Temperature regulates transcription in the zebrafish circadian clock

Histology, fatty acid composition, antioxidant and glycolipid metabolism, and transcriptome analyses of the acute cold stress response in Phoxinus lagowskii

Water temperature is a crucial environmental factor that significantly affects the physiological and biochemical processes of fish. Due to the occurrence of cold events in aquaculture, it is imperative to investigate how fish respond to cold stress. This study aims to uncover the mechanisms responds to acute cold stress by conducting a comprehensive analysis of the histomorphology, glycolipid metabolic and antioxidant enzymes, fatty acid composition and transcriptome at three temperatures (16 °C, 10 °C and 4 °C) in Phoxinus lagowskii. Our results showed that cold stress not damaged muscle microstructure but caused autophagy (at 10 °C). In addition, serum glucose (Glu) and triglycerides (TG) increased during cold stress. The activities of reactive oxygen species (ROS), superoxide dismutase (SOD), catalase (CAT), fructose phosphokinase (PFK), hexokinase (HK), pyruvate kinase (PK), and malondialdehyde (MDA) content in muscle were measured and analyzed. During cold stress, superoxide dismutase and catalase activities increased, reactive oxygen species content decreased. No significant difference in Glutathione peroxidase (GPx) activity, malondialdehyde and total cholesterol (T-CHO) contents among groups. Phosphokinase and pyruvate kinase activities decreased, and HK activity increased during cold stress. Our study resulted in the identification of a total of 25,400 genes, with 2524 genes showing differential expression across different temperature treatments. Furthermore, KEGG pathway indicated that some pathways upregulated during light cold stress (at 10 °C, including autophagy, and AMP-activated protein kinase (AMPK) signaling pathway. Additionally, circadian rhythm is among the most enriched pathways in genes up-regulated during severe cold stress (at 4 °C). Our findings offer valuable insights into how cold-water fish respond to cold stress.

Klebsiella pneumoniae alters zebrafish circadian rhythm via inflammatory pathways and is dependent on light cues

Klebsiella pneumoniae is an opportunistic pathogen causing severe infections. The circadian rhythm is the internal rhythm mechanism of an organism and plays an important role in coping with changes in the 24-h circadian rhythm. Disruption of the circadian rhythm can lead to immune, behavioral, mental, and other related disorders. Whether K. pneumoniae can disrupt the circadian rhythm after infection remains unclear. Here, we examined the effects of K. pneumoniae NTUH-K2044 infection on biological rhythm and inflammation in zebrafish using behavioral assays, quantitative real-time reverse transcription PCR, neutrophil and macrophage transgenic fish, and drug treatment. The results showed that K. pneumoniae infection decreased the motor activity of zebrafish and reduced the circadian rhythm amplitude, phase, and period. The expression of core circadian rhythm-associated genes increased under light-dark conditions, whereas they were downregulated under continuous darkness. Analysis of Klebsiella pneumoniae-mediated inflammation using Tg(mpx:EGFP) and Tg(mpeg:EGFP) transgenic zebrafish, expressing fluorescent neutrophils and macrophages, respectively, showed increased induction of inflammatory cells, upregulated expression of inflammatory factor genes, and stronger inflammatory responses under light-dark conditions. These effects were reversed by the anti-inflammatory drug G6PDi-1, and the expression of clock genes following K. pneumoniae treatment was disrupted. We determined the relationship among K. pneumoniae, inflammation, and the circadian rhythm, providing a theoretical reference for studying circadian rhythm disorders caused by inflammation.

Role of melanocortin system in the locomotor activity rhythms and melatonin secretion as revealed by agouti-signalling protein (asip1) overexpression in zebrafish

Temporal signals such as light and temperature cycles profoundly modulate animal physiology and behaviour. Via endogenous timing mechanisms which are regulated by these signals, organisms can anticipate cyclic environmental changes and thereby enhance their fitness. The pineal gland in fish, through the secretion of melatonin, appears to play a critical role in the circadian system, most likely acting as an element of the circadian clock system. An important output of this circadian clock is the locomotor activity circadian rhythm which is adapted to the photoperiod and thus determines whether animals are diurnal or nocturnal. By using a genetically modified zebrafish strain known as Tg (Xla.Eef1a1:Cau.asip1)iim04, which expresses a higher level of the agouti signalling protein 1 (Asip1), an endogenous antagonist of the melanocortin system, we observed a complete disruption of locomotor activity patterns, which correlates with the ablation of the melatonin daily rhythm. Consistent with this, in vitro experiments also demonstrated that Asip1 inhibits melatonin secretion from the zebrafish pineal gland, most likely through the melanocortin receptors expressed in this gland. Asip1 overexpression also disrupted the expression of core clock genes, including per1a and clock1a, thus blunting circadian oscillation. Collectively, these results implicate the melanocortin system as playing an important role in modulating pineal physiology and, therefore, circadian organisation in zebrafish.

Circadian rhythm of preferred temperature in fish: Behavioural thermoregulation linked to daily photocycles in zebrafish and Nile tilapia

Ectothermic vertebrates, e.g. fish, maintain their body temperature within a specific physiological range mainly through behavioural thermoregulation. Here, we characterise the presence of daily rhythms of thermal preference in two phylogenetically distant and well-studied fish species: the zebrafish (Danio rerio), an experimental model, and the Nile tilapia (Oreochromis niloticus), an aquaculture species. We created a non-continuous temperature gradient using multichambered tanks according to the natural environmental range for each species. Each species was allowed to freely choose their preferred temperature during the 24h cycle over a long-term period. Both species displayed strikingly consistent temporal daily rhythms of thermal preference with higher temperatures being selected during the second half of the light phase and lower temperatures at the end of the dark phase, with mean acrophases at Zeitgeber Time (ZT) 5.37 h (zebrafish) and ZT 12.5 h (tilapia). Interestingly, when moved to the experimental tank, only tilapia displayed consistent preference for higher temperatures and took longer time to establish the thermal rhythms. Our findings highlight the importance of integrating both light-driven daily rhythm and thermal choice to refine our understanding of fish biology and improve the management and welfare of the diversity of fish species used in research and food production.

The cold-sensing ion channel TRPM8 regulates central and peripheral clockwork and the circadian oscillations of body temperature

AIM

Physiological functions in mammals show circadian oscillations, synchronized by daily cycles of light and temperature. Central and peripheral clocks participate in this regulation. Since the ion channel TRPM8 is a critical cold sensor, we investigated its role in circadian function.

METHODS

We used TRPM8 reporter mouse lines and TRPM8-deficient mice. mRNA levels were determined by in situ hybridization or RT-qPCR and protein levels by immunofluorescence. A telemetry system was used to measure core body temperature (Tc).

