The brain–pituitary–gonad axis in male teleosts, with special emphasis on flatfish (Pleuronectiformes)

Effects of glycerol monolaurate on estradiol and follicle-stimulating hormones, offspring quality, and mRNA expression of reproductive-related genes of zebrafish (Danio rerio) females

This study aims to explore whether glycerol monolaurate (GML) can improve reproductive performance of female zebrafish (Danio rerio) and the survival percentage of their offspring. Three kinds of isonitrogenous and isolipid diets, including basal diet (control) and basal diet containing 0.75 g/kg GML (L_GML) and 1.5 g/kg GML (H_GML), were prepared for 4 weeks feeding trial. The results show that GML increased the GSI of female zebrafish. GML also enhanced reproductive performance of female zebrafish. Specifically, GML increased spawning number and hatching rate of female zebrafish. Moreover, GML significantly increased the levels of triglycerides (TG), lauric acid, and estradiol (E2) in the ovary (P < 0.05). Follicle-stimulating hormone (FSH) levels in the ovary and brain also significantly increased in the L_GML group (P < 0.05). Besides, dietary GML regulated the hypothalamus-pituitary-gonad (HPG) axis evidenced by the changed expression levels of HPG axis-related genes in the brain and ovary of the L_GML and H_GML groups compared with the control group. Furthermore, compared with the control group, the expression levels of HPG axis-related genes (kiss2, kiss1r, kiss2r, gnrh3, gnrhr1, gnrhr3, lhβ, and esr2b) in the brain of the L_GML group were significantly increased (P < 0.05), and the expression levels of HPG axis-related genes (kiss1, kiss2, kiss2r, gnrh2, gnrh3, gnrhr4, fshβ, lhβ, esr1, esr2a, and esr2b) in the brain of the H_GML group were significantly increased (P < 0.05). These results suggest that GML may stimulate the expression of gnrh2 and gnrh3 by increasing the expression level of kiss1 and kiss2 genes in the hypothalamus, thus promoting the synthesis of FSH and E2. The expression levels of genes associated with gonadotropin receptors (fshr and lhr) and gonadal steroid hormone synthesis (cyp11a1, cyp17, and cyp19a) in the ovary were also significantly upregulated by dietary GML (P < 0.05). The increasing expression level of cyp19a also may promote the FSH synthesis. Particularly, GML enhanced the richness and diversity and regulated the species composition of intestinal microbiota in female zebrafish. Changes in certain intestinal microorganisms may be related to the expression of certain genes involved in the HPG axis. In addition, L_GML and H_GML both significantly decreased larvae mortality at 96 h post fertilization and their mortality during the first-feeding period (P < 0.05), revealing the enhanced the starvation tolerance of zebrafish larvae. In summary, dietary GML regulated genes related to HPG axis to promote the synthesis of E2 and FSH and altered gut microbiota in female zebrafish, and improved the survival percentage of their offspring.

Decoding the effect of photoperiodic cues in transducing kisspeptin-melatonin circuit during the pubertal onset in common carp

This study unravels the intricate interplay between photoperiod, melatonin, and kisspeptin to orchestrate the pubertal onset of Common carp. Female fingerlings exposed to long days (LD) exhibited a hormonal crescendo, with upregulated hypothalamic-pituitary-ovarian (HPO) axis genes (kiss1, kiss1r, kiss2, gnrh2, gnrh3) and their downstream targets (lhr, fshr, ar1, esr1). However, the expression of the melatonin receptor (mtnr1a) diminished in LD, suggesting a potential inhibitory role. This hormonal symphony was further amplified by increased activity of key transcriptional regulators (gata1, gata2, cdx1, sp1, n-myc, hoxc8, plc, tac3, tacr3) and decreased expression of delayed puberty genes (mkrn1, dlk1). In contrast, short days (SD) muted this hormonal chorus, with decreased gnrh gene and regulator expression, elevated mtnr1a, and suppressed gonadal development. In in-vitro, estradiol mimicked the LD effect, boosting gnrh and regulator genes while dampening mtnr1a and melatonin-responsive genes. Conversely, melatonin acted as a conductor, downregulating gnrh and regulator genes and amplifying mtnr1a. Our findings illuminate the crucial roles of melatonin and kisspeptin as opposing forces in regulating pubertal timing. LD-induced melatonin suppression allows the kisspeptin symphony to flourish, triggering GnRH release and, ultimately, gonadal maturation. This delicate dance between photoperiod, melatonin, and kisspeptin orchestrates common carp's transition from juvenile to reproductive life.

Artificial light at night disrupts male dominance relationships and reproductive success in a model fish species

Environmental light is perceived and anticipated by organisms to synchronize their biological cycles. Therefore, artificial light at night (ALAN) disrupts both diurnal and seasonal biological rhythms. Reproduction is a complex physiological process involving integration of environmental signals by the brain, and release of endocrine signals by the pituitary that regulate gametogenesis and spawning. In addition, males from many species form a dominance hierarchy that, through a combination of aggressive and protective behavior, influences their reproductive success. In this study, we investigated the effect of ALAN and continuous daylight on the behavior and fitness of male fish within a dominance hierarchy using a model fish, the Japanese medaka. In normal light/dark cycles, male medaka establish a hierarchy with the dominant males being more aggressive and remaining closer to the female thus limiting the access of subordinate males to females during spawning. However, determination of the paternity of the progeny revealed that even though subordinate males spend less time with the females, they are, in normal light conditions, equally successful at producing progeny due to an efficient sneaking behavior. Continuous daylight completely inhibited the establishment of male hierarchy, whereas ALAN did not affect it. Nonetheless, when exposed to ALAN, subordinate males fertilize far fewer eggs. Furthermore, we found that when exposed to ALAN, subordinate males produced lower quality sperm than dominant males. Surprisingly, we found no differences in circulating sex steroid levels, pituitary gonadotropin levels, or gonadosomatic index between dominant and subordinate males, neither in control nor ALAN condition. This study is the first to report an effect of ALAN on sperm quality leading to a modification of male fertilization success in any vertebrate. While this work was performed in a model fish species, our results suggest that in urban areas ALAN may impact the genetic diversity of species displaying dominance behavior.

Exploring the Effects of Rearing Densities on Epigenetic Modifications in the Zebrafish Gonads

Rearing density directly impacts fish welfare, which, in turn, affects productivity in aquaculture. Previous studies have indicated that high-density rearing during sexual development in fish can induce stress, resulting in a tendency towards male-biased sex ratios in the populations. In recent years, research has defined the relevance of the interactions between the environment and epigenetics playing a key role in the final phenotype. However, the underlying epigenetic mechanisms of individuals exposed to confinement remain elucidated. By using zebrafish (Danio rerio), the DNA methylation promotor region and the gene expression patterns of six genes, namely dnmt1, cyp19a1a, dmrt1, cyp11c1, hsd17b1, and hsd11b2, involved in the DNA maintenance methylation, reproduction, and stress were assessed. Zebrafish larvae were subjected to two high-density conditions (9 and 66 fish/L) during two periods of overlapping sex differentiation of this species (7 to 18 and 18 to 45 days post-fertilization, dpf). Results showed a significant masculinization in the populations of fish subjected to high densities from 18 to 45 dpf. In adulthood, the dnmt1 gene was differentially hypomethylated in ovaries and its expression was significantly downregulated in the testes of fish exposed to high-density. Further, the cyp19a1a gene showed downregulation of gene expression in the ovaries of fish subjected to elevated density, as previously observed in other studies. We proposed dnmt1 as a potential testicular epimarker and the expression of ovarian cyp19a1a as a potential biomarker for predicting stress originated from high densities during the early stages of development. These findings highlight the importance of rearing densities by long-lasting effects in adulthood conveying cautions for stocking protocols in fish hatcheries.

Effects of mifepristone, a model compound with anti-progestogenic activity, on the development of African clawed frog (Xenopus laevis)

The objective of this study was to assess the effects of a model substance with anti-progestogenic activity on development of African clawed frog (Xenopus laevis) from tadpole to juvenile stage. Mifepristone, a synthetic progesterone receptor-blocking steroid hormone used in medicine as an abortifacient, was chosen as a model compound with anti-progestogenic activity. In the experiment, African clawed frog tadpoles were exposed to mifepristone at three concentrations (2, 21, and 215 ng L-1). A control group was exposed to dimethyl sulfoxide (DMSO; 0.001 %). The experiment started when tadpoles reached stages 47-48 according to Nieuwkoop and Faber (NF; 1994) and continued until stage NF 66, when metamorphosis was complete. Exposure to mifepristone had no significant effect on the rate of tadpole development, occurrence of morphological anomalies, weight, body length, or sex ratio. Mortality was within an acceptable range of 0-3.6 % throughout the test and did not differ among the groups. Histopathological examination of the gonads and thyroid gland revealed no significant changes. Therefore, we can conclude that mifepristone had no negative effect on development of the African clawed frog up to juvenile stage. Nevertheless, at the highest tested mifepristone concentration (215 ng L-1), gene expression analysis revealed up-regulation of mRNA expression of nuclear progesterone receptor (npr), membrane progesterone receptor (mpr), estrogen receptor beta (esrβ), and luteinizing hormone (lh) in the brain-pituitary complex of exposed frogs at stage NF 66. Higher mRNA expression of npr was also found in frogs exposed to 22 ng L-1 mifepristone compared to the solvent control. These findings confirmed the anti-progestogenic activity of mifepristone in frogs because the up-regulation of progesterone receptors occurs if progesterone availability in the body is reduced. All the observed changes in combination may have negative consequences for reproduction and reproductive behavior later in life.

