on the human oocyte itself is unknown. Therefore, we wanted to determine whether human oocytes express AMH receptor 2 (AMHR2) for this hormone. Recombinant AMH was added to the IVM medium to determine whether it affected oocyte maturation.

METHODS

In total, 247 human oocytes (171 immature and 76 mature) were collected from patients enrolled in the intracytoplasmic sperm injection (ICSI) programme who were aged 20 to 43 years and underwent a short antagonist protocol of ovarian stimulation. The expression of AMHR2 protein and AMHR2 gene was analysed in immature and mature oocytes. Additionally, maturation of GV oocytes was performed in vitro in different maturation media with or without added AMH to evaluate the effect of AMH on the oocyte maturation rate.

RESULTS

Immunocytochemistry and confocal microscopy revealed that AMHR2 protein is expressed in both immature and mature human oocytes. AMHR2 was expressed in a spotted pattern throughout the whole oocyte. The IVM procedure revealed that AMH in maturation medium improved GV oocyte maturation in vitro, as all oocytes were successfully matured in maturation medium containing recombinant AMH only. Furthermore, antagonism between AMH and follicle-stimulating hormone (FSH) during the maturation process was observed, with fewer oocytes maturing when both AMH and FSH were added to the maturation medium. Finally, AMHR2 gene expression was found in immature and in vitro matured oocytes but absent in mature oocytes.

CONCLUSIONS

The positive AMHR2 protein and AMHR2 gene expression in human oocytes shows that AMH could directly act on human oocytes. This was further functionally confirmed by the IVM procedure. These findings suggest the potential clinical application of recombinant AMH to improve IVM of human oocytes in the future.

Genetics of anti-Müllerian hormone and its signaling pathway

Anti-Müllerian hormone (AMH) is a member of the TGF-β family produced essentially by the supporting somatic cells of the testis. Initially known for its inhibiting role upon the development of female internal organs, AMH has been shown to exert many other effects namely upon germ cells. Circulating AMH reflects the ovarian reserve of young developing follicles and is used to evaluate the fertility potential in assisted reproduction. The signaling pathway of AMH is both similar and different from that of other members of the TGF-β family. Like these, it signals through two distinct serine/threonine receptors, type 1 and type 2, that phosphorylate cytoplasmic effectors, the Smads. It also shares type 1 receptors and Smads with other members of the family. However, AMH is the only family member with its own, dedicated, ligand-specific type 2 receptor, AMHR2. The monogamic relationship between AMH and AMHR2 is supported by molecular studies of the Persistent Müllerian Duct Syndrome, characterized by the presence of Müllerian derivatives in otherwise normally virilized males: mutations of AMH or AMHR2 are clinically indistinguishable.

Roles of anti-Müllerian hormone and its duplicates in sex determination and germ cell proliferation of Nile tilapia

Duplicates of amh are crucial for fish sex determination and differentiation. In Nile tilapia, unlike in other teleosts, amh is located on X chromosome. The Y chromosome amh (amhΔ-y) is mutated with 5 bp insertion and 233 bp deletion in the coding sequence, and tandem duplicate of amh on Y chromosome (amhy) has been identified as the sex determiner. However, the expression of amh, amhΔ-y, and amhy, their roles in germ cell proliferation and the molecular mechanism of how amhy determines sex is still unclear. In this study, expression and functions of each duplicate were analyzed. Sex reversal occurred only when amhy was mutated as revealed by single, double, and triple mutation of the 3 duplicates in XY fish. Homozygous mutation of amhy in YY fish also resulted in sex reversal. Earlier and higher expression of amhy/Amhy was observed in XY gonads compared with amh/Amh during sex determination. Amhy could inhibit the transcription of cyp19a1a through Amhr2/Smads signaling. Loss of cyp19a1a rescued the sex reversal phenotype in XY fish with amhy mutation. Interestingly, mutation of both amh and amhy in XY fish or homozygous mutation of amhy in YY fish resulted in infertile females with significantly increased germ cell proliferation. Taken together, these results indicated that up-regulation of amhy during the critical period of sex determination makes it the sex-determining gene, and it functions through repressing cyp19a1a expression via Amhr2/Smads signaling pathway. Amh retained its function in controlling germ cell proliferation as reported in other teleosts, while amhΔ-y was nonfunctionalized.