RESULTS

TRPM8 is expressed in the retina, specifically in cholinergic amacrine interneurons and in a subset of melanopsin-positive ganglion cells which project to the central pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. TRPM8-positive fibres were also found innervating choroid and ciliary body vasculature, with a putative function in intraocular temperature, as shown in TRPM8-deficient mice. Interestingly, Trpm8^-/- animals displayed increased expression of the clock gene Per2 and vasopressin (AVP) in the SCN, suggesting a regulatory role of TRPM8 on the central oscillator. Since SCN AVP neurons control body temperature, we studied Tc in driven and free-running conditions. TRPM8-deficiency increased the amplitude of Tc oscillations and, under dim constant light, induced a greater phase delay and instability of Tc rhythmicity. Finally, TRPM8-positive fibres innervate peripheral organs, like liver and white adipose tissue. Notably, Trpm8^-/- mice displayed a dysregulated expression of Per2 mRNA in these metabolic tissues.

CONCLUSION

Our findings support a function of TRPM8 as a temperature sensor involved in the regulation of central and peripheral clocks and the circadian control of Tc.

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.

Global warming and glyphosate toxicity (I): Adult zebrafish modelling with behavioural, immunohistochemical and metabolomic approaches

Global warming further increases the toxic threat of environmental pollutants on organisms. In order to reveal the dimensions of this threat more clearly, it is of great importance that the studies be carried out with temperature differences as close as possible to the temperature values that will represent the global climate projection. In our study, how the toxicity of glyphosate, which is widely used around the world, on zebrafish changes with temperature increases of 0.5° was investigated on behavioral and molecular basis. For this purpose, adult zebrafish were exposed to glyphosate at concentrations of 1 ppm and 5 ppm for 96 h in four environments with a temperature difference of 0.5° (28.5; 29.0; 29.5; 30.0 °C). At the end of the exposure, half of the zebrafish were sampled and remaining half were left for a 10-day recovery process. At the end of the trials, zebrafish were subjected to circadian rhythm and anxiety tests. In addition, histopathological, immunohistochemical and metabolome analyses were performed on brain tissues. As a result, it has been detected that anxiety and circadian rhythm were disrupted in parallel with the increased temperature and glyphosate concentration, and increased histopathological findings and 5-HT4R and GNAT2 immunopositivity in the brain. As a result of metabolome analysis, more than thirty annotated metabolites have been determined due to the synergistic effect of temperature increase and glyphosate exposure. As a conclusion, it was concluded that even a temperature increase of 0.5° caused an increasing effect of glyphosate toxicity in the zebrafish model.

Genome-Wide Association and Expression Analysis Revealed the Candidate Variants and Molecular Underpinnings of Cold-Stress Response in Large Yellow Croaker

Large yellow croaker (Larimichthys crocea) is one of the most economically important fish in China. Recently, global climate change has caused more and more intense and extreme low temperature weathers, resulting in huge losses to the large yellow croaker industry. Therefore, it is essential to understand the mechanisms of low-temperature tolerance in large yellow croaker. Here, we conducted an integrative analysis of genome-wide association study (GWAS) and transcriptome analysis to identify candidate variants and reveal the molecular underpinning of cold-stress response in large yellow croaker. A total of 8 significant single nucleotide polymorphisms (SNPs) loci on 6 chromosomes were identified in the GWAS analysis, and 5764 (gill) and 3588 (liver) differentially expressed genes (DEGs) were detected in cold-stressed large yellow croaker, respectively. Further comparative and functional analysis of the candidate genes and DEGs highlighted the importance of pathways/genes related to immune response, cellular stress response, lipid transport, and metabolism in the cold-stress response of large yellow croaker. Our results provide insights into the cold tolerance of large yellow croaker and contribute to genomic-based selection for low-temperature-resistant large yellow croaker.

Immunity, Infection, and the Zebrafish Clock

Circadian clocks are universally used to coordinate biological processes with the Earth's 24-h solar day and are critical for the health and environmental success of an organism. Circadian rhythms in eukaryotes are driven by a cell-intrinsic transcription-translation feedback loop that controls daily oscillations in gene expression which regulate diverse physiological functions. Substantial evidence now exists demonstrating that immune activation and inflammatory responses during infection are under circadian control, however, the cellular mechanisms responsible for this are not well understood. The zebrafish (Danio rerio) is a powerful model organism to study vertebrate circadian biology and immune function. Zebrafish contain homologs of mammalian circadian clock genes which, to our current knowledge, function similarly to impart timekeeping ability. Consistent with studies in mammalian models, several studies in fish have now demonstrated a bidirectional relationship between the circadian clock and inflammation: the circadian clock regulates immune activity, and inflammation can alter circadian rhythms. This review summarizes our current understanding of the molecular mechanisms of the zebrafish clock and the bi-directional relationship between the circadian clock and inflammation in fish.

The Zebrafish, an Outstanding Model for Biomedical Research in the Field of Melatonin and Human Diseases

The zebrafish has become an excellent model for the study of human diseases because it offers many advantages over other vertebrate animal models. The pineal gland, as well as the biological clock and circadian rhythms, are highly conserved in zebrafish, and melatonin is produced in the pineal gland and in most organs and tissues of the body. Zebrafish have several copies of the clock genes and of aanat and asmt genes, the latter involved in melatonin synthesis. As in mammals, melatonin can act through its membrane receptors, as with zebrafish, and through mechanisms that are independent of receptors. Pineal melatonin regulates peripheral clocks and the circadian rhythms of the body, such as the sleep/wake rhythm, among others. Extrapineal melatonin functions include antioxidant activity, inducing the endogenous antioxidants enzymes, scavenging activity, removing free radicals, anti-inflammatory activity through the regulation of the NF-κB/NLRP3 inflammasome pathway, and a homeostatic role in mitochondria. In this review, we introduce the utility of zebrafish to analyze the mechanisms of action of melatonin. The data here presented showed that the zebrafish is a useful model to study human diseases and that melatonin exerts beneficial effects on many pathophysiological processes involved in these diseases.

