Current knowledge on the photoneuroendocrine regulation of reproduction in temperate fish species

Effects of advanced and continuous photoperiod regimes on maturation control and profiles of sex steroids in brook trout (Salvelinus fontinalis)

Maturation of salmonid species reduces growth, flesh quality and immunocompetency and has inhibited, in part, the commercial production of brook trout in Tasmania. Photoperiod manipulation is routinely used to inhibit or reduce the incidence of maturation in salmonids, so an experimental adaptation of this approach was trialled experimentally for brook trout. Mixed-sex fish (age = 14 months) were subjected to simulated natural photoperiod (NP), advanced photoperiod (AP) and continuous photoperiod (CP) to investigate the response of endogenous circannual rhythm upon sexual maturity in this species. Light treatments commenced on the first day of the last month of the southern hemisphere’s winter and concluded towards the end of autumn when fish were ovulating. Maturation was observed in 100% of females and 96% of males held under NP. Fish exposed to AP corrected their maturation cycle by advanced phase shift of their endogenous rhythm for eight weeks and achieved final maturation during March–April. Plasma profiles of testosterone and oestradiol-17β were also adjusted according to advancement of photoperiod. Exposure to AP inhibited maturation by 6% and 8% in males and females, respectively. CP treatment for 10 consecutive months failed to inhibit gonadal development; however, this regime did delay or inhibit spermiation and ovulation.

Expression profiles of types 2 and 3 iodothyronine deiodinase genes in relation to vitellogenesis in a tropical damselfish, Chrysiptera cyanea

Thyroid hormone (TH) is involved in regulating the reproduction of vertebrates. Its physiological action in the target tissues is due to the conversion of TH by iodothyronine deiodinases. In this study, we aimed to clone and characterize type 2 (sdDio2) and type 3 (sdDio3) of the sapphire devil Chrysiptera cyanea, a tropical damselfish that undergoes active reproduction under long-day conditions, and to study the involvement of THs in the ovarian development of this species. When the cDNAs of sdDio2 and sdDio3 were partially cloned, they had deduced amino acid sequences of lengths 271 and 267, respectively, both of which were characterized by one selenocysteine residue. Real-time quantitative PCR (qPCR) revealed that both genes are highly expressed in the whole brain, and sdDio2 and sdDio3 are highly transcribed in the liver and ovary, respectively. In situ hybridization analyses showed positive signals of sdDio2 and sdDio3 transcripts in the hypothalamic area of the brain. Little change in mRNA abundance of sdDio2 and sdDio3 in the brain was observed during the vitellogenic phases. It is assumed that simultaneous activation and inactivation of THs occur in this area because oral administration of triiodothyronine (T3), but not of thyroxine (T4), upregulated mRNA abundance of both genes in the brain. The transcript levels of sdDio2 in the liver and sdDio3 in the ovary increased as vitellogenesis progressed, suggesting that, through the metabolism of THs, sdDio2 and sdDio3 play a role in vitellogenin synthesis in the liver and yolk accumulation/E2 synthesis in the ovary. Taken together, these results suggest that iodothyronine deiodinases act as a driver for vitellogenesis in tropical damselfish by conversion of THs in certain peripheral tissues.

Novel signals of adaptive genetic variation in northwestern Atlantic cod revealed by whole-genome sequencing

Selection can create complex patterns of adaptive differentiation among populations in the wild that may be relevant to management. Atlantic cod in the Northwest Atlantic are at a fraction of their historical abundance and a lack of recovery within the Gulf of Maine has created concern regarding the misalignment of fisheries management structures with biological population structure. To address this and investigate genome-wide patterns of variation, we used low-coverage sequencing to perform a region-wide, whole-genome analysis of fine-scale population structure. We sequenced 306 individuals from 20 sampling locations in U.S. and Canadian waters, including the major spawning aggregations in the Gulf of Maine in addition to spawning aggregations from Georges Bank, southern New England, the eastern Scotian Shelf, and St. Pierre Bank. With genotype likelihoods estimated at almost 11 million loci, we found large differences in haplotype frequencies of previously described chromosomal inversions between Canadian and U.S. sampling locations and also among U.S. sampling locations. Our whole-genome resolution also revealed novel outlier peaks, some of which showed significant genetic differentiation among sampling locations. Comparisons between allochronic winter- and spring-spawning populations revealed highly elevated relative (FST ) and absolute (dxy ) genetic differentiation near genes involved in reproduction, particularly genes associated with the brain-pituitary-gonadal axis, which likely control timing of spawning, contributing to prezygotic isolation. We also found genetic differentiation associated with heat shock proteins and other genes of functional relevance, with complex patterns that may point to multifaceted selection pressures and local adaptation among spawning populations. We provide a high-resolution picture of U.S. Atlantic cod population structure, revealing greater complexity than is currently recognized in management. Our genome-scan approach likely underestimates the full suite of adaptive differentiation among sampling locations. Nevertheless, it should inform the revision of stock boundaries to preserve adaptive genetic diversity and evolutionary potential of cod populations.

Involvement of melatonin in transducing moon-related signals into the reproductive network of the female honeycomb grouper Epinephelus merra

Most groupers (genus Epinephelus) inhabiting tropical and subtropical waters exhibit lunar-related reproductive cycles. Their gametes develop synchronously toward and are released around the species-selected moon phase. Periodical changes in cues from the moon are likely used as zeitgeber, and the hypothalamic-pituitary-gonadal (HPG) axis may be activated after cues are perceived by the sensory organ and transduced as internal signals. The objective of this study was to examine weekly changes in mRNA expression profiles of gonadotropin-releasing hormones (gnrh1 and gnrh2) and the β-subunit of gonadotropins (fshβ and lhβ) during the spawning season (May to June) of the female honeycomb grouper Epinephelus merra, which spawns around the full moon period. When mature females were collected based on the lunar cycle, the gonadosomatic index peaked around the full moon. Ovarian histology revealed that oocytes laden with yolk developed toward the full moon and, subsequently, ovulatory follicles appeared around the last quarter moon, confirming lunar-related spawning with a full moon preference. Real-time quantitative polymerase chain reaction analyses revealed high abundances of fshβ and lhβ toward the first quarter moon, whereas concentrations of gnrh1 and gnrh2 increased around the last quarter moon and the first quarter moon, respectively, suggesting that transcription levels of these hormones fluctuate with the lunar cycle. The measurement of melatonin in the eye around the new moon and the full moon revealed that the ocular melatonin content was higher around the new moon than around the full moon, suggesting that the honeycomb grouper can perceive changes in moonlight. In addition, implantation of an osmotic pump containing melatonin into the body cavity of E. merra reduced the transcription levels of gonadotropins, suggesting that melatonin negatively affects hormonal synthesis at the HPG axis. We concluded that melatonin plays an essential role in transducing periodical changes in moonlight and that decreases in melatonin levels from the new moon to the full moon activate the HPG axis for entrainment of gonadal development and spawning.