A new experimental model for the investigation of sequential hermaphroditism

The stunning sexual transformation commonly triggered by age, size or social context in some fishes is one of the best examples of phenotypic plasticity thus far described. To date our understanding of this process is dominated by studies on a handful of subtropical and tropical teleosts, often in wild settings. Here we have established the protogynous New Zealand spotty wrasse, Notolabrus celidotus, as a temperate model for the experimental investigation of sex change. Captive fish were induced to change sex using aromatase inhibition or manipulation of social groups. Complete female-to-male transition occurred over 60 days in both cases and time-series sampling was used to quantify changes in hormone production, gene expression and gonadal cellular anatomy. Early-stage decreases in plasma 17β-estradiol (E2) concentrations or gonadal aromatase (cyp19a1a) expression were not detected in spotty wrasse, despite these being commonly associated with the onset of sex change in subtropical and tropical protogynous (female-to-male) hermaphrodites. In contrast, expression of the masculinising factor amh (anti-Müllerian hormone) increased during early sex change, implying a potential role as a proximate trigger for masculinisation. Collectively, these data provide a foundation for the spotty wrasse as a temperate teleost model to study sex change and cell fate in vertebrates.

Involvement of Transforming Growth Factor Beta Family Genes in Gonadal Differentiation in Japanese Eel, Anguilla japonica, According to Sex-Related Gene Expressions

The gonochoristic feature with environmental sex determination that occurs during the yellow stage in the eel provides an interesting model to investigate the mechanisms of gonadal development. We previously studied various sex-related genes during gonadal sex differentiation in Japanese eels. In the present study, the members of transforming growth factor beta (TGF-β) superfamily were investigated. Transcript levels of anti-Müllerian hormone, its receptor, gonadal soma-derived factor (amh, amhr2, and gsdf, respectively) measured by real-time polymerase chain reaction (qPCR) showed a strong sexual dimorphism. Transcripts were dominantly expressed in the testis, and their levels significantly increased with testicular differentiation. In contrast, the expressions of amh, amhr2, and gsdf transcripts were low in the ovary of E2-feminized female eels. In situ hybridization detected gsdf (but not amh) transcript signals in undifferentiated gonads. amh and gsdf signals were localized to Sertoli cells and had increased significantly with testicular differentiation. Weak gsdf and no amh signals were detected in early ovaries of E2-feminized female eels. Transcript levels of amh and gsdf (not amhr2) decreased during human chorionic gonadotropin (HCG)-induced spermatogenesis in males. This study suggests that amh, amhr2, and especially gsdf might be involved in the gene pathway regulating testicular differentiation of Japanese eels.

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.

Functional Activity of Recombinant Forms of Amh and Synergistic Action with Fsh in European Sea Bass Ovary

Although anti-Müllerian hormone (AMH) has classically been correlated with the regression of Müllerian ducts in male mammals, involvement of this growth factor in other reproductive processes only recently come to light. Teleost is the only gnathostomes that lack Müllerian ducts despite having amh orthologous genes. In adult teleost gonads, Amh exerts a role in the early stages of germ cell development in both males and females. Mechanisms involving the interaction of Amh with gonadotropin- and growth factor-induced functions have been proposed, but our overall knowledge regarding Amh function in fish gonads remains modest. In this study, we report on Amh actions in the European sea bass ovary. Amh and type 2 Amh receptor (Amhr2) are present in granulosa and theca cells of both early and late-vitellogenic follicles and cannot be detected in previtellogenic ovaries. Using the Pichia pastoris system a recombinant sea bass Amh has been produced that is endogenously processed to generate a 12–15 kDa bioactive mature protein. Contrary to previous evidence in lower vertebrates, in explants of previtellogenic sea bass ovaries, mature Amh has a synergistic effect on steroidogenesis induced by the follicle-stimulating hormone (Fsh), increasing E2 and cyp19a1a levels.

A sex-linked SNP mutation in amhr2 is responsible for male differentiation in obscure puffer (Takifugu obscurus)

BACKGROUND

Anti-Mullerian hormone receptor type II (Amhr2) is a key receptor of Amh signaling in regulating gonad development. The amhr2 gene has been identified in numerous species, including a few teleost fishes. However, the roles of Amhr2 in Amh signaling in fish are poorly studied.