Effects of copper oxide nanoparticles on reproductive system of zebrafish

Copper oxide nanoparticles (CuO NPs) were regarded as the versatile materials in daily life and the in-depth evaluation of their biological effects is of great concern. Herein the female and male zebrafishes were chosen as the model animals to analyze the reproductive toxicity caused by CuO NPs at low concentration (10, 50 and 100 μg/L) After 20-days exposure, the structure of zebrafish ovary and testis were impaired. Moreover, the contents of 17β-estradiol (E2) in both females and males were increased, while the contents of testosterone (T) were decreased, indicating the imbalanced sex hormones caused by CuO NPs. The expression of genes along the hypothalamic pituitary-gonad (HPG) axis, were examined with quantitative real-time PCR to further evaluate the toxic mechanisms. Meanwhile, the levels of erα/er2β and cyp19a in female zebrafishes and erα/er2β, lhr, hmgra/hmgrb, 3βhsd and 17βhsd in male zebrafishes were obviously up-regulated. While, the level of αr was obviously down-regulated in female and male zebrafishes. Thus, the obtained data uncovered that long-term exposure of CuO NPs with low dose could trigger the endocrine disorder, resulting in the disturbance of E2 and T level, inhibition of gonad development, and alteration of HPG axis genes. In brief, this study enriched the toxicological data of NPs on aquatic vertebrates and provided the theoretical support for assessing the environmental safety of NPs.

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.

Protein-protein networks analysis of differentially expressed genes unveils the key phenomenon of biological process with respect to reproduction in endangered catfish, C. Magur

Clarias magur (magur) is an important freshwater catfish with high potential in the aquaculture sector in its geographical ranges of distribution. One of the impediments to realise its full aquaculture potential is the lack of understanding key genes involved in its reproduction pathways. Nonetheless, very limited information is available on brain and gonads, with respect to reproduction related issues of magur at molecular level. The present study was aimed at understanding the interaction of the brain-gonad system by analysing differentially expressed genes (DEG) in brains and gonads of male and female magur using a protein-protein network interaction study. In brief, 641, 541, 225 and 245 DEGs, respectively, in ovary, testis and female brain and male-brain of magur were used as input in String database 11.0 and Cytoscape v 3.8.0 plug-in Network Analyzer for PPI network construction followed by network superimposition, network merging and analysis. A total of 13 key genes in female brain & ovary and 12 key genes in male brain & testis were obtained based on the network topological parameter betweenness centrality and nodes degree. Among them, cyp19a1b and amh genes in male brain-testis and Tp53 and exo1 genes in female brain-ovary were identified as hub genes having a high level of interaction and expression with other key genes in the network. Further, functional annotation study of these genes revealed their active involvement in important pathways related to reproduction. This is the first report exploring the interaction of brain and gonads in the regulation of magur reproduction through a protein-protein interaction network. The 25 key genes identified in the combined network are involved in various pathways, like neuropeptide signalling pathway, oxytocin receptor-mediated signalling pathway, corticotrophin-releasing factor receptor signalling pathway and reproduction process, which could lead to a better understanding of the magur reproductive system.

Effects of weathered polyethylene microplastic ingestion on sexual maturation, fecundity and egg quality in maturing broodstock Atlantic cod Gadus morhua

Microplastics (MPs) have become a global issue as they are omnipresent in the ocean. Fish ingesting MPs through feed could be affected in their physiological function, e.g., disrupted enzyme production and function, reduction of feeding and reproductive failure. This study assessed the effects of feed containing naturally weathered MPs from the Oslofjord (Norway) on the reproductive physiology of Atlantic cod (Gadus morhua). Farmed cod broodstock were fed either control (C-diet) or feeds containing 1% microplastic (MP-diet) starting nine months prior to spawning, from June until May. No major differences were found between diet groups in overall biometrics or gonad histology. Sex steroid levels (testosterone, 11-ketotestosterone and 17β-estradiol) resulted in expected profiles increasing over time without any significant differences between treatments. Gene expression levels of the steroidogenic enzyme 20β-hydroxysteroid dehydrogenase (20β-hsd) and vitellogenin1 (vtg1) showed significant differences between dietary treatments with lower expression in the control group. This can be a direct effect of MPs, but endocrine disrupting effects of potentially leachable plastic additives cannot be completely ruled out. Thus, these enzymes could be indicators of exposure to contaminants that disrupt sexual maturation by affecting the production of primarily maturation-inducing steroid. Although the concentration of MPs employed in this study may not be high enough to elicit any observable short-term biological effects, the observed gene expression suggests that long-term consequences should be considered caused by an expected increase of MPs in marine environments.

An RNA-seq time series of the medaka pituitary gland during sexual maturation

Directing both organismal homeostasis and physiological adaptation, the pituitary is a key endocrine gland in all vertebrates. One of its major tasks is to coordinate sexual maturation through the production and release of hormones stimulating gonad development. In order to study its developmental dynamics in the model fish medaka (Oryzias latipes), we sampled both the pituitary and the ovaries of 68 female fish. Of these, 55 spanned the entire course of sexual maturation from prepubertal juveniles to spawning adults. An additional 13 showed either considerably faster or slower growth and development than the majority of fish. We used histological examination of the ovaries to determine a histological maturation stage, and analyzed the pituitary glands using RNA-seq optimized for low input. Taken together, these data reveal the timing of hormone production priorities, and form a comprehensive resource for the study of their regulation.

Use of male-to-female sex reversal as a welfare scoring system in the protandrous farmed gilthead sea bream (Sparus aurata)

Gilthead sea bream is a highly cultured marine fish throughout the Mediterranean area, but new and strict criteria of welfare are needed to assure that the intensification of production has no negative effects on animal farming. Most welfare indicators are specific to a given phase of the production cycle, but others such as the timing of puberty and/or sex reversal are of retrospective value. This is of particular relevance in the protandrous gilthead sea bream, in which the sex ratio is highly regulated at the nutritional level. Social and environmental factors (e.g., contaminant loads) also alter the sex ratio, but the contribution of the genetic component remains unclear. To assess this complex issue, five gilthead sea bream families representative of slow/intermediate/fast growth were grown out with control or a plant-based diet in a common garden system from early life to the completion of their sexual maturity in 3-year-old fish. The plant-based diet highly enhanced the male-to-female sex reversal. This occurred in parallel with the progressive impairment of growth performance, which was indicative of changes in nutrient requirements as the result of the different energy demands for growth and reproduction through development. The effect of a different nutritional and genetic background on the reproductive performance was also assessed by measurements of circulating levels of sex steroids during the two consecutive spawning seasons, varying plasma levels of 17β-estradiol (E₂) and 11-ketotestosterone (11-KT) with age, gender, diet, and genetic background. Principal component analysis (PCA) of 3-year-old fish displayed a gradual increase of the E₂/11-KT ratio from males to females with the improvement of nutritional/genetic background. Altogether, these results support the use of a reproductive tract scoring system for leading farmed fish toward their optimum welfare condition, contributing to improving the productivity of the current gilthead sea bream livestock.

Enhancing Reproductive Performance of Freshwater Finfish Species through Dietary Lipids

Dietary lipid manipulation in the feed of commercially cultured finfish is used not only to improve production and culture but also to enhance their reproductive performances. The inclusion of lipid in broodstock diet positively affects growth, immunological responses, gonadogenesis, and larval survival. In this review, existing literature on the importance of freshwater finfish species to aquaculture and the inclusion of dietary lipid compounds in freshwater fish feed to accelerate the reproduction rate is being summarized and discussed. Although lipid compounds have been confirmed to improve reproductive performance, only a few members of the most economically important species have reaped benefits from quantitative and qualitative lipid studies. There is a knowledge gap on the effective inclusion and utilization of dietary lipids on gonad maturation, fecundity, fertilization, egg morphology, hatching rate, and consequently, larval quality contributing to the survival and good performance of freshwater fish culture. This review provides a baseline for potential future research for optimizing dietary lipid inclusion in freshwater broodstock diets.

Pituitary multi-hormone cells in mammals and fish: history, origin, and roles

The vertebrate pituitary is a dynamic organ, capable of adapting its hormone secretion to different physiological demands. In this context, endocrinologists have debated for the past 40 years if endocrine cells are mono- or multi-hormonal. Since its establishment, the dominant "one cell, one hormone" model has been continuously challenged. In mammals, the use of advanced multi-staining approaches, sensitive gene expression techniques, and the analysis of tumor tissues have helped to quickly demonstrate the existence of pituitary multi-hormone cells. In fishes however, only recent advances in imaging and transcriptomics have enabled the identification of such cells. In this review, we first describe the history of the discovery of cells producing multiple hormones in mammals and fishes. We discuss the technical limitations that have led to uncertainties and debates. Then, we present the current knowledge and hypotheses regarding their origin and biological role, which provides a comprehensive review of pituitary plasticity.

