Salinity and photoperiod modulate pubertal development in Atlantic salmon (Salmo salar)

Loss of Fshr Prevents Testicular Maturation in Atlantic Salmon (Salmo salar L.)

Early puberty poses a significant challenge for male Atlantic salmon in aquaculture due to its negative impact on growth and welfare. The regulation of puberty in vertebrates involves 2 key reproductive hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH) and their gonadal receptors. In male mice lacking FSH receptor, testes size is reduced, but fertility is maintained, while medaka and zebrafish with a disrupted fshr gene exhibit near normal testis size and fertility. In these fishes both Fsh and Lh are present during puberty and Lh may rescue fertility, while in salmonid fish only Fsh is present in the circulation during puberty. Using CRISPR-Cas9, we produced crispants with a high prevalence of fshr mutations at the target site, which remained fertile, although more than half showed a testis development deviating from wild-type (wt) males. Crossing out these F0 crispants to each other produced a viable F1 generation showing frameshift (fshr-/-) or in-frame mutations (fshrif/if). Nearly all wt males matured while all fshr-/- males remained immature with small testes containing A spermatogonia as the furthest developed germ cell type and prepubertal plasma androgen levels. Also, the pituitary transcript levels of gnrhr2bba and lhb, but not for fshb, were reduced in the fshr-/- males compared with maturing males. More than half of the fshrif/if mutant males showed no or a delayed maturation. In conclusion, Atlantic salmon show the unique characteristic that loss of Fshr function alone results in male infertility, offering new opportunities to control precocious puberty or fertility in salmon.

The Ideal Diet for Humans to Sustainably Feed the Growing Population - Review, Meta-Analyses, and Policies for Change

INTRODUCTION

As of now, no study has combined research from different sciences to determine the most suitable diet for humans. This issue is urgent due to the predicted population growth, the effect of this on the environment, and the deterioration of human health and associated costs.

METHODS

A literature review determined whether an optimal diet for humans exists and what such a diet is, followed by six meta-analyses. The standard criteria for conducting meta-analyses of observational studies were followed. A review of literature reporting Hazard Ratios with a 95% confidence interval for red meat intake, dairy intake, plant-based diet, fiber intake, and serum IGF-1 levels were extracted to calculate effect sizes.

RESULTS

Results calculated using NCSS software show that high meat consumption increases mortality probability by 18% on average and increases diabetes risk by 50%. Plant-based and high-fiber diets decrease mortality by 15% and 20% respectively ( p < .001). Plant-based diets decreased diabetes risk by 27%, and dairy consumption (measured by increased IGF-1 levels) increased cancer probability by 48% ( p < 0.01). A vegetarian or Mediterranean diet was not found to decrease the probability of heart disease. A vegetarian diet can be healthy or not, depending on the foods consumed. A Mediterranean diet with high quantities of meat and dairy products will not produce the health effects desired. The main limitations of the study were that observational studies were heterogeneous and limited by potential confounders.

DISCUSSION

The literature and meta-analyses point to an optimal diet for humans that has followed our species from the beginnings of humankind. The optimal diet is a whole food, high fiber, low-fat, 90+% plant-based diet. This diet allowed humans to become the most developed species on Earth. To ensure people's nutritional needs are met healthily and sustainably, governmental dietary interventions are necessary.

Adaptive photoperiod interpretation modulates phenological timing in Atlantic salmon

Photoperiod, the portion of 24-h cycle during which an organism is exposed to illumination, is an important phenological cue in many animals. However, despite its influence on critical biological processes, there remain many unknowns regarding how variations in light intensity translate into perceived photoperiod. This experiment examined how light intensity variations affect perceived photoperiod in Atlantic salmon (Salmo salar) to determine whether photoperiod interpretation is, a) fixed such that anything above a minimum detection threshold is regarded as 'illumination', or b) adaptive and varies with recent light exposure. To do this we compared the frequency of smoltification and sexual maturation between groups of male parr which were exposed to one of eight light regimes on a 12:12 cycling regime (12-hour day/12-hour night). The eight regimes were divided into two treatments, four with 'High' daytime light intensity and four with 'Low' daytime light intensity. The 'High' and 'Low' intensity treatments were each sub-divided into four groups for which the subjective 'night' light intensity was 100%, 10%, 1% and 0% of the daytime light intensity, with four replicate tanks of each treatment. The results show that above a minimum detection threshold, Atlantic salmon have adaptive photoperiod interpretation which varies with recent light exposure, and that adaptive photoperiod interpretation modulates the timing of the parr-smolt transformation and sexual maturation. Further, we show that photoperiod interpretation varies between closely related families. Given the influence of phenological timing on species survival, our results reveal a critical role for integration of photoperiod interpretation in attempts to understand how geographically shifting thermal niches due to climate change will affect future populations.

