Regulation of oocyte maturation in fish

Norethindrone alters mating behaviors, ovary histology, hormone production and transcriptional expression of steroidogenic genes in zebrafish (Danio rerio)

The impact of progestins (i.e. synthetic forms of progesterone) on aquatic organisms has drawn increasing attention due to their widespread occurrence in the aquatic environments and potential effects on the endocrine system of fish. In this study, the effects of norethindrone (NET, a progestin) on the reproductive behavior, sex hormone production and transcriptional expressions were evaluated by exposing female zebrafish to NET at 0, 3.1, 36.2 and 398.6 ng L^-1 for 60 days. Results showed that NET impaired the mating behaviors of female at 36.2 and 398.6 ng L^-1 exhibited by males and increased the frequency of atretic follicular cells in the ovary exposed to NET at 398.6 ng L^-1. As for sex hormones, plasma testosterone concentration in zebrafish increased, while estradiol concentration decreased. Up-regulation of genes (Npr, Mpra, Mprβ, Fshβ, Lβ, Tshb, Nis and Dio2) was detected in the brain of fish exposed to NET at 398.6 ng L^-1. The transcriptional levels of genes (Esr1, Vtg1, Ar, Cyp19a, Cyp11b and Ptgs2) were generally inhibited in the ovary of zebrafish by NET at 398.6 ng L^-1. Moreover, the transcripts of genes (Vtg1, Esr1, Ar and Pgr) in the liver were reduced by NET at 36.2 and 398.6 ng L^-1. Our findings suggest that NET can potentially diminish the of fish populations not only by damaging their reproductive organs, but also by altering their mating behavior through the changes in the expressions of genes responsible for the production of sex hormones.

Induction of oocyte development in previtellogenic eel, Anguilla australis

The role of gonadotropins during early ovarian development in fish remains little understood. Concentrations of gonadotropins were therefore experimentally elevated in vivo by administration of recombinant follicle-stimulating hormone (rec-Fsh) or human chorionic gonadotropin (hCG) and the effects on ovarian morphology, sex steroid levels and mRNA levels of genes expressed in pituitary and ovary examined. Hormones were injected thrice at weekly intervals in different doses (20, 100 or 500 µg/kg BW for rec-Fsh and 20, 100 or 500 IU/kg BW for hCG). All treatments, especially at the highest doses of either rec-Fsh or hCG, induced ovarian development, reflected in increased oocyte size and lipid uptake. Both gonadotropins up-regulated follicle-stimulating hormone receptor (fshr) mRNA levels and plasma levels of estradiol-17β (E2). Exogenous gonadotropins largely decreased the expression of follicle-stimulating hormone β-subunit (fshb) and had little effect on those of luteinizing hormone β-subunit (lhb) in the pituitary. It is proposed that the effects of hCG on ovarian development in previtellogenic eels could be indirect as a significant increase in plasma levels of 11-ketotestosterone (11-KT) was found in eels treated with hCG. Using rec-Fsh and hCG has potential for inducing puberty in eels in captivity, and indeed, in teleost fish at large.

Managing the Oocyte Meiotic Arrest-Lessons from Frogs and Jellyfish

During oocyte development, meiosis arrests in prophase of the first division for a remarkably prolonged period firstly during oocyte growth, and then when awaiting the appropriate hormonal signals for egg release. This prophase arrest is finally unlocked when locally produced maturation initiation hormones (MIHs) trigger entry into M-phase. Here, we assess the current knowledge of the successive cellular and molecular mechanisms responsible for keeping meiotic progression on hold. We focus on two model organisms, the amphibian Xenopus laevis, and the hydrozoan jellyfish Clytia hemisphaerica. Conserved mechanisms govern the initial meiotic programme of the oocyte prior to oocyte growth and also, much later, the onset of mitotic divisions, via activation of two key kinase systems: Cdk1-Cyclin B/Gwl (MPF) for M-phase activation and Mos-MAPkinase to orchestrate polar body formation and cytostatic (CSF) arrest. In contrast, maintenance of the prophase state of the fully-grown oocyte is assured by highly specific mechanisms, reflecting enormous variation between species in MIHs, MIH receptors and their immediate downstream signalling response. Convergence of multiple signalling pathway components to promote MPF activation in some oocytes, including Xenopus, is likely a heritage of the complex evolutionary history of spawning regulation, but also helps ensure a robust and reliable mechanism for gamete production.

Zebrafish as a model for studying ovarian development: Recent advances from targeted gene knockout studies

Ovarian development is a complex process controlled by precise coordination of multiple factors. The targeted gene knockout technique is a powerful tool to study the functions of these factors. The successful application of this technique in mice in the past three decades has significantly enhanced our understanding on the molecular mechanism of ovarian development. Recently, with the advent of genome editing techniques, targeted gene knockout research can be carried out in many species. Zebrafish has emerged as an excellent model system to study the control of ovarian development. Dozens of genes related to ovarian development have been knocked out in zebrafish in recent years. Much new information and perspectives on the molecular mechanism of ovarian development have been obtained from these mutant zebrafish. Some findings have challenged conventional views. Several genes have been identified for the first time in vertebrates to control ovarian development. Focusing on ovarian development, the purpose of this review is to briefly summarize recent findings using these gene knockout zebrafish models, and compare these findings with mammalian models. These established mutants and rapid development of gene knockout techniques have prompted zebrafish as an ideal animal model for studying ovarian development.

17β-Hydroxysteroid dehydrogenase type 12 is responsible for maturation-inducing steroid synthesis during oocyte maturation in Nile tilapia

17α, 20β-Dihydroxy-4-pregnen-3-one (DHP) is a maturation-inducing steroid in many teleost fish. Carbonyl reductase-like 20β-hydroxysteroid dehydrogenase (CR/20β-HSD) is a candidate enzyme responsible for DHP production during oocyte maturation in various fish, including Nile tilapia. However, a novel type of 17β-hydroxysteroid dehydrogenase, type 12-like (17β-HSD12L), is responsible for DHP production during oocyte maturation in masu salmon. 17β-HSD12 (presumably orthologous to salmon 17β-HSD12L) has been detected in Nile tilapia; however, its enzymatic activity and specific ability to convert the DHP substrate 17α-hydroxyprogesterone (17OHP) have not been examined. This study aimed to determine whether CR/20β-HSD or 17β-HSD12 is responsible for DHP production during oocyte maturation in the Nile tilapia. Mammalian expression vectors containing tilapia hsd17b12 or CR/20bhsd were transfected into HEK293T cells, followed by incubation with 17OHP. HEK293T cells transfected with hsd17b12 exhibited a strong ability to convert exogenous 17OHP to DHP (73.8% yield). Cells transfected with CR/20bhsd or the control vector converted only 7.4% and 7.5% of 17OHP to DHP, respectively. In addition, based on LC-MS/MS analyses, 17β-HSD12 did not convert any substrates other than 17OHP, including DHP, adrenosterone, androstenedione, estrone, testosterone, 11-ketotestosterone, and estradiol-17β. CR/20β-HSD showed strong 17β-HSD oxidoreductase activity especially with adrenosterone and androstenedione. Tissue-specific hsd17b12 expression analyzed by RT-PCR showed that hsd17b12 mRNA was strongest amplification in full-grown follicles. Finally, full-grown ovarian follicles were incubated with salmon pituitary extract (SPE, 100 µg/mL) or human chorionic gonadotropin (HCG, 100 IU/mL) to induce 20β-HSD activity in vitro, and enzyme activity was assessed by co-incubation with 100 ng/mL 17OHP for 2, 4, 8, and 16 h. Conversion of 17OHP to DHP by ovarian follicles incubated with SPE and HCG peaked at 16 h, subsequent with increased follicular hsd17b12 mRNA levels, which were significantly higher than those in control incubations. However, the levels of CR/20bhsd mRNA remained low and did not differ among time points. The present study strongly suggests that 17β-HSD12, and not CR/20β-HSD, is the 20β-HSD responsible for DHP production by ovarian follicles during oocyte maturation in Nile tilapia.