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.

Life Cycle Exposure to Cyhalofop-Butyl Induced Reproductive Toxicity in Zebrafish

Cyhalofop-butyl (CyB) is a herbicide widely used in paddy fields that may transfer to aquatic ecosystems and cause harm to aquatic organisms. In this study, zebrafish (Danio rerio) were exposed to CyB at environmental concentrations (0.1, 1 and 10 µg/L) throughout their adult life cycle, from embryo to sexual maturity. The effects of CyB on zebrafish growth and reproduction were studied. It was found that female spawning was inhibited, and adult male fertility decreased. In addition, we examined the expression of sex steroid hormones and genes related to the hypothalamus-pituitary-gonad-liver (HPGL) axis. After 150 days of exposure, the hormone balance in zebrafish was disturbed, and the concentrations of 17β-estradiol (E2) and vitellogenin (VTG) were decreased. Changes in sex hormone were regulated by the expression of genes related to the HPGL axis. These results confirmed that long-term exposure to CyB at environmental concentrations can damage the reproductive capacity of zebrafish by disrupting the transcription of genes related to the HPGL axis. Overall, these data may provide a new understanding of the reproductive toxicity of long-term exposure to CyB in zebrafish parents and offspring.

Cloning of gonadotropin Gph-alpha, FSH-beta and LH-beta subunits and seasonal profiles of steroid hormones in wild-caught Nile perch, Lates niloticus

The Nile perch (np; Lates niloticus) is a freshwater teleost species with a potential for aquaculture in freshwater surroundings. However, wild-caught breeders have persistently failed to spawn spontaneously in captivity. Cloning of the gonadotropin subunits and analysing seasonal variation in reproductive hormone levels for a 1-year period were done to gain knowledge on the physiological basis underlying the reproductive biology of np. The β-follicle-stimulating hormone (FSH-β) and β-luteinizing hormone (LH-β) subunits and their common α-glycoprotein (Gph-α) subunit were cloned using 3' and 5' RACE-PCR. The nucleotide sequences of the npgph-α, npfsh-β, and nplh-β subunits were 664, 580 and 675 nucleotides in length, encoding peptides of 124, 120 and 148 amino acids, respectively. The deduced amino acid sequence of each mature subunit showed high similarity with its counterparts in other teleost. Sequence analysis showed that npFSH-β is more similar to higher vertebrate FSH-βs than to higher vertebrate LH-βs. Heterologous immunoassay was calibrated to analyse pituitary LH levels. While the LH immunoassay showed parallelism of npLH with that of tilapia (ta), no parallelism for FSH was found. Levels of pituitary LH were higher in females at gonadal stages of vitellogenic oocytes, mature secondary oocytes and mature tertiary oocytes with migrating nucleus than in pre-vitellogenic oocytes and early and late perinucleolus oocytes. Using competitive steroid ELISA, variations in the levels of the steroid hormones 11-ketotestosterone (11-KT) in males and E2 in females were characterized in relation to month and reproductive index of Nile perch. Our findings show that in females, gonadosomatic index and plasma E2 were highly correlated (R² = 0.699, n = 172) and peaked from September to November while in males, the gonadosomatic index and plasma 11-KT peaked from October to November. In female fish, both steroid hormones were detected in the plasma but greatly varied in concentrations. E2 in particular, increased with the developmental stage of the gonads. The levels of steroid hormones, E2 and 11-KT in females and males respectively increased with fish size (total lengths) and suggest that females mature at a body length of 40-59 cm than their counter part males that mature at a total length of 60-70 cm. Taken together, we describe seasonal endocrine differences in wild-caught adult Nile perch which could potentially be exploited to manipulate the reproductive axis in cultured breeders.

Evolutionary dynamics and conserved function of the Tudor domain-containing (TDRD) proteins in teleost fish

Tudor domain-containing (TDRD) proteins, the germline enriched protein family, play essential roles in the process of gametogenesis and genome stability through their interaction with the PIWI-interacting RNA (piRNA) pathway. Several studies have suggested the rapid evolution of the piRNA pathway in teleost lineages with striking reproductive diversity. However, there is still limited information about the function and evolution of Tdrd genes in teleost species. In this study, through genome wide screening, 13 Tdrd family genes were identified in economically important aquaculture fish, including spotted sea bass (Lateolabrax maculatus), Asian sea bass (Lates calcarifer), and tongue sole (Cynoglossus semilaevis). With copy number, structure, phylogeny, and synteny analysis, duplication of Tdrd6 and Tdrd7, as well as loss of Stk31 and Tdrd10, were characterized in teleost lineages. Codon based molecular evolution analysis indicated faster evolution of teleost Tdrd genes than that in mammals, potentially associated with the accelerated evolution of the piRNA pathway in teleost lineages. The evolutionary diversity of Tdrd genes was also detected between different teleost lineages. RNA-seq analysis showed that most teleost Tdrd genes were dominantly expressed in gonads, particularly highly expressed in testis, such as Tdrd6, Tdrd7a, Tdrd9, Ecat8, and Tdrd15. The varied expression and evolutionary pattern between the duplicated Tdrd6 and Tdrd7 in teleosts may indicate their functional diversification. All these results suggest a conserved function of teleost Tdrd family in gametogenesis and the piRNA pathway, which could lay a foundation for the evolution of Tdrd genes and be helpful for further deciphering of Tdrd functions in teleosts.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-021-00118-7.

Expression Profile Analysis to Identify Circular RNA Expression Signatures in the Prolificacy Trait of Yunshang Black Goat Pituitary in the Estrus Cycle

The pituitary gland is an important organ. It is a complex area of the brain involved in endocrine function and reproductive regulation. However, the function of the pituitary in goat reproduction is still unclear. Herein, RNA sequencing was used to explore the expression patterns of circle RNAs (circRNAs) in the pituitary of Yunshang black goats during the various estrus phases. Then the host genes of the circRNAs were predicted, and a competing endogenous RNA (ceRNA) network was constructed. The results showed a total of 6,705 circRNAs in the pituitary of Yunshang black goats, among which 388 differentially expressed (DE) circRNAs (214 were upregulated, while 174 were downregulated) were identified between high- and low-yield Yunshang black goats in the follicular phase (HF vs. LF); moreover, 361 DE circRNAs (136 were upregulated, while 225 were downregulated) were identified between high- and low-yield Yunshang black goats in the luteal phase (HL vs. LL). There were 65 DE circRNAs targeting 40 miRNAs in the HF vs. LF comparison and 46 DE circRNAs targeting 31 miRNAs in the HL vs. LL comparison. We identified chi_circ_0030920, chi_circ_0043017, chi_circ_0008353, chi_circ_0041580, and chi_circ_0016478 as the key circRNAs through functional enrichment analysis. The ceRNA network analysis showed that chi_circ_0031209 and chi_circ_0019448 might play an important role in reproduction by influencing the expression of prolactin receptor (PRLR) in high- and low-yielding goats during the luteal phase, whereas chi_circ_0014542 regulates the expression of WNT5A during the follicular phase. Our study provided the overall expression profiles of circRNAs in the goat pituitary during the estrus phase, which provides new insight into the mechanism of high-yield goats, which can be helpful to guide goat breeding.

TAC3/TACR3 System Function in the Catadromous Migration Teleost, Anguilla japonica

Neurokinin B (NKB), a member of the tachykinin (TAC) family, plays important roles in mammalian neuropeptide secretion in related to reproduction. However, its potential role in spawning migration teleost is less clear. In the present study, Japanese eel (Anguilla japonica) was employed to study the performance of NKB in regulating reproduction. Results showed that two tac3 and one tacr3 genes were identified in Japanese eel. Sequence analysis showed that two tac3 transcripts, tac3a and tac3b, encode four NKBs: NKBa-13, NKBa-10, NKBb-13, and NKBb-10. However, compared with other species, a mutation caused early termination of TACR3 protein was confirmed, leading to the loss of the 35 amino acid (aa) C-terminal of the receptor. Expression analysis in different tissues showed that both tac3a and tac3b mRNAs were highly expressed in the brain. In situ hybridization localized both tac3a and tac3b mRNAs to several brain regions, mainly in the telencephalon and hypothalamus. Because of the mutation in TACR3 of Japanese eel, we further analyzed whether it could activate the downstream signaling pathway. Luciferase assay results showed the negative regulation of cAMP Response Element (CRE) and Sterol Response Element (SRE) signal pathways by Japanese eel NKBs. Intraperitoneal injection of four different NKB mature peptides at 100 ng/g had negative effect on either gnrh or gth gene expression. However, the high concentration of NKBa-10 and NKBb-13 (1,000 ng/g) upregulated mgnrh and fshb or lhb expression level significantly, which may be mediated by other receptors. In general, the NKBs/NK3Rs system has important functions in regulating eel puberty onset.