METHODS AND RESULTS

In this study, an amhr2 homolog from obscure puffer (Takifugu obscurus) was identified, and its molecular characteristics were systematically analyzed. Expression analysis revealed that amhr2 was highly expressed in the gonads of adult pufferfish and significantly upregulated during sex differentiation. Significantly, a sex-linked SNP site was verified in obscure puffer amhr2. Females exhibited a homozygous genotype (C/C), while males possessed a heterozygous genotype (C/G), resulting in an amino acid variation (His/Asp384) in the kinase domain of Amhr2. Then, the functions of the different Amhr2 genotypes were further investigated. The male genotype protein (Amhr2^D384) showed an enhanced ability to interact with the type I receptor (Bmpr1a) compared to the female genotype (Amhr2^H384). The phosphorylation levels of Smads and activity of the target gene (id3) induced by the male genotype were also much higher than those induced by the female genotype. These results confirmed that the male genotype had an enhanced effect on the Amh signaling pathway compared with the female genotype.

CONCLUSIONS

This study provides direct experimental evidence for the roles of different Amhr2 genotypes in pufferfish and suggests that amhr2 is responsible for male sex differentiation in obscure puffer.

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.

Genetic Drift Dominates Genome-Wide Regulatory Evolution Following an Ancient Whole-Genome Duplication in Atlantic Salmon

Whole-genome duplications (WGD) have been considered as springboards that potentiate lineage diversification through increasing functional redundancy. Divergence in gene regulatory elements is a central mechanism for evolutionary diversification, yet the patterns and processes governing regulatory divergence following events that lead to massive functional redundancy, such as WGD, remain largely unknown. We studied the patterns of divergence and strength of natural selection on regulatory elements in the Atlantic salmon (Salmo salar) genome, which has undergone WGD 100-80 Ma. Using ChIPmentation, we first show that H3K27ac, a histone modification typical to enhancers and promoters, is associated with genic regions, tissue-specific transcription factor binding motifs, and with gene transcription levels in immature testes. Divergence in transcription between duplicated genes from WGD (ohnologs) correlated with difference in the number of proximal regulatory elements, but not with promoter elements, suggesting that functional divergence between ohnologs after WGD is mainly driven by enhancers. By comparing H3K27ac regions between duplicated genome blocks, we further show that a longer polyploid state post-WGD has constrained regulatory divergence. Patterns of genetic diversity across natural populations inferred from resequencing indicate that recent evolutionary pressures on H3K27ac regions are dominated by largely neutral evolution. In sum, our results suggest that post-WGD functional redundancy in regulatory elements continues to have an impact on the evolution of the salmon genome, promoting largely neutral evolution of regulatory elements despite their association with transcription levels. These results highlight a case where genome-wide regulatory evolution following an ancient WGD is dominated by genetic drift.

Molecular characterization and expression analysis of anti-Müllerian hormone in common carp (Cyprinus carpio) adult testes

Anti-Müllerian hormone (Amh) is a member of the transforming growth factor-β (Tgf-β) superfamily required in the regression of Müllerian ducts during gonadal sex differentiation of higher vertebrates. Teleost fish lack Müllerian ducts, but identified Amh orthologs have been shown to exert crucial functions during sex determination and differentiation of several species of teleosts. However, the function of Amh during gametogenesis in adult fish remains poorly investigated. Therefore, to expand present knowledge on the role of Amh in teleosts, the present study aimed to isolate and clone full-length amh cDNA in the common carp, Cyprinus carpio, and examine its expression levels throughout the male reproductive cycle and in response to different hormone treatments of testicular explants. Molecular cloning and characterization showed that the common carp Amh precursor amino acid sequence shared common features to other fish Amh precursors, including a conserved C-terminus (Tgf-β domain) and a double proteolytic cleavage site (R-X-X-R-X-X-R) upstream to the Tgf-β domain. Expression analysis showed amh dimorphic expression in the adult gonads with higher expression in the testes than ovaries. In testes, amh mRNA was detected in Sertoli cells contacting different types of germ cells, although the expression was greatest in Sertoli cells associated with type A undifferentiated spermatogonia. Expression analysis during the reproductive cycle showed that amh transcripts were down-regulated during the developing phase, which is characterized by an increased proliferation of type A undifferentiated spermatogonia and Sertoli cells and appearance of spermatocytes (meiosis) in the testes. Furthermore, ex vivo experiments showed that a 7 day exposure to Fsh or estrogens was required to decrease amh mRNA levels in common carp testicular explants. In summary, this study provided information on the molecular characterization and transcript abundance of amh in common carp adult testes. Altogether, these data will be useful for further investigations on sex determination and differentiation in this species, and also to improved strategies for improved carp aquaculture, such as inhibiting precocious maturation of males.