Mummichog (Fundulus heteroclitus) are less sensitive to 17α-ethinylestradiol (EE2) than other common model teleosts: A comparative review of reproductive effects

The environmental estrogen 17α-ethinylestradiol (EE₂) will depress or completely inhibit egg production in many common model teleosts at low concentrations (≤0.5 ng/L; Runnalls et al., 2015). This inhibition is not seen in the estuarine killifish, or mummichog (Fundulus heteroclitus), even when exposed to 100 ng/L EE₂. This relative insensitivity to EE₂ exposure indicates species-specific mechanisms for compensating for exogenous estrogenic exposure. This review compares various reproductive responses elicited by EE₂ in mummichog to other common model teleosts, such as zebrafish (Danio rerio) and fathead minnow (Pimephales promelas), identifying key endpoints where mummichog differ from other studied fish. For example, EE₂ accumulates primarily in the liver/gall bladder of mummichog, which is different than zebrafish and fathead minnow in which accumulation is predominantly in the carcass. Despite causing species-specific differences in fecundity, EE₂ has been shown to consistently induce hepatic vitellogenin in males and cause feminization/sex reversal during gonadal differentiation in larval mummichog, similar to other species. In addition, while gonadal steroidogenesis and plasma steroid levels respond to exogenous EE_2, it is generally at higher concentrations than observed in other species. In mummichog, production of 17β-estradiol (E₂) by full grown ovarian follicles remains high; unlike other teleost models where E₂ synthesis decreases as 17α,20β-dihydroxy-4-prenen-3-on levels increase to induce oocyte maturation. New evidence in mummichog indicates some dissimilarity in gonadal steroidogenic gene expression responses compared to gene expression responses in zebrafish and fathead minnow exposed to EE₂. The role of ovarian physiology continues to warrant investigation regarding the tolerance of mummichog to exogenous EE₂ exposure. Here we present a comprehensive review, highlighting key biological differences in response to EE₂ exposure between mummichog and other commonly used model teleosts.

Microcystin-LR promotes zebrafish (Danio rerio) oocyte (in vivo) maturation by activating ERK1/2-MPF signaling pathways, and cAMP is involved in this process

Cyanobacterial blooms and their secondary metabolites, microcystins (MCs), are not only toxic to aquatic organisms, but also to humans. MCs exert reproductive toxicity in female fish by affecting the oocyte development. However, the mechanism behind MC-LR interference in oocyte development remains largely unknown. In our study, adult female zebrafish were exposed to MC-LR (0, 1, 5, 20 μg/L) for 30 d. After exposure to MC-LR for 30 d, fertilized eggs from the treated females and healthy males were collected and cultured in water without MC-LR. Histomorphological observations showed pathological damage in the ovary after MC-LR exposure, which was mainly characterized by enlarged intercellular spaces, detachment of follicular cells from oocytes, and vacuolation of parenchymal tissues. The 20 μg/L MC-LR treatment caused a remarkable increase in the rate of the zebrafish oocytes germinal vesicle breakdown (GVBD) and a significant decrease in the levels of cyclic adenosine monophosphate (cAMP) and vitellogenin (VTG). In addition, the phosphorylation levels of the extracellular signal-regulated kinases (ERK) were elevated in ovaries from zebrafish exposed to 5 and 20 μg/L MC-LR, and cyclinB phosphorylation levels were also upregulated notably in the 20 μg/L MC-LR group. However, MC-LR exposure did not cause any change in the levels of cAMP-dependent protein kinase (PKA) protein and cdc2 phosphorylation in all the treatments. All the doses of MC-LR reduced the number of eggs, prematurely hatched the fertilized eggs and increased the abnormal rate of offspring generation. In summary, the present study demonstrates that MC-LR promotes oocyte maturation by activating the ERK1/2 and MPF signaling pathways, and cAMP is involved in this process.

Screening of Biomarkers Related to Ovarian Maturation and Spawning in Blunt Snout Bream (Megalobrama amblycephala) Based on Metabolomics and Transcriptomics

In fish breeding practices, gamete maturity of females is vital to reproductive success. For some species, it is possible to estimate the female maturation status based on abdomen observation, but quite difficult for some species which mature at big size. To screen out the potential biomarker in fish blood relating to female maturation, we employed the approach integrating the UPLC-MS/MS and RNA-seq techniques to investigate the metabolites and genes reflecting the sexual maturation and spawning of female blunt snout bream Megalobrama amblycephala. The study included four groups, 1-year-old immature female individuals, 2-year-old immature female individuals, 2-year-old sexually mature female individuals, and 2-year-old sexually mature female individuals after 24 h of successful spawning. The upregulated metabolites in mature females were involved in "steroid hormone biosynthesis," "metabolic pathways," "glycerophospholipid metabolism," etc. compared with those of immature individuals. As the key intermediate of steroid hormone biosynthesis, 17α-hydroxypregnenolone exhibited the highest level in 2-year-old mature females than in the immature females. Meanwhile, the metabolites (i.e., dodecanoic acid and myristic acid) participating in fatty acid synthesis exhibited much lower levels in the females after spawning than those before spawning. In addition to the metabolites, the genes involved in ovarian steroidogenesis were significantly upregulated in the 2-year-old immature females compared to the 1-year-old immature females, indicating that the ovarian steroidogenesis plays important roles in ovarian development of M. amblycephala at the early stages. The significant upregulation of genes (i.e., itpr1, camk2, and mekk2) involved in the "GnRH signaling pathway" was observed in the mature females compared with the immature females, which indicated that the estrogen levels increased after female maturation in M. amblycephala. Moreover, many genes (e.g., gck, creb1, tf2-9, ryr2, asgr1, and creb1) regulating insulin secretion and thyroid hormone synthesis were significantly downregulated after female spawning. The dynamics of gene expression and metabolites observed in this study provide novel cues for guiding fish practical artificial reproduction.

A G protein-coupled receptor mediates neuropeptide-induced oocyte maturation in the jellyfish Clytia

The reproductive hormones that trigger oocyte meiotic maturation and release from the ovary vary greatly between animal species. Identification of receptors for these maturation-inducing hormones (MIHs) and understanding how they initiate the largely conserved maturation process remain important challenges. In hydrozoan cnidarians including the jellyfish Clytia hemisphaerica, MIH comprises neuropeptides released from somatic cells of the gonad. We identified the receptor (MIHR) for these MIH neuropeptides in Clytia using cell culture–based “deorphanization” of candidate oocyte-expressed G protein–coupled receptors (GPCRs). MIHR mutant jellyfish generated using CRISPR-Cas9 editing had severe defects in gamete development or in spawning both in males and females. Female gonads, or oocytes isolated from MIHR mutants, failed to respond to synthetic MIH. Treatment with the cAMP analogue Br-cAMP to mimic cAMP rise at maturation onset rescued meiotic maturation and spawning. Injection of inhibitory antibodies to the alpha subunit of the G_s heterodimeric protein (Gα_S) into wild-type oocytes phenocopied the MIHR mutants. These results provide the molecular links between MIH stimulation and meiotic maturation initiation in hydrozoan oocytes. Molecular phylogeny grouped Clytia MIHR with a subset of bilaterian neuropeptide receptors, including neuropeptide Y, gonadotropin inhibitory hormone (GnIH), pyroglutamylated RFamide, and luqin, all upstream regulators of sexual reproduction. This identification and functional characterization of a cnidarian peptide GPCR advances our understanding of oocyte maturation initiation and sheds light on the evolution of neuropeptide-hormone systems.

A study of jellyfish oocytes identifies the receptor for Maturation-Inducing Hormone, the neuropeptide hormone that triggers oocyte maturation and spawning via GαS and cyclic AMP. This receptor defines a superfamily of hormone-receptor systems involved in regulating sexual reproduction across animal species.

Inhibition of medaka ovulation by gap junction blockers due to its disrupting effect on the transcriptional process of LH-induced Mmp15 expression

Using medaka, we found that in vitro follicle ovulation, but not germinal vesicle breakdown, was inhibited by three gap junction blockers, carbenoxolone, mefloquine, and flufenamic acid. The blockers specifically inhibited follicular expression of matrix metalloproteinase-15 mRNA and the protein (mmp15/Mmp15), a protease indispensable for medaka ovulation, indicating that gap junctional communication may be required for successful ovulation and mmp15/Mmp15 expression. Further experiments using carbenoxolone as the representative of the gap junction blockers showed that expression of nuclear progestin receptor (Pgr), a transcription factor required for mmp15 expression, was not affected by carbenoxolone treatment, but the formation of phosphorylated Pgr was considerably suppressed. Carbenoxolone treatment caused a decrease in the Pgr binding to the promoter region of mmp15. mRNA expression of cyclin-dependent protein kinase-9 (cdk9) and cyclin I (ccni), whose translation products are demonstrated to be involved in Pgr phosphorylation in the medaka ovulating follicles, was suppressed by carbenoxolone treatment. Transcripts of connexin 34.5 (cx34.5) and connexin 35.4 (cx35.4) were dominantly expressed in the follicle cells of ovulating follicles. The results indicate that gap junctional communication plays an important role in medaka ovulation.