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

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

Daily expression of a clock gene in the brain and pituitary of the Malabar grouper (Epinephelus malabaricus)

Recent studies have revealed that, in addition to regulating the circadian system, clock genes such as cryptochrome (Cry) genes are involved in seasonal and lunar rhythmicity in fish. This study clarified the transcriptional characteristics of a Cry subtype (mgCry2) in the brain of the Malabar grouper, Epinephelus malabaricus, which is an important aquaculture species that spawns around the new moon. The cDNA sequence of mgCry2 showed high identity (97-99%) with fish Cry2 and had an open reading frame encoding a protein with 170 amino acids. Phylogenetic analyses revealed that mgCRY2 had high identity with CRY in other fish species. Real-time quantitative polymerase chain reaction (qPCR) showed the widespread distribution of mgCry2 in neural (brain, pituitary, and retina) and peripheral (heart, liver, kidney, spleen, gill, intestine, and ovary) tissues. When immature Malabar groupers were reared under a light-dark cycle (LD = 12:12) and the amounts of mgCry2 mRNA in the telencephalon and diencephalon were measured at 4-h intervals, the levels increased during photophase and decreased during scotophase. Day-night variation in mgCry2 mRNA abundance was also observed in the pituitary. These daily profiles suggest that mgCry2 is a light-responsive gene in neural tissues. In situ hybridization analyses showed that mgCry2 was strongly transcribed in the nucleus lateralis tuberis of the ventral hypothalamus, peripheral area of the proximal pars distalis, and the pars intermedia of the pituitary. We conclude that clock genes expressed in the pituitary and diencephalon play a role in entraining the endocrine network of the Malabar grouper to periodic changes in external cues.

Effects of photoperiod on growth, lipid metabolism and oxidative stress of juvenile gibel carp (Carassius auratus)

A 58-day cultivation experiment was carried out to investigate the effects of photoperiods on growth, lipid metabolism and oxidative stress of juvenile gibel carp. Juveniles (5.41 ± 0.01 g) were cultured under seven light photoperiods (0 h of light (L):24 h of darkness (D), 4L:20D (12:00-16:00 light), 8L:16D (10:00-18:00 light), 12L:12D (8:00-20:00 light), 16L:8D (6:00-22:00 light), 20L:4D (4:00-24:00 light) and 24L:0D) in an indoor recirculating aquaculture system. The light intensity was 1.02 μmol·m^-2·s^-1 (at the tank bottom in a 0.5-m water depth). The fish were fed to satiety three times daily (8:30, 14:30 and 18:30). At the end of the experiment, final body weight, specific growth rate, feed efficiency and feed intake were significantly higher in 16L:8D, 20L:4D and 24L:0D groups than those in other groups (P < 0.05). Long-day photoperiods (16L:8D, 20L:4D and 24L:0D) simultaneously promoted lipogenesis, lipolysis and fatty acid oxidation. The increases in lipid retention efficiency, whole body lipid concentration and liver lipid content (P < 0.05) indicated that lipogenesis exceeded fatty acid oxidation. Liver oxidative stress was induced in juvenile gibel carp by short day lengths. The hepatic total antioxidant capacity, superoxide dismutase, glutathione peroxidase and the contents of metabolite glutathione were the highest in the short-day-length groups (0L:24D, 4L:20D and 8L:16D) (P < 0.05). Based on the growth performance and health status in the long-term cultivation experiment, the optimal photoperiods were 16L:8D, 20L:4D and 24L:0D in juvenile gibel carp.

The gonadotropin-inhibitory hormone system of fish: The case of sea bass (Dicentrarchus labrax)

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide belonging to the RFamide peptide family that was first discovered in quail by Tsutsui and co-workers in the year 2000. Since then, different GnIH orthologues have been identified in all vertebrate groups, from agnathans to mammals. These GnIH genes synthesize peptide precursors that encompass two to four C-terminal LPXRFamide peptides. Functional and behavioral studies carried out in birds and mammals have demonstrated a clear inhibitory role of GnIH on GnRH and gonadotropin synthesis and secretion as well as on aggressive and sexual behavior. However, the effects of Gnih orthologues in reproduction remain controversial in fish with both stimulatory and inhibitory actions being reported. In this paper, we will review the main findings obtained in our laboratory on the Gnih system of the European sea bass, Dicentrarchus labrax. The sea bass gnih gene encodes two putative Gnih peptides (sbGnih1 and sbGnih2), and is expressed in the olfactory bulbs/telencephalon, diencephalon, midbrain tegmentum, rostral rhombencephalon, retina and testis. The immunohistochemical study performed using specific antibodies developed in our laboratory revealed Gnih-immunoreactive (ir) perikarya in the same central areas and Gnih-ir fibers that profusely innervated the brain and pituitary of sea bass. Moreover, in vivo studies revealed the inhibitory role of centrally- and peripherally-administered Gnih in the reproductive axis of male sea bass, by acting at the brain (on gnrh and kisspeptin expression), pituitary (on gnrh receptors and gonadotropin synthesis and release) and gonadal (on androgen secretion and gametogenesis) levels. Our results have revealed the existence of a functional Gnih system in sea bass, and have provided evidence of the differential actions of the two Gnih peptides on the reproductive axis of this species, the main inhibitory role in the brain and pituitary being exerted by the sbGnih2 peptide. Recent studies developed in our laboratory also suggest that Gnih might be involved in the transduction of photoperiod and temperature information to the reproductive axis, as well as in the modulation of daily and seasonal rhythmic processes in sea bass.

Effects of different light spectra on embryo development and the performance of newly hatched turbot (Scophthalmus maximus) larvae

Light is a key environmental factor that synchronizes various life stages from embryo development to sexual maturation in fish. For turbot, light spectra have the most influence at the larval and juvenile stages. In the current study, differences in the development of embryos and the performance of newly hatched turbot larvae exposed to five different spectra: full spectrum (LDF), blue (LDB, peak at 450 nm), green (LDG, peak at 533 nm), orange (LDO, peak at 595 nm) and red (LDR, peak at 629 nm), were examined. At 62.8 h post fertilization, a higher number of embryos exposed to short-wavelengths (LDG and LDB) had developed a heartbeat in comparison with embryos exposed to other wavelengths. Larvae exposed to the green spectrum had higher malformation rates than larvae exposed to the other spectra, indicating that larvae exposed to green light may have significantly reduced survival rates. The results of non-specific immunity parameters showed that the mRNA expression levels of cathepsin D (CTSD), cathepsin F (CTSF), catalase (CAT) and metallothionein (MT) in larvae exposed to LDB were significantly higher than those exposed to other spectra, but CAT activity in larvae exposed to LDB was significantly lower than larvae exposed to the other spectra. There was no significant difference in MT activity in larvae exposed to the five different spectra. The mRNA expression level of lysozyme (LZM) in larvae exposed to LDR was significantly higher than other spectra, while there was no significant difference in LZM activity observed in larvae exposed to LDR, LDG, LDB and LDF. The difference of the enzyme activity of total superoxide dismutase (T-SOD) was not significant among larvae exposed to the five spectra. mRNA expression of the heat shock protein 70 (HSP70) was significantly higher in newly hatched larvae exposed to LDB, LDR and LDG, indicating that larvae exposed to LDB, LDG and LDR exhibited a stress response. The mRNA expression level of the insulin-like growth factor-1 (IGF-1) and growth parameters in the newly hatched larvae exposed to the different spectra were not significantly different. The results of the present study indicate that LDO and LDF should be used for embryo incubation and newly hatched larvae when rearing turbot. This study provides a theoretical basis for optimizing the incubation light environment for fertilized turbot eggs, promoting immunity and reducing stress responses in newly hatched larvae.

