The circadian clock machinery during early development of Senegalese sole (Solea senegalensis): effects of constant light and dark conditions

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.

Ontogenetic expression rhythms of visual opsins in senegalese sole are modulated by photoperiod and light spectrum

In the fish retina, rods and cones are responsible for nocturnal vision and colour perception, respectively, and exhibit a repertoire of light-sensitive opsin photopigments that permits the adaptation to different photic environment. The metamorphosis of Senegalese sole determines a migration from pelagic to benthic environments, which is accompanied by essential changes in light intensity and spectrum. In this paper, we analysed the daily expression rhythms of rod opsin and five cone opsins during sole ontogeny in animals maintained under light-dark cycles of white (LDW), blue (LDB), red (LDR) and continuous white (LL) lights. We showed that the expression of visual opsins at early stages of development was enhanced under LDB in relation to LDW, LDR and LL. Moreover, daily rhythms of opsins were more robust under LDW and LDB conditions, in particular, before and after metamorphosis. A shift in the phase of opsin rhythms was observed between hatching and pre-metamorphosis. Metamorphosis was accompanied by a transient loss in the expression rhythms for most of the opsins, which were significantly influenced by light photoperiod and spectrum. In LDR, transcript levels and rhythms were markedly affected for the majority of the opsins analysed. Under LL, most of the opsins examined exhibited endogenous rhythms, although amplitudes and acrophases changed considerably. To the best of our knowledge, this is the first study on the daily expression rhythms of visual opsins during the ontogeny of a metamorphic flatfish and further emphasises the importance of using natural lighting conditions for proper development of Senegalese sole.

Development of the Astyanax mexicanus circadian clock and non-visual light responses

Most animals and plants live on the planet exposed to periods of rhythmic light and dark. As such, they have evolved endogenous circadian clocks to regulate their physiology rhythmically, and non-visual light detection mechanisms to set the clock to the environmental light-dark cycle. In the case of fish, circadian pacemakers are not only present in the majority of tissues and cells, but these tissues are themselves directly light-sensitive, expressing a wide range of opsin photopigments. This broad non-visual light sensitivity exists to set the clock, but also impacts a wide range of fundamental cell biological processes, such as DNA repair regulation. In this context, Astyanax mexicanus is a very intriguing model system with which to explore non-visual light detection and circadian clock function. Previous work has shown that surface fish possess the same directly light entrainable circadian clocks, described above. The same is true for cave strains of Astyanax in the laboratory, though no daily rhythms have been observed under natural dark conditions in Mexico. There are, however, clear alterations in the cave strain light response and changes to the circadian clock, with a difference in phase of peak gene expression and a reduction in amplitude. In this study, we expand these early observations by exploring the development of non-visual light sensitivity and clock function between surface and cave populations. When does the circadian pacemaker begin to oscillate during development, and are there differences between the various strains? Is the difference in acute light sensitivity, seen in adults, apparent from the earliest stages of development? Our results show that both cave and surface populations must experience daily light exposure to establish a larval gene expression rhythm. These oscillations begin early, around the third day of development in all strains, but gene expression rhythms show a significantly higher amplitude in surface fish larvae. In addition, the light induction of clock genes is developmentally delayed in cave populations. Zebrafish embryonic light sensitivity has been shown to be critical not only for clock entrainment, but also for transcriptional activation of DNA repair processes. Similar downstream transcriptional responses to light also occur in Astyanax. Interestingly, the establishment of the adult timing profile of clock gene expression takes several days to become apparent. This fact may provide mechanistic insight into the key differences between the cave and surface fish clock mechanisms.

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Non-visual light sensitivity is developmentally delayed in cave strains of Astyanax.

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Neither strains of Astyanax show clock gene oscillations in dark raised larvae.

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The timing of the circadian molecular clock rhythm is delayed by 6-h in Pachon.

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Expression CPD photolyase is raised in constant darkness in Pachon.

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The clock has gained greater regulation over light-responsive genes in cave strains.

Daily feeding and protein metabolism rhythms in Senegalese sole post-larvae

Fish hatcheries must adapt larval feeding protocols to feeding behavior and metabolism patterns to obtain more efficient feed utilization. Fish larvae exhibit daily ingesting rhythms rather than ingesting food continuously throughout the day. The aim of this study was to determine the daily patterns of feed intake, protein digestibility, protein retention and catabolism in Senegalese sole post-larvae (Solea senegalensis; 33 days post-hatching) using 14C-labeled Artemia protein and incubation in metabolic chambers. Sole post-larvae were fed at 09:00, 15:00, 21:00, 03:00 and 09:00+1 day; and those fed at 09:00, 21:00, 03:00 and 09:00+1 day showed significantly higher feed intake than post-larvae fed at 15:00 h (P=0.000). Digestibility and evacuation rate of ingested protein did not change during the whole cycle (P=0.114); however, post-larvae fed at 21:00 and 03:00 h showed the significantly highest protein retention efficiency and lowest catabolism (P=0.002). Therefore, results confirm the existence of daily rhythmicity in feeding activity and in the utilization of the ingested nutrients in Senegalese sole post-larvae.