The opioid peptide dynorphin suppresses pituitary-ovary axis in the tilapia Oreochromis mossambicus

The opioid peptides are involved in the regulation of neuroendocrine functions in vertebrates. Nonetheless, the influence of an opioid peptide, dynorphin A (DYN), on reproduction in fish is understudied. The aim of this work was to study the influence of DYN on the pituitary-ovary axis in Oreochromis mossambicus. Daily injections (ip) of 250 μg DYN kg^-1 body weight for 22 days during the ovarian cycle caused a reduction in the intensity and the per cent area of luteinizing hormone (LH) immunoreactive content in the proximal pars distalis region of the pituitary gland compared with an intense immunostaining in time-matched controls. In the ovary, DYN treatment caused a decrease in the number of stage I (previtellogenic) follicles compared with time-matched controls. No difference was observed in the number of stage IV (vitellogenic) follicles among different experimental groups, whereas the numbers of stage II and stage III follicles (previtellogenic) were higher in DYN-treated fish than in time-matched controls. Nonetheless, there was a reduction in the number of stage V (preovulatory) follicles in DYN-treated fish compared with time-matched controls. Taken together, these results indicate that DYN exerts an inhibitory effect on follicular recruitment at the late vitellogenic stage, through the suppression of LH secretion in fish.

Wild Leporinus friderici induced spawning with different dose of mGnRHa and metoclopramide or carp pituitary extract

Breeding technology is of utmost importance for reproduction of wild fish in captivity for the reintroduction and selective breeding programs purposes. The main challenge is that when applied to wild undomesticated specimens, conventional protocols often cause breeders and/or embryo mortality and spawning failure. In this study, we evaluated the reproductive performance of wild Leporinus friderici, a great importance fish for subsistence fishing in South American rivers, applying conventional and lower-dose hormonal therapies by means of two consecutive experiments. In the first, a conventional (0.5 and 5.5 mg/kg) and a lower carp pituitary extract (CPE) dose (0.5 and 1.0 mg/kg) were applied. In the second, a conventional mammalian GnRH analogue associated with metoclopramide (mGnRHa + MET) (40 µg mGnRHa + 20 mg MET/kg) and a lower dose (4 μg mGnRHa + 2 mg MET/kg and 8 µg + 4 mg of mGnRHa + MET/kg) were applied. Ovulation was observed in all treatments, however, only lower CPE protocol provided viable embryos. High levels of 17α,20β-dihydroxy-4-pregnen-3-one (DHP) and 17β estradiol (E₂) detected in conventional, but not in lower CPE dose, at ovulation, might be associated to the mortality of the embryos. The use of lower CPE dose applied here was the best way to obtain L. friderici viable embryos. These results directly contribute to the knowledge about poorly explored effects of reproductive management and hormonal therapies in wild-type breeders, showing that the use of reduced doses may be an alternative to reproductive success.

Disruption of dmrt1 rescues the all-male phenotype of the cyp19a1a mutant in zebrafish - a novel insight into the roles of aromatase/estrogens in gonadal differentiation and early folliculogenesis

Sex determination and differentiation are complex processes controlled by many different factors; however, the relationships among these factors are poorly understood. Zebrafish gonadal differentiation exhibits high plasticity involving multiple factors and pathways, which provides an excellent model for investigating the interactions between them. Ovarian aromatase (cyp19a1a) and dmrt1 are key factors in directing vertebrate ovary and testis differentiation, respectively. Knockout of zebrafish cyp19a1a leads to all-male offspring, whereas the loss of dmrt1 results in a female-biased sex ratio. In the present study, we established dmrt1^-/- ;cyp19a1a^-/- double mutant zebrafish and discovered that the introduction of the dmrt1 mutation into the cyp19a1a mutant could rescue the all-male phenotype of the latter. Interestingly, despite the lack of aromatase/estrogens, the follicles in the ovary of the rescued cyp19a1a mutant could develop normally up to the previtellogenic stage. Further evidence suggested the ovarian aromatase directed ovarian differentiation by suppressing dmrt1 expression via nuclear estrogen receptors (nERs). Our results provide solid evidence for an interaction between cyp19a1a and dmrt1 in zebrafish gonadal differentiation, and for the dispensability of estrogens in controlling early folliculogenesis.

Life cycle exposure to propiconazole reduces fecundity by disrupting the steroidogenic pathway and altering DNA methylation in zebrafish (Danio rerio)

Propiconazole is fungicide widely used in agriculture, which may enter aquatic ecosystems and affect organisms. In this study, zebrafish (Danio rerio) were exposed to environmentally relevant levels of propiconazole throughout the life cycle, from embryo to sexually mature adults, and the effects on growth, reproduction, and offspring viability were investigated. To investigate the mechanisms of propiconazole action, the sex steroid hormones and the expression of genes related to the hypothalamus-pituitary-gonad-liver (HPGL) axis and DNA methylation were examined. Growth decreased in the parental zebrafish (F0) after exposure to propiconazole for 120 days. In males, increases in the levels of 17β-estradiol and vitellogenin were observed. The alterations in sex steroid hormones were regulated by the expression of genes involved with the HPGL axis. The decreases in fecundity and fertilization of the F0 was induced by the global DNA methylation, and then may result in the abnormal development of the F1. Therefore, propiconazole disrupted the steroidogenic pathway and caused changes in global DNA methylation that induced reproductive toxicity.

The role of PDE5a in oocyte maturation of zebrafish

The importance of cyclic guanosine monophosphate (cGMP) signaling pathway in oocyte maturation has recently attracted much attention in vertebrates. Previously, using zebrafish as a model, we have revealed the role of cGMP and the action of cGMP protein kinase (PKG) in oocyte maturation. In the present study, the function of a cGMP specific phosphodiesterase (PDE5a) is further analyzed in oocyte maturation in zebrafish. Two distinct PDE5a coding genes (named PDE5aa and PDE5ab) were identified in zebrafish, and expressed in most adult tissues including ovary. Both pde5aa and pde5ab mRNA are predominantly expressed in the oocyte but not in follicular cells. Two commercial antibodies targeted to mammalian PDE5a and phosphorylated PDE5a were validated in zebrafish, and we found both antibodies can be used to detect PDE5ab and phosphorylated PDE5ab of zebrafish, respectively. Using both antibodies, we found PDE5ab is only expressed in the oocyte and the phosphorylation of PDE5ab in oocyte could be activated during oocyte maturation induced by human chronic gonadotropin. Intriguingly, we found that the oocyte maturation could be stimulated by treatment of either two different PDE5a inhibitors, sildenafil or tadalafil, and such effects could be completely blocked by a PKG inhibitor KT5823 and two gap junction blockers, respectively. All of these results clearly demonstrate the importance of PDE5a in maintaining the oocyte maturation of zebrafish. When compared with mammals, the functional model of PDE5a is different in zebrafish, suggesting the function of PDE5a might shift from the oocyte in fish to the granulosa cell in mammals during evolution.

Gonadal recrudescence and annual reproductive hormone pattern of captive female Spotted Scats (Scatophagus argus)

The Spotted Scat (Scatophagus argus), an important euryhaline fish inhabiting mangrove and coastal regions of Indo-Pacific waters, is both an ornamental and food fish in India. Detailed insight into maturation of Spotted Scat when maintained in aquaculture systems, therefore, needs to be elucidated. Lack of information on annual maturation dynamics of female scat collected from their natural habitat and reared in earthen ponds is the basis of this study. Oocytes were classified into five developmental stages: pre-vitellogenic, vitellogenic, late-vitellogenic, ripe, and follicular atresia. Ovarian maturity stages were subsequently categorized as immature (Stage 1), vitellogenesis (Stage 2), maturing (Stage 3), mature (Stage 4), and spent (Stage 5). In oocytes in primary, secondary and tertiary yolk stages, there are greater concentrations of E2 in vitellogenic females between March and June. Significant increases of E2, T, and 17-OHP paralleled the increase of diameter of late-vitellogenic oocytes in maturing females during July. The completion of vitellogenesis and initiation of germinal vesicle migration in the cytoplasm were evident in mature females (Stage 4) with a decreasing trend of sex steroids in and subsequent to the month of August. There were 50 % of oocytes in the final oocyte maturation stage (FOM) (490-620 μm) until completion of Stage 4 in September. The results of this study indicate there is complete ovarian maturation in female scats captured in their natural habitat and maintained in an earthen pond, which may be important information for hatchery management for induction of spawning of Spotted Scat in aquaculture systems.

