Seasonal changes in expression of LH-beta and FSH-beta in male and female three-spined stickleback, Gasterosteus aculeatus

Differences in the contributions of sex linkage and androgen regulation to sex-biased gene expression in juvenile and adult sticklebacks

Different evolutionary interests between males and females can lead to the evolution of sexual dimorphism. However, intersex genetic correlations due to the shared genome can constrain the evolution of sexual dimorphism, resulting in intra-locus sexual conflict. One of the mechanisms resolving this conflict is sex linkage, which allows males and females to carry different alleles on sex chromosomes. Another is a regulatory mutation causing sex-biased gene expression, which is often mediated by gonadal steroids in vertebrates. How do these two mechanisms differ in the contributions to the resolution of intra-locus sexual conflict? The magnitude of sexual conflict often varies between the juvenile and adult stages. Because gonadal steroids change in titre during development, we hypothesized that gonadal steroids play a role in sexual dimorphism expression only at certain developmental stages, whereas sex linkage is more important for sexual dimorphism expressed throughout life. Our brain transcriptome analysis of juvenile and adult threespine sticklebacks showed that the majority of genes that were sex-biased in both stages were sex-linked. The relative contribution of androgen-dependent regulation to the sex-biased transcriptome increased and that of sex linkage declined in adults compared to juveniles. The magnitude of the sex differences was greater in sex-linked genes than androgen-responsive genes, suggesting that sex linkage is more effective than androgen regulation in the production of large sex differences in gene expression. Overall, our data are consistent with the hypothesis that sex linkage is effective in resolving sexual conflict throughout life, whereas androgen-dependent regulation can contribute to temporary resolution of sexual conflict.

Gonad development and reproductive hormones of invasive silver carp (Hypophthalmichthys molitrix) in the Illinois River

Reproduction is a major component of an animal's life history strategy. Species with plasticity in their reproductive biology are likely to be successful as an invasive species, as they can adapt their reproductive effort during various phases of a biological invasion. Silver carp (Hypophthalmicthys molitrix), an invasive cyprinid in North America, display wide variation in reproductive strategies across both their native and introduced ranges, though the specifics of silver carp reproduction in the Illinois River have not been established. We assessed reproductive status using histological and endocrinological methods in silver carp between April and October 2018, with additional histological data from August to October 2017. Here, we show that female silver carp are batch spawners with asynchronous, indeterminate oocyte recruitment, while male silver carp utilize a determinate pattern of spermatogenesis which ceases in the early summer. High plasma testosterone levels in females could be responsible for regulating oocyte development. Our results suggest that silver carp have high spawning activity in the early summer (May-June), but outside of the peak spawning period, female silver carp can maintain spawning-capable status by adjusting rates of gametogenesis and atresia in response to environmental conditions, while males regress their gonads as early as July. The results of this study are compared to reports of silver carp reproduction in other North American rivers as well as in Asia.

Diversity in reproductive seasonality in the three-spined stickleback, Gasterosteus aculeatus

The annual timing of reproduction is a key life history trait with a large effect on fitness. Populations often vary in the timing and duration of reproduction to adapt to different seasonality of ecological and environmental variables between habitats. However, little is known about the molecular genetic mechanisms underlying interpopulation variation in reproductive seasonality. Here, we demonstrate that the three-spined stickleback (Gasterosteus aculeatus) is a good model for molecular genetic analysis of variations in reproductive seasonality. We first compiled data on reproductive seasons of diverse ecotypes, covering marine-anadromous, lake and stream ecotypes, of three-spined stickleback inhabiting a wide range of latitudes. Our analysis showed that both ecotype and latitude significantly contribute to variation in reproductive seasons. Stream ecotypes tend to start breeding earlier and end later than other ecotypes. Populations from lower latitudes tend to start breeding earlier than those from higher latitudes in all three ecotypes. Additionally, stream ecotypes tend to have extended breeding seasons at lower latitudes than at higher latitudes, leading to nearly year-round reproduction in the most southern stream populations. A review of recent progress in our understanding of the physiological mechanisms underlying seasonal reproduction in the three-spined stickleback indicates that photoperiod is an important external cue that stimulates and/or suppresses reproduction in this species. Taking advantage of genomic tools available for this species, the three-spined stickleback will be a good model to investigate what kinds of genes and mutations underlie variations in the physiological signalling pathways that regulate reproduction in response to photoperiod.

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.

