Diurnal expression of clock genes in pineal gland and brain and plasma levels of melatonin and cortisol in Atlantic salmon parr and smolts

Cloning, tissue distribution, and effects of different circadian rhythms on the mRNA expression levels of circadian clock genes Per1a and Per1b in Phoxinus lagowskii

This study describes the cloning and characterization of Period 1a and Period 1b genes and the analysis of their mRNA and protein expression in Amur minnow (Phoxinus lagowskii) after exposure to different light cycles. The full-length P. lagowskii Per1a and Per1b genes encode proteins consisting of 1393 and 1409 amino acids, and share high homology with the per1 genes of other freshwater fish species. The Per1a and Per1b genes were widely expressed within the brain, eye, and peripheral tissues. The acrophase of the Per1a gene in the pituitary gland occurred during the dark phase at ZT15 (zeitgeber time 15, 12 L: 12 D) and ZT18 (8 L, 16 D), whereas the acrophase of the Per1b gene in the pituitary gland was observed during the light phase. Our study suggests that the expression of Per1a and Per1b in P. lagowskii varied depending on differences in circadian rhythm patterns. The results of our dual-luciferase reporter assays demonstrated that the P. lagowskii Per1b gene enhances the activation of NF-κB. This study is the first to examine the circadian clock gene Per1a and Per1b in the high-latitude fish P. lagowskii, offering valuable insights into the effects of different light periods on this fish species.

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

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

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.

Changes of the clock gene expression in central and peripheral organs of common carp exposed to constant lighting conditions

In both mammals and fish, the circadian system is composed of oscillators that function at the cellular, tissue, and system levels and show the cyclic expression of clock genes. The organization and functioning of the biological clock in fish has not yet been characterized in detail, therefore, in the present study, an extensive analysis of the rhythmic expression of the main components of the biological clock in the central and peripheral organs of common carp was performed. The diurnal changes in clock gene expression were determined with respect to the subjective light cycle in fish exposed to constant light or darkness. It was found that the pattern of expression of clock, bmal, per and cry genes in carp was highest in the brain, pituitary gland, and retina. The peak clock and bmal expression was phase aligned with the lights off, whereas both per genes show similar phasing with acrophase close to light onset. The expression of cry genes varied depending on the type of tissue and the subtype of gene. The diurnal changes in the expression of clock genes demonstrates that, in particular, the expression of the clock in the retina shows endogenous oscillations independent of the influence of light. The data suggest that in carp, the time-varying expression of individual genes allows for a diverse and tissue-specific response to secure oscillations with variable phase and period.

Rhythmic Clock Gene Expression in Atlantic Salmon Parr Brain

To better understand the complexity of clock genes in salmonids, a taxon with an additional whole genome duplication, an analysis was performed to identify and classify gene family members (clock, arntl, period, cryptochrome, nr1d, ror, and csnk1). The majority of clock genes, in zebrafish and Northern pike, appeared to be duplicated. In comparison to the 29 clock genes described in zebrafish, 48 clock genes were discovered in salmonid species. There was also evidence of species-specific reciprocal gene losses conserved to the Oncorhynchus sister clade. From the six period genes identified three were highly significantly rhythmic, and circadian in their expression patterns (per1a.1, per1a.2, per1b) and two was significantly rhythmically expressed (per2a, per2b). The transcriptomic study of juvenile Atlantic salmon (parr) brain tissues confirmed gene identification and revealed that there were 2,864 rhythmically expressed genes (p < 0.001), including 1,215 genes with a circadian expression pattern, of which 11 were clock genes. The majority of circadian expressed genes peaked 2 h before and after daylight. These findings provide a foundation for further research into the function of clock genes circadian rhythmicity and the role of an enriched number of clock genes relating to seasonal driven life history in salmonids.

Melatonin and osmoregulation in fish: A focus on Atlantic salmon Salmo salar smoltification