Transcriptomic analysis of pituitary in female and male spotted scat (Scatophagus argus) after 17β-estradiol injection

Spotted scat (Scatophagus argus) is a popular species of marine fish cultured in China. It shows normal sexual growth dimorphism. Female spotted scat grows quicker and bigger than males. Growth and reproduction are the most important traits in aquaculture. In vertebrates, the pituitary gland occupies an important position in the growth and reproduction axis. Estrogen is involved in regulating growth and reproduction in the pituitary gland in an endocrine fashion. Transcriptome sequencing of the pituitary was performed in female and male fish at 6 h after 17β-estradiol injection (4.0 μg E₂/g body weight, BW). Compared with the pituitary of female and male groups, 144 and 64 genes [|log₂^{(fold change)}| ≥ 1.0 and false discovery rate (FDR) < 0.05] were significantly differentially expressed in E₂-injected females and males, respectively (p < 0.05). Of these, 59 and 48 were up-regulated, and 85 and 16 were down-regulated. According to the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) pathway analyses, DEGs were involved in signal pathways, such as growth, reproduction, oocyte meiosis and steroid biosynthesis. Of these, estrogen affected the expression of some sex steroid synthesis and receptor genes in the pituitary gland through feedback, such as hsd17b7, pgr and cyp19a1b, regulating the reproductive activities. Besides, some growth-related genes, such as gap43, junbb, mstn2 and insm1a responded to estrogen. E₂ might affect the expression level of gh mRNA by regulating the expression levels of growth-related genes. Our results provide a theoretical basis for studying the molecular mechanism of growth and reproduction regulation at the pituitary level of spotted scat responded to E₂.

Characterization of hormone-producing cell types in the teleost pituitary gland using single-cell RNA-seq

The pituitary is the vertebrate endocrine gland responsible for the production and secretion of several essential peptide hormones. These, in turn, control many aspects of an animal’s physiology and development, including growth, reproduction, homeostasis, metabolism, and stress responses. In teleost fish, each hormone is presumably produced by a specific cell type. However, key details on the regulation of, and communication between these cell types remain to be resolved. We have therefore used single-cell sequencing to generate gene expression profiles for 2592 and 3804 individual cells from the pituitaries of female and male adult medaka (Oryzias latipes), respectively. Based on expression profile clustering, we define 15 and 16 distinct cell types in the female and male pituitary, respectively, of which ten are involved in the production of a single peptide hormone. Collectively, our data provide a high-quality reference for studies on pituitary biology and the regulation of hormone production, both in fish and in vertebrates in general.

Measurement(s)	RNA-seq gene expression profiling assay
Technology Type(s)	tag based single cell RNA sequencing
Factor Type(s)	sex
Sample Characteristic - Organism	Oryzias latipes
Sample Characteristic - Environment	fresh water aquarium

Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.16592621

Biomarker responses induced by bisphenol A on spermatogenesis in a Neotropical teleost fish are temperature-dependent

Bisphenol A (BPA) is an organic synthetic compound used in the plastic industry with endocrine disrupting activity. Although it is frequently found in surface waters, few studies have investigated its impact on fish gametogenesis, particularly when associated with natural stressors. In this regard, the present study evaluated BPA toxicity on spermatogenesis in the lambari Astyanax bimaculatus under controlled conditions and its interactive effects with water temperature. Adult specimens were exposed in duplicate to 40 μg/L and 400 μg/L BPA at 23 °C and 28 °C for 21 days; the control group did not receive BPA. Testicular samples were collected and analyzed using different cellular and molecular techniques. The results showed a significant reduction in the gonadosomatic index in the BPA-treated groups at both temperatures. A decrease in the testicular levels of 11-ketotestosterone was observed in the 400 μg/L BPA group at 23 °C, 17β-estradiol increased significantly in the treated groups at 28 °C, and vitellogenin showed no difference between the treatments. The morphometric analysis of spermatogenesis revealed a significant increase in the proportion of spermatogonia, spermatocytes, and Sertoli cells in the treated groups, with a higher proportion at 23 °C than at 28 °C. Otherwise, the proportion of spermatozoa was significantly lower in the BPA-treated groups, with a greater reduction at 23 °C. In addition, BPA also stimulated spermatogonial proliferation in the treated groups, but apoptosis was significantly increased in spermatids at 23 °C. Testis-ova, cell degeneration, and chromatin alterations in spermatids and Sertoli cells were observed in the germinal epithelium of the BPA-treated groups. The integrated biomarker response (IBR) index revealed that the analyzed endpoints are suitable for assessing estrogenic contamination. Taken together, our results indicate that the interactive effects of BPA and temperature contribute to the impairment of spermatogenesis in A. bimaculatus with more severe effects observed on sperm production at 23 °C than at 28 °C.

Hormonal and neuroendocrine control of reproductive function in teleost fish

Reproduction is one of the important physiological events for the maintenance of the species. Hormonal and neuroendocrine regulation of teleost requires multiple and complex interactions along the hypothalamic-pituitary-gonad (HPG) axis. Within this axis, gonadotropin-releasing hormone (GnRH) regulates the synthesis and release of gonadotropins, follicle-stimulating hormone (FSH), and luteinizing hormone (LH). Steroidogenesis drives reproduction function in which the development and differentiation of gonads. In recent years, new neuropeptides have become the focus of reproductive physiology research as they are involved in the different regulatory mechanisms of these species' growth, metabolism, and reproduction. However, especially in fish, the role of these neuropeptides in the control of reproductive function is not well studied. The study of hormonal and neuroendocrine events that regulate reproduction is crucial for the development and success of aquaculture.

Relation of gilthead seabream (Sparus aurata) seasonal reproductive activity to hematology, serum biochemistry, histopathology, and Brdt gene expression

This study aimed to find the relation of Sparus aurata (gilthead seabream) reproductive activities to some blood parameters as complete blood count, liver enzymes, some hormones related to reproduction process and microscopic findings of gonads, as well as expression of Bromodomain testis-specific gene. Eighty-eight sexually mature seabream were collected and investigated through the four seasons. Red blood cells were higher in autumn and spring. Hemoglobin was high in summer, MCV highest values ​​were seen in winter and summer, while MCHC was highest in summer. The values ​​of white blood cells increased significantly in spring, summer, and autumn compared with winter. The highest value of lymphocytes was recorded in spring and autumn. Eosinophil was recorded the highest value in the spring. The highest value of segmented neutrophils was recorded in summer. The highest value of band neutrophil was recorded in summer and winter. Alanine aminotransferase and aspartate aminotransferase showed high values in the winter. Luteinizing hormone (LH) was higher in females, males, and hermaphrodites during winter. Follicle-stimulating hormone (FSH) was higher in females during spring. The highest value of estradiol 17-β and progesterone was recorded in summer. The highest value of total testosterone was recorded in spring. Microscopically, ovaries were immature and inactive during spring and summer but well developed in autumn and winter. During spring and summer, testes were immature and began spermatogenesis process but well developed with the appearance of spermatids and spermatozoa during autumn and winter. The expression of Brdt was higher in testes than ovary. Brdt recorded high expression in autumn and spring than in summer and winter.

Effect of Fishmeal Content in the Diet on the Growth and Sexual Maturation of Olive Flounder (Paralichthysolivaceus) at a Typical Fish Farm

Simple Summary

Increasing demand for an efficient and economic fishmeal feed for sustainable aquaculture has urged the aquafeed sector to seek an optimum fish-feed formulation. This study investigated the physiological response in olive flounder fed various fishmeal diets in a typical fish farm. The fish were farmed for 20 weeks, using the following experimental feeds: a control feed (CON), a replacement by 20% (F20), and 30% (F30) of the fish meal content of the CON. All groups showed no significant difference in growth and survival rates. However, due to investigating hormone expression associated with maturation, high expression of PSS-I and low expression of FSH-β, ER-α, and ER-β in FM30 compared to other experimental groups were observed. Therefore, up to 30% fishmeal replacement does not affect growth, but it appears to have a slight effect on the sexual development of olive flounder.

Abstract

Olive flounder (Paralichthys olivaceus) is a commercially important and valuable species for aquaculture in Korea. Due to the unstable supply of fishmeal for farmed fish, an optimum fish-feed formulation should be researched to ensure the sustainability of P. olivaceus aquaculture. This study investigated the effect of three experimental diets: Con (basal diet); FM₂₀ (20% fishmeal replacement of CON); and FM₃₀ (30% fishmeal replacement of CON) on P. olivaceus over 20 weeks at a typical farm by monitoring the growth and factors relating to sexual maturation. The results showed that no differences in growth were observed between the CON and diet-replacement groups. Gonadal oocyte development was similar between the CON and diet-replacement groups. Moreover, sbGnRH and GH expression did not differ between the CON and diet-replacement groups. The levels of Erβ and Vtg expression were significantly higher in the FM₂₀ group than in the CON and FM₃₀ groups after the experimental period. The expression of PSS-I was significantly higher in the FM₃₀ group than in the CON and FM₂₀ groups. Therefore, although growth occurred when 30% of the fishmeal was replaced, such high dietary protein replacement may be ill-advised during the maturation of olive flounder at the commercial fish farm.