Proteomics Recapitulates Ovarian Proteins Relevant to Puberty and Fertility in Brahman Heifers (Bos indicus L.)

High fertility and early puberty in Bos indicus heifers are desirable and genetically correlated traits in beef production. The hypothalamus-pituitary-ovarian (HPO) axis synthesizes steroid hormones, which contribute to the shift from the pre-pubertal state into the post-pubertal state and influence subsequent fertility. Understanding variations in abundance of proteins that govern steroid synthesis and ovarian signaling pathways remains crucial to understanding puberty and fertility. We used whole ovaries of six pre-pubertal and six post-pubertal Brahman heifers to conduct differential abundance analyses of protein profiles between the two physiological states. Extracted proteins were digested into peptides followed by identification and quantification with massspectrometry (MS) by sequential window acquisition of all instances of theoretical fragment ion mass spectrometry (SWATH-MS). MS and statistical analysis identified 566 significantly differentially abundant (DA) proteins (adjusted p < 0.05), which were then analyzed for gene ontology and pathway enrichment. Our data indicated an up-regulation of steroidogenic proteins contributing to progesterone synthesis at luteal phase post-puberty. Proteins related to progesterone signaling, TGF-β, retinoic acid, extracellular matrix, cytoskeleton, and pleiotrophin signaling were DA in this study. The DA proteins probably relate to the formation and function of the corpus luteum, which is only present after ovulation, post-puberty. Some DA proteins might also be related to granulosa cells signaling, which regulates oocyte maturation or arrest in ovaries prior to ovulation. Ten DA proteins were coded by genes previously associated with reproductive traits according to the animal quantitative trait loci (QTL) database. In conclusion, the DA proteins and their pathways were related to ovarian activity in Bos indicus cattle. The genes that code for these proteins may explain some known QTLs and could be targeted in future genetic studies.

Ontogeny of the specificity of gonadotropin receptors and gene expression in carp

The pituitary gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), are the principle endocrine drivers of reproductive processes in the gonads of jawed vertebrates. Canonically, FSH recruits and maintains selected ovarian follicles for maturation and LH induces the stages of germinal vesicle breakdown and ovulation. In mammals, LH and FSH specifically activate cognate G-protein-coupled receptors that affect the proteins involved in steroidogenesis, protein hormone synthesis, and gametogenesis. This dual-gonadotropin model also exists in some fish species, but not in all. In fact, due to their diverse number of species, extended number of ecological niches, and remarkably flexible reproductive strategies, fish are appropriate as models to understand the co-evolution of gonadotropins and their receptors. In this study, we cloned and characterized the expression profile over the final stages of ovarian maturation of carp (Cyprinus carpio) LHCGR and FSHR. Expression of both gonadotropin receptors increased in the later stage of early vitellogenesis, suggesting that both LH and FSH play a role in the development of mature follicles. We additionally tested the activation of cLHCGR and cFSHR using homologous and heterologous recombinant gonadotropins in order to gain insight into an evolutionary model of permissive gonadotropin receptor function. These data suggest that carp (Cyprinus carpio) gonad development and maturation depends on a specific gonadotropin profile that does not reflect the temporally distinct dual-gonadotropin model observed in salmonids or mammals, and that permissive gonadotropin receptor activation is a specific feature of Ostariophysi, not all teleosts.

Investigating the role of prostaglandin receptor isoform EP4b in zebrafish ovulation

Prostaglandins (PGs) are a class of fatty acid-derived hormones that play an essential role in the regulation of ovulation of teleosts. This study investigated the various isoforms of ovarian PG receptors in the zebrafish ovary and their role in ovulation. Using real time qPCR, six PG receptor isoforms (ptger1a, ptger1b, ptger2a, ptger4a, ptger4b, and ptgfr) were shown to be expressed in the ovary. Only the PG receptor isoform ptger4b was upregulated at the time of ovulation in vivo, or following treatment in vivo with Ovaprim, which contains a gonadotropin releasing hormone analogue and a dopamine receptor antagonist and stimulates ovulation. Treatment of full-grown follicles with the maturation-inducing hormone 17α,20β-dihydroxy-4-pregnen-3-one (17,20βP) in vitro also induced expression of EP4b mRNA. Females ovulate in vivo after injection with Ovaprim, or injection with Ovaprim and inhibitors of EP1 (ONO-8130) or EP2 (TG4-155) function; they do not ovulate when injected with Ovaprim and an EP4 inhibitor (GW237368x). These findings suggest that the EP4 receptor, in particular the EP4b isoform, is essential for ovulation.

Chronic exposure to environmental concentrations of phenanthrene impairs zebrafish reproduction

Phenanthrene (PHE) is a tricyclic polycyclic aromatic hydrocarbon which distributed extensively in the aquatic environment. However, the knowledge about its impact on fish reproduction is still limited, particularly under a chronic exposure regime. In this study, we exposed zebrafish (Danio rerio) embryos to environmentally relevant concentrations (0.2, 1.0, and 5.0 μg/L) of PHE for 4 months and assessed the impact on reproduction. The results demonstrated that egg production was decreased in fish exposed to PHE, with a significant reduction at 5.0 μg/L. The exposure significantly decreased the circulating concentrations of estradiol (E2) and testosterone (T) in female fish or E2 in male fish. In addition, plasma vitellogenin levels were significantly inhibited after PHE exposure in female fish. The transcription of hypothalamic-pituitary-gonadal (HPG) axis related genes (GnRH2, FSHβ, LHβ, 17β-HSD, CYP11A1, and CYP19a) were significantly altered in a sex-specific manner. In addition, embryos derived from exposed parents exhibited increased malformation and decreased hatching success in the F1 generation. Taken together, these results demonstrate that chronic exposure to environmentally relevant concentration of PHE could cause adverse effects on reproduction and impair the development of offspring, ultimately leading to fish population decline in aquatic environment.

Reproductive cycle of northern largemouth bass Micropterus salmoides salmoides

The seasonal dynamics of the northern largemouth bass (Micropterus salmoides salmoides) reproductive cycle was investigated in a Midwestern United States population. Mature largemouth bass (n = five males and five females) were collected each month over a one-year period. Environmental cues (water temperature and photoperiod) were recorded in situ using data loggers. Temporal changes in plasma sex steroid hormones (testosterone [T], 11-ketotestosterone [11-KT], and estradiol-17β [E₂ ]) and vitellogenin (Vtg), gamete development (histology), and organosomatic indices were highly associated with temporal water temperature and photoperiod dynamics. Female plasma concentrations of T and E₂ and organosomatic indices exhibited similar temporal patterns, with peaks occurring in early summer. Gamete development in females was associated with increases in gonadosomatic index (GSI) and Vtg. In males, plasma concentrations of T and organosomatic indices exhibited bimodal peaks; high peaks occurred in the spring for T and GSI and in the fall for viscerosomatic and hepatosomatic indices. Male gamete development and GSI closely tracked temporal patterns in plasma T, with GSI lagging behind the T cycle by approximately one sampling period. Plasma concentrations of 11-KT were elevated in late winter and spring and were considerably higher than T throughout the reproductive cycle. Unlike the Florida largemouth bass (M. salmoides floridanus) advanced gamete development was observed in both sexes before winter, which may reduce the immediate energy required for gamete development in the spring. Results from this study also provide baseline information necessary for off-season spawning procedures and endocrine-disruption studies in northern largemouth bass.

Expression of nitric oxide synthase (NOS) in Anabas testudineus ovary and participation of nitric oxide-cyclic GMP cascade in maintenance of meiotic arrest

Participation of cyclic nucleotide-mediated signaling in nitric oxide/soluble guanylate cyclase (NO/sGC) regulation of oocyte maturation (OM) in perch (Anabas testudineus) follicle-enclosed oocytes has been investigated. Congruent with sharp decline in follicular cyclic GMP (cGMP) level, nitric oxide synthase (NOS)-inhibitor (L-NAME) attenuates protein kinase A (PKA) phosphorylation but promotes p-ERK1/2 and p-p34Cdc2 (Thr-161) in maturing oocytes. Conversely, NO donor (SNP) prevents OM, potentially through elevated cGMP synthesis. Expression and localization of Nos2 and Nos3 immunoreactivity in perch ovary varied considerably at progressively higher stages of folliculogenesis. While sGC inhibitor (ODQ) alone could induce OM, 8-bromo-cGMP attenuates 17,20β-P-induced OM indicating functional significance of NO/sGC/cGMP in perch ovary. Interestingly, high NO/cGMP inhibition of OM shows positive relation with elevated cAMP level. MIS induced OM is more susceptible to the oocyte-specific phosphodiesterase (PDE) 3 than PDE4 inhibition. Collectively, high NO/cGMP attenuation of OM potentially involves PDE3 inhibition, cAMP accumulation and PKA activation.