Part of the life cycle of several fish species includes important salinity changes, as is the case for the sea bass (Dicentrarchus labrax) or the Atlantic salmon (Salmo salar). Salmo salar juveniles migrate downstream from their spawning sites to reach seawater, where they grow and become sexually mature. The process of preparation enabling juveniles to migrate downstream and physiologically adapt to seawater is called smoltification. Daily and seasonal variations of photoperiod and temperature play a role in defining the timing of smoltification, which may take weeks to months, depending on the river length and latitude. Smoltification is characterised by a series of biochemical, physiological and behavioural changes within the neuroendocrine axis. This review discusses the current knowledge and gaps related to the neuroendocrine mechanisms that mediate the effects of light and temperature on smoltification. Studies performed in S. salar and other salmonids, as well as in other species undergoing important salinity changes, are reviewed, and a particular emphasis is given to the pineal hormone melatonin and its possible role in osmoregulation. The daily and annual variations of plasma melatonin levels reflect corresponding changes in external photoperiod and temperature, which suggests that the hormonal time-keeper melatonin might contribute to controlling smoltification. Here, we review studies on (i) the impact of pinealectomy and/or melatonin administration on smoltification; (ii) melatonin interactions with hormones involved in osmoregulation (e.g., prolactin, growth hormone and cortisol); (iii) the presence of melatonin receptors in tissues involved in osmoregulation; and (iv) the impacts of salinity changes on melatonin receptors and circulating melatonin levels. Altogether, these studies show evidence indicating that melatonin interacts with the neuroendocrine pathways controlling smoltification, although more information is needed to clearly decipher its mechanisms of action.

Diversified regulation of circadian clock gene expression following whole genome duplication

Across taxa, circadian control of physiology and behavior arises from cell-autonomous oscillations in gene expression, governed by a networks of so-called ‘clock genes’, collectively forming transcription-translation feedback loops. In modern vertebrates, these networks contain multiple copies of clock gene family members, which arose through whole genome duplication (WGD) events during evolutionary history. It remains unclear to what extent multiple copies of clock gene family members are functionally redundant or have allowed for functional diversification. We addressed this problem through an analysis of clock gene expression in the Atlantic salmon, a representative of the salmonids, a group which has undergone at least 4 rounds of WGD since the base of the vertebrate lineage, giving an unusually large complement of clock genes. By comparing expression patterns across multiple tissues, and during development, we present evidence for gene- and tissue-specific divergence in expression patterns, consistent with functional diversification of clock gene duplicates. In contrast to mammals, we found no evidence for coupling between cortisol and circadian gene expression, but cortisol mediated non-circadian regulated expression of a subset of clock genes in the salmon gill was evident. This regulation is linked to changes in gill function necessary for the transition from fresh- to sea-water in anadromous fish. Overall, this analysis emphasises the potential for a richly diversified clock gene network to serve a mixture of circadian and non-circadian functions in vertebrate groups with complex genomes.

The generation of daily (circadian) rhythms in behaviour and physiology depends on the activities of networks of so-called clock genes. In vertebrates, these have become highly complex due to a process known as whole genome duplication, which has occurred repeatedly during evolutionary history, giving rise to additional copies of key elements of the clock gene network. It remains unclear whether this results in functional redundancy, or whether it has permitted new roles for clock genes to emerge. Here, based on studies in the Atlantic salmon, a species with an unusually large complement of clock genes, we present evidence in favour of the latter scenario. We observe marked tissue-specific, and developmentally-dependent differences in the expression patterns of duplicated copies of key clock genes, and we identify a subset of clock genes whose expression is associated with the physiological preparation to migrate to sea, but is independent of circadian regulation. Associated with this, cortisol secretion is uncoupled from circadian organisation, contrasting with the situation in mammals. Our results indicate that whole genome duplication has permitted clock genes to diversify into non-circadian functions, and raise interesting questions about the ubiquity of mammal-like coupling between circadian and endocrine function.

Effects of a temperature rise on melatonin and thyroid hormones during smoltification of Atlantic salmon, Salmo salar

Smoltification prepares juvenile Atlantic salmon (Salmo salar) for downstream migration. Dramatic changes characterize this crucial event in the salmon's life cycle, including increased gill Na⁺/K⁺-ATPase activity (NKA) and plasma hormone levels. The triggering of smoltification relies on photoperiod and is modulated by temperature. Both provide reliable information, to which fish have adapted for thousands of years, that allows deciphering daily and calendar time. Here we studied the impact of different photoperiod (natural, sustained winter solstice) and temperature (natural, ~ + 4° C) combinations, on gill NKA, plasma free triiodothyronine (T3) and thyroxine (T4), and melatonin (MEL; the time-keeping hormone), throughout smoltification. We also studied the impact of temperature history on pineal gland MEL production in vitro. The spring increase in gill NKA was less pronounced in smolts kept under sustained winter photoperiod and/or elevated temperature. Plasma thyroid hormone levels displayed day-night variations, which were affected by elevated temperature, either independently from photoperiod (decrease in T3 levels) or under natural photoperiod exclusively (increase in T4 nocturnal levels). Nocturnal MEL secretion was potentiated by the elevated temperature, which also altered the MEL profile under sustained winter photoperiod. Temperature also affected pineal MEL production in vitro, a response that depended on previous environmental acclimation of the organ. The results support the view that the salmon pineal is a photoperiod and temperature sensor, highlight the complexity of the interaction of these environmental factors on the endocrine system of S. salar, and indicate that climate change might compromise salmon's time "deciphering" during smoltification, downstream migration and seawater residence.