Molecular and cellular regulation on sex change in hermaphroditic fish, with a special focus on protandrous black porgy, Acanthopagrus schlegelii

In teleost fish, sex can be determined by genetic factors, environmental factors, or both. Unlike in gonochoristic fish, in which sex is fixed in adults, sex can change in adults of hermaphroditic fish species. Thus, sex is generated during the initial gonadal differentiation stage (primary sex differentiation) and later during sexual fate alternation (secondary sex differentiation) in hermaphroditic fish species. Depending on the species, sex phase alternation can be induced by endogenous cues (such as individual age and body size) or by social cues (such as sex ratio or relative body size within the population). In general, the fluctuation in plasma estradiol-17β (E2) levels is correlated with the sexual fate alternation in hermaphroditic fish. Hormonal treatments can artificially induce sexual phase alternation in sequential hermaphroditic fishes, but in a transient and reversible manner. This is the case for the E2-induced female phase in protandrous black porgy and the methyltestosterone (MT)- or aromatase inhibitor (AI)-induced male phase in protogynous grouper. Recent reviews have focused on the different forms of sex change in fish who undergo sequential sex change, especially in terms of gene expression and the role of hormones. In this review, we use the protandrous black porgy, a nonsocial cue-influenced hermaphroditic species, with digonic gonads (ovarian and testis separated by a connective tissue), as a model to describe our findings and discuss the molecular and cellular regulation of sexual fate determination in hermaphroditic fish.

Comparative transcriptomic analysis for identification of candidate sex-related genes and pathways in Crimson seabream (Parargyrops edita)

Teleost fishes display the largest array of sex-determining systems among animals, resulting in various reproductive strategies. Research on sex-related genes in teleosts will broaden our understanding of the process, and provide important insight into the plasticity of the sex determination process in vertebrates in general. Crimson seabream (Parargyrops edita Tanaka, 1916) is one of the most valuable and abundant fish resources throughout Asia. However, little genomic information on P. edita is available. In the present study, the transcriptomes of male and female P. edita were sequenced with RNA-seq technology. A total of 388,683,472 reads were generated from the libraries. After filtering and assembling, a total of 79,775 non redundant unigenes were obtained with an N50 of 2,921 bp. The unigenes were annotated with multiple public databases, including NT (53,556, 67.13%), NR (54,092, 67.81%), Swiss-Prot (45,265, 56.74%), KOG (41,274, 51.74%), KEGG (46,302, 58.04%), and GO (11,056, 13.86%) databases. Comparison of the unigenes of different sexes of P. edita revealed that 11,676 unigenes (9,335 in females, 2,341 in males) were differentially expressed between males and females. Of these, 5,463 were specifically expressed in females, and 1,134 were specifically expressed in males. In addition, the expression levels of ten unigenes were confirmed to validate the transcriptomic data by qRT-PCR. Moreover, 34,473 simple sequence repeats (SSRs) were identified in SSR-containing sequences, and 50 loci were randomly selected for primer development. Of these, 36 loci were successfully amplified, and 19 loci were polymorphic. Finally, our comparative analysis identified many sex-related genes (zps, amh, gsdf, sox4, cyp19a, etc.) and pathways (MAPK signaling pathway, p53 signaling pathway, etc.) of P. edita. This informative transcriptomic analysis provides valuable data to increase genomic resources of P. edita. The results will be useful for clarifying the molecular mechanism of sex determination and for future functional analyses of sex-associated genes.