Microcystin-LR influences the in vitro oocyte maturation of zebrafish by activating the MAPK pathway

Harmful cyanobacteria and their production of microcystins (MCs) exert significant toxicity on reproduction of fish, especially the process of oogenesis. Our previous studies demonstrated that MCs have negative impacts on the quantity and quality of mature oocytes in female zebrafish. However, the underlying mechanisms of MCs disrupting oocyte maturation (OM) have been rarely reported. In the present study, in vitro oocytes (immature) were separated from zebrafish and treated with 1, 10, 100 μg/L MC-LR. The serine/threonine protein phosphatase 2A (PP2A) activity was downregulated significantly in oocytes exposed to 10 and 100 μg/L MC-LR for both 2 and 4 h. The phosphorylation levels of mitogen-activated protein kinase (MAPK) were detected without noticeable change in all oocytes treated with MC-LR for 2 h, whereas the activated levels of MAPK subtypes (ERK, p38 and JNK) increased remarkably in the 100 μg/L MC-LR treatment of 4 h. In the oocytes exposed to 100 μg/L MC-LR for 4 h, germinal vesicle breakdown (GVBD) rates changed abnormally and maturation-promoting factor (MPF) activity increased significantly, in accordance with the upregulation of Cyclin B protein levels. Moreover, the MAPK inhibitors (10 μM) were applied to explore the role of MAPK subtypes during MC-LR influencing OM and results showed that ERK inhibitor U0126 and p38 inhibitor SB203580 mitigated the effects of 100 μg/L MC-LR-induced MAPK hyper-phosphorylation and elevated GVBD in the oocytes. In conclusion, the present study indicates that microcystins disrupt the meiotic maturation by the pathway of MC-PP2A-MAPK-OM due to the phosphorylation disorder in oocytes.

The regulation of oocyte maturation and ovulation in the closest sister group of vertebrates

Ascidians are the closest living relatives of vertebrates, and their study is important for understanding the evolutionary processes of oocyte maturation and ovulation. In this study, we first examined the ovulation of Ciona intestinalis Type A by monitoring follicle rupture in vitro, identifying a novel mechanism of neuropeptidergic regulation of oocyte maturation and ovulation. Ciona vasopressin family peptide (CiVP) directly upregulated the phosphorylation of extracellular signal-regulated kinase (CiErk1/2) via its receptor. CiVP ultimately activated a maturation-promoting factor, leading to oocyte maturation via germinal vesicle breakdown. CiErk1/2 also induced expression of matrix metalloproteinase (CiMMP2/9/13) in the oocyte, resulting in collagen degradation in the outer follicular cell layer and liberation of fertile oocytes from the ovary. This is the first demonstration of essential pathways regulating oocyte maturation and ovulation in ascidians and will facilitate investigations of the evolutionary process of peptidergic regulation of oocyte maturation and ovulation throughout the phylum Chordata.

Effects of long term antiprogestine mifepristone (RU486) exposure on sexually dimorphic lncRNA expression and gonadal masculinization in Nile tilapia (Oreochromis niloticus)

Mifepristone (RU486), a clinical abortion agent and potential endocrine disruptor, binds to progestin and glucocorticoid receptors and has multiple functional importance in reproductive physiology. A long-term exposure of RU486 resulted in masculinization of female fish, however, the epigenetic landscape remains elusive. Recent studies demonstrated that long non-coding RNAs (lncRNAs) might play potential roles in epigenetic modulation of sex differentiation, ovarian cancer and germline stem cell survival. To further understand the influence of RU486 exposure on epigenetic regulation, we performed a comparative investigation on sex-biased gonadal lncRNAs profiles using control XX/XY and RU486-induced sex reversed XX Nile tilapia (Oreochromis niloticus) by RNA-seq. In total, 962 sexually differentially expressed lncRNAs and their target genes were screened from the gonads of control and sex reversed fish. In comparison with the control XX group, sex reversal induced by RU486 treatment led to significant up-regulation of 757 lncRNAs and down-regulation of 221 lncRNAs. Hierarchical clustering analysis revealed that global lncRNA expression profiles in RU486-treated XX group clustered into the same branch with the control XY, whereas XX control group formed a separate branch. The KEGG pathway enrichment analysis showed that the cis-target genes between RU486-XX and control-XX were concentrated in NOD - like receptor signaling pathway, Cell adhesion molecules (CAMs) and Biosynthesis of amino acids. Real-time PCR and in situ hybridization experiments demonstrate that lncRNAs showing intense fluctuation during RU486 treatment are also sexually dimorphic during early sex differentiation, which further proves the intimate relationship between lncRNAs and sex differentiation and sexual transdifferentiation. Taken together, our data strongly indicates that a long-term exposure of RU486 resulted in sex reversal of XX female fish and the altered expression of sexually dimorphic lncRNAs might partially account for the sex reversal via epigenetic modification.

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.

Insulin-like growth factor binding proteins inhibit oocyte maturation of zebrafish

The function of insulin-like growth factor (Igf) system in ovary has attracted much attention, but the role of Igf binding proteins (Igfbps) in ovary is still largely unknown. In this study, the role of Igfbps in oocyte maturation was investigated in zebrafish. The expression of all eight identified Igfbps except Igfbp6b could be detected in the adult ovary and exhibited differential expression profiles during folliculogenesis. The expression of several Igfbps is dynamically changed during oocyte maturation induced by human chorionic gonadotropin (hCG). By treatment of an Igfbps inhibitor NBI-31772 in vitro, the oocyte maturation could be stimulated in a clear dose-, time- and stage-dependent manner. Such effects were also observed by administration of NBI-31772 in vivo. Igfbps are differentially expressed in both follicular cells and oocytes, but the effect of NBI-31772 could only be found in intact follicles and not in the denuded oocytes. Previous studies have demonstrated that Igf3 is the major Igf member in regulating oocyte maturation of zebrafish. Interestingly, NBI-31772 could increase the effect of Igf3 on oocyte maturation. Furthermore, we found the effect of NBI-31772 on oocyte maturation could be blocked by an Igf type 1 receptor inhibitor BMS-536924 in vitro, suggesting the Igfbps can inhibit the oocyte maturation via Igf/Igf1r pathway. Together, we provided the first evidence in fish that Igfbps inhibit oocyte maturation of zebrafish.

Differential alteration in reproductive toxicity of medaka fish on exposure to nanoscale zerovalent iron and its oxidation products

Nanoscale zerovalent iron (nZVI) is a redox-active nanomaterial commonly used in remediation of soil and groundwater pollution and wastewater treatment processes. A large quantity of nZVI (e.g., >100 mg/L) accidentally released from in situ sites to nearby oxygenized aquifers could be rapidly oxidized to iron oxides (e.g., Fe₃O₄ or Fe₂O₃) and ions (e.g., Fe²⁺), for acute hypoxia effects to aquatic life. However, we do not know the ecotoxicological fate of nZVI and its oxidation products at lower, environmentally concentrations in surface water receiving waterborne transportation or effluent discharge in terms of exposure to aquatic vertebrate species. This study assessed the causal effect on reproductive toxicity in medaka adults (Oryzias latipes) of carboxymethyl cellulose-stabilized nZVI (CMC-nZVI), Fe²⁺ and iron oxide nanoparticles (nFe₃O₄) with 21-day aqueous exposure at 5 and 20 mg/L (Fe-equivalent). Such concentrations did not significantly change the dissolved oxygen, oxidation-reduction potential or pH values in the 3 iron solutions during the fish exposure period. Neither CMC-nZVI nor Fe²⁺ treated adults showed altered daily egg production (fecundity) and oxidative stress responses in observed tissues, as compared to controls. However, the fecundity in nFe₃O₄ (20 mg/L)-treated pairs was significantly decreased, with increased incidence of abnormal immature oocytes in the ovary. As well, nFe₃O₄ treatment suppressed activities of the antioxidants superoxide dismutase and expression of glutathione peroxidase (gpx) in the brain and ovary. Although nFe₃O₄ or Fe²⁺ treatments inhibited mRNA expression of hepatic estrogen receptor (er-α) in females, plasma levels of sex hormones and (Na, K)-ATPase activity in gills of treated fish did not differ from controls for both sexes. Hence, oxidation products (e.g., nFe₃O₄) from nZVI at lower milligram-per-liter levels may be potent in inducing nanoparticle-specific reproductive toxicity in medaka fish by inducing oxidative stress in female gonads. MAIN FINDING: nZVI oxidation product nFe₃O₄ at lower mg/L induces nanoparticle-specific reproductive toxicity in medaka fish.