Melatonin synthesis and clock gene regulation in the pineal organ of teleost fish compared to mammals: Similarities and differences

The pineal organ of all vertebrates synthesizes and secretes melatonin in a rhythmic manner due to the circadian rhythm in the activity of arylalkylamine N-acetyltransferase (AANAT) - the rate-limiting enzyme in melatonin synthesis pathway. Nighttime increase in AANAT activity and melatonin synthesis depends on increased expression of aanat gene (a clock-controlled gene) and/or post-translation modification of AANAT protein. In mammalian and avian species, only one aanat gene is expressed. However, three aanat genes (aanat1a, aanat1b, and aanat2) are reported in fish species. While aanat1a and aanat1b genes are expressed in the fish retina, the nervous system and other peripheral tissues, aanat2 gene is expressed exclusively in the fish pineal organ. Clock genes form molecular components of the clockwork, which regulates clock-controlled genes like aanat gene. All core clock genes (i.e., clock, bmal1, per1, per2, per3, cry1 and cry2) and aanat2 gene (a clock-controlled gene) are expressed in the pineal organ of several fish species. There is a large body of information on regulation of clock genes, aanat gene and melatonin synthesis in the mammalian pineal gland. However, the information available on clock genes, aanat genes and melatonin synthesis in photoreceptive pineal organ of teleosts is fragmentary and not well documented. Therefore, we have reviewed published information on rhythmic expression of clock genes, aanat genes as well as synthesis of melatonin, and their regulation by photoperiod and temperature in teleostean pineal organ as compared to mammalian pineal gland. A critical analysis of the literature suggests that in contrast to the mammalian pineal gland, the pineal organ of teleosts (except salmonids) possesses a well developed indigenous clock composed of clock genes for regulation of rhythmic expression of aanat2 gene and melatonin synthesis. Further, the fish pineal organ also possesses essential molecular components for responding to light and temperature directly. The fish pineal organ seems to act as a potential master biological clock in most of the teleosts.

Testing the expression of circadian clock genes in the tissues of Chinook salmon, Oncorhynchus tshawytscha

Animals have an endogenous circadian clock that temporally regulates 24 hour (h) oscillations in behavior and physiology. This highly conserved mechanism consists of two positive regulators, Bmal and Clock, and two negative regulators, Cry and Per, that run with a 24-h cycle that synchronizes itself with environmental changes in light, food, and temperature. We examined the circadian clock in Chinook salmon (Oncorhynchus tshawytscha), a non-model organism in which the function of the clock has not been studied. Recent studies indicate that clock genes in Chinook salmon play a role in its evolution of local adaptation, possibly by influencing migration timing. We designed real-time quantitative PCR (RT-qPCR) assays to quantify the transcription of components of the clock system, and validated these for PCR efficiency and specificity in detecting Chinook target genes. Chinook salmon tissue samples were collected in 3-h intervals, over the course of 24 h, from five different organs. Our data indicate that the circadian clock functions differently in each of these tissues. In the liver, positive and negative regulators exhibit anti-phasic peaking in the evening and morning, respectively. However, in the heart, these same regulators peak and trough with a different timing, indicating that the liver and heart are not synchronous. The digestive tract displays yet another difference: simultaneous phases in the expression of positive and negative clock regulators, and we do not observe significant rhythms in clock gene expression in the retina. Our data show that there is a functional clock in Chinook salmon tissues, but that this clock behaves in a tissue-specific manner, regardless of the whole animal being exposed to the same environmental cues. These results highlight the adaptive role of the clock in Chinook salmon and that it may have different positive and negative effects depending on tissue function.

Circadian rhythms and environmental disturbances – underexplored interactions

Biological rhythms control the life of virtually all organisms, impacting numerous aspects ranging from subcellular processes to behaviour. Many studies have shown that changes in abiotic environmental conditions can disturb or entrain circadian (∼24 h) rhythms. These expected changes are so large that they could impose risks to the long-term viability of populations. Climate change is a major global stressor affecting the fitness of animals, partially because it challenges the adaptive associations between endogenous clocks and temperature – consequently, one can posit that a large-scale natural experiment on the plasticity of rhythm–temperature interactions is underway. Further risks are posed by chemical pollution and the depletion of oxygen levels in aquatic environments. Here, we focused our attention on fish, which are at heightened risk of being affected by human influence and are adapted to diverse environments showing predictable changes in light conditions, oxygen saturation and temperature. The examined literature to date suggests an abundance of mechanisms that can lead to interactions between responses to hypoxia, pollutants or pathogens and regulation of endogenous rhythms, but also reveals gaps in our understanding of the plasticity of endogenous rhythms in fish and in how these interactions may be disturbed by human influence and affect natural populations. Here, we summarize research on the molecular mechanisms behind environment–clock interactions as they relate to oxygen variability, temperature and responses to pollutants, and propose ways to address these interactions more conclusively in future studies.