The study of ameliorative effect of dietary supplementation of vitamin C, vitamin E, and tryptophan on Labeo rohita (Cyprinidae) fry exposed to intense light

The stress ameliorating effect of dietary supplementation of vitamin C, vitamin E, and tryptophan on rohu Labeo rohita fry was evaluated. Rohu fry (1.1 ± 0.03 g) were cultured under five different feeding regimes: enriched with 0.08% vitamin C (D1), 0.02% vitamin E (D2), 1.42% tryptophan (D3), a combination of these three ingredients at similar doses (D4), and control diet (D5). Rohu fry of D5 were divided into two groups-exposed to experimental light (D5FL) and ambient light (114 ± 4 lx, D5AL). All fry (except D5AL) were exposed at light intensity of 3442 ± 648 lx. Feeding of rohu with enriched diets significantly (P < 0.05) enhanced the survival rate and average weight. A 15-25% higher survival and 1.3-1.8-fold higher average weight were recorded in rohu fed with enriched diet compared to D5FL treatment. Supplementation of vitamin C in diet (D1) of rohu resulted in 4.1-fold and 6.9-fold higher nitric oxide synthase and reduced glutathione (GSH) levels, respectively compared to the D5FL treatment. The tryptophan-enriched diet (D3) showed 5.8-fold higher melatonin and 4.4-fold lower cortisol levels in rohu compared to the D5FL treatment. Significantly (P < 0.05) higher nitric oxide synthase, GSH and melatonin, and lower cortisol, glucose, thiobarbituric acid reactive substances, carbonyl protein, glutathione S-transferase, and glutathione peroxidase levels were found in D4 diet fed rohu compared to the other treatments. Reduced level of stress in D4 treatment resulted in best performance of rohu in terms of less swimming activity and higher survival and growth compared to the other treatments.

Advanced puberty triggered by bi-weekly changes in reproductive factors during the photolabile period in a male teleost fish, Dicentrarchus labrax L

This study evaluated the impact of continuous light (LL) within the photolabile period on advanced puberty in juvenile male European sea bass. The exposure to an LL regime for 1 month, from August 15 to September 15 (LLa/s), was compared to a constant simulated natural photoperiod (NP) and constant continuous light conditions year-round (LLy). Somatic growth, hormone plasma levels, rates of testicular maturation and spermiation, as well as the mRNA levels of some reproductive genes were analyzed. Our results demonstrated that both LLa/s and LLy treatments, which include LL exposure during the photolabile period, were highly effective in inhibiting the gametogenesis process that affects testicular development, and clearly reduced the early sexual maturation of males. Exposure to an LL photoperiod affected body weight and length of juvenile fish during early gametogenesis and throughout the first year of life. Interestingly, LL induced bi-weekly changes in some reproductive factors affecting Gnrh1 and Gnrh2 content in the brain, and also reduced pituitary fshβ expression and plasmatic levels of 11-KT, E₂, Fsh throughout early gametogenesis. We suggest that low levels of E₂ in early September in the LL groups, which would be concomitant with the reduced number of spermatogonial mitoses in these groups, might indicate a putative role for estrogens in spermatogonial proliferation during the early gonadal development of this species. Furthermore, a significant decrease in amh expression was observed, coinciding with low plasma levels of 11-KT under LL regimes, which is consistent with the idea that this growth factor may be crucial for the progress of spermatogenesis in male sea bass.

Transcriptomic profiling reveals molecular regulation of seasonal reproduction in Tibetan highland fish, Gymnocypris przewalskii

BACKGROUND

The Tibetan highland fish, Gymnocypris przewalskii, migrates from Lake Qinghai to its spawning grounds every summer. This seasonal reproduction is critically regulated by intrinsic and extrinsic signals. However, the molecular mechanisms that process environmental oscillations to initiate the seasonal mating are largely unknown.

RESULTS

A transcriptomic analysis was conducted on the brain and gonad of male and female G. przewalskii in reproductive and nonreproductive seasons. We obtained 2034, 760, 1158 and 17,856 differentially expressed genes between the reproductively active and dormant female brain, male brain, ovary and testis. Among these genes, DIO2 was upregulated in the reproductively active brain and gonad of both males and females. Neuroactive ligand-receptor genes were activated in male and female brain. Functional enrichment analysis suggested that retinol metabolism was uniquely stimulated in reproductively active males. Genes involved in GnRH signaling and sex hormone synthesis exhibited higher expression levels in brain and gonad during the reproductive season. A co-expression network classified all the genes into 9 modules. The network pinpointed CDC42 as the hub gene that connected the pathways in responsible for modulating reproduction in G. przewalskii. Meanwhile, the sex pheromone receptor gene prostaglandin receptor was identified to link to multiple endocrine receptors, such as GnRHR2 in the network.

CONCLUSIONS

The current study profiled transcriptomic variations between reproductively active and dormant fish, highlighting the potential regulatory mechanisms of seasonal reproduction in G. przewalskii. Our data suggested that the seasonal regulation of reproduction in G. przewalskii was controlled by the external stimulation of photoperiodic variations. The activated transcription of neuroendocrine and sex hormone synthesis genes contributed to seasonal reproduction regulation in G. przewalskii, which was presumably influenced by the increased day-length during the breeding season.