Spexin and a Novel Cichlid-Specific Spexin Paralog Both Inhibit FSH and LH Through a Specific Galanin Receptor (Galr2b) in Tilapia

Spexin (SPX) is a 14 amino acid peptide hormone that has pleiotropic functions across vertebrates, one of which is involvement in the brain-pituitary-gonad axis of fish. SPX(1) has been identified in each class of vertebrates, and a second SPX (named SPX2) has been found in some non-mammalian species. We have cloned two spexin paralogs, designated as Spx1a and Spx1b, from Nile tilapia (Oreochromis niloticus) that have varying tissue distribution patterns. Spx1b is a novel peptide only identified in cichlid fish, and is more closely related to Spx1 than Spx2 homologs as supported by phylogenetic, synteny, and functional analyses. Kisspeptin, Spx, and galanin (Gal) peptides and their corresponding kiss receptors and Gal receptors (Galrs), respectively, are evolutionarily related. Cloning of six tilapia Galrs (Galr1a, Galr1b, Galr2a, Galr2b, Galr type 1, and Galr type 2) and subsequent in vitro second-messenger reporter assays for Gα_s, Gα_q, and Gα_i suggests that Gal and Spx activate Galr1a/Galr2a and Galr2b, respectively. A decrease in plasma follicle stimulating hormone and luteinizing hormone concentrations was observed with injections of Spx1a or Spx1b in vivo. Additionally, application of Spx1a and Spx1b to pituitary slices decreased the firing rate of LH cells, suggesting that the peptides can act directly at the level of the pituitary. These data collectively suggest an inhibitory mechanism of action against the secretion of gonadotropins for a traditional and a novel spexin paralog in cichlid species.

Hormonal changes over the spawning cycle in the female three-spined stickleback, Gasterosteus aculeatus

Female three-spined sticklebacks are batch spawners laying eggs in a nest built by the male. We sampled female sticklebacks at different time points, when they were ready to spawn and 6, 24, 48 and 72h post-spawning (hps) with a male. Following spawning, almost all females (15 out of 19) had ovulated eggs again at Day 3 post-spawning (72hps). At sampling, plasma, brain and pituitaries were collected, and the ovary and liver were weighed. Testosterone (T) and estradiol (E2) were measured by radioimmunoassay. Moreover, the mRNA levels of follicle-stimulating hormone (fsh-β) and luteinizing hormone (lh-β) in the pituitary, and of the gonadotropin-releasing hormones (GnRHs: gnrh2, gnrh3) and kisspeptin (kiss2) and its G protein-coupled receptor (gpr54) in the brain were measured by real-time qPCR. Ovarian weights peaked in "ready to spawn" females, dropped after spawning, before again progressively increasing from 6 to 72hps. Plasma T levels showed peaks at 24 and 48hps and decreased at 72hps, while E2 levels increased already at 6hps and remained at high levels up to 48hps. There was a strong positive correlation between T and E2 levels over the spawning cycle. Pituitary lh-β mRNA levels showed a peak at 48hps, while fsh-β did not change. The neuropeptides and gpr54 did not show any changes. The changes in T and E2 over the stickleback spawning cycle were largely consistent with those found in other multiple-spawning fishes whereas the marked correlation between T and E2 does not support T having other major roles over the cycle than being a precursor for E2.

Effects of re-stripping on the seminal characteristics of pacu (Piaractus mesopotamicus) during the breeding season

Seminal characteristics in teleost fish with an annual reproductive period, such as pacu (Piaractus mesopotamicus), may vary during the breeding season. The sperm formed before the beginning of the spawning period may be stored for a long time, causing damage to the cells. Therefore, re-stripping may be an important way to eliminate the "old" and allow for the collection of "new" spermatozoids. In this study, we analyzed the seminal characteristics of hormonally induced pacu at the beginning, middle and end of the breeding season, and we analyzed samples from re-stripped males (stripped first at the beginning, re-stripped in the middle, and re-stripped again at the end of the season) during two breeding seasons. The sperm density, ionic composition, pH, and osmolality were similar among the groups. The semen volume, seminal plasma protein concentration and incidence of morphologically anomalous sperm increased over time. In addition, some parameters that are associated with good-quality semen decreased, such as sperm motility, viability and DNA integrity. Moreover, we observed a positive association among motility, viability and DNA integrity for sperm with elevated 11-ketotestosterone, but there was no such association for fshb or lhb mRNA levels in the pituitary. The semen that was obtained earlier (at the beginning) or from re-stripped males exhibited better characteristics than the other samples collected. In conclusion, collecting semen from pacu at the end of breeding season should be avoided; it is preferable to strip early and then re-strip later in the season, and this approach may be used for diverse aquaculture purposes.