Pituitary Gonadotropin Gene Expression During Induced Onset of Postsmolt Maturation in Male Atlantic Salmon: In Vivo and Tissue Culture Studies

Precocious male maturation causes reduced welfare and increased production costs in Atlantic salmon (Salmo salar) aquaculture. The pituitary produces and releases follicle-stimulating hormone (Fsh), the gonadotropin triggering puberty in male salmonids. However, little is known about how Fsh production is regulated in Atlantic salmon. We examined, in vivo and ex vivo, transcriptional changes of gonadotropin-related genes accompanying the initial steps of testis maturation, in pituitaries of males exposed to photoperiod and temperature conditions promoting maturation (constant light and 16°C). Pituitary fshb, lhb and gnrhr2bba transcripts increased in vivo in maturing males (gonado-somatic index > 0.1%). RNA sequencing (RNAseq) analysis using pituitaries from genetically similar males carrying the same genetic predisposition to mature, but differing by responding or not responding to stimulatory environmental conditions, revealed 144 differentially expressed genes, ~2/3rds being up-regulated in responders, including fshb and other pituitary hormones, steroid-related and other puberty-associated transcripts. Functional enrichment analyses confirmed gene involvement in hormone/steroid production and gonad development. In ex vivo studies, whole pituitaries were exposed to a selection of hormones and growth factors. Gonadotropin-releasing hormone (Gnrh), 17β-estradiol (E₂) and 11-ketotestosterone (11-KT) up-regulated gnrhr2bba and lhb, while fshb was up-regulated by Gnrh but down-regulated by 11-KT in pituitaries from immature males. Also pituitaries from maturing males responded to Gnrh and sex steroids by increased gnrhr2bba and lhb transcript levels, but fshb expression remained unchanged. Growth factors (inhibin A, activin A and insulin-like growth factor 1) did not change gnrhr2bba, lhb or fshb transcript levels in pituitaries either from immature or maturing males. Additional pituitary ex vivo studies on candidates identified by RNAseq showed that these transcripts were preferentially regulated by Gnrh and sex steroids, but not by growth factors, and that Gnrh/sex steroids were less effective when incubating pituitaries from maturing males. Our results suggest that a yet to be characterized mechanism up-regulating fshb expression in the salmon pituitary is activated in response to stimulatory environmental conditions prior to morphological signs of testis maturation, and that the transcriptional program associated with this mechanism becomes unresponsive or less responsive to most stimulators ex vivo once males had entered pubertal developmental in vivo.

Development of supermale and all-male Atlantic salmon to research the vgll3 allele - puberty link

BACKGROUND

Farmed Atlantic salmon are one of the most economically significant global aquaculture products. Early sexual maturation of farmed males represents a significant challenge to this industry and has been linked with the vgll3 genotype. However, tools to aid research of this topic, such as all-male and clonal fish, are still lacking. The present 6-year study examined if all-male production is possible in Atlantic salmon, a species with heteromorphic sex chromosomes (males being XY, females XX), and if all-male fish can be applied to further explore the vgll3 contribution on the likelihood of early maturation.

RESULTS

Estrogen treatment of mixed sex yolk sac larvae gave rise to one sexually mature hermaphrodite with a male genotype (XY) that was used to produce both self-fertilized offspring and androgenetic double haploid (dh) offspring following egg activation with UV treated sperm and pressure shock to block the first mitotic division. There were YY supermales among both offspring types, which were crossed with dh females. Between 1 and 8% of the putative all-male offspring from the eight crosses with self-fertilized supermales were found to have ovaries, and 95% of these phenotypic females were also genetically female. None of the offspring from the one dh supermale cross had ovaries. When assessing the general contribution of the vgll3 locus on the likelihood of early post-smolt sexual maturation (jacking) in the all-male populations we found individuals that were homozygous for the early maturing genotype (97%) were more likely to enter puberty than individuals that were homozygous for the late maturing genotype (26%). However, the likelihood of jacking within individuals with an early/late heterozygous genotype was higher when the early allele came from the dam (94%) compared to the sire (45%).