Environmental Cycles, Melatonin, and Circadian Control of Stress Response in Fish

Fish have evolved a biological clock to cope with environmental cycles, so they display circadian rhythms in most physiological functions including stress response. Photoperiodic information is transduced by the pineal organ into a rhythmic secretion of melatonin, which is released into the blood circulation with high concentrations at night and low during the day. The melatonin rhythmic profile is under the control of circadian clocks in most fish (except salmonids), and it is considered as an important output of the circadian system, thus modulating most daily behavioral and physiological rhythms. Lighting conditions (intensity and spectrum) change in the underwater environment and affect fish embryo and larvae development: constant light/darkness or red lights can lead to increased malformations and mortality, whereas blue light usually results in best hatching rates and growth performance in marine fish. Many factors display daily rhythms along the hypothalamus-pituitary-interrenal (HPI) axis that controls stress response in fish, including corticotropin-releasing hormone (Crh) and its binding protein (Crhbp), proopiomelanocortin A and B (Pomca and Pomcb), and plasma cortisol, glucose, and lactate. Many of these circadian rhythms are under the control of endogenous molecular clocks, which consist of self-sustained transcriptional-translational feedback loops involving the cyclic expression of circadian clock genes (clock, bmal, per, and cry) which persists under constant light or darkness. Exposing fish to a stressor can result in altered rhythms of most stress indicators, such as cortisol, glucose, and lactate among others, as well as daily rhythms of most behavioral and physiological functions. In addition, crh and pomca expression profiles can be affected by other factors such as light spectrum, which strongly influence the expression profile of growth-related (igf1a, igf2a) genes. Additionally, the daily cycle of water temperature (warmer at day and cooler at night) is another factor that has to be considered. The response to any acute stressor is not only species dependent, but also depends on the time of the day when the stress occurs: nocturnal species show higher responses when stressed during day time, whereas diurnal fish respond stronger at night. Melatonin administration in fish has sedative effects with a reduction in locomotor activity and cortisol levels, as well as reduced liver glycogen and dopaminergic and serotonergic activities within the hypothalamus. In this paper, we are reviewing the role of environmental cycles and biological clocks on the entrainment of daily rhythms in the HPI axis and stress responses in fish.

Increasing the reliability and reproducibility of aquatic ecotoxicology: Learn lessons from aquaculture research

Regulatory ecotoxicology highly relies on aquatic toxicity studies carried out under controlled conditions. Researchers recently expressed increasing concern about their possible lack of repeatability/reproducibility in many cases. Poor experimental designs, inappropriate statistics and lack of accurate reporting are often pointed out. However, I believe that there is also insufficient attention paid to the various experimental conditions under which fish studies are conducted. These conditions encompass numerous factors (temperature, photoperiod, food, stressors…) which modulate fish response to chemicals. Their effects are poorly studied in ecotoxicology but have been investigated for decades in aquaculture research. It is therefore proposed herein to consider experimental ecotoxicology from an aquaculture perspective. An overview of modulating factors and plausible associated experimental flaws is presented, with emphasis to fish health, growth and reproduction which are the most common regulatory endpoints. Photoperiod and temperature mainly determine growth/reproductive status for which fish also have species and stage-specific nutritional requirements. Stressors, sex ratio, density, water quality and factorial interactions may induce experimental bias. Modulating factors can strongly limit findings applicability and might explain the lack of reproducibility in some cases. Aquaculture knowledge/experience can already allow avoiding some experimental flaws (e.g., stress) while further research is warranted for some other aspects (e.g., nutrition). Detailed reporting of fish husbandry and experimental conditions is of utmost importance for study quality assessment.

Dose-response effects of light at night on the reproductive physiology of great tits (Parus major): Integrating morphological analyses with candidate gene expression

Artificial light at night (ALAN) is increasingly recognized as a potential threat to wildlife and ecosystem health. Among the ecological effects of ALAN, changes in reproductive timing are frequently reported, but the mechanisms underlying this relationship are still poorly understood. Here, we experimentally investigated these mechanisms by assessing dose‐dependent photoperiodic responses to ALAN in the great tit (Parus major). We individually exposed photosensitive male birds to one of three nocturnal light levels (0.5, 1.5, and 5 lux), or to a dark control. Subsequent histological and molecular analyses on their testes indicated a dose‐dependent reproductive response to ALAN. Specifically, different stages of gonadal growth were activated after exposure to different levels of light at night. mRNA transcript levels of genes linked to the development of germ cells (stra8 and spo11) were increased under 0.5 lux compared to the dark control. The 0.5 and 1.5 lux groups showed slight increases in testis size and transcript levels associated with steroid synthesis (lhr and hsd3b1) and spermatogenesis (fshr, wt1, sox9, and cldn11), although spermatogenesis was not detected in histological analysis. In contrast, all birds under 5 lux had 10 to 30 times larger testes than birds in all other groups, with a parallel strong increase in mRNA transcript levels and clear signs of spermatogenesis. Across treatments, the volume of the testes was generally a good predictor of testicular transcript levels. Overall, our findings indicate that even small changes in nocturnal light intensity can increase, or decrease, effects on the reproductive physiology of wild organisms.

Characterization of basal gene expression trends over a diurnal cycle in Xiphophorus maculatus skin, brain and liver

Evolutionarily conserved diurnal circadian mechanisms maintain oscillating patterns of gene expression based on the day-night cycle. Xiphophorus fish have been used to evaluate transcriptional responses after exposure to various light sources and it was determined that each source incites distinct genetic responses in skin tissue. However, basal expression levels of genes that show oscillating expression patterns in day-night cycle, may affect the outcomes of such experiments, since basal gene expression levels at each point in the circadian path may influence the profile of identified light responsive genes. Lack of knowledge regarding diurnal fluctuations in basal gene expression patterns may confound the understanding of genetic responses to external stimuli (e.g., light) since the dynamic nature of gene expression implies animals subjected to stimuli at different times may be at very different stages within the continuum of genetic homeostasis. We assessed basal gene expression changes over a 24-hour period in 200 select Xiphophorus gene targets known to transcriptionally respond to various types of light exposure. We identified 22 genes in skin, 36 genes in brain and 28 genes in liver that exhibit basal oscillation of expression patterns. These genes, including known circadian regulators, produced the expected expression patterns over a 24-hour cycle when compared to circadian regulatory genes identified in other species, especially human and other vertebrate animal models. Our results suggest the regulatory network governing diurnal oscillating gene expression is similar between Xiphophorus and other vertebrates for the three Xiphophorus organs tested. In addition, we were able to categorize light responsive gene sets in Xiphophorus that do, and do not, exhibit circadian based oscillating expression patterns.

The pars tuberalis: The site of the circannual clock in mammals?

Accurate timing and physiological adaptation to anticipate seasonal changes are an essential requirement for an organism's survival. In contrast to all other environmental cues, photoperiod offers a highly predictive signal that can be reliably used to activate a seasonal adaptive programme at the correct time of year. Coupled to photoperiod sensing, it is apparent that many organisms have evolved innate long-term timekeeping systems, allowing reliable anticipation of forthcoming environmental changes. The fundamental biological processes giving rise to innate long-term timing, with which the photoperiod-sensing pathway engages, are not known for any organism. There is growing evidence that the pars tuberalis (PT) of the pituitary, which acts as a primary transducer of photoperiodic input, may be the site of the innate long-term timer or "circannual clock". Current research has led to the proposition that the PT-specific thyrotroph may act as a seasonal calendar cell, driving both hypothalamic and pituitary endocrine circuits. Based on this research we propose that the mechanistic basis for the circannual rhythm appears to be deeply conserved, driven by a binary switching cell based accumulator, analogous to that proposed for development. We review the apparent conservation of function and pathways to suggest that these broad principles may apply across the vertebrate lineage and even share characteristics with processes driving seasonal adaptation in plants.