Fibronectin and Integrin α5 play overlapping and independent roles in regulating the development of pharyngeal endoderm and cartilage

Craniofacial skeletal elements are derived from cranial neural crest cells (CNCCs), which migrate along discrete paths and populate distinct pharyngeal arches, structures that are separated by the neighboring endodermal pouches (EPs). Interactions between the CNCCs and the endoderm are critical for proper craniofacial development. In zebrafish, integrin α5 (Itga5) functions in the endoderm to regulate formation of specifically the first EP (EP1) and the development of the hyoid cartilage. Here we show that fibronectin (Fn), a major component of the extracellular matrix (ECM), is also required for these developmental processes, and that the penetrance of defects in mutants is temperature-dependent. fn1a^-/- embryos exhibited defects that are similar to, but much more severe than, those of itga5^-/- embryos, and a loss of integrin av (itgav) function enhanced both endoderm and cartilage defects in itga5^-/- embryos, suggesting that Itga5 and Itgav cooperate to transmit signals from Fn to regulate the development of endoderm and cartilage. Whereas the endodermal defects in itga5; itga5v^-/- double mutant embryos were comparable to those of fn1a^-/- mutants, the cartilage defects were much milder. Furthermore, Fn assembly was detected in migrating CNCCs, and the epithelial organization and differentiation of CNCC-derived arches were impaired in fn1a^-/- embryos, indicating that Fn1 exerts functions in arch development that are independent of Itga5 and Itgav. Additionally, reduction of itga5 function in fn1a^-/- embryos led to profound defects in body axis elongation, as well as in endoderm and cartilage formation, suggesting that other ECM proteins signal through Itga5 to regulate development of the endoderm and cartilage. Thus, our studies reveal that Fn1a and Itga5 have both overlapping and independent functions in regulating development of the pharyngeal endoderm and cartilage.

Genomic insights into variation in thermotolerance between hybridizing swordtail fishes

Understanding how organisms adapt to changing environments is a core focus of research in evolutionary biology. One common mechanism is adaptive introgression, which has received increasing attention as a potential route to rapid adaptation in populations struggling in the face of ecological change, particularly global climate change. However, hybridization can also result in deleterious genetic interactions that may limit the benefits of adaptive introgression. Here, we used a combination of genome-wide quantitative trait locus mapping and differential gene expression analyses between the swordtail fish species Xiphophorus malinche and X. birchmanni to study the consequences of hybridization on thermotolerance. While these two species are adapted to different thermal environments, we document a complicated architecture of thermotolerance in hybrids. We identify a region of the genome that contributes to reduced thermotolerance in individuals heterozygous for X. malinche and X. birchmanni ancestry, as well as widespread misexpression in hybrids of genes that respond to thermal stress in the parental species, particularly in the circadian clock pathway. We also show that a previously mapped hybrid incompatibility between X. malinche and X. birchmanni contributes to reduced thermotolerance in hybrids. Together, our results highlight the challenges of understanding the impact of hybridization on complex ecological traits and its potential impact on adaptive introgression.

Magnetic Fluctuations Entrain the Circadian Rhythm of Locomotor Activity in Zebrafish: Can Cryptochrome Be Involved?

Simple Summary

Most physiological processes are subject to biological circadian rhythms maintained by a complex cascade of biochemical events. The circadian rhythmicity of behavior allows organisms to use energy and resources optimally under changing environmental conditions. To that end, endogenous circadian rhythms are synchronized with external pacemakers (zeitgebers), especially daily changes in illumination. In the 1960s, it was assumed that, in addition to this primary photic cue, animals can use diurnal geomagnetic variation as a secondary zeitgeber. Earlier research found that slow magnetic fluctuations can affect some behavioral endpoints of circadian rhythms by modulating an organism’s physiological state. However, no direct experiments to test such an entrainment of biological clocks by artificial magnetic fields were performed due to the technical difficulty of eliminating natural geomagnetic variation. For the first time, we carried out such tests in a fully controlled magnetic environment using zebrafish as a research model. The experimental treatments included various light/dark cycles and continuous illumination coupled with pre-recorded natural geomagnetic variations. The obtained results indicate that slow magnetic fluctuations can entrain endogenous rhythmical activity in vertebrates. Probably, cryptochromes play a key role in this process. This research provides promising opportunities for the magnetic control of circadian processes, e.g., correcting circadian dysfunctions.

Abstract

In the 1960s, it was hypothesized that slow magnetic fluctuations could be a secondary zeitgeber for biological circadian rhythms. However, no comprehensive experimental research has been carried out to test the entrainment of free-running circadian rhythms by this zeitgeber. We studied the circadian patterns of the locomotor activity of zebrafish (Danio rerio) under different combinations of light regimes and slow magnetic fluctuations, based on a record of natural geomagnetic variation. A rapid synchronization of activity rhythms to an unusual 24:12 light/dark cycle was found under magnetic fluctuations with a period of 36 h. Under constant illumination, significant locomotor activity rhythms with 26.17 h and 33.07 h periods were registered in zebrafish exposed to magnetic fluctuations of 26.8 h and 33.76 h, respectively. The results reveal the potential of magnetic fluctuations for entrainment of circadian rhythms in zebrafish and genuine prospects to manipulate circadian oscillators via magnetic fields. The putative mechanisms responsible for the entrainment are discussed, including the possible role of cryptochromes.

The synergic toxicity of temperature increases and nanopolystrene on zebrafish brain implies that global warming may worsen the current risk based on plastic debris

Global warming and plastic pollution are among the most important environmental problems today. Unfortunately, our world is warming more than expected and biological life, especially in the oceans, has come to the limit of the struggle for survival with the nano-scale plastic pollution that is constantly released from the main material. In this study, the synergic effect of one-degree temperature increase (28, 29, 30 °C) and 100 nm size polystyrene plastic nanoparticles on circadian rhythm, brain damage and metabolomics in zebrafish were investigated in an environment where temperature control with 0.05-degree precision is provided. A temperature increase of 1°, together with nanoplastic exposure, affected the circadian rhythm in zebrafish, caused damage to the brain and caused significant changes in the intensity of a total of 18 metabolites in different pathways. It was also detected Raman signals of polystyrene in the brain homogenate. As a consequence, it is suggested that one degree of temperature increase pave the way for degeneration in the brain by disrupting some metabolic pathways, thereby significantly increasing the negative effects of nano-plastic on behavior.

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.

Low Temperature Effect on the Endocrine and Circadian Systems of Adult Danio rerio

The control of the biological rhythms begins with the activation of photo- and thermosensitive cells located in various organs of the fish such as brain, eye, and skin, but a central clock is still to be identified in teleosts. Thermal changes are stressors which increase cortisol and affect the rhythm of other hormones such as melatonin and growth hormone (GH), in both endo- and ectothermic organisms. Our aim was to investigate how temperature (23°C for 6 days) lower than the optimal (28°C) modulates expression of several gene pathways including growth hormone (gh1) and its receptors (ghra, ghrb), insulin-like growth factor1 (igf1a, igf1b) and its receptors (igf1ra, igf1rb), cortisol and its receptor (gr), the limiting enzyme of melatonin synthesis (arylalkylamine N-acetyltransferase, aanat) and melatonin receptors (mtnr1aa, mtnr1bb), as well as their relationship with clock genes in Danio rerio in early light and early dark phases of the day. Lower temperature reduced the expression of the hormone gene gh1, and of the related receptors ghra, ghrb, igf1ra, and igf1rb. Cortisol levels were higher at the lower temperature, with a decrease of its receptor (gr) transcripts in the liver. Interestingly, we found higher levels of aanat transcripts in the brain at 23°C. Overall, lower temperature downregulated the transcription of hormone related genes and clock genes. The results suggest a strong correlation of temperature challenge with the clock molecular mechanism and the endocrine systems analyzed, especially the growth hormone and melatonin axes, in D. rerio tissues.