The involvement of gonadotropins and gonadal steroids in the ovulatory dysfunction of the potamodromous Salminus hilarii (Teleostei: Characidae) in captivity

Potamodromous teleosts that require migration to reproduce show dysfunctions that block ovulation and spawning while in captivity. To understand the physiological basis of these reproductive dysfunctions, follicle-stimulating hormone b subunit (fshb) and luteinizing hormone b subunit (lhb) gene expression analyses by real-time quantitative PCR, together with measurements of estradiol (E 2), 17α-hydroxyprogesterone (17α-OHP) and 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-DHP) levels, were carried out throughout the reproductive cycle of the potamodromous Salminus hilarii. The following reproductive stages were evaluated in captive and wild females: previtellogenic (PV), advanced maturation/mature (AM) and regression/spent (REG/SPENT). In the wild females, fshb expression decreased from the PV to the AM stage, and the opposite pattern was detected for E 2, which increased from the PV to the AM stage. fshb was expressed at lower levels in captive than in wild females, and this difference did not change during the reproductive cycle. lhb expression also increased from the PV to the AM stage in both groups, but the wild females at the AM and REG/SPENT stages showed higher lhb expression levels than the captive females. The concentrations of 17α-OHP did not change during the reproductive cycle, and the levels were higher in the captive than in the wild females at all reproductive stages. 17α,20β-DHP levels did not change between wild and captive females. However, in captive females, the transition from PV to AM stage was followed by an increase in 17α,20β-DHP levels. These data indicate that dysfunctions in the gonadotropins and steroids synthesis pathways cause the ovulation failure in captive S. hilarii.

Seasonal changes and endocrine regulation of pejerrey (Odontesthes bonariensis) spermatogenesis in the wild

The endocrine mechanisms that regulate spermatogenesis and their interaction with environmental cues have been poorly studied compared with oogenesis in fish. The aim of this work was to study the spermatogenesis in pejerrey under the influence of photoperiod and water temperature fluctuation in the wild, evaluating the transcript levels of brain Gnrh variants and cyp19a1b, pituitary Gth subunits, gonadal Gth receptors, 11β-hsd, and 11-KT plasma levels. Males at spermiogenic stage were observed during spring and autumn, under a photoperiod above 11h of light and a water temperature below 23 °C. Most arrested males were observed in summer when water temperatures increased above 23 °C. Males at spermatogonial stage were mainly observed in autumn, while most males at spermatocytary stage were caught in winter. An increase of gnrh-I, cyp19a1b, fshb, gpha and 11β-hsd transcripts and 11-KT plasma levels was observed during spermatogonial and/or spermatocytary stage (early spermatogenesis). The spermiogenic stage was associated to the maximum gnrh-I gene expression level and a significant increase of Gth receptors transcripts, being this fact more evident for lhcgr. During this last gonadal stage, cyp19a1b transcript level remained high, while fshb mRNA and 11-KT plasma levels showed a significant decreased compared to that occurred at the spermatocytary stage. Also, gphα and 11β-hsd gene expression levels fell during spermiation up to similar values to those observed in arrested males. A significant correlation between 11-KT and gnrh-I, cyp19a1b, gphα, fshb, 11β-hsd transcripts, and the number of spermatocytes was observed during spermatogenesis. All these findings suggested that in pejerrey, the spermatocyte proliferation occurs mainly during winter under the stimulation of 11-KT induced by FSH through the stimulation of specific enzymes, including the 11β-hsd while spermiation occurs after photoperiod increase and with temperatures of the water below 23 °C, through the stimulation of gnrh-I, cyp19a1b and lhcgr.

Molecular characterization of three gonadotropin subunits and their expression patterns during ovarian maturation in Cynoglossus semilaevis

The endocrine regulation of reproduction in a multiple spawning flatfish with an ovary of asynchronous development remains largely unknown. The objectives of this study were to monitor changes in mRNA expression patterns of three gonadotropin hormone (GTH) subunits (FSHβ, LHβ and CGα) and plasma GTH levels during ovarian maturation of half-smooth tongue sole Cynoglossus semilaevis. Cloning and sequence analysis revealed that the cDNAs of FSHβ, LHβ and CGα were 541, 670 and 685 bp in length, and encode for peptides of 130, 158 and 127 amino acids, respectively. The number of cysteine residues and potential N-linked glycosylation sites of the flatfish GTHs were conserved among teleosts. However, the primary structure of GTHs in Pleuronectiformes appeared to be highly divergent. The FSHβ transcriptional level in the pituitary remained high during the vitellogenic stage while plasma levels of FSH peaked and oocyte development was stimulated. The LHβ expression in the pituitary and ovary reached the maximum level during oocyte maturation stages when the plasma levels of LH peaked. The brain GTHs were expressed at the different ovarian stages. These results suggested that FSH and LH may simultaneously regulate ovarian development and maturation through the brain-pituitary-ovary axis endocrine system in tongue sole.