CONCLUSIONS

The present results show that supermale Atlantic salmon are viable and fertile and can be used as a research tool to study important aspects of sexual maturation, such as to further explore the sex dependent parental genetic contribution to age at puberty in Atlantic salmon. In addition, we report the production of viable double haploid supermale fish.

Ultrasound as a noninvasive tool for monitoring reproductive physiology in male Atlantic salmon (Salmo salar)

We examined the potential for ultrasound as a noninvasive tool for maturation monitoring in Atlantic salmon (Salmo salar) males. Ultrasound examination and measurements were compared to common practices for maturation monitoring such as gonadosomatic index (GSI), sex hormone analysis, and histological analysis of spermatogenesis. There were significant correlations (R² = 0.68, P < 0.01) between ultrasound‐based measurements of the left testis and total testes weight and GSI, and ultrasound could be used for noninvasive GSI measurements. Echogenicity of ultrasound images corresponded to the histological stages observed, which added nuance to ultrasound‐based GSI measurements during final weeks preceding stripping. We propose that ultrasound can be used as an alternative to more invasive methods for sexual maturation monitoring in wild and farmed Atlantic salmon males. Using ultrasound technology, we have established a quick and noninvasive method that could reduce the number of stressful handlings and unwanted sacrifice of broodfish required for maturation monitoring in Atlantic salmon males.

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

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

Transcription Profiles of Age-at-Maturity-Associated Genes Suggest Cell Fate Commitment Regulation as a Key Factor in the Atlantic Salmon Maturation Process

Despite recent taxonomic diversification in studies linking genotype with phenotype, follow-up studies aimed at understanding the molecular processes of such genotype-phenotype associations remain rare. The age at which an individual reaches sexual maturity is an important fitness trait in many wild species. However, the molecular mechanisms regulating maturation timing processes remain obscure. A recent genome-wide association study in Atlantic salmon (Salmo salar) identified large-effect age-at-maturity-associated chromosomal regions including genes vgll3, akap11 and six6, which have roles in adipogenesis, spermatogenesis and the hypothalamic-pituitary-gonadal (HPG) axis, respectively. Here, we determine expression patterns of these genes during salmon development and their potential molecular partners and pathways. Using Nanostring transcription profiling technology, we show development- and tissue-specific mRNA expression patterns for vgll3, akap11 and six6. Correlated expression levels of vgll3 and akap11, which have adjacent chromosomal location, suggests they may have shared regulation. Further, vgll3 correlating with arhgap6 and yap1, and akap11 with lats1 and yap1 suggests that Vgll3 and Akap11 take part in actin cytoskeleton regulation. Tissue-specific expression results indicate that vgll3 and akap11 paralogs have sex-dependent expression patterns in gonads. Moreover, six6 correlating with slc38a6 and rtn1, and Hippo signaling genes suggests that Six6 could have a broader role in the HPG neuroendrocrine and cell fate commitment regulation, respectively. We conclude that Vgll3, Akap11 and Six6 may influence Atlantic salmon maturation timing via affecting adipogenesis and gametogenesis by regulating cell fate commitment and the HPG axis. These results may help to unravel general molecular mechanisms behind maturation.

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

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

Evaluation of endocrine and transcriptomic markers of male maturation in winter-run Steelhead Trout (Oncorhynchus mykiss)