Transcriptome analysis provides the first insight into the molecular basis of temperature plasticity in Banggai cardinalfish, Pterapogon kauderni

Banggai cardinalfish, Pterapogon kauderni, is a tropical fish listed as an endangered species by IUCN. Its distribution and survival condition are extremely limited, and the changes of living environment caused by global warming may seriously threaten its geographical distribution. In order to understand the survival temperature range and the potential mechanism of temperature plasticity of P. kauderni, transcriptome analysis was performed under five temperature conditions (18 °C, 22 °C, 26 °C, 30 °C and 34 °C). A total of 432,444,497 clean reads were obtained from the mix tissues of whole head, viscera (except intestine), and muscle. All clean data were spliced into 194,832 unigenes. Compared with 26 °C, 57, 107, 187 and 174 differentially expressed genes (DEGs) were obtained at 18 °C, 22 °C, 30 °C and 34 °C, respectively. Gene Ontology (GO) analysis showed the most highly enriched in the DEGs were cellular processes, binding, metabolic processes and biological regulation. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis indicated circadian rhythm, protein processing in endoplasmic reticulum, influenza A and prion disease were significantly enriched. 47 genes that may be related to temperature stress were identified, such as Per1, MLP, IGFBP1, HSP70, HSP90α, HSPA4, DNAJB1, CALR. This is the first RNA-Seq study of P. kauderni. This information should be valuable for further targeted studies on temperature tolerance, thereby assisting the protection and development of P. kauderni.

Circadian Rhythm in Feeding Behavior of Daphnia dentifera

Circadian rhythms enable organisms to mediate their molecular and physiological processes with changes in their environment. Although feeding behavior directly affects within-organism processes, there are few examples of a circadian rhythm in this key behavior. Here, we show that Daphnia have a nocturnal circadian rhythm in feeding behavior that corresponds with their diel vertical migration (DVM), an important life history strategy for predator and UV avoidance. In addition, this feeding rhythm appears to be temperature compensated, which suggests that feeding behavior is robust to seasonal changes in water temperature. A circadian rhythm in feeding behavior can impact energetically demanding processes like metabolism and immunity, which may have drastic effects on susceptibility to disease, starvation risk, and ultimately, fitness.

Anthropogenic alteration of flow, temperature, and light as life-history cues in stream ecosystems

Life history events, from mating and voltinism to migration and emergence, are governed by external and historically predictable environmental factors. The ways humans have altered natural environments during the Anthropocene have created myriad and compounding changes to these historically predictable environmental cues. Over the past few decades, there has been an increased interest in the control temperature exerts on life history events as concern over climate change has increased. However, temperature is not the only life history cue that humans have altered. In stream ecosystems, flow and light serve as important life history cues in addition to temperature. The timing and magnitude of peak flows can trigger migrations, decreases in stream temperature may cause a stream insect to enter diapause, and photoperiod appears to prompt spawning in some species of fish. Two or more of these cues may interact with one another in complex and sometimes unpredictable ways. Large dams and increasing impervious cover in urban ecosystems have modified flows and altered the timing of spawning and migration in fish. Precipitation draining hot impervious surfaces increases stream temperature and adds variability to the general pattern of stream warming from climate change. The addition of artificial light in urban and suburban areas is bright enough to eliminate or dampen the photoperiod signal and has resulted in caddisfly emergence becoming acyclical. The resulting changes in the timing of life history events also have the potential to influence the evolutionary trajectory of an organism and its interactions with other species. This paper offers a review and conceptual framework for future research into how flow, temperature, and light interact to drive life history events of stream organisms and how humans have changed these cues. We then present some of the potential evolutionary and ecological consequences of altered life history events, and conclude by highlighting what we perceive to be the most pressing research needs.

Environmental chemicals affect circadian rhythms: An underexplored effect influencing health and fitness in animals and humans

Circadian rhythms control the life of virtually all organisms. They regulate numerous aspects ranging from cellular processes to reproduction and behavior. Besides the light-dark cycle, there are additional environmental factors that regulate the circadian rhythms in animals as well as humans. Here, we outline the circadian rhythm system and considers zebrafish (Danio rerio) as a representative vertebrate organism. We characterize multiple physiological processes, which are affected by circadian rhythm disrupting compounds (circadian disrupters). We focus on and summarize 40 natural and anthropogenic environmental circadian disrupters in fish. They can be divided into six major categories: steroid hormones, metals, pesticides and biocides, polychlorinated biphenyls, neuroactive drugs and other compounds such as cyanobacterial toxins and bisphenol A. Steroid hormones as well as metals are most studied. Especially for progestins and glucocorticoids, circadian dysregulation was demonstrated in zebrafish on the molecular and physiological level, which comprise mainly behavioral alterations. Our review summarizes the current state of knowledge on circadian disrupters, highlights their risks to fish and identifies knowledge gaps in animals and humans. While most studies focus on transcriptional and behavioral alterations, additional effects and consequences are underexplored. Forthcoming studies should explore, which additional environmental circadian disrupters exist. They should clarify the underlying molecular mechanisms and aim to better understand the consequences for physiological processes.

Adaptation and convergence in circadian-related genes in Iberian freshwater fish

BACKGROUND

The circadian clock is a biological timing system that improves the ability of organisms to deal with environmental fluctuations. At the molecular level it consists of a network of transcription-translation feedback loops, involving genes that activate (bmal and clock - positive loop) and repress expression (cryptochrome (cry) and period (per) - negative loop). This is regulated by daily alternations of light but can also be affected by temperature. Fish, as ectothermic, depend on the environmental temperature and thus are good models to study its integration within the circadian system. Here, we studied the molecular evolution of circadian genes in four Squalius freshwater fish species, distributed across Western Iberian rivers affected by two climatic types with different environmental conditions (e.g., light and temperature). S. carolitertii and S. pyrenaicus inhabit the colder northern region under Atlantic climate type, while S. torgalensis, S. aradensis and some populations of S. pyrenaicus inhabit the warmer southern region affected by summer droughts, under Mediterranean climate type.

RESULTS

We identified 16 circadian-core genes in the Squalius species using a comparative transcriptomics approach. We detected evidence of positive selection in 12 of these genes using methods based on dN/dS. Positive selection was mainly found in cry and per genes of the negative loop, with 55 putatively adaptive substitutions, 16 located on protein domains. Evidence for positive selection is predominant in southern populations affected by the Mediterranean climate type. By predicting protein features we found that changes at sites under positive selection can impact protein thermostability by changing their aliphatic index and isoelectric point. Additionally, in nine genes, the phylogenetic clustering of species that belong to different clades but inhabit southern basins with similar environmental conditions indicated evolutionary convergence. We found evidence for increased nonsynonymous substitution rate in convergent lineages, likely due to positive selection at 27 sites, mostly in cry genes.