Molecular characterization of the Chinese alligator follicle-stimulating hormone β subunit (FSHβ) and its expression during the female reproductive cycle

The Chinese alligator Alligator sinensis is an endangered species endemic to China, it has a highly specialized reproductive pattern with low fecundity. Up to date, little is known about the regulation of its female reproductive cycle. Follicle-stimulating hormone (FSH), a glycoprotein hormone, plays a key role in stimulating and regulating ovarian follicular development and egg production. In this study, the complete FSHβ cDNA from the ovary of the Chinese alligator was obtained for the first time, it consists of 843-bp nucleotides, including 120-bp nucleotides of the 5'-untranslated region (UTR), 396-bp of the open reading frame, and 3'-UTR of 327-bp nucleotides. It encodes a 131-amino acid precursor molecule of FSHβ with a signal peptide of 18 amino acids followed by a mature protein of 113 amino acids. Its deduced amino acid sequence shares high identities with the American alligator (100%) and birds (89-92%). Phylogenetic tree analysis of the FSHβ amino acid sequence indicated that alligators cluster into the bird branch. Tissue distribution analyses indicated that FSHβ mRNA is expressed in ovary, intestine and liver with the highest level in the ovary, while not in stomach, pancreas, heart, thymus and thyroid. Expression of FSHβ in ovary increases in May (breeding prophase) and peaks in July (breeding period), it is maintained at high levels through September, then decreases significantly in November (post-reproductive period) and remains relatively low from January to March (hibernating period). These temporal changes of FSHβ expression implicated that it might play an important role in promoting ovarian development during the female reproductive cycle.

Androgens increase lws opsin expression and red sensitivity in male three-spined sticklebacks

Optomotor studies have shown that three-spined sticklebacks (Gasterosteus aculeatus) are more sensitive to red during summer than winter, which may be related to the need to detect the red breeding colour of males. This study aimed to determine whether this change of red light sensitivity is specifically related to reproductive physiology. The mRNA levels of opsin genes were examined in the retinae of sexually mature and immature fish, as well as in sham-operated males, castrated control males, or castrated males implanted with androgen 11-ketoandrostenedione (11 KA), maintained under stimulatory (L16:D8) or inhibitory (L8:D16) photoperiods. In both sexes, red-sensitive opsin gene (lws) mRNA levels were higher in sexually mature than in immature fish. Under L16:D8, lws mRNA levels were higher in intact than in castrated males, and were up-regulated by 11 KA treatment in castrated males. Moreover, electroretinogram data confirmed that sexual maturation resulted in higher relative red spectral sensitivity. Mature males under L16:D8 were more sensitive to red light than males under L8:D16. Red light sensitivity under L16:D8 was diminished by castration, but increased by 11 KA treatment. Thus, in sexually mature male sticklebacks, androgen is a key factor in enhancing sensitivity to red light via regulation of opsin gene expression. This is the first study to demonstrate that sex hormones can regulate spectral vision sensitivity.

Environmental concentrations of an androgenic progestin disrupts the seasonal breeding cycle in male three-spined stickleback (Gasterosteus aculeatus)

Synthetic steroid hormones from contraceptive pharmaceuticals have become global aquatic contaminants. Progestins, the synthetic analogs to progesterone, are receiving increasing attention as contaminants and have been shown to impair reproduction in fish and amphibians at low ng L(-1) concentrations. Certain progestins, such as levonorgestrel have androgenic properties and seem to be several orders of magnitude more potent in terms of reproductive impairment in fish than non-androgenic progestins and progestagens. We recently reported that levonorgestrel has strong androgenic effects in female three-spined sticklebacks (Gasterosteus aculeatus), including induction of the normally male-specific glue protein spiggin and suppression of vitellogenesis. In light of this we investigated if exposure to levonorgestrel could disrupt the highly androgen-dependent seasonal reproductive cycle in male sticklebacks. Male sticklebacks that were in the final stage of a breeding period were exposed to various concentrations of levonorgestrel for six weeks in winter conditions in terms of light and temperature, after which reproductive status was evaluated from gross morphology, histology and key gene transcript levels. During the experimental period the controls had transitioned from full breeding condition into the non-breeding state, including regression of secondary sex characteristics, cessation of spiggin production in the kidney, and resumption of spermatogenesis in the testes. This is ascribed to the natural drop in plasma androgen levels after breeding. However, in the groups concurrently exposed to levonorgestrel, transition to the non-breeding condition was dose-dependently inhibited. Our results show that levonorgestrel can disrupt the seasonal breeding cycle in male sticklebacks. The fitness costs of such an effect could be detrimental to natural stickleback populations. Some effects occurred at a levonorgestrel concentration of 6.5 ng L(-1), well within the range of levonorgestrel levels in surface waters and may therefore occur in progestin-contaminated waters. Furthermore, the effects by levonorgestrel in the present study were likely mediated mainly by its androgenic activity, and the low concentration at which they occurred makes levonorgestrel one of the most potent androgenic contaminants known.