Steelhead Trout (Oncorhynchus mykiss) display a varied life-history, including precocious male maturation at age-1 or age-2. In wild fish, precocious male maturation represents an important component of a diverse life-history portfolio. In hatchery programs, however, it is undesirable if rearing practices increase rates of early male maturation and reduce numbers of anadromous male adults. Our study aimed to develop endocrine and molecular markers for identifying males at early stages of maturation in the spring (prior to smolt release) and evaluated the potential use of these markers for quantifying early male maturation rates at a hatchery scale. In a laboratory study, Skookumchuck winter-run Steelhead Trout were reared at a high growth rate in order to increase the occurrence of precocious male maturation. Fish were lethally sub-sampled in February, prior to the time of smolt release; in May, at the time of smolt release; and in September, when 1+ age maturing males that would spawn the following spring were clearly identifiable based solely on gonadosomatic index (GSI). In February and May samples, we measured GSI, plasma 11-ketotestosterone (11KT), mRNAs for pituitary follicle stimulating hormone (fshb) and luteinizing hormone (lhb) beta subunits, and analyzed stage of spermatogenesis by testis histology. Additionally, in May, we measured testis anti-Müllerian hormone (amh) and insulin-like growth factor 3 (igf3) mRNA. Our primary goal was to evaluate the aforementioned maturation indices for their efficacy in forecasting the proportion of fish initiating early male maturation in the spring (approximately 1 year prior to spermiation), compared to the proportion that actually matured. Combining measures of GSI, plasma 11KT, and pituitary fshb and lhb mRNA expression provided a useful, but conservative, estimate of the proportion of males initiating maturation in the spring (21%) compared to the proportion that were ultimately destined to mature (37%) the following spring. These results suggest that maturation may be less synchronous than previously appreciated and some males may have initiated maturation after our census in May.

Entry into puberty is reflected in changes in hormone production but not in testicular receptor expression in Atlantic salmon (Salmo salar)

BACKGROUND

Puberty in male Atlantic salmon in aquaculture can start as early as after the first winter in seawater, stunts growth and entails welfare problems due to the maturation-associated loss of osmoregulation capacity in seawater. A better understanding of the regulation of puberty is the basis for developing improved cultivation approaches that avoid these problems. Our aim here was to identify morphological and molecular markers signaling the initiation of, and potential involvement in, testis maturation.

METHODS

In the first experiment, we monitored for the first time in large Atlantic salmon males several reproductive parameters during 17 months including the first reproductive cycle. Since testicular growth accelerated after the Winter solstice, we focused in the second experiment on the 5 months following the winter solstice, exposing fish from February 1 onwards to the natural photoperiod (NL) or to continuous additional light (LL).

RESULTS

In the first experiment, testis weight, plasma androgens and pituitary gonadotropin transcript levels increased with the appearance of type B spermatogonia in the testis, but testicular transcript levels for gonadotropin or androgen receptors did not change while being clearly detectable. In the second experiment, all males kept under NL had been recruited into puberty until June. However, recruitment into puberty was blocked in ~ 40% of the males exposed to LL. The first morphological sign of recruitment was an increased proliferation activity of single spermatogonia and Sertoli cells. Irrespective of the photoperiod, this early sign of testis maturation was accompanied by elevated pituitary gnrhr4 and fshb and testicular igf3 transcript levels as well as increased plasma androgen levels. The transition into puberty occurred again with stable testicular gonadotropin and androgen receptor transcript levels.

CONCLUSIONS

The sensitivity to reproductive hormones is already established before puberty starts and up-regulation of testicular hormone receptor expression is not required to facilitate entry into puberty. The increased availability of receptor ligands, on the other hand, may result from an up-regulation of pituitary Gnrh receptor expression, eventually activating testicular growth factor and sex steroid release and driving germ and Sertoli cell proliferation and differentiation.

The initiation of puberty in Atlantic salmon brings about large changes in testicular gene expression that are modulated by the energy status

Background

When puberty starts before males reach harvest size, animal welfare and sustainability issues occur in Atlantic salmon (Salmo salar) aquaculture. Hallmarks of male puberty are an increased proliferation activity in the testis and elevated androgen production. Examining transcriptional changes in salmon testis during the transition from immature to maturing testes may help understanding the regulation of puberty, potentially leading to procedures to modulate its start. Since differences in body weight influence, via unknown mechanisms, the chances for entering puberty, we used two feed rations to create body weight differences.

Results

Maturing testes were characterized by an elevated proliferation activity of Sertoli cells and of single undifferentiated spermatogonia. Pituitary gene expression data suggest increased Gnrh receptor and gonadotropin gene expression, potentially responsible for the elevated circulating androgen levels in maturing fish. Transcriptional changes in maturing testes included a broad variety of signaling systems (e.g. Tgfβ, Wnt, insulin/Igf, nuclear receptors), but also, activation of metabolic pathways such as anaerobic metabolism and protection against ROS. Feed restriction lowered the incidence of puberty. In males maturing despite feed restriction, plasma androgen levels were higher than in maturing fish receiving the full ration. A group of 449 genes that were up-regulated in maturing fully fed fish, was up-regulated more prominently in testis from fish maturing under caloric restriction. Moreover, 421 genes were specifically up-regulated in testes from fish maturing under caloric restriction, including carbon metabolism genes, a pathway relevant for nucleotide biosynthesis and for placing epigenetic marks.