An extended family of novel vertebrate photopigments is widely expressed and displays a diversity of function

Light affects animal physiology and behavior more than simply through classical visual, image-forming pathways. Nonvisual photoreception regulates numerous biological systems, including circadian entrainment, DNA repair, metabolism, and behavior. However, for the majority of these processes, the photoreceptive molecules involved are unknown. Given the diversity of photophysiological responses, the question arises whether a single photopigment or a greater diversity of proteins within the opsin superfamily detect photic stimuli. Here, a functional genomics approach identified the full complement of photopigments in a highly light-sensitive model vertebrate, the zebrafish (Danio rerio), and characterized their tissue distribution, expression levels, and biochemical properties. The results presented here reveal the presence of 42 distinct genes encoding 10 classical visual photopigments and 32 nonvisual opsins, including 10 novel opsin genes comprising four new pigment classes. Consistent with the presence of light-entrainable circadian oscillators in zebrafish, all adult tissues examined expressed two or more opsins, including several novel opsins. Spectral and electrophysiological analyses of the new opsins demonstrate that they form functional photopigments, each with unique chromophore-binding and wavelength specificities. This study has revealed a remarkable number and diversity of photopigments in zebrafish, the largest number so far discovered for any vertebrate. Found in amphibians, reptiles, birds, and all three mammalian clades, most of these genes are not restricted to teleosts. Therefore, nonvisual light detection is far more complex than initially appreciated, which has significant biological implications in understanding photoreception in vertebrates.

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

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

The Light Wavelength Affects the Ontogeny of Clock Gene Expression and Activity Rhythms in Zebrafish Larvae

Light plays a key role in synchronizing rhythms and setting the phase of early development. However, to date, little is known about the impact of light wavelengths during the ontogeny of the molecular clock and the behavioural rhythmicity. The aim of this research was to determine the effect of light of different wavelengths (white, blue and red) on the onset of locomotor activity and clock gene (per1b, per2, clock1, bmal1 and dbp) expression rhythms. For this purpose, 4 groups of zebrafish embryo/larvae were raised from 0 to 7 days post-fertilization (dpf) under the following lighting conditions: three groups maintained under light:dark (LD) cycles with white (full visible spectrum, LDW), blue (LDB), or red light (LDR), and one group raised under constant darkness (DD). The results showed that lighting conditions influenced activity rhythms. Larvae were arrhythmic under DD, while under LD cycles they developed wavelength-dependent daily activity rhythms which appeared earlier under LDB (4 dpf) than under LDW or LDR (5 dpf). The results also revealed that development and lighting conditions influenced clock gene expression. While clock1 rhythmic expression appeared in all lighting conditions at 7 dpf, per1b, per2 and dbp showed daily variations already at 3 dpf. Curiously, bmal1 showed consistent rhythmic expression from embryonic stage (0 dpf). Summarizing, the data revealed that daily rhythms appeared earlier in the larvae reared under LDB than in those reared under LDW and LDR. These results emphasize the importance of lighting conditions and wavelengths during early development for the ontogeny of daily rhythms of gene expression and how these rhythms are reflected on the behavioural rhythmicity of zebrafish larvae.

Differential maturation of rhythmic clock gene expression during early development in medaka (Oryzias latipes)

One key challenge for the field of chronobiology is to identify how circadian clock function emerges during early embryonic development. Teleosts such as the zebrafish are ideal models for studying circadian clock ontogeny since the entire process of development occurs ex utero in an optically transparent chorion. Medaka (Oryzias latipes) represents another powerful fish model for exploring early clock function with, like the zebrafish, many tools available for detailed genetic analysis. However, to date there have been no reports documenting circadian clock gene expression during medaka development. Here we have characterized the expression of key clock genes in various developmental stages and in adult tissues of medaka. As previously reported for other fish, light dark cycles are required for the emergence of clock gene expression rhythms in this species. While rhythmic expression of per and cry genes is detected very early during development and seems to be light driven, rhythmic clock and bmal expression appears much later around hatching time. Furthermore, the maturation of clock function seems to correlate with the appearance of rhythmic expression of these positive elements of the clock feedback loop. By accelerating development through elevated temperatures or by artificially removing the chorion, we show an earlier onset of rhythmicity in clock and bmal expression. Thus, differential maturation of key elements of the medaka clock mechanism depends on the developmental stage and the presence of the chorion.