Influence of light intensity and spectral composition of artificial light at night on melatonin rhythm and mRNA expression of gonadotropins in roach Rutilus rutilus

In this study we investigated the influence of artificial light at night (ALAN) of different intensities (0, 1, 10, 100 lx) and different colours (blue, green, red) on the daily melatonin rhythm and mRNA expression of gonadotropins in roach Rutilus rutilus, a ubiquitous cyprinid, which occur in standing and moderately flowing freshwater habitats of central Europe. Melatonin concentrations were significantly lowered under nocturnal white light already at 1 lx. Low intensity blue, green and red ALAN lowered the melatonin levels significantly in comparison to a dark control. We conclude that ALAN can disturb melatonin rhythms in roach at very low intensities and at different wavelengths and thus light pollution in urban waters has the potential to impact biological rhythms in fish. However, mRNA expression of gonadotropins was not affected by ALAN during the period of the experiments. Thus, suspected implications of ALAN on reproduction of roach could not be substantiated.

Does constant photoperiod inhibit the onset of the reproductive cycle in northern pike (Esox lucius) males?

For most temperate and arctic fish species, photoperiod is the key abiotic factor determining the onset of the reproductive cycle, and, for many species, constant long photoperiod seems to have an inhibitory effect. Yet, there is no knowledge about the effect of photoperiod on the onset of the reproductive cycle in northern pike males. The aim of this work was to investigate the potential inhibitory effect of constant long photoperiod on pike males. Batches of fish were reared in triplicate under natural simulated photoperiod (NSP) or constant long photoperiod (CP; 16L:8D) from 14 October 2013 to 14 March 2014. Fish were sampled three times (at the beginning of the photoperiod decrease, at the beginning of the temperature decrease and at the end of the temperature decrease). Morphological parameters, sexual steroid levels and spermatogenetic stages were investigated. Our results showed that CP partially inhibited gonadal development (gonado-somatic index or GSI, %; 2.5% under NSP and 1.2% under CP at day 152), spermatic development (96.2% of spermatozoa under NSP and 69.3% under CP at day 152) and the plasmatic levels of testosterone (7.20 ng/mL under NSP and 2.37 ng/mL under CP at day 152). In conclusion, photoperiod may be one of the determinant factors that control the onset of the reproductive cycle in pike males, but temperature and endogenous rhythms also need to be investigated.

Identification and characterization of opsin gene and its role in ovarian maturation in the oriental river prawn Macrobrachium nipponense

Opsins are photoreceptors with important roles in reproductive regulation in birds and fishes. In the present study, we identified an opsin gene from the eyes of the oriental river prawn Macrobrachium nipponense using expressed sequence tag analysis and rapid amplification of cDNA ends. The full-length transcript contained 1382 base pairs, encoding 375 amino acids. It was classified into the long-wavelength opsin group by phylogenetic analysis, and designated Mn-LW. Mn-LW expression demonstrated significant seasonal variation in somatic tissues from both male and female prawns, with the highest expression in the eyes, and expression also shown in the ovary. The expression profiles of Mn-LW in eyes and ovary were positively related to ovarian development. In situ hybridization showed that Mn-LW was present in retinular cells in the eye and oocytes in the ovary. Injection of Mn-LW dsRNA in vivo effectively down-regulated Mn-LW expression levels compared with control levels. Mn-LW dsRNA injection also significantly reduced vitellogenin (Vg) expression, indicating a close relationship between Mn-LW and Vg in ovarian development. These results suggest that Mn-LW may play an important role in Vg synthesis and accumulation during ovarian maturation in M. nipponense.

Influence of artificially induced light pollution on the hormone system of two common fish species, perch and roach, in a rural habitat

Almost all life on earth has adapted to natural cycles of light and dark by evolving circadian and circannual rhythms to synchronize behavioural and physiological processes with the environment. Artificial light at night (ALAN) is suspected to interfere with these rhythms. In this study we examined the influence of ALAN on nocturnal melatonin and sex steroid blood concentrations and mRNA expression of gonadotropins in the pituitary of European perch (Perca fluviatilis) and roach (Rutilus rutilus). In a rural experimental setting, fish were held in net cages in drainage channels experiencing either additional ALAN of ~15 lx at the water surface or natural light conditions at half-moon. No differences in melatonin concentrations between ALAN and natural conditions were detected. However, blood concentration of sex steroids (17β-estradiol; 11-ketotestosterone) as well as mRNA expression of gonadotropins (luteinizing hormone, follicle stimulating hormone) was reduced in both fish species. We conclude that ALAN can disturb biological rhythms in fish in urban waters. However, impacts on melatonin rhythm might have been blurred by individual differences, sampling methods and moonlight. The effect of ALAN on biomarkers of reproduction suggests a photo-labile period around the onset of gonadogenesis, including the experimental period (August). Light pollution therefore has a great potential to influence crucial life history traits with unpredictable outcome for fish population dynamics.

Analyses of the cellular clock gene expression in peripheral tissue, caudal fin, in the Japanese flounder, Paralichthys olivaceus

Understanding the systems for maintaining the circadian rhythms that give organisms the flexibility to adapt to environmental changes is important in both aquaculture and fish chronobiology, because nursery lighting conditions can affect the survival and growth rates of larvae. We previously demonstrated that in flounder, the suprachiasmatic nucleus (SCN) exhibits daily rhythm in per2 expression, in sharp contrast to zebrafish, in which the SCN does not exhibit clear per2 expression rhythm. To examine whether a hierarchy exists in systems that maintain the expression rhythm of peripheral clock genes in flounder, in the present study we analyzed the in vivo and in vitro expression of three clock genes, per2, per1, and cry1, in the caudal fin and the effects of cortisol and melatonin administration on the expression of each clock gene. In vivo, the fin maintained a daily expression rhythm of all three genes, even in 24-h darkness (DD) when shifted from 12-h light:12-h dark (LD) conditions, but fin explants lost the expression rhythm after a short time of tissue culture, even under LD conditions. Cortisol, but not melatonin, significantly upregulated the expression of the three clock genes in fin both in vitro and in vivo. Therefore, we hypothesize that the SCN-pituitary-adrenal cortex pathway plays a role in the oscillation of the peripheral clock in flounder. However, in vivo, peak expression of per2 and cry1 was shifted 2-4h earlier under DD conditions, and their expression was upregulated in response to short exposures to light when larvae were kept under DD conditions. Therefore, we also hypothesize that in addition to the SCN, a light-responsive coordinating factor also functions in photo-entrainment of the peripheral clock in flounder.