The role of pituitary gonadotropins in gonadal sex differentiation in the protogynous Malabar grouper, Epinephelus malabaricus

The aim of this study was to clarify the roles of 2 gonadotropins (GTHs), follicle-stimulating hormone (FSH) and luteinizing hormone (LH), on sex differentiation in the protogynous Malabar grouper, Epinephelus malabaricus. To do this, the mRNA expression patterns of GTH subunits (cga, fshb, and lhb) in the fish pituitary throughout gonadal sex differentiation were investigated. Real-time reverse transcriptase (RT)-PCR showed that cga and fshb were present in the undifferentiated and ovarian differentiation stages, and that the expression levels significantly increased after ovarian differentiation (AOD). However, lhb was not expressed before ovarian differentiation (BOD) and was first detected AOD. Next, to investigate the differentiation and distribution of Fshb and Lhb-producing cells in the pituitary of fish throughout gonadal sex differentiation, immunohistochemical analysis was used to detect teleost GTH subunits. Positive immunoreactivity against Fshb and Lhb was not detected in the pituitary BOD; Fshb and Lhb-positive cells first appeared in the pituitary AOD. It therefore seems unlikely that pituitary gonadotropins play a major role in the control of gonadal sex differentiation in the Malabar grouper.

Conservation of the photoperiodic neuroendocrine axis among vertebrates: evidence from the teleost fish, Gasterosteus aculeatus

Photoperiod, or length of day, has a predictable annual cycle, making it an important cue for the timing of seasonal behavior and development in many organisms. Photoperiod is widely used among temperate and polar animals to regulate the timing of sexual maturation. The proper sensing and interpretation of photoperiod can be tightly tied to an organism's overall fitness. In photoperiodic mammals and birds the thyroid hormone pathway initiates sexual maturation, but the degree to which this pathway is conserved across other vertebrates is not well known. We use the threespine stickleback Gasterosteus aculeatus, as a representative teleost to quantify the photoperiodic response of key genes in the thyroid hormone pathway under controlled laboratory conditions. We find that the photoperiodic responses of the hormones are largely consistent amongst multiple populations, although differences suggest physiological adaptation to various climates. We conclude that the thyroid hormone pathway initiates sexual maturation in response to photoperiod in G. aculeatus, and our results show that more components of this pathway are conserved among mammals, birds, and teleost fish than was previously known. However, additional endocrinology, cell biology and molecular research will be required to define precisely which aspects of the pathway are conserved across vertebrates.

Exposure to the pesticide linuron affects androgen-dependent gene expression in the three-spined stickleback (Gasterosteus aculeatus)

Previous research demonstrated that exposure to exogenous androgens and effluents with androgenic activity can induce spiggin mRNA production in the kidney of the three-spined stickleback (Gasterosteus aculeatus). In the present study, we determine whether a short-term exposure to a known antiandrogenic pesticide, linuron (LN), suppresses spiggin mRNA in male stickleback and in androgenized female stickleback. Primers were designed from previously characterized sequences for each androgen receptor (AR) isoform in stickleback, arα and arβ, to assess whether these receptors are differentially regulated by androgen or antiandrogen exposure. Fish were exposed for 72 h to one of four treatments: control, LN (250 µg/L), 17α-methyltestosterone (MT, 500 ng/L), and an LN-MT mixture at those same concentrations. There was no effect of LN on spiggin and arβ mRNA levels in male kidney, while levels of arα were significantly increased twofold. Exposure to LN significantly inhibited MT-induced spiggin RNA production in female kidney with no effect on expression of arα and arβ. The present study is the first to demonstrate the antiandrogenic effect of LN at the transcript level and to examine androgenic/antiandrogenic responsiveness of the two ARs in the stickleback. From the present study, it was determined that measurement of spiggin RNA is a reliable and sensitive screening tool for the detection of both androgenic and antiandrogenic compounds.