Igf3 is essential for ovary differentiation in zebrafish†

Zebrafish gonadal sexual differentiation is an important but poorly understood subject. Previously, we have identified a novel insulin-like growth factor (Igf) named insulin-like growth factor 3 (Igf3) in teleosts. The importance of Igf3 in oocyte maturation and ovulation has been recently demonstrated by us in zebrafish. In this study, we have further found the essential role of Igf3 in gonadal sexual differentiation of zebrafish. A differential expression pattern of igf3 between ovary and testis during sex differentiation (higher level in ovary than in testis) was found in zebrafish. An igf3 knockout zebrafish line was established using TALENs-mediated gene knockout technique. Intriguingly, all igf3 homozygous mutants were males due to the female-to-male sex reversal occurred during sex differentiation. Further analysis showed that Igf3 did not seem to affect the formation of so-called juvenile ovary and oocyte-like germ cells. Oocyte development was arrested at primary growth stage, and the ovary was gradually sex-reversed to testis before 60 day post fertilization (dpf). Such sex reversal was likely due to decreased germ cell proliferation by suppressing PI3K/Akt pathway in early ovaries of igf3 mutants. Estrogen is considered as a master regulator in fish sex differentiation. Here, we found that igf3 expression could be upregulated by estrogen in early stages of ovarian follicles as evidenced in in vitro treatment assays and cyp19a1a mutant zebrafish, and E2 failed to rescue the defects of igf3 mutants in ovarian development, suggesting that Igf3 may serve as a downstream factor of estrogen signaling in sex differentiation. Taken together, we demonstrated that Igf3 is essential for ovary differentiation in zebrafish.

Genetic evidence for Amh modulation of gonadotropin actions to control gonadal homeostasis and gametogenesis in zebrafish and its noncanonical signaling through Bmpr2a receptor

Anti-Müllerian hormone (Amh) plays an important role in gonadal function. Amh deficiency causes severe gonadal dysgenesis and dysfunction in zebrafish, with gonadal hypertrophy in both sexes. However, its mechanism of action remains unknown. Intriguingly, the Amh cognate type II receptor (Amhr2) is missing in the zebrafish genome, in sharp contrast to other species. Using a series of zebrafish mutants (amh, fshb, fshr and lhcgr), we provided unequivocal evidence for actions of Amh, via modulation of gonadotropin signaling, on both germ cell proliferation and differentiation. The gonadal hypertrophy in amh mutants was abolished in the absence of Fshr in females or Fshr/Lhcgr in males. Furthermore, we demonstrated that knockout of bmpr2a, but not bmpr2b, phenocopied all phenotypes of the amh mutant in both sexes, including gonadal hypertrophy, hyperproliferation of germ cells, retarded gametogenesis and reduced fshb expression. In summary, the present study provided comprehensive genetic evidence for an intimate interaction of gonadotropin and Amh pathways in gonadal homeostasis and gametogenesis and for Bmpr2a as the possible missing link for Amh signaling in zebrafish.

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.

Anti-Müllerian hormone (Amh/amh) plays dual roles in maintaining gonadal homeostasis and gametogenesis in zebrafish

Anti-Müllerian hormone (AMH/Amh) plays a role in gonadal differentiation and function across vertebrates. In zebrafish we demonstrated that Amh deficiency caused severe gonadal dysgenesis and dysfunction. The mutant gonads showed extreme hypertrophy with accumulation of early germ cells in both sexes, namely spermatogonia in the testis and primary growth oocytes in the ovary. In amh mutant females, the folliculogenesis was normal in young fish but receded progressively in adults, which was accompanied by progressive decrease in follicle-stimulating hormone (fshb) expression. Interestingly the expression of fshb increased in the pituitary of juvenile amh mutant males but decreased in adults. The upregulation of fshb in mutant male juveniles was likely one of the mechanisms for triggering gonadal hypergrowth, whereas the downregulation of fshb in adults might involve a negative feedback by gonadal inhibin. Further analysis using mutants of fshb and growth differentiation factor 9 (gdf9) provided evidence for a role of FSH in triggering ovarian hypertrophy in young female amh mutant as well. In summary, the present study provided comprehensive genetic evidence for dual roles of Amh in controlling zebrafish gonadal homeostasis and gametogenesis in both sexes. Amh suppresses proliferation or accumulation of early germ cells (spermatogonia in testis and primary growth oocytes in ovary) while promoting their exit to advanced stages, and its action may involve both endocrine and paracrine pathways.

Cis-regulatory differences in isoform expression associate with life history strategy variation in Atlantic salmon