CONCLUSIONS

Our results support that temperature may be a selective pressure driving the evolution of genes involved in the circadian system. By integrating sequence-based functional protein prediction with dN/dS-based methods to detect selection we uncovered adaptive convergence in the southern populations, probably related to their similar thermal conditions.

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.

Circadian rhythms in zebrafish (Danio rerio) behaviour and the sources of their variability

Over recent decades, changes in zebrafish (Danio rerio) behaviour have become popular quantitative indicators in biomedical studies. The circadian rhythms of behavioural processes in zebrafish are known to enable effective utilization of energy and resources, therefore attracting interest in zebrafish as a research model. This review covers a variety of circadian behaviours in this species, including diurnal rhythms of spawning, feeding, locomotor activity, shoaling, light/dark preference, and vertical position preference. Changes in circadian activity during zebrafish ontogeny are reviewed, including ageing-related alterations and chemically induced variations in rhythmicity patterns. Both exogenous and endogenous sources of inter-individual variability in zebrafish circadian behaviour are detailed. Additionally, we focus on different environmental factors with the potential to entrain circadian processes in zebrafish. This review describes two principal ways whereby diurnal behavioural rhythms can be entrained: (i) modulation of organismal physiological state, which can have masking or enhancing effects on behavioural endpoints related to endogenous circadian rhythms, and (ii) modulation of period and amplitude of the endogenous circadian rhythm due to competitive relationships between the primary and secondary zeitgebers. In addition, different peripheral oscillators in zebrafish can be entrained by diverse zeitgebers. This complicated orchestra of divergent influences may cause variability in zebrafish circadian behaviours, which should be given attention when planning behavioural studies.

Transcriptome analysis reveals molecular mechanisms responsive to acute cold stress in the tropical stenothermal fish tiger barb (Puntius tetrazona)

BACKGROUND

Tropical stenothermal fish exhibit special tolerance and response to cold stress. However current knowledge of the molecular mechanisms response to cold stress in aquatic ectotherms is largely drawn from eurythermal or extreme stenothermal species. The tiger barb Puntius tetrazona is a tropical stenothermal fish, with great popularity in aquarium trade and research.

RESULTS

To investigate the response mechanism of P. tetrazona to low temperature, fish were exposed to increasing levels of acute cold stress. Histopathological analysis showed that the brain, gill, liver and muscle tissues appeared serious damage after cold stress (13 °C). Brain, gill, liver and muscle tissues from control (CTRL) groups (27 °C) and COLD stress groups (13 °C) of eight-month fish (gender-neutral) were sampled and assessed for transcriptomic profiling by high-throughput sequencing. 83.0 Gb of raw data were generated, filtered and assembled for de novo transcriptome assembly. According to the transcriptome reference, we obtained 392,878 transcripts and 238,878 unigenes, of which 89.29% of the latter were annotated. There were 23,743 differently expressed genes (DEGs) been filtered from four pairs of tissues (brain, gill, liver and muscle) between these cold stress and control groups. These DEGs were mainly involved in circadian entrainment, circadian rhythm, biosynthesis of steroid and fatty acid. There were 64 shared DEGs between the four pairs of groups, and five were related to ubiquitylation/deubiquitylation. Our results suggested that ubiquitin-mediated protein degradation might be necessary for tropical stenothermal fish coping with acute cold stress. Also, the significant cold-induced expression of heat shock 70 kDa protein (HSP70) and cold-induced RNA-binding protein (CIRBP) was verified. These results suggested that the expression of the molecular chaperones HSP70 and CIRBP in P. tetrazona might play a critical role in coping with acute cold stress.

CONCLUSIONS

This is the first transcriptome analysis of P. tetrazona using RNA-Seq technology. Novel findings about tropical stenothermal fish under cold stress (such as HSP70 and CIRBP genes) are presented here. This study contributes new insights into the molecular mechanisms of tropical stenothermal species response to acute cold stress.

Simultaneous eco-toxicity assessment technique using an online monitoring system: effects of different environmental factors on swimming behavior of zebrafish (Danio rerio)

Environmental factors, such as photoperiod and temperature were the main limiting factors for the survival of organisms in the nature environment. Changes in environmental factors are well predicted but determining their effects on organisms are challenging hot topic in the field of eco-toxicology. Thus, technology based eco-toxicity assessment was focused worldwide. In this research, the effects of different temperatures (15 °C, 22 °C, 30 °C, 32 °C, and 35 °C) and photoperiods (dark and light periods) on the continuous behavior responses of Zebrafish (Danio rerio) were investigated using an online monitoring system (OMS). We designed a new fish chamber with sensors to measure the behavior responses of zebrafish under different conditions. Data obtained from the OMS could be assessed for factors such as difference in swimming behavior, circadian rhythm, and avoidance behavior using latest software (MATLAB). The observed behavior anomalies on zebrafish under different temperatures and continuous photoperiods were statically significant (p < 0.05). We conclude that the new designed fish chamber (behavior sensors) is good in sensing behavioral responses of zebrafish under different conditions. The fish behavior strength could be a potential biomarker to assess the effects of environmental factors. The present study would be a basic platform for assessing the effects of different stressors simultaneously on swimming behavior of zebrafish.

Modeling Cancer Using Zebrafish Xenografts: Drawbacks for Mimicking the Human Microenvironment

The first steps towards establishing xenografts in zebrafish embryos were performed by Lee et al., 2005 and Haldi et al., 2006, paving the way for studying human cancers using this animal species. Since then, the xenograft technique has been improved in different ways, ranging from optimizing the best temperature for xenografted embryo incubation, testing different sites for injection of human tumor cells, and even developing tools to study how the host interacts with the injected cells. Nonetheless, a standard protocol for performing xenografts has not been adopted across laboratories, and further research on the temperature, microenvironment of the tumor or the cell-host interactions inside of the embryo during xenografting is still needed. As a consequence, current non-uniform conditions could be affecting experimental results in terms of cell proliferation, invasion, or metastasis; or even overestimating the effects of some chemotherapeutic drugs on xenografted cells. In this review, we highlight and raise awareness regarding the different aspects of xenografting that need to be improved in order to mimic, in a more efficient way, the human tumor microenvironment, resulting in more robust and accurate in vivo results.