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.

Molecular cloning, immunological characterization, and expression analysis of gonadotropin-releasing hormone (GnRH) in the brain of the Chinese alligator during different stages of reproductive cycle

The neurohormone gonadotropin-releasing hormone (GnRH) plays an essential role in the control of reproductive functions in vertebrates. However, the full-length complementary DNA (cDNA) encoding the GnRHs precursor and it role in the reproductive cycles regulating has not been illustrated in crocodilian species. In the present study, full-length cDNAs encoding GnRH1 forms, its predominant localization within brain and peripheral tissues, and GnRH1 peptide concentrations in the hypothalamus and pituitary in relation to seasonal gonadal development of Chinese alligator were investigated. The cDNA of GnRH1 is consisted of 282 bp open reading frame encoding 93 amino acids. The deduced amino acid sequence of alligator GnRH1 contains several conserved regions and shows a closer genetic relationship to the avian species than to other reptile species. The GnRH1 immunopositive cells were not only detected widely in cerebrum, diencephalon, medulla oblongata but also observed in peripheral tissues, these widespread distribution characteristics indicated that GnRH1 possibly possess the multi-functionality in Chinese Alligator. GnRH1 peptide concentration within hypothalamus were observed be the highest in RP group (P < 0.05), in association with an peak value in GSI and emerging of late vitellogenic follicles in the ovary. Taken together, our results suggested that GnRH1 was predominantly involved in the vitellogenesis process of seasonal gonadal development of Chinese Alligator.

The reproductive regulation of LPXRFa and its receptor in the hypothalamo-pituitary-gonadal axis of the spotted scat (Scatophagus argus)

Gonadotropin-inhibitory hormone (GnIH) plays a critical role in regulating gonadotropin-releasing hormone (GnRH), gonadotropin hormone (GtH), and steroidogenesis. The Lpxrfa (the piscine ortholog of GnIH) system has been found to regulate fish reproduction. To gain insight into the role of Lpxrfa in the regulation of spotted scat (Scatophagus argus) reproduction, spotted scat Lpxrfa (ssLpxrfa), and its receptor (ssLpxrfa-r) were cloned and analyzed. Tissue distribution and expression patterns at the hypothalamo-pituitary-gonadal axis (HPG axis) of sslpxrfa and sslpxrfa-r mRNA were also investigated during gonadal development of spotted scat. The open reading frame (ORF) of the sslpxrfa was 606 bp encoding 201 amino acids and includes a putative signal peptide and two mature ssLpxrfa peptides with LPXRFamide motif at their C-terminus. The sslpxrfa-r ORF was 1449 bp encoding 482 amino acids and contracted a seven-hydrophobic transmembrane (TM) domain structure. The tissue distribution showe d that the sslpxrfa was highly expressed in hypothalami, gill, and the gonads. In addition, sslpxrfa-r was highly expressed in hypothalami, pituitaries, and the gonads. Quantitative real-time polymerase chain reaction (qPCR) revealed that sslpxrfa had the highest expression in the hypothalami and pituitaries, and the lowest expression in the gonads in stage V. During gonadal development, the expression of sslpxrfa-r was gradually increased in the hypothalami but reduced in the gonads. However, no obvious trend was observed in the pituitaries. The expression of sslpxrfa and sslpxrfa-r decreased significantly after injection with 17β-estradiol (E₂). However, the expression of both sslpxrfa and sslpxrfa-r was not changed after injection with 17α-methyltestosterone(17α-MT) in the hypothalami. In addition, no changes were observed in the expression of fshβ and lhβ in the pituitaries after injecting ssLpxrfa-1. However, ssLpxrfa-2 could downregulate the expression of sbgnrh and fshβ in the hypothalami and pituitaries, respectively. Taken together, these findings suggested that ssLpxrfa may participate in E₂ feedback in reproduction and regulate the reproductive axis of spotted scat.

Long extensions with varicosity-like structures in gonadotrope Lh cells facilitate clustering in medaka pituitary culture

Accumulating evidence indicates that some pituitary cell types are organized in complex networks in both mammals and fish. In this study, we have further investigated the previously described cellular extensions formed by the medaka (Oryzias latipes) luteinizing hormone gonadotropes (Lh cells). Extensions, several cell diameters long, with varicosity-like swellings, were common both in vitro and in vivo. Some extensions approached other Lh cells, while others were in close contact with blood vessels in vivo. Gnrh further stimulated extension development in vitro. Two types of extensions with different characteristics could be distinguished, and were classified as major or minor according to size, origin and cytoskeleton protein dependance. The varicosity-like swellings appeared on the major extensions and were dependent on both microtubules and actin filaments. Immunofluorescence revealed that Lhβ protein was mainly located in these swellings and at the extremity of the extensions. We then investigated whether these extensions contribute to network formation and clustering, by following their development in primary cultures. During the first two days in culture, the Lh cells grew long extensions that with time physically attached to other cells. Successively, tight cell clusters formed as cell somas that were connected via extensions migrated towards each other, while shortening their extensions. Laser photolysis of caged Ca2+ showed that Ca2+ signals originating in the soma propagated from the soma along the major extensions, being particularly visible in each swelling. Moreover, the Ca2+ signal could be transferred between densely clustered cells (sharing soma-soma border), but was not transferred via extensions to the connected cell. In summary, Lh gonadotropes in medaka display a complex cellular structure of hormone-containing extensions that are sensitive to Gnrh, and may be used for clustering and possibly hormone release, but do not seem to contribute to communication between cells themselves.

Special features of neuroendocrine interactions between stress and reproduction in teleosts

Stress and reproduction are both essential functions for vertebrate survival, ensuring on one side adaptative responses to environmental changes and potential life threats, and on the other side production of progeny. With more than 25,000 species, teleosts constitute the largest group of extant vertebrates, and exhibit a large diversity of life cycles, environmental conditions and regulatory processes. Interactions between stress and reproduction are a growing concern both for conservation of fish biodiversity in the frame of global changes and for the development of sustainability of aquaculture including fish welfare. In teleosts, as in other vertebrates, adverse effects of stress on reproduction have been largely documented and will be shortly overviewed. Unexpectedly, stress notably via cortisol, may also facilitate reproductive function in some teleost species in relation to their peculiar life cyles and this review will provide some examples. Our review will then mainly address the neuroendocrine axes involved in the control of stress and reproduction, namely the corticotropic and gonadotropic axes, as well as their interactions. After reporting some anatomo-functional specificities of the neuroendocrine systems in teleosts, we will describe the major actors of the corticotropic and gonadotropic axes at the brain-pituitary-peripheral glands (interrenals and gonads) levels, with a special focus on the impact of teleost-specific whole genome duplication (3R) on the number of paralogs and their potential differential functions. We will finally review the current knowledge on the neuroendocrine mechanisms of the various interactions between stress and reproduction at different levels of the two axes in teleosts in a comparative and evolutionary perspective.

Direct and Indirect Effects of Sex Steroids on Gonadotrope Cell Plasticity in the Teleost Fish Pituitary

The pituitary gland controls many important physiological processes in vertebrates, including growth, homeostasis, and reproduction. As in mammals, the teleost pituitary exhibits a high degree of plasticity. This plasticity permits changes in hormone production and secretion necessary to meet the fluctuating demands over the life of an animal. Pituitary plasticity is achieved at both cellular and population levels. At the cellular level, hormone synthesis and release can be regulated via changes in cell composition to modulate both sensitivity and response to different signals. At the cell population level, the number of cells producing a given hormone can change due to proliferation, differentiation of progenitor cells, or transdifferentiation of specific cell types. Gonadotropes, which play an important role in the control of reproduction, have been intensively investigated during the last decades and found to display plasticity. To ensure appropriate endocrine function, gonadotropes rely on external and internal signals integrated at the brain level or by the gonadotropes themselves. One important group of internal signals is the sex steroids, produced mainly by the gonadal steroidogenic cells. Sex steroids have been shown to exert complex effects on the teleost pituitary, with differential effects depending on the species investigated, physiological status or sex of the animal, and dose or method of administration. This review summarizes current knowledge of the effects of sex steroids (androgens and estrogens) on gonadotrope cell plasticity in teleost anterior pituitary, discriminating direct from indirect effects.

Exposure of zebrafish to environmentally relevant concentrations of mercury during early life stages impairs subsequent reproduction in adults but can be recovered in offspring

Mercury (Hg) is a global pollutant that poses potential threats to health of fishes. Although effects of Hg on reproduction of fishes have been documented, little is known about effects of exposure to Hg²⁺ during early life stages on subsequent reproductive fitness of adults or whether these effects can be transferred to offspring. In this study, zebrafish embryos were exposed to environmentally relevant concentrations of Hg²⁺ (0.6, 3 or 15 μg/L) for 5 days and then depurated in clean water for another 115 days. Exposure to Hg²⁺ during early life stages disturbed the balance of sex hormones and gametogenesis by altering expression of mRNA for genes involved in the hypothalamic-pituitary-gonadal axis, which resulted in delayed gonadal development and lesser gonado-somatic index, thereby resulting in lesser fecundity. A similar, but less pronounced effect was observed in F₁ females that were not exposed directly to Hg, whereas such damage was neither observed in F₁ males nor either sex during the F₂ generation. Exposure to Hg²⁺ during early life can impair subsequent reproduction in adults and has intergenerational effects on F₁ females, but this reproductive damage can be recovered in F₁ males and in F₂ females.