Recovery of reproductive function of female zebrafish from the toxic effects of microcystin-LR exposure

Fish has a strong resistance to microcystins (MCs), cyclic heptapeptide cyanotoxins, known as endocrine disrupting chemicals (EDCs) which are released during cyanobacterial blooms and many laboratory and field studies have found the hepatic recovery of fish from the MCs exposure. The aim of the present study was to investigate the recovery mechanisms of reproductive function of adult zebrafish (Danio rerio) from microcystin-LR (MC-LR) exposure. Therefore, adult female zebrafish were exposed to 0, 1 or 50 μg/L of MC-LR for 21days and transferred to MC free water for another 21 days to investigate the recovery. After MC-LR exposure, marked histological lesions in the gonads, decreased the percentage of mature oocytes, decreased number of spawned eggs, decreased fertilization and hatching rates were observed. MC-LR exposure increased the concentration of 17β-estradiol (E2), testosterone (T) and vitellogenin (VTG) in female zebrafish. Some gene transcriptions of the hypothalamic-pituitary-gonad (HPG) axis significantly changed. The protein levels of 17βhsd and cyp19a remarkably increased in the MC-LR exposure groups. However, our laboratory observation also indicates that zebrafish transferred from microcystin exposure to toxin-free water and reared for 21 days exhibited a nearly complete recovery of reproductive functions, including histological structure, increased the percentage of matured oocytes and spawned eggs, stable hormone levels, well-balanced transcriptional and translational levels. These results indicate that after MC-LR exposure, the reproductive impairments in zebrafish are also reversible likewise hepatic recovery seen by different studies in fish. Future studies should be conducted to explore a better understanding of the recovery mechanisms of fish from microcystins exposure.

Thermal manipulation and GnRHa therapy applied to the reproduction of lambari-do-rabo-amarelo, Astyanax altiparanae females (Characiformes: Characidae) during the non-breeding season

Lambari-do-rabo-amarelo Astyanax altiparanae in the wild reproduce during spring and summer, but females undergo vitellogenesis throughout the year, including the non-spawning winter period when water temperatures are low. The present study investigated the physiological role of temperature modulation on the hypothalamus-pituitary-gonads axis of lambari during winter, as well as the effects of gonadotropin releasing hormone agonist (GnRHa) therapy. Captive females were exposed to two different temperatures (20 °C and 27 °C) and were injected weekly with GnRHa for 21 days during winter (Control, CTR; Low dose; LD and high dose of GnRHa, HD). At the end of the 21-days period gonadosomatic index (GSI), oocyte stage of development and theoretical fecundity were evaluated, together with plasma levels of 17β-estradiol (E₂). Gene expression of the two pituitary gonadotropins follicle-stimulating hormone (fshβ) and luteinizing hormone (lhβ), as well as hepatic vitellogenin-A (vtgA) expression were also analyzed. At the end of the experimental period, females from the six different experimental conditions were induced to spawn using human chorionic gonadotropin (hCG). Spawning performance parameters and plasma levels of the maturation inducing steroid (MIS) were analyzed. Gene expression of fshβ did not change with temperature manipulation, but females exposed to 27 °C and supplemented with a HD of GnRHa exhibited an increased fshβ gene expression, associated with higher E₂ levels. The higher water temperature alone was able to increase E₂ levels. At both water temperatures GnRHa injections induced a decrease in E₂ levels. GnRHa injected females had a lower vtgA gene expression levels at 20 °C. Even with differences in the gene expression of gonadotropins among the various temperature/GnRHa treatments, GSI and oocyte diameter did not change, but GnRHa enhanced the number of vitellogenic oocytes at 20 °C. The reproductive performance of lambari induced to spawn with hCG was better after the combined treatment with GnRHa and summer temperature.

Environmental exposure to oestrogenic endocrine disruptors mixtures reflecting on gonadal sex steroids and gametogenesis of the neotropical fishAstyanax rivularis

Discharge of municipal wastewater promotes the entry of diverse oestrogenic compounds into the water bodies. This complex mixture of substances interferes in the steroidogenic pathway, being able to promote severe reproductive impairment in freshwater fish populations. The purpose of the present study was to evaluate the effects of oestrogenic endocrine disruptors (EDCs) mixture on gonadal sex steroids (testosterone, T; 11-ketotestosterone, 11-KT; 17β-oestradiol, E2; 17-hydroxyprogesterone, 17-OHP) in the peak of the reproductive season of Astyanax rivularis, correlating the results obtained with the proportion of germ cells and gonadal histopathology. Three sampling sites were chosen to conduct the study, one reference site (S1), without contamination by municipal wastewater and two sites (S2 and S3) receiving discharge of municipal wastewater. Males of A. rivularis presented higher concentrations of E2, lower androgens (T and 11-KT) in gonads when compared to males from site S1. Concentrations of 17-OHP did not present significant difference among sites. In sites S2 and S3, the proportion of early spermatocytes, spermatids and Leydig cells increased while spermatozoa decreased compared to fish from S1. The following gonadal histopathologies were detected in the male fishes: intersex gonads (28% in S3) and testicular degeneration with germinal epithelium exhibiting agglutinated germ cells masses and empty cysts (57% in S2 and 71% in S3). In females, concentrations of T, E2 and 17-OHP did not present significant difference among the sites, however higher 11-KT concentrations were detected in females from sites S2 and S3. A lower proportion of perinucleolar follicles and a higher incidence of vitellogenic follicles, besides, aged oocytes and the presence of eosinophilic proteinaceous fluid in the interstitial compartment were also found in females from impacted sites. These results indicate that the urbanization and consequent release of municipal wastewater containing oestrogenic compounds in the headwater creeks are altering the levels of sex hormones and gametogenesis of A. rivularis. Further studies should be performed to determine whether oestrogenic endocrine disrupters are disrupting the reproduction of A. rivularis.

Transcriptomic Analysis Revealed the Regulatory Mechanisms of Oocyte Maturation and Hydration in Orange-Spotted Grouper (Epinephelus coioides)

Oocyte maturation and hydration are regulated by a complex interplay of various hormones and local factors. We have investigated the morphological changes of follicles and serum steroid levels during the HCG (human choionic gonadotophin)-induced oocyte maturation in the orange-spotted grouper. For the first time, a large-scale transcriptomic analysis of follicles during the maturation has been conducted in a fish species which produce pelagic oocytes. Eight cDNA libraries of follicle samples, from full-grown immature follicles to mature follicles, were constructed. A total of 402,530,284 high-quality clean reads were obtained after filtering, 79.66% of which perfectly mapped to the orange-spotted grouper genome. Real-time PCR results of 12 representative genes related to oocyte maturation and hydration verified the reliability of the RNA-seq data. A large number of genes related to oocyte maturation and hydration were identified in the transcriptome dataset. And the transcriptomic analysis revealed the dynamic changes of the steroid synthesis pathway and the pathway of hydration during oocyte maturation. The present study will facilitate future study on the oocyte maturation and hydration in the orange-spotted grouper and other marine pelagic egg spawner.

Physiological relevance of nitric oxide in ovarian functions: An overview

Nitric oxide (NO, nitrogen monoxide), a short-lived, free radical carrying an unpaired electron, is one of the smallest molecules synthesized in the biological system. In addition to its role in angiogenesis, neuronal function and inflammatory response, NO has wide-spread significance in regulation of ovarian function in vertebrates. Based on tissue-specific expression, three different nitric oxide synthase (NOS) isoforms, neuronal (nNOS) or NOS1, inducible (iNOS) or NOS2 and endothelial (eNOS) or NOS3 have been identified. While expression of both inducible (iNOS) and constitutive NOS (eNOS) isoforms varies considerably in the ovary at various stages of follicular growth and development, selective binding of NO with proteins containing heme moieties have significant influence on ovarian steroidogenesis. Besides, NO modulation of ovulatory response suggests physiological significance of NO/NOS system in mammalian ovary. Compared to the duality of NO action on follicular development, steroidogenesis and meiotic maturation in mammalian models, participation of NO/NOS system in teleost ovary is less investigated. Genes encoding nos1 and nos2 have been identified in fish; however, presence of nos3 is still ambiguous. Interestingly, two distinct nos2 genes, nos2a and nos2b in zebrafish, possibly arose through whole genome duplication. Differential expression of major NOS isoforms in catfish ovary, NO inhibition of meiosis resumption in Anabas testudineus follicle-enclosed oocytes and NO/sGC/cGMP modulation of oocyte maturation in zebrafish are some of the recent advancements. The present overview is an update on the advancements made and shortfalls still remaining in NO/NOS modulation of intercellular communication in teleost vis-à-vis mammalian ovary.