Zebrafish Circadian Clock Entrainment and the Importance of Broad Spectral Light Sensitivity

One of the key defining features of an endogenous circadian clock is that it can be entrained or set to local time. Though a number of cues can perform this role, light is the predominant environmental signal that acts to entrain circadian pacemakers in most species. For the past 20 years, a great deal of work has been performed on the light input pathway in mammals and the role of intrinsically photosensitive retinal ganglion cells (ipRGCs)/melanopsin in detecting and sending light information to the suprachiasmatic nucleus (SCN). In teleost fishes, reptiles and birds, the biology of light sensitivity is more complicated as cells and tissues can be directly light responsive. Non-visual light signalling was described many years ago in the context of seasonal, photoperiodic responses in birds and lizards. In the case of teleosts, in particular the zebrafish model system, not only do peripheral tissues have a circadian pacemaker, but possess clear, direct light sensitivity. A surprisingly wide number of opsin photopigments have been described within these tissues, which may underpin this fundamental ability to respond to light, though no specific functional link for any given opsin yet exists. In this study, we show that zebrafish cells show wide spectral sensitivities, as well as express a number of opsin photopigments – several of which are under direct clock control. Furthermore, we also show that light outside the visual range, both ultraviolet and infrared light, can induce clock genes in zebrafish cells. These same wavelengths can phase shift the clock, except infrared light, which generates no shift even though genes such as per2 and cry1a are induced.

A Robust and Self-Sustained Peripheral Circadian Oscillator Reveals Differences in Temperature Compensation Properties with Central Brain Clocks

Circadian clocks are characterized by three properties: they run in constant conditions with a period of ∼24 h, synchronize to the environmental cycles of light and temperature, and are temperature compensated, meaning they do not speed up with temperature. Central brain clocks regulate daily activity rhythms, whereas peripheral clocks are dispersed throughout the body of insects and vertebrates. Using a set of luciferase reporter genes, we show that Drosophila peripheral clocks are self-sustained but over-compensated, i.e., they slow down with increasing temperature. In contrast, central clock neurons in the fly brain, both in intact flies and in cultured brains, show accurate temperature compensation. Although this suggests that neural network properties contribute to temperature compensation, the circadian neuropeptide Pigment Dispersing Factor (PDF) is not required for temperature-compensated oscillations in brain clock neurons. Our findings reveal a fundamental difference between central and peripheral clocks, which likely also applies for vertebrate clocks.

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Drosophila halteres contain a robust circadian oscillator

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Circadian clocks of halteres and antennae are over-compensated

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The Drosophila central brain clock is temperature compensated

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The neuropeptide PDF is not required for temperature compensation of clock neurons

Influence of light and temperature cycles on the expression of circadian clock genes in the mussel Mytilus edulis

Clock genes and environmental cues regulate essential biological rhythms. The blue mussel, Mytilus edulis, is an ecologically and economically important intertidal bivalve undergoing seasonal reproductive rhythms. We previously identified seasonal expression differences in M. edulis clock genes. Herein, the effects of light/dark cycles, constant darkness, and daily temperature cycles on the circadian expression patterns of such genes are characterised. Clock genes Clk, Cry1, ROR/HR3, Per and Rev-erb/NR1D1, and Timeout-like, show significant mRNA expression variation, persisting in darkness indicating endogenous control. Rhythmic expression was apparent under diurnal temperature cycles in darkness for all except Rev-erb. Temperature cycles induced a significant expression difference in the non-circadian clock-associated gene aaNAT. Furthermore, Suppression Subtractive Hybridisation (SSH) was used to identify seasonal genes with potential links to molecular clock function and revealed numerous genes meriting further investigation. Understanding the relationship between environmental cues and molecular clocks is crucial in predicting the outcomes of environmental change on fundamental rhythmic processes.

metaRE R Package for Meta-Analysis of Transcriptome Data to Identify the cis-Regulatory Code behind the Transcriptional Reprogramming

At the molecular level, response to an external factor or an internal condition causes reprogramming of temporal and spatial transcription. When an organism undergoes physiological and/or morphological changes, several signaling pathways are activated simultaneously. Examples of such complex reactions are the response to temperature changes, dehydration, various biologically active substances, and others. A significant part of the regulatory ensemble in such complex reactions remains unidentified. We developed metaRE, an R package for the systematic search for cis-regulatory elements enriched in the promoters of the genes significantly changed their transcription in a complex reaction. metaRE mines multiple expression profiling datasets generated to test the same organism’s response and identifies simple and composite cis-regulatory elements systematically associated with differential expression of genes. Here, we showed metaRE performance for the identification of low-temperature-responsive cis-regulatory code in Arabidopsis thaliana and Danio rerio. MetaRE identified potential binding sites for known as well as unknown cold response regulators. A notable part of cis-elements was found in both searches discovering great conservation in low-temperature responses between plants and animals.

Climate change impacts on fish reproduction are mediated at multiple levels of the brain-pituitary-gonad axis

Anthropogenic emissions of carbon dioxide in the atmosphere have generated rapid variations in atmospheric composition which drives major climate changes. Climate change related effects include changes in physico-chemical proprieties of sea and freshwater, such as variations in water temperature, salinity, pH/pCO₂ and oxygen content, which can impact fish critical physiological functions including reproduction. In this context, the main aim of the present review is to discuss how climate change related effects (variation in water temperature and salinity, increases in duration and frequency of hypoxia events, water acidification) would impact reproduction by affecting the neuroendocrine axis (brain-pituitary-gonad axis). Variations in temperature and photoperiod regimes are known to strongly affect sex differentiation and the timing and phenology of spawning period in several fish species. Temperature mainly acts at the level of gonad by interfering with steroidogenesis, (notably on gonadal aromatase activity) and gametogenesis. Temperature is also directly involved in the quality of released gametes and embryos development. Changes in salinity or water acidification are especially associated with reduction of sperm quality and reproductive output. Hypoxia events are able to interact with gonad steroidogenesis by acting on the steroids precursor cholesterol availability or directly on aromatase action, with an impact on the quality of gametes and reproductive success. Climate change related effects on water parameters likely influence also the reproductive behavior of fish. Although the precise mechanisms underlying the regulation of these effects are not always understood, in this review we discuss different hypothesis and propose future research perspectives.