Multimodal hypothalamo-hypophysial communication in the vertebrates

The vertebrate pituitary is arguably one of the most complex endocrine glands from the evolutionary, anatomical and functional perspectives. The pituitary plays a master role in endocrine physiology for the control of growth, metabolism, reproduction, water balance, and the stress response, among many other key processes. The synthesis and secretion of pituitary hormones are under the control of neurohormones produced by the hypothalamus. Under this conceptual framework, the communication between the hypophysiotropic brain and the pituitary gland is at the foundation of our understanding of endocrinology. The anatomy of the connections between the hypothalamus and the pituitary gland has been described in different vertebrate classes, revealing diverse modes of communication together with varying degrees of complexity. In this context, the evolution and variation in the neuronal, neurohemal, endocrine and paracrine modes will be reviewed in light of recent discoveries, and a re-evaluation of earlier observations. There appears to be three main hypothalamo-pituitary communication systems: 1. Diffusion, best exemplified by the agnathans; 2. Direct innervation of the adenohypophysis, which is most developed in teleost fish, and 3. The median eminence/portal blood vessel system, most conspicuously developed in tetrapods, showing also considerable variation between classes. Upon this basic classification, there exists various combinations possible, giving rise to taxon and species-specific, multimodal control over major physiological processes. Intrapituitary paracrine regulation and communication between folliculostellate cells and endocrine cells are additional processes of major importance. Thus, a more complex evolutionary picture of hypothalamo-hypophysial communication is emerging. There is currently little direct evidence to suggest which neuroendocrine genes may control the evolution of one communication system versus another. However, studies at the developmental and intergenerational timescales implicate several genes in the angiogenesis and axonal guidance pathways that may be important.

The gonadotropin-releasing hormones: Lessons from fish

Fish have been of paramount importance to our understanding of vertebrate comparative neuroendocrinology and the mechanisms underlying the physiology and evolution of gonadotropin-releasing hormones (GnRH) and their genes. This review integrates past and recent knowledge on the Gnrh system in the fish model. Multiple Gnrh isoforms (two or three forms) are present in all teleosts, as well as multiple Gnrh receptors (up to five types), which differ in neuroanatomical localization, pattern of projections, ontogeny and functions. The role of the different Gnrh forms in reproduction seems to also differ in teleost models possessing two versus three Gnrh forms, Gnrh3 being the main hypophysiotropic hormone in the former and Gnrh1 in the latter. Functions of the non-hypothalamic Gnrh isoforms are still unclear, although under suboptimal physiological conditions (e.g. fasting), Gnrh2 may increase in the pituitary to ensure the integrity of reproduction under these conditions. Recent developments in transgenesis and mutagenesis in fish models have permitted the generation of fish lines expressing fluorophores in Gnrh neurons and to elucidate the dynamics of the elaborate innervations of the different neuronal populations, thus enabling a more accurate delineation of their reproductive roles and regulations. Moreover, in combination with neuronal electrophysiology, these lines have clarified the Gnrh mode of actions in modulating Lh and Fsh activities. While loss of function and genome editing studies had the premise to elucidate the exact roles of the multiple Gnrhs in reproduction and other processes, they have instead evoked an ongoing debate about these roles and opened new avenues of research that will no doubt lead to new discoveries regarding the not-yet-fully-understood Gnrh system.

Plasticity in medaka gonadotropes via cell proliferation and phenotypic conversion

Follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) produced by the gonadotropes play a major role in control of reproduction. Contrary to mammals and birds, Lh and Fsh are mostly produced by two separate cell types in teleost. Here, we investigated gonadotrope plasticity, using transgenic lines of medaka (Oryzias latipes) where DsRed2 and hrGfpII are under the control of the fshb and lhb promotors respectively. We found that Fsh cells appear in the pituitary at 8 dpf, while Lh cells were previously shown to appear at 14 dpf. Similar to Lh cells, Fsh cells show hyperplasia from juvenile to adult stages. Hyperplasia is stimulated by estradiol. Both Fsh and Lh cells show hypertrophy during puberty with similar morphology. They also share similar behavior, using their cellular extensions to make networks. We observed bi-hormonal gonadotropes in juveniles and adults but not in larvae where only mono-hormonal cells are observed, suggesting the existence of phenotypic conversion between Fsh and Lh in later stages. This is demonstrated in cell culture, where some Fsh cells start to produce Lhβ, a phenomenon enhanced by gonadotropin-releasing hormone (Gnrh) stimulation. We have previously shown that medaka Fsh cells lack Gnrh receptors, but here we show that with time in culture, some Fsh cells start responding to Gnrh, while fshb mRNA levels are significantly reduced, both suggestive of phenotypic change. All together, these results reveal high plasticity of gonadotropes due to both estradiol-sensitive proliferation and Gnrh promoted phenotypic conversion, and moreover, show that gonadotropes lose part of their identity when kept in cell culture.

Gonadotrope plasticity at cellular, population and structural levels: A comparison between fishes and mammals

Often referred to as "the master gland", the pituitary is a key organ controlling growth, maturation, and homeostasis in vertebrates. The anterior pituitary, which contains several hormone-producing cell types, is highly plastic and thereby able to adjust the production of the hormones governing these key physiological processes according to the changing needs over the life of the animal. Hypothalamic neuroendocrine control and feedback from peripheral tissues modulate pituitary cell activity, adjusting levels of hormone production and release according to different functional or environmental requirements. However, in some physiological processes (e.g. growth, puberty, or metamorphosis), changes in cell activity may be not sufficient to meet the needs and a general reorganization of cell composition and pituitary structure may occur. Focusing on gonadotropes, this review examines plasticity at the cellular level, which allows precise and rapid control of hormone production and secretion, as well as plasticity at the population and structural levels, which allows more substantial changes in hormone production. Further, we compare current knowledge of the anterior pituitary plasticity in fishes and mammals in order to assess what has been conserved or not throughout evolution, and highlight important remaining questions.

Transcription pattern of reproduction relevant genes along the brain-pituitary-gonad axis of female, male and intersex thicklip grey mullets, Chelon labrosus, from a polluted harbor

The reproductive cycle of teleost fishes is regulated by the brain-pituitary-gonad (BPG) axis. The transcription profile of genes involved in the reproduction signalling in the BPG-axis differs in females and males during the gametogenic cycle. Impacts of endocrine disrupting chemicals on these signalling pathways in fish are known, but the participation of the BPG-axis in the development of the intersex condition is not well understood. Intersex thicklip grey mullets (Chelon labrosus) have been identified in several estuaries from the SE Bay of Biscay, revealing the presence of feminizing contaminants in the area. In previous studies, transcription patterns of genes related with steroidogenesis and gamete growth have been shown to differ among female, male and intersex mullets. However, many components of the reproduction control have not been studied yet. The aim of this study was to assess the transcription levels of target BPG-axis genes in female, male and intersex mullets captured in the polluted harbour of Pasaia, during their gametogenic cycle. After histologically examining the gonads, the transcription levels of previously sequenced target genes were measured by qPCR: kiss2, gpr54 and gnrh1 in brain, fshβ and lhβ in pituitary and fshr and lhr in gonads. In both females and males, brain genes were most transcribed in early gametogenesis, proving their relevance in the onset of both oogenesis and spermatogenesis. Pituitary gonadotropins in females showed upregulation as oogenesis progressed, reaching the highest transcription levels at vitellogenic stage, while in males transcript levels were constant during spermatogenesis. Transcription levels of gonadotropin receptors showed different patterns in ovaries and testes, suggesting differing function in relation to gametogenesis and maturation. Intersex mullets showed transcription levels of brain target genes similar to those observed in females at cortical alveoli stage and to those in mid spermatogenic males. In intersex testes the transcription pattern of gonadotropin receptor fshr was downregulated in comparison to non-intersex testes.

Effects of Melatonin on Anterior Pituitary Plasticity: A Comparison Between Mammals and Teleosts

Melatonin is a key hormone involved in the photoperiodic signaling pathway. In both teleosts and mammals, melatonin produced in the pineal gland at night is released into the blood and cerebrospinal fluid, providing rhythmic information to the whole organism. Melatonin acts via specific receptors, allowing the synchronization of daily and annual physiological rhythms to environmental conditions. The pituitary gland, which produces several hormones involved in a variety of physiological processes such as growth, metabolism, stress and reproduction, is an important target of melatonin. Melatonin modulates pituitary cellular activities, adjusting the synthesis and release of the different pituitary hormones to the functional demands, which changes during the day, seasons and life stages. It is, however, not always clear whether melatonin acts directly or indirectly on the pituitary. Indeed, melatonin also acts both upstream, on brain centers that control the pituitary hormone production and release, as well as downstream, on the tissues targeted by the pituitary hormones, which provide positive and negative feedback to the pituitary gland. In this review, we describe the known pathways through which melatonin modulates anterior pituitary hormonal production, distinguishing indirect effects mediated by brain centers from direct effects on the anterior pituitary. We also highlight similarities and differences between teleosts and mammals, drawing attention to knowledge gaps, and suggesting aims for future research.