Defining global gene expression changes of the hypothalamic-pituitary-gonadal axis in female sGnRH-antisense transgenic common carp (Cyprinus carpio)

Background

The hypothalamic-pituitary-gonadal (HPG) axis is critical in the development and regulation of reproduction in fish. The inhibition of neuropeptide gonadotropin-releasing hormone (GnRH) expression may diminish or severely hamper gonadal development due to it being the key regulator of the axis, and then provide a model for the comprehensive study of the expression patterns of genes with respect to the fish reproductive system.

Methodology/Principal Findings

In a previous study we injected 342 fertilized eggs from the common carp (Cyprinus carpio) with a gene construct that expressed antisense sGnRH. Four years later, we found a total of 38 transgenic fish with abnormal or missing gonads. From this group we selected the 12 sterile females with abnormal ovaries in which we combined suppression subtractive hybridization (SSH) and cDNA microarray analysis to define changes in gene expression of the HPG axis in the present study. As a result, nine, 28, and 212 genes were separately identified as being differentially expressed in hypothalamus, pituitary, and ovary, of which 87 genes were novel. The number of down- and up-regulated genes was five and four (hypothalamus), 16 and 12 (pituitary), 119 and 93 (ovary), respectively. Functional analyses showed that these genes involved in several biological processes, such as biosynthesis, organogenesis, metabolism pathways, immune systems, transport links, and apoptosis. Within these categories, significant genes for neuropeptides, gonadotropins, metabolic, oogenesis and inflammatory factors were identified.

Conclusions/Significance

This study indicated the progressive scaling-up effect of hypothalamic sGnRH antisense on the pituitary and ovary receptors of female carp and provided comprehensive data with respect to global changes in gene expression throughout the HPG signaling pathway, contributing towards improving our understanding of the molecular mechanisms and regulative pathways in the reproductive system of teleost fish.

Molecular cloning and localization of the luteinizing hormone beta subunit and glycoprotein hormone alpha subunit from Japanese anchovy Engraulis japonicus

Although Clupeiformes contain many economically important species, there is limited information on their reproductive physiology. To obtain more insight into reproductive mechanisms in clupeiform fishes, molecular cloning of the Japanese anchovy Engraulis japonicus luteinizing hormone beta (LHbeta) and glycoprotein hormone alpha (GPHalpha) subunits, and immunocytochemistry of gonadotrophs in the pituitary using antisera raised against the synthetic peptides for both subunits were carried out. The cDNAs for LHbeta and GPHalpha subunits consisted of 963 and 535 nucleotides encoding 141 and 122 amino acids, respectively. The deduced amino acid sequences of the E. japonicus LHbeta subunit showed a 60% similarity to the Pacific herring Clupea pallasii LHbeta subunit and 24-31% similarities to FSHbeta subunits of other fish species. The E. japonicus GPHalpha subunit showed 52-57% similarities to anguilliform and cypriniform GPHalpha subunits. Both the subunits have typical structural characteristics of each subunit such as N-linked glycosylation sites, conserved cysteine residues and highly conserved short amino acid sequences. These results indicate that cDNAs cloned in this study encode the E. japonicus LHbeta and GPHalpha subunits. Reverse-transcription polymerase chain reaction (RT-PCR) revealed that both the LHbeta and GPHalpha subunit genes were abundantly expressed in the pituitary, and the GPHalpha subunit was observed to be weakly expressed in the extrapituitary tissues. Immunocytochemistry of the E. japonicus pituitary showed that cells that immunoreacted with antiserum against the LHbeta subunit were distributed in the peripheral regions of proximal pars distalis, and these cells were also immunoreactive to antiserum against the GPHalpha subunit. An abundant number of both LHbeta and GPHalpha cells in the pituitary of matured fish were observed, in comparison with immature fish. These results indicate that the E. japonicus LH is involved in the final reproductive maturation as well as those of other teleosts.