Steroidogenic acute regulatory protein and luteinizing hormone are required for normal ovarian steroidogenesis and oocyte maturation in zebrafish†

In recent studies, luteinizing hormone (LH) was reported to play important roles in oocyte maturation. However, the mechanism by which LH signaling, especially regarding the steroidogenesis process, affects oocyte maturation has not been clarified. In this study, zebrafish models with a functional deficiency in luteinizing hormone beta (Lhb) or steroidogenic acute regulatory protein (Star), an enzyme that promotes the transport of cholesterol into the inner mitochondrial membrane for maturation-induced hormone (MIH) production, were generated using transcription activator-like effector nucleases (TALENs). Similar phenotypes of the maturation-arrested oocytes in both female mutants have been observed. The levels of MIH in the oocytes of the female mutants were clearly decreased in both the lhb and star knockout zebrafish. The expression of star was dramatically down-regulated in the lhb mutant follicles and was clearly promoted by forskolin and hCG in vitro. Furthermore, treatment with the MIH precursors, pregnenolone or progesterone, as well as with MIH itself rescued the maturation-arrested oocyte phenotypes in both lhb and star mutants. The plasma levels of other steroids, including testosterone, estradiol, and cortisol, were not affected in the lhb mutants, while the levels of gonad hormones testosterone and estradiol were significantly increased in the star mutants. The cortisol levels were decreased in the star mutants. Collectively, our results confirm that LH plays important roles in the initiation of MIH synthesis from cholesterol and maintains oocyte maturation in zebrafish, as well as provide evidence that Star might act downstream of LH signaling in steroidogenesis.

Prostaglandin affects in vitro ovulation and 17α, 20β-Dihydroxy- 4-pregnen-3-one production in longchin goby, Chasmichthys dolichognathus oocytes

This study focused on the association of prostaglandins and a progestin, 17α, 20β-dihydroxy-4-pregnen-3-one (17α20βP) during the ovulation process in longchin goby, Chasmichthys dolichognathus. We performed several in vitro experiments using 850–920 μm diameter oocytes which were at the migratory nucleus stage. With the 890–920 μm diameter oocytes, no significant difference in ovulation was observed in any of the prostaglandins (PGE1, PGE2, and PGF2α) treated groups although PGE2 and PGF2α at concentrations of 50 ng/mL increased ovulation slightly compared with controls; however, 17α20βP production was stimulated with PGE1 alone at low concentrations (5 ng/mL). In 850 μm diameter oocytes, PGF2α at concentrations of 50 and 500 ng/ml resulted in a significant increase in ovulation. 17α20βP (50 ng/ml) alone had no observable effect on ovulation, but in the combined of PGF2α 50 or 500 ng/ml it caused the greatest effect on ovulation. The sensitivity of oocytes to the induction of ovulation varies between 850 and 890–920 μm, it appeared to vary depending on the migration status of nucleus. These results suggest that PGF2α (or combined of 17α20βP) was more potent in inducing ovulation of the longchin goby.

Copper caused reproductive endocrine disruption in zebrafish (Danio rerio)

Cu in surface waters has been demonstrated to affect aquatic animals at ecologically relevant concentrations. However, its effects on reproductive endocrine system and the underlying toxicological mechanisms are largely unknown. In this study, zebrafish (Danio rerio) were exposed to 0, 10, 20, 40 μg/L of Cu for 30 days. Growth, gonad histopathology, the hormone levels and the transcriptional profiles of genes in the hypothalamic-pituitary-gonadal (HPG) axis in both sexes were examined. The results indicated that body weight was significantly reduced, the gonadal development was affected, and the levels of E₂, T and 11-KT were remarkably disturbed in Cu-exposed fish. Moreover, the expression profiles of steroidogenesis-related genes in gonad (3βhsd, 17βhsd, cyp11a1, cyp17, cyp19a, lhr, fshr, hmgra and star) and in brains (ar, cyp19b, erα, er2β, lhβ, fshβ, gnrh2, gnrh3, gnrhr1, gnrh2 and gnrh4) displayed alterations after exposure to Cu. These results demonstrated that Cu could suppress the growth of zebrafish and significantly affect the reproductive biology in both sexes by damaging the structure of the gonads, altering the steroid hormone levels and the expressions of endocrine-related genes in HPG of zebrafish. This study suggests that Cu adversely affects the reproductive endocrine system in zebrafish and could pose a potential threat to fish populations inhabiting Cu-contaminated waters.

microRNA-mRNA analysis in pituitary and hypothalamus of sterile Japanese flounder

Double haploidy is an advantageous situation for genetic mapping and genome sequencing studies. In the present study, the hypothalamus and pituitary gland from sterile and fertile double-haploid (DH) Japanese flounders (aged 5 years) were used as experimental materials for studying the expression of genes in individuals with reproductive disorders, using high-throughput sequencing technology. The results revealed abnormal levels of some hormones in sterile DHs during the breeding season. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analysis showed that the significantly different microRNAs and messenger RNAs were related to metabolism, signal transduction, and melanogenesis; those related to steroid hormone synthesis and secretion related pathways were not detected. Our results suggest that the key to sterility in DHs was the arrested ovary development. However, the reason for arrested ovary development was mainly related to the lower levels of expression of genes involved in steroid biosynthesis in gonads, and was not related to the pituitary. For maintaining homeostasis, the hypothalamus and pituitary would have large differences in several processes, including signal transduction, metabolism, and immune response. The present study provides primary data for further studies on sterility in fish, and even in other animals.

Nuclear Progestin Receptor Phosphorylation by Cdk9 Is Required for the Expression of Mmp15, a Protease Indispensable for Ovulation in Medaka

Ovulation denotes the discharge of fertilizable oocytes from ovarian follicles. Follicle rupture during ovulation requires extracellular matrix (ECM) degradation at the apex of the follicle. In the teleost medaka, an excellent model for vertebrate ovulation studies, LH-inducible matrix metalloproteinase 15 (Mmp15) plays a critical role during rupture. In this study, we found that follicle ovulation was inhibited not only by roscovitine, the cyclin-dependent protein kinase (CDK) inhibitor, but also by CDK9-inhibitor II, a specific CDK9 inhibitor. Inhibition of follicle ovulation by the inhibitors was accompanied by the suppression of Mmp15 expression in the follicle. In follicles treated with the inhibitors, the formation of the phosphorylated nuclear progestin receptor (Pgr) was inhibited. Roscovitine treatment caused a reduction in the binding of Pgr to the promoter region of mmp15. The expression of Cdk9 and cyclin I (Ccni), and their association in the follicle was demonstrated, suggesting that Cdk9 and Ccni may be involved in the phosphorylation of Pgr in vivo. LH-induced follicular expression of ccni/Ccni was also shown. This study is the first to report the involvement of CDK in ECM degradation during ovulation in a vertebrate species.

Developmental toxicity of kresoxim-methyl during zebrafish (Danio rerio) larval development

Kresoxim-methyl (KM) is a broad spectrum strobilurin fungicide that has been used widely on crops around the world. In the present study, we aimed to investigate the toxic effects of KM using various sublethal endpoints during zebrafish (Danio rerio) larval development. Results showed that the LC₅₀ values of KM to zebrafish at multiple life stages (embryo, larvae, juvenile and adult) were 0.340, 0.224, 0.328 and 0.436 mg/L, respectively. The transcription patterns of 45 genes involved in hypothalamic-pituitary-thyroid/gonadal (HPT/HPG) axis, oxidative stress and apoptosis revealed KM could affect zebrafish larval development at multiple pathways. The activities of aromatase, superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), caspase 3 (Cas3) and caspase 9 (Cas9), and the levels of estradiol (E₂), vitellogenin (VTG), thyroid hormones (T₃ and T₄), reactive oxygen species (ROS) and ATP after embryos exposed to KM for 3 d, 6 d and 10 d were correlated well with the transcription of the corresponding molecules involved in these pathways. In addition to providing the first description of the toxic effects induced by KM during larval development, the results of present study also provided the potential mechanisms of KM on multi-level biomarker responses in larval zebrafish.