Conclusions

Undifferentiated spermatogonia and Sertoli cell populations increased at the beginning of puberty, which was associated with the up-regulation of metabolic pathways (e.g. anaerobic and ROS pathways) known from other stem cell systems. The higher androgen levels in males maturing under caloric restriction may be responsible for the stronger up-regulation of a common set of (449) maturation-associated genes, and the specific up-regulation of another set of (421) genes. The latter opened regulatory and/or metabolic options for initiating puberty despite feed restriction. As a means to reduce the incidence of male puberty in salmon, however, caloric restriction seems unsuitable.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5869-9) contains supplementary material, which is available to authorized users.

Influence of salinity on spermatogenesis in adult Nile tilapia (Oreochromis niloticus) testis

Continental waters salinisation is a global threat that has grown because of climate change and human activities, but little is known about how and what biological tracts are affected. The aim of this study was to investigate the influence of different water salinities on the expression of HSP70, PCNA and caspase-3 during spermatogenesis of Nile tilapia. Adult males were submitted to four salinity treatments: (S₀) fresh water, (S₇) 7 g L^-1, (S₁₄) 14 g L^-1, and (S₂₁) 21 g L^-1 for 1, 4, and 9 days. All specimens were in spermatogenic activity and the highest values of the gonadosomatic index (GSI) occurred in the S₀ and S₇. In the morphometric analysis, spermatocytes were the most frequent germ cell detected in all treatments (>50%) and spermatids achieved about 20% of the testicular proportion, with few variations among treatments. Spermatozoa were significantly reduced only in S14 compared to S7. Leydig cells were significantly increased in S₁₄ when compared to S7 but plasma concentrations of 11-KT showed no significant difference among treatments. ELISA assay showed higher testicular expression of HSP70 at 1 day in all groups, followed by a significant decrease at days 4 and 9 in S₁₄ and S₂₁. The expression of PCNA was significantly lower while the activity of caspase-3 was higher in S₁₄ and S₂₁ when compared to S₀ and S₇. These results indicate that higher salinities in S₁₄ and S₂₁ interfere with the relationship between testicular HSP70, PCNA, and caspase-3, but with few effects over spermatogenesis dynamics of Nile tilapia.

Duration of spermatogenesis and identification of spermatogonial stem cell markers in a Neotropical catfish, Jundiá (Rhamdia quelen)

Spermatogenesis is a process driven by stem cell, where germ cell cycle is under the control of a specific genotype species. Considering that Jundiá (Rhamdia quelen) is a Neotropical catfish with great economical importance and useful experimental model, little information is available on basic aspects of its reproductive biology, especially on spermatogenesis. As a result, this study aimed to characterize the male germ cells, estimate the duration of spermatogenesis and evaluate the expression of selected stem cell genes in Jundiá testis. Similar to other fish species, our results showed a remarkable decrease of germ cell nuclear volume during Jundiá spermatogenesis, particularly from type A undifferentiated to late type B spermatogonia and from diplotene to late spermatids. Using a S-phase marker, bromodeoxyuridine (BrdU), the combined duration of meiotic and spermiogenic phases in this species was estimated in approximately 7 days. This is considered very short when compared to mammals, where spermatogenesis last from 30 to 74 days. Selected stem cell genes were partially sequenced and characterized in Jundiá testis. Expression analysis showed higher plzf and pou5f3 mRNA levels in the cell fractions enriched by type A undifferentiated spermatogonia. These results were further confirmed by in situ hybridization that showed strong signal of plzf and pou5f3 mRNA in type A undifferentiated spermatogonia. Altogether, these information will expand our knowledge of the reproductive biology of this species, contributing to improve its production and management, and also for biotechnological applications, such as germ cell transplantation.

Sexual maturation and smoltification in domesticated Atlantic salmon (Salmo salar L.) - is there a developmental conflict?