Sexually dimorphic expression of gonadotropin subunits in the pituitary of protogynous honeycomb grouper (Epinephelus merra): evidence that follicle-stimulating hormone (FSH) induces gonadal sex change

Recent studies have suggested that the hypothalamic-pituitary-gonadal axis is involved in gonadal sex change in sex-changing teleosts. However, its underlying mechanism remains largely unknown. In this study, we focused on the distinct roles of two gonadotropins (GTHs), follicle-stimulating hormone (FSH) and luteinizing hormone (LH), in the protogynous hermaphrodite teleost, honeycomb grouper (Epinephelus merra). First, we investigated the expression pattern of mRNAs for GTH subunits (cga, fshb, and lhb) in the pituitaries from fish at the different sexual phases. Real-time RT-PCR analyses showed that fhsb mRNA levels in the female pituitary were low. However, fshb transcripts increased dramatically in association with testis development. In contrast, levels of cga and lhb mRNAs did not significantly vary during sex change. In addition, immunohistochemical observations of Fshb- and Lhb-producing cells in the pituitary, through the use of specific antibodies for detections of teleost GTH subunits, were consistent with sexually dimorphic expression of Fshb. In order to identify the role of GTH in gonad of honeycomb grouper, we treated females with bovine FSH (50 or 500 ng/fish) or LH (500 ng/fish) in vivo. After 3 wk, FSH treatments induced female-to-male sex change and up-regulated endogenous androgen levels and fshb transcripts, whereas LH treatment had no effect on sex change. These results suggest that FSH may trigger the female-to-male sex change in honeycomb grouper.

Perspectives on fish gonadotropins and their receptors

Teleosts lack a hypophyseal portal system and hence neurohormones are carried by nerve fibers from the preoptic region to the pituitary. The various cell types in the teleost pituitary are organized in discrete domains. Fish possess two gonadotropins (GtH) similar to FSH and LH in other vertebrates; they are heterodimeric hormones that consist of a common alpha subunit non-covalently associated with a hormone-specific beta subunit. In recent years the availability of molecular cloning techniques allowed the isolation of the genes coding for the GtH subunits in 56 fish species representing at least 14 teleost orders. Advanced molecular engineering provides the technology to produce recombinant GtHs from isolated cDNAs. Various expression systems have been used for the production of recombinant proteins. Recombinant fish GtHs were produced for carp, seabream, channel and African catfish, goldfish, eel, tilapia, zebrafish, Manchurian trout and Orange-spotted grouper. The hypothalamus in fishes exerts its regulation on the release of the GtHs via several neurohormones such as GnRH, dopamine, GABA, PACAP, IGF-I, norepinephrine, NPY, kisspeptin, leptin and ghrelin. In addition, gonadal steroids and peptides exert their effects on the gonadotropins either directly or via the hypothalamus. All these are discussed in detail in this review. In mammals, the biological activities of FSH and LH are directed to different gonadal target cells through the cell-specific expression of the FSH receptor (FSHR) and LH receptor (LHR), respectively, and the interaction between each gonadotropin-receptor couple is highly selective. In contrast, the bioactivity of fish gonadotropins seems to be less specific as a result of promiscuous hormone-receptor interactions, while FSHR expression in Leydig cells explains the strong steroidogenic activity of FSH in certain fish species.

Seasonal relationship between gonadotropin, growth hormone, and estrogen receptor mRNA expression in the pituitary gland of largemouth bass

The objectives of this study were to investigate the seasonal changes in pituitary gonadotropins, growth hormone (GH), and estrogen receptor (ER) isoform mRNA in wild female and male largemouth bass (LMB) (Micropterus salmoides) from an unpolluted habitat to better understand reproductive physiology in this ecologically important species. Female pituitary luteinizing hormone (LH) beta subunit and follicle stimulating hormone (FSH) beta subunit mRNA showed significant seasonal variation with levels peaking from January to April and were lowest from May to August. Male LMB showed more variation in gonadotropin subunit expression from month to month. Females had approximately 2-3 times higher gonadotropin mRNA levels in the pituitary when compared to males. All three gonadotropin mRNAs in females were positively correlated to gonadosomatic index (GSI), but only LHbeta mRNA was correlated to GSI in males. Gonadotropin mRNA expression also increased with increasing oocyte and sperm maturation. Gonadotropin beta subunit mRNA expression was positively correlated to GH mRNA in both sexes. The expression of all three ER isoforms was significantly correlated to each other in both sexes. The concurrent increase in all three ER mRNA isoforms with increasing gonadotropin mRNA in females and males suggests a prominent role for E2 feedback on pituitary gonadotropin synthesis in both sexes and that each of the three ER isoforms are likely to play a role in the pituitary during teleost reproduction.