Ovarian fluid in the three-spined stickleback Gasterosteus aculeatus: effects of egg overripening and sex steroid treatment

The ovarian fluid properties of the three-spined stickleback Gasterosteus aculeatus were studied in overripe and non-overripe ovulated female sticklebacks and in females that were implanted with Silastic capsules containing testosterone (T), oestradiol (E2), 17,20β-dihydroxypregn-4-en-3-one (17,20β-P) or progesterone (P4) into the abdominal cavity. Overripe females had less ovarian fluid than non-overripe females, but with higher dry mass, higher protein concentration and lower viscosity. T and 17,20β-P increased the amount of ovarian fluid and the fluid protein concentration was increased by 17,20β-P. 1-D sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) showed that ovarian fluid contains several proteins, with high individual variability but with no consistent differences between groups. Some of the ovarian fluid proteins appeared to correspond to proteins from the eggs. The results suggest that secretion of ovarian fluid may be influenced by steroid hormones and that changes in its properties are related to the overripening of ovulated eggs. In at least some respects it appears that the changes in the ovarian fluid is a result, rather than the cause of overripening.

Disruption of oocyte maturation by selected environmental chemicals in zebrafish

Oocyte maturation can be a target of endocrine disruption by environmental chemicals capable of acting as hormone mimics, receptor blockers, and/or enzyme inhibitors. Six environmental chemicals (genistein, endosulfan, malathion, iprodione, carbaryl, and glyphosate) were selected to determine their ability to interfere with oocyte maturation in zebrafish. The translucent oocytes undergoing germinal vesicle (nucleus) breakdown (GVBD) were counted and expressed as a ratio of oocytes undergoing GVBD and total oocytes exposed. The GVBD increased significantly in oocytes exposed to 10 IU/ml to 100 IU/ml human chorionic gonadotropin (hCG). The lowest effective concentration of genistein that inhibited hCG-induced GVBD was 30 μM, while endosulfan inhibited it at 0.03 μM concentration. In addition, malathion inhibited hCG-induced GVBD at the lowest concentration of 60 μM. These inhibitory effects were likely due to the chemicals acting as estrogen mimics, induction of estrogen receptors, or increase in aromatase activity resulting in enhanced estrogen action. Fungicide iprodione, possibly acting as a progestin mimic, promoted hCG-induced GVBD at the lowest concentration of 20 μM, while the weed killer glyphosate inhibited hCG-induced GVBD starting at the 50 μM concentration. These results demonstrate the feasibility of using fully grown zebrafish oocytes arrested at the prophase I stage in an in vitro incubation system to evaluate the effects of a variety of environmental chemicals on oocyte maturation.

Sex hormone binding globulin during an annual reproductive cycle in the hepatopancreas and ovary of pejerrey (Odontesthes bonariensis)

In the present study, we determined the hepatopancratic shbg transcript abundance and ovarian immunoreactive Shbg (ir-Shbg) localization in pejerrey females at different gonadal stages during an annual ovarian cycle. In the hepatopancreas, shbg expression remains was constant in pre-vitellogenic stages, decreased at final vitellogenesis to increase again in final maturation and atretic stages to previous levels at post-vitellogenic stages. Related to this, also we found a negative significant relation between sex steroid and shbg expression. On the other hand, in the ovary we found ir-Shbg inside of cortical alveoli, from previtellogenic stages to final maturation. This localization of Shbg in a teleost fish ovary suggests a new role for Shbg in oocytes, that may also extend the oocyte fertilization/development process.

Expression of gonadotropin subunit and gonadotropin receptor genes in wild female New Zealand shortfinned eel (Anguilla australis) during yellow and silver stages

Despite tremendous importance of follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) as primary controllers of reproductive development, information on the expression profiles of the genes encoding gonadotropin subunits and gonadotropin receptors (Fshr and Lhr) in wild eels are essentially non-existent. This study investigated pituitary fshb and lhb mRNA levels and ovarian fshr and lhr mRNA levels of wild shortfinned eels, Anguilla australis at different stages of oogenesis. Protein expression of Fsh in the pituitary was also quantified and visualized using slot blot and immunohistochemistry. Pituitary fshb and lhb mRNA levels showed a differential expression pattern, fshb mRNA levels increasing significantly from the perinucleolus (PN) to the oil droplet stage (OD) before slightly decreasing (not significantly) in the early vitellogenic stage (EV). A similar trend was observed in relative Fsh protein levels analyzed by slot blot and immunohistochemistry, but this trend was not reflected in the plasma levels of sex steroids. In contrast, pituitary lhb mRNA levels increased significantly from the PN to EV stage. A higher expression of Fsh at both mRNA and protein levels in the pituitary of eels at the OD stage compared to other investigated stages suggests that synthesis of Fsh production in the pituitary may reach a peak at the OD stage. In the ovary, transcript abundances of fshr and lhr gradually increased during previtellogenic follicle growth, but markedly and significantly increased thereafter. Taken together, our data suggest i) that Fsh release may be very limited, or absent, prior to onset of puberty in shortfinned eels and ii) that Lh is not functionally important in this fish during the EV stage.

Differential expression of HPG-axis genes in autotetraploids derived from red crucian carp Carassius auratus red var., ♀ × blunt snout bream Megalobrama amblycephala, ♂

Autotetraploid fish (4n = 200, abbreviated as 4nRR), which reach sexual maturity at 1 year of age, were derived from the whole genome duplication of red crucian carp Carassius auratus red var. (RCC; 2n = 100) and possess four sets of chromosomes from RCC. The histological features of the gonads showed that the RCC and 4nRR both possessed normal gonadal structure and could arrive at maturation. To understand the expression characteristics of genes related to reproductive development in the autotetraploid fish, we analysed the nucleotide sequence and expression characteristics of the gnrh2, gthb and gthr genes, which are the pivotal genes of the hypothalamic-pituitary-gonadal (HPG) axis. We found that the gnrh2, gthb and gthr genes in 4nRR share remarkable homology with RCC, but there were obvious differences in expression levels between 4nRR and RCC. These results demonstrate that autotetraploidization can lead to gene expression changes. This study provides insights into the molecular mechanism underlying the reproductive development of autotetraploid fish and is expected to be of great significance for subsequent research on polyploidization.

The genetic program of oocytes can be modified in vivo in the zebrafish ovary

Oocytes, the irreplaceable gametes for generating a new organism, are matured in the ovary of living female animals. It is unknown whether any genetic manipulations can be applied to immature oocytes inside the living ovaries. As a proof-of-concept, we here demonstrate genetic amendments of zebrafish immature oocytes within the ovary. Oocyte microinjection in situ (OMIS) stimulates tissue repair responses, but some of the microinjected immature oocytes are matured, ovulated and fertilizable. By OMIS-mediated Cas9 approach, ntla and gata5 loci of oocytes arrested at prophase I of meiosis are successfully edited before fertilization. Through OMIS, high efficiency of biallelic mutations in single or multiple loci using Cas9/gRNAs allows immediate manifestation of mutant phenotypes in F0 embryos and multiple transgenes can co-express the reporters in F0 embryos with patterns similar to germline transgenic embryos. Furthermore, maternal knockdown of dnmt1 by antisense morpholino via OMIS results in a dramatic decrease of global DNA methylation level at the dome stage and causes embryonic lethality prior to segmentation period. Therefore, OMIS opens a door to efficiently modify the genome and provides a possibility to repair genetically abnormal oocytes in situ.

Photoperiodic regulation of plasma gonadotropin levels in previtellogenic greater amberjack (Seriola dumerili)

In Seriola species, exposure to a long photoperiod regime is known to induce ovarian development. This study examined photoperiodic effects on pituitary gene expression and plasma levels of follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) in previtellogenic greater amberjack (Seriola dumerili). The fish were exposed to short (8L:16D) or long (18L:6D) photoperiod. The water temperature was maintained at 22 °C. Compared with the short-photoperiod group, plasma Fsh levels were higher on days 10 and 30 in the long-photoperiod group, but plasma Lh levels did not significantly differ. On day 30, pituitary Fsh- and Lh-β subunit gene expressions were also higher in the long-photoperiod group than the short-photoperiod group, whereas α-subunit gene expressions were higher on days 20 and 30. Throughout the experiment, average gonadosomatic index and plasma E2 levels did not significantly differ between the two groups. This study clearly demonstrated that a long photoperiod induced Fsh release in the previtellogenic fish followed by upregulation of pituitary Fsh and Lh subunit gene expressions. An increase in plasma Fsh levels may be a key factor that mediates the photoperiodic effect on the initiation of ovarian development.