Nanoplastics Cause Neurobehavioral Impairments, Reproductive and Oxidative Damages, and Biomarker Responses in Zebrafish: Throwing up Alarms of Wide Spread Health Risk of Exposure

Plastic pollution is a growing global emergency and it could serve as a geological indicator of the Anthropocene era. Microplastics are potentially more hazardous than macroplastics, as the former can permeate biological membranes. The toxicity of microplastic exposure on humans and aquatic organisms has been documented, but the toxicity and behavioral changes of nanoplastics (NPs) in mammals are scarce. In spite of their small size, nanoplastics have an enormous surface area, which bears the potential to bind even bigger amounts of toxic compounds in comparison to microplastics. Here, we used polystyrene nanoplastics (PS-NPs) (diameter size at ~70 nm) to investigate the neurobehavioral alterations, tissue distribution, accumulation, and specific health risk of nanoplastics in adult zebrafish. The results demonstrated that PS-NPs accumulated in gonads, intestine, liver, and brain with a tissue distribution pattern that was greatly dependent on the size and shape of the NPs particle. Importantly, an analysis of multiple behavior endpoints and different biochemical biomarkers evidenced that PS-NPs exposure induced disturbance of lipid and energy metabolism as well as oxidative stress and tissue accumulation. Pronounced behavior alterations in their locomotion activity, aggressiveness, shoal formation, and predator avoidance behavior were exhibited by the high concentration of the PS-NPs group, along with the dysregulated circadian rhythm locomotion activity after its chronic exposure. Moreover, several important neurotransmitter biomarkers for neurotoxicity investigation were significantly altered after one week of PS-NPs exposure and these significant changes may indicate the potential toxicity from PS-NPs exposure. In addition, after ~1-month incubation, the fluorescence spectroscopy results revealed the accumulation and distribution of PS-NPs across zebrafish tissues, especially in gonads, which would possibly further affect fish reproductive function. Overall, our results provided new evidence for the adverse consequences of PS-NPs-induced behavioral dysregulation and changes at the molecular level that eventually reduce the survival fitness of zebrafish in the ecosystem.

Doing the locomotion: Insights and potential pitfalls associated with using locomotor activity as a readout of the circadian rhythm in larval zebrafish

BACKGROUND

Zebrafish have been used as a model to study circadian rhythms (CRs) for over 20 years by analyzing various endpoints including locomotor activity. Such studies often utilize high-throughput analysis monitoring activity of larvae placed in well plates numbering >48 wells per plate. Although the CR can be influenced by numerous factors, it is not clear if such effects are permanent. Here, we investigated the variability of CRs of larvae analyzed in different types of well plates and determined the permanency of experimentally-induced aberrations in CRs.

NEW METHOD

Utilized the tracking software Ethovision XT to investigate how different well plate sizes influence the CR. Re-tested subjects for recovery from long-term CR disruptions and evaluated CR patterns at the individual level.

RESULTS

CR tracking using locomotion as a readout is best in 24 well plates. CR consistency is not maintained in larvae tracked in 48 or 96 well plates. A perturbed CR due to constant light recovered after just 3 days of a normal light/dark cycle.

COMPARISON WITH EXISTING METHODS

Unlike other CR locomotor-based assays, our approach allowed for a medium-throughput analysis of individual CRs, minimized variability and allowed for the re-evaluation of larval CRs 4-5 days later.

CONCLUSIONS

This medium-throughput locomotor CR analysis allows for a standardized, less variable approach whereby larvae can be re-tested to identify potential long-term changes after experimental manipulations. Long-term behavioral experiments in 48 or 96 well plates may impart stress on the larvae due to space constraints which could impact nervous system function and/or behavior.

Genomics of Developmental Plasticity in Animals

Developmental plasticity refers to the property by which the same genotype produces distinct phenotypes depending on the environmental conditions under which development takes place. By allowing organisms to produce phenotypes adjusted to the conditions that adults will experience, developmental plasticity can provide the means to cope with environmental heterogeneity. Developmental plasticity can be adaptive and its evolution can be shaped by natural selection. It has also been suggested that developmental plasticity can facilitate adaptation and promote diversification. Here, we summarize current knowledge on the evolution of plasticity and on the impact of plasticity on adaptive evolution, and we identify recent advances and important open questions about the genomics of developmental plasticity in animals. We give special attention to studies using transcriptomics to identify genes whose expression changes across developmental environments and studies using genetic mapping to identify loci that contribute to variation in plasticity and can fuel its evolution.

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.

Temperature alters the hypothalamic transcription of photoperiod responsive genes in induction of seasonal response in migratory redheaded buntings

We investigated the temperature effects on hypothalamic transcription of genes involved in the induction of photoperiodic response in redheaded buntings. Birds were exposed at 22 and 38 °C to 13-h long photoperiods (LP), with controls at 22 °C on 8-h short photoperiods (SP). At 22 °C, compared to SP, we found higher tshb, eya3 and dio2 and low dio3 and gnih mRNA expressions after a week of LP; concomitant with testis recrudescence this confirmed buntings' responsiveness to LP-induced photostimulation. tshb, dio2 and gnrh mRNA levels were further increased by 2.5 weeks of LP at 38 °C. Temperature sensitive trpm8, but not trpv4, bdnf or adcyap1 also showed LP-induced expression at 22 °C. Concomitant changes in dnmt3b and tet2 mRNA expressions further suggested epigenetic modification of temperature influence on photoperiodic responses. These results demonstrate the role of temperature in hypothalamic molecular regulation of the photoperiodic gonadal response in seasonally breeding birds.

Cold seawater induces early sexual developmental stages in the BPG axis of European eel males

BACKGROUND

The impossibility of closing the life cycle of the European eel (Anguilla anguilla) in captivity troubles the future of this critically endangered species. In addition, the European eel is a highly valued and demanded resource, thus the successful closing of its life cycle would have a substantial economic and ecological impact. With the aim of obtaining the highest gamete quality, the study of the effects of environmental factors, such as temperature, on reproductive performance may prove valuable. This is especially true for the exposure to cold water, which has been reported to improve sexual development in multiple other Actinopterygii species.

RESULTS

European eel males treated with cold seawater (10 °C, T10) for 2 weeks showed an increase in the proliferation and differentiation of spermatogonial cells until the differentiated spermatogonial type A cell stage, and elevated testosterone and 11-ketotestosterone plasma levels. Transcriptomes from the tissues of the brain-pituitary-gonad (BPG) axis of T10 samples revealed a differential gene expression profile compared to the other experimental groups, with clustering in a principal component analysis and in heat maps of all differentially expressed genes. Furthermore, a functional analysis of differentially expressed genes revealed enriched gene ontology terms involved in the regulation of circadian rhythm, histone modification, meiotic nuclear division, and others.

CONCLUSIONS

Cold seawater treatment had a clear effect on the activity of the BPG-axis of European eel males. In particular, our cold seawater treatment induces the synchronization and increased proliferation and differentiation of specific spermatogonial cells. In the transcriptomic results, genes related to thermoception were observed. This thermoception may have caused the observed effects through epigenetic mechanisms, since all analysed tissues further revealed differentially expressed genes involved in histone modification. The presented results support our hypothesis that a low temperature seawater treatment induces an early sexual developmental stage in European eels. This hypothesis is logical given that the average temperature experienced by eels in the early stages of their oceanic reproductive migration is highly similar to that of this cold seawater treatment. Further studies are needed to test whether a cold seawater treatment can improve the response of European eels to artificial hormonal treatment, as the results suggest.