Characterization of antibodies for quantitative determination of spiggin protein levels in male and female three-spined stickleback (Gasterosteus aculeatus)

Spiggin is an adhesive glycoprotein produced in the kidney of sticklebacks during the breeding season and is subsequently secreted into the urinary bladder from where it is employed for nest building. Since the production of the protein has been shown to be under androgenic control, spiggin has been suggested to be a useful biomarker for androgenic substances in the environment. In this study, two polyclonal spiggin antibodies based on synthetic peptides and one polyclonal antibody directed against native spiggin have been characterized. The antibodies ability to identify spiggin was investigated by quantitative immunoassay. For both peptide antibodies the quantification range was determined to be between 1 and 80 ng spiggin and determination of renal spiggin levels from immature and mature males displayed a 15-fold increase in total spiggin content of the kidney resulting in a 6-fold increase in male kidney weight due to hypertrophy. The kidney somatic index (KSI) was found to correlate well with the total renal spiggin content and therefore it appears that KSI in sticklebacks could be used as an initial method to identify substances displaying androgenic effects. Furthermore, western blot analysis revealed that the polyclonal antibodies recognize different spiggin isoforms and that spiggin can be detected in the urinary bladder and kidney of both males and female sticklebacks. In order to develop a quantitative detection method for native spiggin it is necessary to produce a standard that can be used in a bioassay. Due to the adhesive and polymerization characteristics of spiggin the protein is difficult to use as a standard in bioassays. So far spiggin has been shown to exist in at least 14 isoforms, all of which contain polymerization domains. To overcome the solubility problem we have produced recombinant spiggin gamma, with only one polymerization domain, that can be expressed in E. coli. Western blot analysis demonstrated that the polyclonal antibodies were able to detect recombinant spiggin gamma protein in bacterial cell lysate, suggesting that it may be developed into a useful source of standard spiggin to be used for quantitative determination of androgen induced spiggin production in sticklebacks.

Comparative gene expression of gonadotropins (FSH and LH) and peptide levels of gonadotropin-releasing hormones (GnRHs) in the pituitary of wild and cultured Senegalese sole (Solea senegalensis) broodstocks

The Senegalese sole (Solea senegalensis) is a valuable flatfish for aquaculture, but it presents important reproductive problems in captivity. Spawning is achieved by wild-caught breeders but cultured broodstocks fail to spawn spontaneously and, when they do, eggs are unfertilized. To gain knowledge on the physiological basis underlying this reproductive dysfunction, this study aimed at analyzing comparative hormone levels between wild and cultured broodstocks at the spawning season. The Senegalese sole gonadotropin (GTH) subunits, FSHbeta, LHbeta and GPalpha, were cloned and qualitative (in situ hybridization) and quantitative (real-time PCR) assays developed to analyze pituitary GTH gene expression. In females, FSHbeta and GPalpha mRNA levels were higher in wild than in cultured broodstocks, whereas in males all three subunits were highest in cultured. By ELISA, three GnRH forms were detected in the pituitary, displaying a relative abundance of GnRH2>GnRH1>GnRH3. All GnRHs were slightly more abundant in wild than cultured females, whereas no differences were observed in males. Plasma levels of vitellogenin and sex steroids were also analyzed. Results showed endocrine differences between wild and cultured broodstocks at the spawning period, which could be related to the endocrine failure of the reproductive axis in cultured breeders.

Effects of castration and androgen-treatment on the expression of FSH-beta and LH-beta in the three-spine stickleback, gasterosteus aculeatus--feedback differences mediating the photoperiodic maturation response?

In many animals, including the three-spine stickleback (Gasterosteus aculeatus), photoperiod strongly influences reproduction. The aim of this study was to investigate if feedback mechanisms on the brain-pituitary-gonadal axis play a role in mediating the photoperiodic response in the stickleback. To that end, stickleback males, exposed to either non-stimulatory short photoperiod (light/dark 8:16) or under stimulatory long photoperiod (LD 16:8), were subjected to either sham-operation, castration, castration combined with treatment with the androgens 11-ketoandrostenedione (11KA) and testosterone (T), and the effects on levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH)-beta mRNA were analyzed. During breeding season the kidney of the stickleback male hypertrophies and produces a glue used for building nests. Kidney weight and expression of both LH-beta and FSH-beta were higher in sham-operated fish kept under long than under short photoperiod. Under both photoperiods, LH-beta mRNA levels were lower in castrated males compared to sham-operated males and treatment with 11KA and T increased expression, indicating a positive feedback. A positive feedback was also found on FSH-beta expression under long photoperiod, where castration decreased, and androgen replacement restored FSH-beta mRNA expression. On the contrary, castration under short photoperiod instead increased FSH-beta levels whereas treatment with 11KA and T decreased FSH-beta expression, indicating a negative feedback on FSH-beta under these conditions. The positive feedback on FSH-beta expression under stimulatory photoperiod may accelerate maturation, whereas the negative feedback under inhibitory photoperiod may suppress maturation. This could be part of the mechanisms by which photoperiod controls maturation.