Sperm in hot water: direct and indirect thermal challenges interact to impact on brown trout sperm quality

Climate change alters the thermal habitat of aquatic species on a global scale, generating novel environmental challenges during all life stages, including reproduction. Changes in water temperature profoundly influence the performance of ectothermic aquatic organisms. This is an especially crucial issue for migratory fish, because they traverse multiple environments in order to reproduce. In externally fertilizing migratory fish, gametes are affected by water temperature indirectly, within the reproductive organ in which they are produced during migration, as well as directly, upon release into the surrounding medium at the spawning grounds. Both direct (after release) and indirect (during production) thermal impacts on gamete quality have been investigated, but never in conjunction. Here, we assessed the cumulative influence of temperature on brown trout, Salmo trutta, sperm quality during sperm production (male acclimation temperature) as well as upon release (sperm activation water temperature) on two consecutive dates during the brown trout spawning season. Early in the season, warm acclimation of males reduced their fertilization probability (lower sperm velocity) when compared with cold-acclimated males, especially when the activation water temperature was also increased beyond the thermal optimum (resulting in a lower proportion of motile sperm with lower velocity). Later in the season, sperm quality was unaffected by acclimation temperature and thermal sensitivity of sperm was reduced. These results give novel insights into the complex impacts of climate change on fish sperm, with implications for the reproduction and management of hatchery and wild trout populations in future climate scenarios.

Interplay between the endocrine and circadian systems in fishes

The circadian system is responsible for the temporal organisation of physiological functions which, in part, involves daily cycles of hormonal activity. In this review, we analyse the interplay between the circadian and endocrine systems in fishes. We first describe the current model of fish circadian system organisation and the basis of the molecular clockwork that enables different tissues to act as internal pacemakers. This system consists of a net of central and peripherally located oscillators and can be synchronised by the light-darkness and feeding-fasting cycles. We then focus on two central neuroendocrine transducers (melatonin and orexin) and three peripheral hormones (leptin, ghrelin and cortisol), which are involved in the synchronisation of the circadian system in mammals and/or energy status signalling. We review the role of each of these as overt rhythms (i.e. outputs of the circadian system) and, for the first time, as key internal temporal messengers that act as inputs for other endogenous oscillators. Based on acute changes in clock gene expression, we describe the currently accepted model of endogenous oscillator entrainment by the light-darkness cycle and propose a new model for non-photic (endocrine) entrainment, highlighting the importance of the bidirectional cross-talking between the endocrine and circadian systems in fishes. The flexibility of the fish circadian system combined with the absence of a master clock makes these vertebrates a very attractive model for studying communication among oscillators to drive functionally coordinated outputs.

Photoperiod regulate gonad development via kisspeptin/kissr in hypothalamus and saccus vasculosus of Atlantic salmon (Salmo salar)

Atlantic salmon exhibit seasonal reproduction. However, the mechanisms governing this are still unclear. Generally speaking, kisspeptin has been recognized as a regulator of reproduction. Here, we report a relationship between kisspeptin, GnRH and photoperiod in Atlantic salmon. The results demonstrated that the expression of the Atlantic salmon kisspeptin-receptor (skissr) was not always consistent with the expression pattern of Atlantic salmon GnRH3 (sGnRH3) during all developmental processes. Kisspeptin may exert its influence primarily in the early and later stages of gonad development by promoting the secretion of sGnRH3. Meanwhile, the expression levels of kissr were higher in fish with gonads at stage II and stage V under the long-day photoperiod regime than under the short-day regime. In addition, both skissr and sGnRH3 were also expressed in the saccus vasculosus (SV), an organ only found in fish. The SV might be a seasonal sensor regulating reproduction in addition to the hypothalamus (Hyp).

Effects of pinealectomy on the neuroendocrine reproductive system and locomotor activity in male European sea bass, Dicentrarchus labrax

The seasonally changing photoperiod controls the timing of reproduction in most fish species, however, the transduction of this photoperiodic information to the reproductive axis is still unclear. This study explored the potential role of two candidate neuropeptide systems, gonadotropin-inhibitory hormone (Gnih) and kisspeptin, as mediators between the pineal organ (a principle transducer of photoperiodic information) and reproductive axis in male European sea bass, Dicentrarchus labrax. Two seven-day experiments of pinealectomy (Px) were performed, in March (end of reproductive season) and August (resting season). Effects of Px and season on the brain expression of gnih (sbgnih) and its receptor (sbgnihr), kisspeptins (kiss1, kiss2) and their receptors (kissr2, kissr3) and gonadotropin-releasing hormone (gnrh1, gnrh2, gnrh3) and the main brain receptor (gnrhr-II-2b) genes, plasma melatonin levels and locomotor activity rhythms were examined. Results showed that Px reduced night-time plasma melatonin levels. Gene expression analyses demonstrated a sensitivity of the Gnih system to Px in March, with a reduction in sbgnih in the mid-hindbrain, a region with bilateral connections to the pineal organ. In August, kiss2 levels increased in Px animals but not in controls. Significant differences in expression were observed for diencephalic sbgnih, sbgnihr, kissr3 and tegmental gnrh2 between seasons. Recordings of locomotor activity following surgery revealed a change from light-synchronised to free-running rhythmic behavior. Altogether, the Gnih and Kiss2 sensitivity to Px and seasonal differences observed for Gnih and its receptor, Gnrh2, and the receptor for Kiss2 (Kissr3), suggested they could be mediators involved in the relay between environment and seasonal reproduction.

Seasonal variation in the profile of sex steroids and histological testicular development of brook trout (Salvelinus fontinalis Mitchill) during the annual reproductive cycle in Tasmania

Early maturation in brook trout males has been a constraint for commercial production of this species in Tasmania. However, control of maturation in males can be achieved by manipulating photoperiod (duration of light phase). To design an appropriate light treatment regime, an assessment of the annual reproductive cycle was initially required under ambient photoperiod as the annual reproductive cycle of male brook trout under ambient Tasmanian conditions of light and temperature was unknown. Here we describe the seasonal variations in testicular development and plasma profiles of testosterone (T), 11-ketotestosterone (11-KT) and oestradiol-17β (E2) during the second year in a male brook trout cohort held under ambient light conditions. Brook trout males were recruited for maturation in December when a reduction in daylength presumably stimulated testicular development. Increases in levels of T, 11-KT and gonadosomatic index (GSI) values were observed soon after the summer solstice, corresponding with maturation. The highest GSI mean value (3.44 ± 0.11%) was observed in April when mature males could be manually stripped of milt and such males were present until July. Profiles of sex steroids and histological development observed in this study were finely synchronised with variations in seasonal photoperiod changes. Photoperiod before recruitment during December should be controlled to manipulate maturation in brook trout males during their second year.