Divergence in expression of candidate genes for the smoltification process between juvenile resident rainbow and anadromous steelhead trout

Rainbow and steelhead trout (Oncorhynchus mykiss), among other salmonid fishes, exhibit tremendous life history diversity, foremost of which is variation in migratory propensity. While some individuals possess the ability to undertake an anadromous marine migration, others remain resident in freshwater throughout their life cycle. Those that will migrate undergo tremendous physiological, morphological, and behavioral transformations in a process called smoltification which transitions freshwater-adapted parr to marine-adapted smolts. While the behavior, ecology, and physiology of smoltification are well described, our understanding of the proximate genetic mechanisms that trigger the process are not well known. Quantitative genetic analyses have identified several genomic regions associated with smoltification and migration-related traits within this species. Here we investigate the divergence in gene expression of 18 functional and positional candidate genes for the smoltification process in the brain, gill, and liver tissues of migratory smolts, resident parr, and precocious mature male trout at the developmental stage of out-migration. Our analysis reveals several genes differentially expressed between life history classes and validates the candidate nature of several genes in the parr-smolt transformation including Clock1α, FSHβ, GR, GH2, GHR1, GHR2, NDK7, p53, SC6a7, Taldo1, THRα, THRβ, and Vdac2.

Daily and seasonal expression of clock genes in the pituitary of the European sea bass (Dicentrarchus labrax)

The expression of select clock genes (clock, bmal, per1, per2, cry1, cry2) was investigated throughout the day and across the four seasons for two consecutive years in the pituitary of adult sea bass (Dicentrarchus labrax). A rhythmic pattern of daily expression was consistently observed in summer and autumn, while arrhythmicity was observed for some clock genes during spring and winter, concomitant with low water temperatures. The expression of clock and bmal showed highest values at the end of the day and during the night, while that of per and cry was mostly antiphasic, with high values during the day. Melatonin affects clock-gene expression in the pituitary of mammals. We therefore sought to test the effect of melatonin on clock-gene expression in the pituitary of sea bass both in vivo and in vitro. Melatonin modestly affected the expression of some clock genes (in particular cry genes) when added to the fish diet or the culture medium of pituitary glands. Our data show that clock genes display rhythmic daily expression in the pituitary of adult sea bass, which are profoundly modified according to the season. We suggest that the effect of photoperiod on clock gene expression may be mediated, at least in part, by melatonin, and that temperature may have a key role adjusting seasonal variations.

Comparative study of pineal clock gene and AANAT2 expression in relation to melatonin synthesis in Atlantic salmon (Salmo salar) and European seabass (Dicentrarchus labrax)

The photoreceptive teleost pineal is considered to be essential to the generation, synchronisation and maintenance of biological rhythms, primarily via melatonin release. The role of internal (circadian clock) and external (light) signals controlling melatonin production in the fish pineal differs between species, yet the reasons underpinning this remain largely unknown. Whilst in salmonids, pineal melatonin is apparently regulated directly by light, in all other studied teleosts, rhythmic melatonin production persists endogenously under the regulation of clock gene expression. To better understand the role of clocks in teleost pineals, this study aimed to characterise the expression of selected clock genes in vitro under different photoperiodic conditions in comparison to in vivo in both Atlantic salmon (Salmo salar) and in European seabass (Dicentrarchus labrax) (in vitro 12L:12D), a species known to display endogenous rhythmic melatonin synthesis. Results revealed no rhythmic clock gene (Clock, Period 1 &2) expression in Atlantic salmon or European seabass (Clock and Period 1) pineal in vitro. However rhythmic expression of Cryptochrome 2 and Period 1 in the Atlantic salmon pineal was observed in vivo, which infers extra-pineal regulation of clocks in this species. No rhythmic arylalkylamine N-acetyltransferase 2 (Aanat2) expression was observed in the Atlantic salmon yet in the European seabass, circadian Aanat2 expression was observed. Subsequent in silico analysis of available Aanat2 genomic sequences reveals that Atlantic salmon Aanat2 promoter sequences do not contain similar regulatory architecture as present in European seabass, and previously described in other teleosts which alludes to a loss in functional connection in the pathway.