A major goal in biology is to understand how evolution shapes variation in individual life histories. Genome-wide association studies have been successful in uncovering genome regions linked with traits underlying life history variation in a range of species. However, lack of functional studies of the discovered genotype-phenotype associations severely restrains our understanding how alternative life history traits evolved and are mediated at the molecular level. Here, we report a cis-regulatory mechanism whereby expression of alternative isoforms of the transcription co-factor vestigial-like 3 (vgll3) associate with variation in a key life history trait, age at maturity, in Atlantic salmon (Salmo salar). Using a common-garden experiment, we first show that vgll3 genotype associates with puberty timing in one-year-old salmon males. By way of temporal sampling of vgll3 expression in ten tissues across the first year of salmon development, we identify a pubertal transition in vgll3 expression where maturation coincided with a 66% reduction in testicular vgll3 expression. The late maturation allele was not only associated with a tendency to delay puberty, but also with expression of a rare transcript isoform of vgll3 pre-puberty. By comparing absolute vgll3 mRNA copies in heterozygotes we show that the expression difference between the early and late maturity alleles is largely cis-regulatory. We propose a model whereby expression of a rare isoform from the late allele shifts the liability of its carriers towards delaying puberty. These results exemplify the potential importance of regulatory differences as a mechanism for the evolution of life history traits.

Alternative life history strategies are an important source of diversity within populations and promote the maintenance of adaptive capacity and population resilience. However, in many cases the molecular basis of different life history strategies remains elusive. Age at maturity is a key adaptive life history trait in Atlantic salmon and has a relatively simple genetic basis. Using salmon age at maturity as a model, we report a mechanism whereby different transcript isoforms of the key age at maturity gene, vestigial-like 3 (vgll3), associate with variation in the timing of male puberty. Our results show how gene regulatory differences in conjunction with variation in gene transcript structure can encode for complex alternative life histories.

Structure and Sequence of the Sex Determining Locus in Two Wild Populations of Nile Tilapia

In domesticated strains of the Nile tilapia, phenotypic sex has been linked to genetic variants on linkage groups 1, 20 and 23. This diversity of sex-loci might reflect a naturally polymorphic sex determination system in Nile tilapia, or it might be an artefact arising from the process of domestication. Here, we searched for sex-determiners in wild populations from Kpandu, Lake Volta (Ghana-West Africa), and from Lake Koka (Ethiopia-East Africa) that have not been subjected to any genetic manipulation. We analysed lab-reared families using double-digest Restriction Associated DNA sequencing (ddRAD) and analysed wild-caught males and females with pooled whole-genome sequencing (WGS). Strong sex-linked signals were found on LG23 in both populations, and sex-linked signals with LG3 were observed in Kpandu samples. WGS uncovered blocks of high sequence coverage, suggesting the presence of B chromosomes. We confirmed the existence of a tandem amh duplication in LG23 in both populations and determined its breakpoints between the oaz1 and dot1l genes. We found two common deletions of ~5 kb in males and confirmed the presence of both amhY and amh∆Y genes. Males from Lake Koka lack both the previously reported 234 bp deletion and the 5 bp frameshift-insertion that creates a premature stop codon in amh∆Y.

Molecular and morphological sex differentiation in sablefish (Anoplopoma fimbria), a marine teleost with XX/XY sex determination

Phenotypic sex of an organism is determined by molecular changes in the gonads, so-called molecular sex differentiation, which should precede the rise of cellular or anatomical sex-distinguishing features. This study characterized molecular and morphological sex differentiation in sablefish (Anoplopoma fimbria), a marine teleost with established XX/XY genotypic sex determination. Next generation sequencing was conducted on sablefish ovarian and testicular mRNAs to obtain sequences for transcripts associated with vertebrate sex determination and differentiation and early reproductive development. Gene-specific PCRs were developed to determine the distribution and ontogenetic gonadal expression of transcription, growth, steroidogenic and germline factors, as well as gonadotropin and steroid receptors. Molecular changes associated with sex differentiation were first apparent in both XY- and XX-genotype sablefish at ~ 60 mm in body length and prior to histological signs of sex differentiation. The earliest and most robust markers of testicular differentiation were gsdf, amh, dmrt1, cyp11b, star, sox9a, and fshr. Markedly elevated mRNA levels of several steroidogenesis-related genes and ar2 in differentiating testes suggested that androgens play a role in sablefish testicular differentiation. The earliest markers of ovarian differentiation were cyp19a1a, lhcgr, foxl2, nr0b1, and igf3. Other transcripts such as figla, zp3, and pou5f3 were expressed predominantly in XX-genotype fish and significantly increased with the first appearance and subsequent development of primary oocytes. This study provides valuable insight to the developmental sequence of events associated with gonadal sex differentiation in marine teleosts with XX/XY sex determination. It also implicates particular genes in processes of male and female development and establishes robust molecular markers for phenotypic sex in sablefish, useful for ongoing work related to sex control and reproductive sterilization.