Deep Brain Photoreceptor (val-opsin) Gene Knockout Using CRISPR/Cas Affects Chorion Formation and Embryonic Hatching in the Zebrafish

Non-rod non-cone photopigments in the eyes and the brain can directly mediate non-visual functions of light in non-mammals. This was supported by our recent findings on vertebrate ancient long (VAL)-opsin photopigments encoded by the val-opsinA (valopa) and val-opsinB (valopb) genes in zebrafish. However, the physiological functions of valop isoforms remain unknown. Here, we generated valop-mutant zebrafish using CRISPR/Cas genome editing, and examined the phenotypes of loss-of-function mutants. F0 mosaic mutations and germline transmission were confirmed via targeted insertions and/or deletions in the valopa or valopb gene in F1 mutants. Based on in silico analysis, frameshift mutations converted VAL-opsin proteins to non-functional truncated forms with pre-mature stop codons. Most F1 eggs or embryos from F0 female valopa/b mutants showed either no or only partial chorion elevation, and the eggs or embryos died within 26 hour-post-fertilization. However, most F1 embryos from F0 male valopa mutant developed but hatched late compared to wild-type embryos, which hatched at 4 day-post-fertilization. Late-hatched F1 offspring included wild-type and mutants, indicating the parental effects of valop knockout. This study shows valop gene knockout affects chorion formation and embryonic hatching in the zebrafish.

Effects of light-emitting diode spectra on the vertebrate ancient long opsin and gonadotropin hormone in the goldfish Carassius auratus

We determined the molecular mechanism underlying the environmental (photoperiodic) regulation of sexual maturation in fish, we examined the expression of sexual maturation-related hormones and vertebrate ancient long opsin (VAL-opsin) in goldfish (Carassius auratus) exposed to different light spectra (red and green light-emitting diodes). We further evaluated the effect of exogenous gonadotropin hormone (GTH) on the expression of VAL-opsin under different light conditions. Our results demonstrated that the expression of GTHs was higher in the fish exposed to green light, and VAL-opsin levels were increased in the fish receiving GTH injection. Therefore, we have uncovered a molecular mechanism underlying the environmental (light)-induced trigger for sexual maturation: VAL-opsin is activated by green light and GTH, which promotes the expression of sexual maturation genes.

Neuronal Organization of Deep Brain Opsin Photoreceptors in Adult Teleosts

Biological impacts of light beyond vision, i.e., non-visual functions of light, signify the need to better understand light detection (or photoreception) systems in vertebrates. Photopigments, which comprise light-absorbing chromophores bound to a variety of G-protein coupled receptor opsins, are responsible for visual and non-visual photoreception. Non-visual opsin photopigments in the retina of mammals and extra-retinal tissues of non-mammals play an important role in non-image-forming functions of light, e.g., biological rhythms and seasonal reproduction. This review highlights the role of opsin photoreceptors in the deep brain, which could involve conserved neurochemical systems that control different time- and light-dependent physiologies in in non-mammalian vertebrates including teleost fish.

Relevance of Light Spectra to Growth of the Rearing Tiger Puffer Takifugu rubripes

In fish, light (photoperiod, intensity and spectra) is main regulator in many physiological actions includinggrowth. We investigate the effect of light spectra on the somatic growth and growth-related gene expression in the rearing tiger puffer. Fish was reared under different light spectra (blue, green and red) for 8 weeks. Fish body weight and total length were promoted when reared under green light condition than red light condition. Expression of somatostatins (ss1 and ss2) in brain were showed higher expression under red light condition than green light condition. The ss3 mRNA was observed only higher expression in blue light condition. Expression of growth hormone (gh) in pituitary was detected no different levels between experimental groups. However, the fish of green light condition group was showed more high weight gain and feed efficiency than other light condition groups. Our present results suggest that somatic growth of tiger puffer is induced under green light condition because of inhibiting ss mRNA expression in brain by effect of green wavelength.

The Role of Melatonin as a Hormone and an Antioxidant in the Control of Fish Reproduction

Reproduction in most fish is seasonal or periodic, and the spawning occurs in an appropriate season to ensure maximum survival of the offspring. The sequence of reproductive events in an annual cycle is largely under the control of a species-specific endogenous timing system, which essentially relies on a well-equipped physiological response mechanism to changing environmental cues. The duration of solar light or photoperiod is one of the most predictable environmental signals used by a large number of animals including fish to coordinate their seasonal breeding. In vertebrates, the pineal gland is the major photoneuroendocrine part of the brain that rhythmically synthesizes and releases melatonin (N-acetyl-5-methoxytryptamine) into the circulation in synchronization with the environmental light-dark cycle. Past few decades witnessed an enormous progress in understanding the mechanisms by which melatonin regulates seasonal reproduction in fish and in other vertebrates. Most studies emphasized hormonal actions of melatonin through its high-affinity, pertussis toxin-sensitive G-protein (guanine nucleotide-binding protein)-coupled receptors on the hypothalamus-pituitary-gonad (HPG) axis of fish. However, the discovery that melatonin due to its lipophilic nature can easily cross the plasma membrane of all cells and may act as a potent scavenger of free radicals and stimulant of different antioxidants added a new dimension to the idea explaining mechanisms of melatonin actions in the regulation of ovarian functions. The basic concept on the actions of melatonin as an antioxidant emerged from mammalian studies. Recently, however, some new studies clearly suggested that melatonin, apart from playing the role of a hormone, may also be associated with the reduction in oxidative stress to augment ovarian functions during spawning. This review thus aims to bring together the current knowledge on the role of melatonin as a hormone as well as an antioxidant in the control of fish reproduction and shape the current working hypotheses supported by recent findings obtained in carp or based on knowledge gathered in mammalian and avian species. In essence, this review highlights potential actions of melatonin as a hormone in determining temporal pattern of spawning and as an antioxidant in regulating oocyte maturation at the downstream of HPG axis in fish.

Influence of tidal cycles on the endocrine control of reproductive activity in common snook (Centropomus undecimalis)

The aim of our study was to confirm the role of tidal pattern on the coordination of oocyte maturation and spawning in common snook Centropomus undecimalis. To do so, we studied oocyte maturation during the spawning season in relation to the tidal pattern in both males and females by means of histology and hormonal profiling along the pituitary-gonadal axis. Plasma LH levels, as well as transcript levels of gonadotropin genes (fshβ and lhβ) from the pituitaries of sexually mature male and female common snook were analyzed using a heterologous ELISA and quantitative RT-PCR, respectively. The fshβ and lhβ cDNAs were isolated and phylogenetic analysis of the deduced amino acid sequences revealed strong identity with other teleosts (75-90%). A strong link was found between tide and follicular development irrespective of the time of the day: female snook sampled on the rising tide were all found to have oocytes in the Secondary Growth Stage whereas females sampled at high tide or on the falling tide had oocytes in the later stages of maturation and ovulation. In addition, LH plasma and mRNA levels of fshβ and lhβ increased during the later stages of vitellogenesis peaking at ovulation in females. Plasma estradiol and testosterone significantly increased in late vitellogenesis (Secondary Growth Stage) and oocyte maturation (Eccentric Germinal Vesicle Step) respectively. Among male common snook sampled, no correlation was identified between tide and gonadal development. In addition, lhβ mRNA expression in males peaked at the mid germinal epithelium stage as for testosterone and 11-KT in the blood while fshβ expression and plasma LH levels peaked at late germinal epithelium stage. This study confirms the role played by tidal cycle on the entrainment of the later stages of oogenesis of common snook and provides a better understanding of the link between environmental and endocrine control of reproduction in this species.