Establishing simple image-based methods and a cost-effective instrument for toxicity assessment on circadian rhythm dysregulation in fish

Analysis of circadian rhythm behavior alteration in fish for toxicity assessment usually requires expensive commercial equipment and laborious and complicated tweaking. Here, we report a simple setup that consists of a custom-made light box equipped with white and 940 nm light-emitting diode (LED) light strips as light sources, where the locomotion activities of zebrafish or catfish are captured using an infrared-sensitive coupled charged device (CCD). The whole setup was housed in a temperature-controlled incubator to isolate external noise and to maintain consistent experimental conditions. The video recording and light triggering were synchronized using Total Recorder, a recording scheduling software. By using the setup mentioned above and open source software such as ImageJ or idTracker, the locomotion activities of diurnal (e.g. zebrafish) and nocturnal (e.g. catfish) fish during day and night cycles can be quantitatively analyzed. We used simple image-based methods and a cost-effective instrument to assess the circadian rhythm of multiple fish species, as well as other parameters such as age, ambient temperature and chemical toxicology with high precision and reproducibility. In conclusion, the instrument setting and analysis methods established in this study provide a reliable and easy entry point for toxicity assessment on circadian rhythm dysregulation in fish.

Summary: A cost-effective device and two methods to measure the circadian rhythm in diurnal and nocturnal fish, providing a reliable entry point for toxicity assessment on circadian rhythm dysregulation in fish.

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.

Workshop Report: Zebrafish and Other Fish Models-Description of Extrinsic Environmental Factors for Rigorous Experiments and Reproducible Results

Current standards for husbandry and maintenance of zebrafish and other aquatic species in the laboratory are diverse, and are subject to laboratory performance, engineering, and practice standards (the Guide), institutional interpretation, national animal welfare laws, and cultural differences. Consequently, it is difficult, and probably not advantageous, to establish a single standard in view of the hardy nature of zebrafish and the diversity of research requirements it is used to address. Based on their natural habitat, zebrafish can thrive in a variety of environmental conditions, which is a specific advantage for working with this laboratory organism. However, it also makes reporting and reproducibility difficult, because variations in the husbandry and environmental conditions, including the environmental conditions before and during experiments, are often underreported in the scientific literature. This lack of consistency presents a potential problem for research reproducibility. To begin addressing this emerging scientific gap, the National Institutes of Health's (NIH) Office of Research Infrastructure Programs (ORIP), Division of Construction and Instruments (DCI), hosted a workshop in late 2017, entitled "Zebrafish and Other Aquatic Models: Reporting of Environmental Husbandry Conditions for Rigorous Experiments and Reproducible Results," that was attended by ∼60 participants. The objectives of the workshop were to bring together a diverse group of stakeholders-researchers, facility managers, veterinarians, journal editors, commercial vendors, and others to (1) review current husbandry and environmental management practices for the care of zebrafish and other aquatic organisms in the laboratory and to (2) propose a process for the development of a minimal set of environmental parameters that should be reported in publications to ensure rigor and robustness of experiments and reproducible outcomes. The participants also discussed how these recommendations, as an initial step, might be collected, disseminated, implemented, and improved upon after future iteration.

Tunable assembly of protein-microdomains in living vertebrate embryos

Subcellular events such as trafficking and signaling are regulated by self-assembled protein complexes inside the cell. The ability to rapidly and reversibly manipulate these protein complexes would likely enhance studies of their mechanisms and their roles in biological function and disease manifestation.^{[1, 2]} This manuscript reports that thermally-responsive elastin-like polypeptides (ELPs) linked to fluorescent proteins can regulate the self-assembly and disassembly of protein microdomains within the individual cells of zebrafish embryos. By exploring a library of fluorescent ELP proteins, this reports demonstrates that ELPs can co-assemble different fluorescent proteins inside of embryos. By tuning ELP length and sequence, fluorescent protein microdomains can be assembled at different temperatures, in varying sizes, or for desired periods of time. For the first time in a multicellular living embryo, these studies demonstrate that temperature-mediated ELP assembly can reversibly manipulate assembly of subcellular protein complexes, which may have applications in the study and manipulation of in vivo biological functions.

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.

Cold temperature represses daily rhythms in the liver transcriptome of a stenothermal teleost under decreasing day length

The climate-change-driven increase in temperature is occurring rapidly and decreasing the predictability of seasonal rhythms at high latitudes. It is therefore urgent to understand how a change in the relationship between photoperiod and temperature can affect ectotherms in these environments. We tested whether temperature affects daily rhythms of transcription in a cold-adapted salmonid using high-throughput RNA sequencing. Arctic char (Salvelinus alpinus) from a subarctic population were reared at a high and a low temperature (15 and 8°C) for 1 month under natural, decreasing day length during late summer. Liver transcriptomes were compared between samples collected in the middle and towards the end of the light period and in the middle of the dark period. Daily variation in transcription was lower in fish from the low temperature compared with strong daily variation in warm-acclimated fish, suggesting that cold temperatures dampen the cycling of transcriptional rhythms under a simultaneously decreasing day length. Different circadian clock genes had divergent expression patterns, responding either by decreased expression or by increased rhythmicity at 15°C compared with 8°C. The results point out mechanisms that can affect the ability of fish to adapt to increasing temperatures caused by climate change.

zHSF1 modulates zper2 expression in zebrafish embryos

HSF1 is a transcription factor that plays a key role in circadian resetting by temperature. We have used zebrafish embryos to decipher the roles of zHsf1, heat and light on zper2 transcription in vivo. Our results show that heat shock (HS) stimulated zper2 expression in the dark but has no cumulative effect combined with light. After light exposition, zper2 expression was 2.7 fold increased threefold in the hsf1-morphants in comparison to control embryos. Our results show that zHsf1 plays a positive role in HS-driven expression of zper2 in the dark but seems to act as an attenuator in the presence light.

Heritable gene expression differences between lake and stream stickleback include both parallel and antiparallel components

The repeated phenotypic patterns that characterize populations undergoing parallel evolution provide support for a deterministic role of adaptation by natural selection. Determining the level of parallelism also at the genetic level is thus central to our understanding of how natural selection works. Many studies have looked for repeated genomic patterns in natural populations, but work on gene expression is less common. The studies that have examined gene expression have found some support for parallelism, but those studies almost always used samples collected from the wild that potentially confounds the effects of plasticity with heritable differences. Here we use two independent pairs of lake and stream threespine stickleback (Gasterosteus aculeatus) raised in common garden conditions to assess both parallel and antiparallel (that is, similar versus different directions of lake–stream expression divergence in the two watersheds) heritable gene expression differences as measured by total RNA sequencing. We find that more genes than expected by chance show either parallel (22 genes, 0.18% of expressed genes) or antiparallel (24 genes, 0.20% of expressed genes) lake–stream expression differences. These results correspond well with previous genomic studies in stickleback ecotype pairs that found similar levels of parallelism. We suggest that parallelism might be similarly constrained at the genomic and transcriptomic levels.