We present data from two experiments that examined how the developmental processes of smoltification and sexual maturation proceed in parallel in domesticated Atlantic salmon. Onset of maturation and smoltification was stimulated using temperature and photoperiod. Our observations on gonadosomatic index (GSI), spermatogenic activity, gill Na+, K+-ATPase enzyme (NKA) activity, and plasma 11-ketotestosterone (11-KT), Na, Cl, and Ca show that smoltification and maturation were both triggered and developed in parallel in male Atlantic salmon, but that the progressing maturation impaired hypoosmoregulation. Female maturation started after completion of smoltification. Furthermore, we present data showing that domesticated salmon can physiologically smoltify-desmoltify-resmoltify within a short period of time, and that development of a secondary sexual characteristic, such as a kype, depends on size in male postsmolts.

Vgll3 and the Hippo pathway are regulated in Sertoli cells upon entry and during puberty in Atlantic salmon testis

Vgll3 is linked to age at maturity in Atlantic salmon (Salmo salar). However, the molecular mechanisms involving Vgll3 in controlling timing of puberty as well as relevant tissue and cell types are currently unknown. Vgll3 and the associated Hippo pathway has been linked to reduced proliferation activity in different tissues. Analysis of gene expression reveals for the first time that vgll3 and several members of the Hippo pathway were down-regulated in salmon testis during onset of puberty and remained repressed in maturing testis. In the gonads, we found expression in Sertoli and granulosa cells in males and females, respectively. We hypothesize that vgll3 negatively regulates Sertoli cell proliferation in testis and therefore acts as an inhibitor of pubertal testis growth. Gonadal expression of vgll3 is located to somatic cells that are in direct contact with germ cells in both sexes, however our results indicate sex-biased regulation of vgll3 during puberty.

Sex steroid production associated with puberty is absent in germ cell-free salmon

In all vertebrates studied so far, germ cells are not required for pubertal maturation of the gonadal steroidogenic system, subsequent development of secondary sex characteristics and reproductive behavior. To explore if the absence of germ cells affects puberty or growth in Atlantic salmon, germ cell-free (GCF), dnd knockout and wild type (WT) postsmolts were stimulated to enter puberty. No GCF fish entered puberty, whereas 66.7% (males) and 30% (females) WT fish completed or entered puberty, respectively. Expression of genes related to steroidogenesis (star, cyp17a1, cyp11β, cyp19a1a), gonadal somatic cells (insl3, amh, igf3), oocytes (bmp15), gonadotropin receptors (fshr, lhcgr), and pituitary gonadotropic cells (fshb, lhb, gnrhr4) showed an immature status and failure to up-regulate gonadal sex steroid production in male and female GCF fish was also reflected in low or undetectable plasma sex steroids (11-ketotestosterone, estradiol-17β and testosterone). A gender difference (high in females, low in males) was found in the expression of star and cyp17a1 in GCF fish. No clear difference in growth was detected between GCF and immature WT fish, while growth was compromised in maturing WT males. We demonstrate for the first time in a vertebrate that germ cells are required for pubertal activation of the somatic steroidogenic cells.

In-vitro and in-vivo biological activity of recombinant yellowtail kingfish (Seriola lalandi) follicle stimulating hormone

Biologically active recombinant yellowtail kingfish follicle stimulating hormone (rytkFsh) was produced in yeast Pichia pastoris and its biological activity was demonstrated by both in-vitro and in-vivo bioassays. Incubation of ovarian and testicular fragments with the recombinant hormone stimulated E₂ and 11-KT secretion, respectively. In-vivo trial in immature female YTK resulted in a significant increase of plasma E₂ levels and development of oocytes. In males at the early stages of puberty, advancement of spermatogenesis was observed, however plasma 11-KT levels were reduced when administered with rytkFsh.