The potential impacts of migratory difficulty, including warmer waters and altered flow conditions, on the reproductive success of salmonid fishes

Climate change and urbanisation of watercourses affect water temperatures and current flow velocities in river systems on a global scale. This represents a particularly critical issue for migratory fish species with complex life histories that use rivers to reproduce. Salmonids are migratory keystone species that provide substantial economical value to ecosystems and human societies. Consequently, a comprehensive understanding of the effects of environmental stressors on their reproductive success is critical in order to ensure their continued abundance during future climatic change. Salmonids are capital breeders, relying entirely on endogenous energy stores to fuel return migration to their natal spawning sites and reproduction upon arrival. Metabolic rates and cost of transport en-route increase with temperature and at extreme temperatures, swimming is increasingly fuelled anaerobically, resulting in an oxygen debt and reduced capacity to recover from exhaustive exercise. Thermally challenged salmonids also produce less viable gametes, which themselves are affected by water temperature after release. Passage through hydrological barriers and temperature changes both affect energy expenditure. As a result, important energetic tradeoffs emerge between extra energy used during migration and that available for other facets of the reproductive cycle, such as reproductive competition and gamete production. However, studies identifying these tradeoffs are extremely sparse. This review focuses on the specific locomotor responses of salmonids to thermal and hydrological challenges, identifying gaps in our knowledge and highlighting the potential implications for key aspects of their reproduction.

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.

Environmental impacts on the gonadotropic system in female Atlantic salmon (Salmo salar) during vitellogenesis: Photothermal effects on pituitary gonadotropins, ovarian gonadotropin receptor expression, plasma sex steroids and oocyte growth

The gonadotropic system and ovarian growth and development were studied during vitellogenesis in female Atlantic salmon subjected to either simulated natural photoperiod and ambient water temperature (NL-amb), or an accelerating photoperiod (short day of LD8:16 from May 10) combined with either warmed (ca 2°C above ambient; 8L-warm) or cooled water (ca 2°C below ambient; 8L-cold) from May to September. Monthly samples were collected from 10 females/group for determination of transcript levels of pituitary gonadotropin subunits (fshb and lhb) and ovarian gonadotropin receptors (fshr and lhr), plasma sex steroids (testosterone: T and estradiol-17β: E2), gonadosomatic index (GSI) and oocyte size. Short day in combination with either warmed or cooled water induced an earlier increase in pituitary fshb and lhb levels compared with NL-amb controls, and advanced ovarian growth and the seasonal profiles of T, E2. By contrast only minor effects were seen of the photothermal treatments on ovarian fshr and lhr. The 8L-cold had earlier increase in fshb, lhb and E2, but similar oocyte and gonadal growth as 8L-warm, suggesting that the 8L-cold group tried to compensate for the lower water temperature during the period of rapid gonadal growth by increasing fshb and E2 production. Both the 8L-warm and 8L-cold groups showed incomplete ovulation in a proportion of the females, possibly due to the photoperiod advancement resulting in earlier readiness of spawning occurring at a higher ambient temperature, or due to some reproductive dysfunction caused by photothermal interference with normal neuroendocrine regulation of oocyte development and maturation.

Updating control of puberty in male European sea bass: A holistic approach

Puberty is the process by which an immature animal acquires the ability to reproduce for the first time; its onset occurs soon after sexual differentiation and is characterized by the beginning of gametogenesis in both sexes. Here we present new insights on when and how the onset of puberty occurs in male European sea bass, its dependence on reaching a critical size, and how it can be controlled by photoperiod, revealing the existence of a photolabile period with important applications in aquaculture. Regarding size, apparently only European sea bass above a certain size threshold attain the ability to carry out gametogenesis during their first year of life, while their smaller counterparts fail to do so. This could imply that fish need to achieve an optimal threshold of hormone production, particularly from the kisspeptin/Gnrh/Gth systems, in order to initiate and conclude puberty. However, a long-term restricted feeding regime during the second year of life did not prevent the onset of puberty, thus suggesting that the fish are able to maintain the reproductive function, even at the expense of other functions. Finally, the study of daily hormonal rhythms under different photoperiod regimes revealed the equivalence between their core values and those of seasonal rhythms, in such a way that the daily rhythms could be considered as the functional units of the seasonal rhythms.

Early development of circadian rhythmicity in the suprachiamatic nuclei and pineal gland of teleost, flounder (Paralichthys olivaeus), embryos

Circadian rhythms enable organisms to coordinate multiple physiological processes and behaviors with the earth's rotation. In mammals, the suprachiasmatic nuclei (SCN), the sole master circadian pacemaker, has entrainment mechanisms that set the circadian rhythm to a 24-h cycle with photic signals from retina. In contrast, the zebrafish SCN is not a circadian pacemaker, instead the pineal gland (PG) houses the major circadian oscillator. The SCN of flounder larvae, unlike that of zebrafish, however, expresses per2 with a rhythmicity of daytime/ON and nighttime/OFF. Here, we examined whether the rhythm of per2 expression in the flounder SCN represents the molecular clock. We also examined early development of the circadian rhythmicity in the SCN and PG. Our three major findings were as follows. First, rhythmic per2 expression in the SCN was maintained under 24 h dark (DD) conditions, indicating that a molecular clock exists in the flounder SCN. Second, onset of circadian rhythmicity in the SCN preceded that in the PG. Third, both 24 h light (LL) and DD conditions deeply affected the development of circadian rhythmicity in the SCN and PG. This is the first report dealing with the early development of circadian rhythmicity in the SCN in fish.

Short winters threaten temperate fish populations

Although climate warming is expected to benefit temperate ectotherms by lengthening the summer growing season, declines in reproductive success following short, warm winters may counter such positive effects. Here we present long-term (1973–2010) field patterns for Lake Erie yellow perch, Perca flavescens, which show that failed annual recruitment events followed short, warm winters. Subsequent laboratory experimentation and field investigations revealed how reduced reproductive success following short, warm winters underlie these observed field patterns. Following short winters, females spawn at warmer temperatures and produce smaller eggs that both hatch at lower rates and produce smaller larvae than females exposed to long winters. Our research suggests that continued climate warming can lead to unanticipated, negative effects on temperate fish populations.

Reductions in winter duration under climate change may have negative consequences for ectotherms adapted to seasonal temperature regimes. Here Farmer et al. show how short winters have, in part, caused failed lake fishery recruitment by altering spawning phenology and lowering the quality of eggs and larvae.