Characterization of gonadotropin receptors Fshr and Lhr in Japanese medaka, Oryzias latipes

Reproduction in vertebrates is controlled by the brain-pituitary-gonad axis, where the two gonadotropins follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) play vital parts by activating their cognate receptors in the gonads. The main purpose of this work was to study intra- and interspecies ligand promiscuity of teleost gonadotropin receptors, since teleost receptor specificity is unclear, in contrast to mammalian receptors. Receptor activation was investigated by transfecting COS-7 cells with either Fsh receptor (mdFshr, tiFshr) or Lh receptor (mdLhr, tiLhr), and tested for activation by recombinant homologous and heterologous ligands (mdFshβα, mdLhβα, tiFshβα, tiLhβα) from two representative fish orders, Japanese medaka (Oryzias latipes, Beloniformes) and Nile tilapia (Oreochromis niloticus, Cichliformes). Results showed that each gonadotropin preferentially activates its own cognate receptor. Cross-reactivity was detected to some extent as mdFshβα was able to activate the mdLhr, and mdLhβα the mdFshr. Medaka pituitary extract (MPE) stimulated CRE-LUC activity in COS-7 cells expressing mdlhr, but could not stimulate cells expressing mdfshr. Recombinant tiLhβα, tiFshβα and tilapia pituitary extract (TPE) could activate the mdLhr, suggesting cross-species reactivity for mdLhr. Cross-species reactivity was also detected for mdFshr due to activation by tiFshβα, tiLhβα, and TPE, as well as for tiFshr and tiLhr due to stimulation by mdFshβα, mdLhβα, and MPE. Tissue distribution analysis of gene expression revealed that medaka receptors, fshr and lhr, are highly expressed in both ovary and testis. High expression levels were found for lhr also in brain, while fshr was expressed at low levels. Both fshr and lhr mRNA levels increased significantly during testis development. Amino acid sequence alignment and three-dimensional modelling of ligands and receptors highlighted conserved beta sheet domains of both Fsh and Lh between Japanese medaka and Nile tilapia. It also showed a higher structural homology and similarity of transmembrane regions of Lhr between both species, in contrast to Fshr, possibly related to the substitution of the conserved cysteine residue in the transmembrane domain 6 in medaka Fshr with glycine. Taken together, this is the first characterization of medaka Fshr and Lhr using homologous ligands, enabling to better understand teleost hormone-receptor interactions and specificities. The data suggest partial ligand promiscuity and cross-species reactivity between gonadotropins and their receptors in medaka and tilapia.

Gnrh receptor gnrhr2bbα is expressed exclusively in lhb-expressing cells in Atlantic salmon male parr

Gonadotropin-releasing hormone (Gnrh) plays a major role in the regulation of physiological and behavioural processes related to reproduction. In the pituitary, it stimulates gonadotropin synthesis and release via activation of Gnrh receptors (Gnrhr), belonging to the G protein-coupled receptor superfamily. Evidence suggests that differential regulation of the two gonadotropins (Fsh and Lh) is achieved through activation of distinct intracellular pathways and, probably, through the action of distinct receptors. However, the roles of the different Gnrhr isoforms in teleosts are still not well understood. This study investigates the gene expression of Gnrhr in the pituitary gland of precociously maturing Atlantic salmon (Salmo salar) male parr. A total of six Gnrhr paralogs were identified in the Atlantic salmon genome and named according to phylogenetic relationship; gnrhr1caα, gnrhr1caβ, gnrhr1cbα, gnrhr1cbβ, gnrhr2bbα, gnrhr2bbβ. All paralogs, except gnrhr1caα, were expressed in male parr pituitary during gonadal maturation as evidenced by qPCR analysis. Only one gene, gnrhr2bbα, was differentially expressed depending on maturational stage (yearly cycle), with high expression levels in maturing fish, increasing in parallel with gonadotropin subunit gene expression. Additionally, a correlation in daily expression levels was detected between gnrhr2bbα and lhb (daily cycle) in immature fish in mid-April. Double fluorescence in situ hybridization showed that gnrhr2bbα was expressed exclusively in lhb gonadotropes in the pituitary, with no expression detected in fshb cells. These results suggest the involvement of receptor paralog gnrhr2bbα in the regulation of lhb cells, and not fshb cells, in sexually maturing Atlantic salmon male parr.

Insights on the seasonal variations of reproductive features in the Eastern Atlantic Bluefin Tuna

The Atlantic Bluefin Tuna (ABFT, Thunnus thynnus) is one of the most intensely exploited fisheries resources in the world. In spite of the years of studies on ABFT, basic aspects of its reproductive biology remain uncertain. To gain insight regarding the seasonal changes of the reproductive characteristics of the eastern stock of ABFT, blood and tissue samples were collected from mature specimens caught in the Mediterranean basin during the reproductive (May-June) and non-reproductive season (Oct-Nov). Histological analysis of the gonads of May-June samples indicated that there were females which were actively spawning (contained post-ovulatory follicles) and females that were not actively spawning that had previtellogenic and fully vitellogenic oocytes. In males, testis were at early or late stage of spermatogenesis during the reproductive season. In Oct-Nov, ovaries contained mostly previtellogenic oocytes as well as β and α atretic follicles while the testis predominantly contained spermatogonia and few cysts with spermatocytes and spermatozoa. Gonadosomatic index (GSI) in females was highest among the actively spawning individuals while in males GSI was higher in early and late spermatogenic individuals compared to those that were spent. Plasma sex steroids levels varied with the reproductive season. In females, estradiol (E₂), was higher in May-June while testosterone (T) and progesterone (P) did not vary. In males, E₂ and T were higher in May-June while P levels were similar at the two sampling points. Circulating follicle stimulating hormone (FSH) was higher in Oct-Nov than in May-June both in males and females. Vitellogenin (VTG) was detected in plasma from both males and females during the reproductive season with levels in females significantly higher than in males. VTG was undetected in Oct-Nov samples. Since choriogenesis is an important event during follicle growth, the expression of three genes involved in vitelline envelope formation and hardening was measured and results showed significantly higher levels in ovaries in fish caught in May-June with respect to those sampled in Oct-Nov. In addition, a set of genes encoding for ion channels that are responsible for oocyte hydration and buoyancy, as well as sperm viability, were characterized at the two time points, and these were found to be more highly expressed in females during the reproductive season. Finally, the expression level of three mRNAs encoding for different lipid-binding proteins was analyzed with significantly higher levels detected in males, suggesting sex-specific expression. Our findings provide additional information on the reproductive biology of ABFT, particularly on biomarkers for the assessment of the state of maturation of the gonad, highlighting gender-specific signals and seasonal differences.

In Vitro Effects of Bisphenol A and Tetrabromobisphenol A on Cell Viability and Reproduction-Related Gene Expression in Pituitaries from Sexually Maturing Atlantic Cod (Gadus morhua L.)

Bisphenol A (BPA) and tetrabromobisphenol A (TBBPA) are widely used industrial chemicals, ubiquitously present in the environment. While BPA is a well-known endocrine disruptor and able to affect all levels of the teleost reproductive axis, information regarding TBBPA on this subject is very limited. Using primary cultures from Atlantic cod (Gadus morhua), the present study was aimed at investigating potential direct effects of acute (72 h) BPA and TBBPA exposure on cell viability and the expression of reproductive-relevant genes in the pituitary. The results revealed that both bisphenols stimulate cell viability in terms of metabolic activity and membrane integrity at environmentally relevant concentrations. BPA had no direct effects on gonadotropin gene expression, but enhanced the expression of gonadotropin-releasing hormone (GnRH) receptor 2a, the main gonadotropin modulator in Atlantic cod. In contrast, TBBPA increased gonadotropin transcript levels but had no effect on GnRH receptor mRNA. In conclusion, both anthropogenic compounds display endocrine disruptive properties and are able to directly interfere with gene expression related to reproductive function in cod pituitary cells at environmentally relevant concentrations in vitro.

The Kiss2/GPR54 system stimulates the reproductive axis in male black porgy, Acanthopagrus schlegelii

Although it is well established that the Kiss1/GPR54 system stimulates the reproductive axis in mammals, its functional roles, especially in male reproduction of non-mammalian species, is less clear. In this study, we have isolated the full-length kiss2 and gpr54 cDNAs from black porgy (Acanthopagrus schlegelii). The Kiss2 precursor expressed from kiss2 comprises 124 amino acids and contains a highly conserved 10-amino acid sequence, Kiss2-10 (FNFNPFGLRF). GPR54 comprises 375 amino acid residues and contains distinct characteristics of G protein-coupled receptors. Real-time PCR analysis indicated that kiss2 and gpr54 were expressed highly in the brain regions. Moreover, intraperitoneal injection of porgy Kiss2-10 could stimulate genes expression of the gpr54, gnrh1, gnrh3, fshβ, lhβ, p450c17, star, and ar, and the serum testerone level in male black porgy. Our findings demonstrate that the Kisspeptin stimulates the male reproductive axis in black porgy.