Developmental exposure to progestins causes male bias and precocious puberty in zebrafish (Danio rerio)

Progestins are aquatic contaminants that in low concentrations can impair fish reproduction. The mechanisms are likely multiple since different progestins interact with other steroid receptors in addition to progesterone receptors. Puberty is the process when animals first acquire the capability to reproduce and it comprises maturation of sperm and eggs. In zebrafish, puberty is initiated around 45days post fertilization (dpf) in females and around 53-55 dpf in males, and is marked by increased production of pituitary gonadotropins. We exposed juvenile zebrafish from 20 to 80 dpf to the androgenic progestin levonorgestrel at concentrations of 5.5, 79 and 834ngL(-1) and to the non-androgenic progestin progesterone at concentrations of 3.7, 77 and 1122ngL(-1), during sexual differentiation and puberty. Levonorgestrel exposure caused 100% males even at the lowest concentration tested whereas progesterone did not affect the sex ratio. Transcript levels of the gonadal genes amh, CYP11B and CYP19a1a indicated that the masculinizing effect of levonorgestrel occurred very rapidly. Transcript concentrations of gonadotropins in pituitaries were low in control fish at 44 dpf, but high at 55 dpf and onward. In fish exposed to levonorgestrel or progesterone gonadotropin transcript concentrations were high already at 44 dpf, indicating that both progestins caused precocious puberty. Gonad histology at 50 dpf confirmed a well advanced sexual maturation, but only in males. Our results show that progestins can affect sexual development in fish and that the androgenic progestin levonorgestrel induces a male phenotype at concentrations similar to those detected in aquatic environments.

Conserved Anti-Müllerian Hormone: Anti-Müllerian Hormone Type-2 Receptor Specific Interaction and Intracellular Signaling in Teleosts

In higher vertebrates, anti-Müllerian hormone (AMH) is required for Müllerian duct regression in fetal males. AMH is also produced during postnatal life in both sexes regulating steroidogenesis and early stages of folliculogenesis. Teleosts lack Müllerian ducts, but Amh has been identified in several species including European sea bass. However, information on Amh type-2 receptor (Amhr2), the specific receptor for Amh binding, is restricted to a couple of fish species. Here, we report on cloning sea bass amhr2, the production of a recombinant sea bass Amh, and the functional analysis of this ligand-receptor couple. Phylogenetic analysis revealed that sea bass amhr2 segregates with Amhr2 from other vertebrates. This piscine receptor is capable of activating Smad proteins. Antibodies raised against sea bass Amh were used to study native and recombinant Amh, revealing proteins in the range of 66-70 kDa corresponding to the full length Amh. Once proteolytically treated, recombinant sea bass Amh generates a 12 kDa C-terminal mature protein, suggesting that contrary to what has been described for other fish Amh proteins, this protein is processed in a similar way as mammalian AMH. The mature sea bass Amh is a biologically active protein able to bind sea bass Amhr2 and, surprisingly, also human AMHR2. In prepubertal sea bass testes, Amh was detected by immunohistochemistry mostly in Sertoli cells surrounding early germ-cell generations. During spermatogenesis, a weaker staining signal could be observed in Sertoli cells surrounding spermatocytes.

Androgens directly stimulate spermatogonial differentiation in juvenile Atlantic salmon (Salmo salar)

We studied the effects of androgens on early stages of spermatogenesis along with androgen receptor binding characteristics and the expression of selected testicular and pituitary genes. To this end, immature Atlantic salmon postsmolts received testosterone (T), adrenosterone (OA, which is converted in vivo into 11-ketotestosterone, 11-KT) or a combination of the two androgens (T+OA). Treatment with OA and T elevated the plasma levels of 11-KT and T, respectively, and co-injection of OA with T lead to high 11-KT levels but prevented plasma T levels to reach the levels observed after injecting T alone. Clear stimulatory effects were recorded as regards pituitary lhb and gnrhr4 transcript levels in fish receiving T, and to a lesser extent in fish receiving OA (but for the lhb transcript only). The two androgen receptors (Ara1 and Ara2) we cloned bound T and 11-KT and responded to these androgens in a similar way. Both androgens down-regulated testicular amh and increased igf3 transcript levels after 1 week of treatment, but effects on growth factor gene expression required sustained androgen stimulation and faded out in the groups with the decreasing T plasma levels. In fish exhibiting a sustained elevation of 11-KT plasma levels (OA and T+OA groups) for 2 weeks, the number of differentiating spermatogonia had increased while the number of undifferentiated spermatogonia decreased. Previous work showed that circulating gonadotropin levels did not increase following androgen treatments of gonad-intact immature male salmonids. Taken together, androgen treatment of immature males modulated testicular growth factor expression that, when sustained for 2 weeks, stimulated differentiation, but not self-renewal, of undifferentiated type A spermatogonia.