Establishment of specific enzyme-linked immunosorbent assay (ELISA) for measuring Fsh and Lh levels in medaka (Oryzias latipes), using recombinant gonadotropins

The paucity of information on understanding the regulatory mechanisms that are involved in the control of piscine Fsh and Lh synthesis, secretion, and function, prompted the present work. Part of the problem is related to the molecular heterogeneity and the unavailability of Fsh and Lh assays for quantifying gonadotropins, in particular assays regarding the measurement of Fsh, and such assays are available today for only a few teleost species. The present study reports the development and validation of competitive ELISAs for quantitative determination of medaka Fsh and Lh by first producing medaka recombinant (md) gonadotropins mdFshβ, mdLhβ, mdFshβα, and mdLhβα by Pichia pastoris, generating specific antibodies against their respective β subunits, and their use within the development of ELISAs.

The advantages of this protocol include:

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The reproducibility of the ELISA demonstrated was relatively high, as shown by reasonably low intra- (Fsh 2.7%, Lh 3%) and interassay CVs (Fsh 5.3%, Lh 5.7%).

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The high degree of parallelism between serial dilutions of the recombinant and native pituitary-derived Fsh and Lh, may be a sign of similar structures and immunologically similarity.

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Two new competitive ELISAs for the quantification of medaka Fsh and Lh were established for the first time.

Metabolic hormones and the regulation of spermatogenesis in fishes

Metabolic hormones play essential regulatory roles in many biological processes, including morphogenesis, growth, and reproduction through the maintenance of energy balance. Various metabolic hormones originally discovered in mammals, including ghrelin, leptin, and nesfatin-1 have been identified and characterized in fish. However, physiological roles of these metabolic hormones in regulating reproduction are largely unknown in fishes, especially in males. While the information available is restricted, this review attempts to summarize the main findings on the roles of metabolic peptides on the reproductive system in male fishes with an emphasis on testicular development and spermatogenesis. Specifically, the primary goal is to review the physiological interactions between hormones that regulate reproduction and hormones that regulate metabolism as a critical determinant of testicular function. A brief introduction to the localization of metabolic hormones in fish testis is also provided. Besides, the consequences of fasting and food deprivation on testicular development and sperm quality will be discussed with a focus on interactions between metabolic and reproductive hormones.

Medaka follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh): Developmental profiles of pituitary protein and gene expression levels

The two gonadotropins follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) are of particular importance within the hypothalamic-pituitary-gonadal (HPG) axis of vertebrates. In the current study, we demonstrate the production and validation of Japanese medaka (Oryzias latipes) recombinant (md) gonadotropins Fshβ (mdFshβ), Lhβ (mdLhβ), Fshβα (mdFshβα), and Lhβα (mdLhβα) by Pichia pastoris, the generation of specific rabbit antibodies against their respective β subunits, and their use within the development and validation of competitive enzyme-linked immunosorbent assays (ELISAs) for quantification of medaka Fsh and Lh. mdFsh and mdLh were produced as single-chain polypeptides by linking the α subunit with mdFshβ or mdLhβ mature protein coding sequences to produce a "tethered" polypeptide with the β-chain at the N-terminal and the α-chain at the C-terminal. The specificity of the antibodies raised against mdFshβ and mdLhβ was determined by immunofluorescence (IF) for Fshβ and Lhβ on medaka pituitary tissue, while comparison with fluorescence in situ hybridization (FISH) for fshb and lhb mRNA was used for validation. Competitive ELISAs were developed using antibodies against mdFshβ or mdLhβ, and the tethered proteins mdFshβα or mdLhβα for standard curves. The standard curve for the Fsh ELISA ranged from 97.6 pg/ml to 50 ng/ml, and for the Lh ELISA from 12.21 pg/ml to 6.25 ng/ml. The sensitivity of the assays for Fsh and Lh was 44.7 and 70.8 pg/ml, respectively. A profile of pituitary protein levels of medaka Fsh and Lh comparing juveniles with adults showed significant increase of protein amount from juvenile group (body length from 12 mm to 16.5 mm) to adult group (body length from 21 mm to 26.5 mm) for both hormones in male medaka. Comparing these data to a developmental profile of pituitary mRNA expression of medaka fshb and lhb, the mRNA expression of lhb also increased during male maturation and a linear regression analysis revealed a significant increase of lhb expression with increased body length that proposes a linear model. However, fshb mRNA expression did not change significantly during male development and therefore was not correlated with body length. In summary, we have developed and validated homologous ELISA assays for medaka Fsh and Lh based on proteins produced in P. pastoris, assays that will be used to study the functions and regulations of Fsh and Lh in more detail.

Natural sex change in fish

Sexual fate can no longer be considered an irreversible deterministic process that once established during early embryonic development, plays out unchanged across an organism's life. Rather, it appears to be a dynamic process, with sexual phenotype determined through an ongoing battle for supremacy between antagonistic male and female developmental pathways. That sexual fate is not final and is actively regulated via the suppression or activation of opposing genetic networks creates the potential for flexibility in sexual phenotype in adulthood. Such flexibility is seen in many fish, where sex change is a usual and adaptive part of the life cycle. Many fish are sequential hermaphrodites, beginning life as one sex and changing sometime later to the other. Sequential hermaphrodites include species capable of female-to-male (protogynous), male-to-female (protandrous), or bidirectional (serial) sex change. These natural forms of sex change involve coordinated transformations across multiple biological systems, including behavioral, anatomical, neuroendocrine and molecular axes. Here we review the biological processes underlying this amazing transformation, focusing particularly on the molecular aspects, where new genomic technologies are beginning to help us understand how sex change is initiated and regulated at the molecular level.

Multi-Tissue Transcriptome Profiling of North American Derived Atlantic Salmon

The availability of a reference genome assembly for Atlantic salmon, Salmo salar, SNP genotyping platforms and low cost sequencing are enhancing the understanding of both life history and production-related traits in this important commercial species. We collected and analyzed transcriptomes from selected tissues of Atlantic salmon to inform future functional and comparative genomics studies. Messenger RNA (mRNA) was isolated from pituitary gland, brain, ovary, and liver before Illumina sequencing produced a total of 640 million 150-bp paired-end reads. Following read mapping, feature counting, and normalization, cluster analysis identified genes highly expressed in a tissue-specific manner. We identified a cluster of 508 tissue specific genes for pituitary gland, 3395 for brain, 2939 for ovary, and 539 for liver. Functional profiling identified gene clusters describing the unique functions of each tissue. Moreover, highly-expressed transcription factors (TFs) present in each tissue-specific gene cluster were identified. TFs belonging to homeobox and bhlh families were identified for pituitary gland, pou and zf-c2h2 families for brain, arid, and zf-c2h2 for ovary and rxr-like family for liver. The data and analysis presented are relevant to the emerging Functional Annotation of All Salmonid Genomes (FAASG) initiative that is seeking to develop a detailed understanding of both salmonid evolution and the genomic elements that drive gene expression and regulation.

Evolutionary conservation and divergence of Vasa, Dazl and Nanos1 during embryogenesis and gametogenesis in dark sleeper (Odontobutis potamophila)

Germline-specific genes, Vasa, Dazl and Nanos1, have highly conserved functions in germline development and fertility across animal phyla. In this study, the full-length sequences of Opvasa, Opdazl and Opnanos1 were cloned and characterized from the dark sleeper (Odontobutis potamophila). Gonad-specific expression patterns of Opvasa and Opdazl were confirmed in adult tissues by quantitative real-time PCR (qRT-PCR). Different from Opvasa and Opdazl, the expression of Opnanos1 was ubiquitously detected in all examined tissues except for the liver and spleen. Time-course dynamic expressions during embryogenesis were assessed, and all three genes (Opvasa, Opdazl and Opnanos1) persisted at a high level until gastrulation. qRT-PCR and Western blotting analyses revealed that all three genes were highly expressed throughout gametogenesis. In testis, the expressions of all three genes at the mRNA and protein levels were down-regulated during spermatogenesis. In ovary, different expression patterns were found, and all three genes had a differential role in translational regulation during oogenesis. The expressions of Opvasa, Opdazl and Opnanos1 at the mRNA but not the protein level were high in stage IV. Different expression patterns were found in premeiotic gonads treated by HPG axis hormones (HCG and LHRH-A). Immunolocalization analysis demonstrated that in testis, Opvasa, Opdazl and Opnanos1 were detected in spermatogonia and spermatocytes but absent in the meiotic products, such as spermatids and spermatozoa. In ovary, Opvasa, Opdazl and Opnanos1 persisted at a high level throughout oogenesis. These findings indicated that Opvasa, Opdazl and Opnanos1 played an important role in mitotic and early meiotic phases of oogenesis and spermatogenesis, and they functioned as maternal factors in early embryogenesis. Their proteins could be used as three new markers for germ cells during gametogenesis in O. potamophila gonad. Our data laid a good foundation for improving the breeding efficiency of O. potamophila.