Lunar phase-dependent expression of cryptochrome and a photoperiodic mechanism for lunar phase-recognition in a reef fish, goldlined spinefoot

Lunar Age-Dependent Oscillations in Expression of the Genes for Kisspeptin, GnIH, and Their Receptors in the Grass Puffer during the Spawning Season

Grass puffer is a semilunar-synchronized spawner: spawning occurs on beaches only for several days of spring tide around new moon (lunar age 0) and full moon (lunar age 15) every 2 weeks from spring to early summer. To investigate the role of kisspeptin and gonadotropin-inhibitory hormone (GnIH) in the semilunar-synchronized spawning, lunar age-dependent expression of the genes encoding kisspeptin (kiss2), kisspeptin receptor (kissr2), GnIH (gnih), GnIH receptor (gnihr), gonadotropin-releasing hormone 1 (GnRH1) (gnrh1), and three gonadotropin (GTH) subunits (gpa, fshb, lhb) was examined in the male grass puffer, which was kept in an aquarium under natural light condition in a lunar month during the spawning period. In the brain, both kiss2 and kissr2 showed lunar variations with a peak at lunar age 10, while both gnih and gnihr showed semilunar variations with two peaks at lunar age 0 and 20. On the other hand, gnrh1 showed semilunar variation with two peaks at lunar age 0 and 15. In the pituitary, kiss2, kissr2, gnih, and gnihr showed similar variations to those shown in the brain. The fshb and lhb mRNA levels showed semilunar variations with two peaks at lunar age 0 and 15. The present study shows lunar and semilunar oscillations of kiss2/kissr2 and gnih/gnihr expressions, respectively, with their peaks around spring tide in the brain and pituitary along with the semilunar expressions of gnrh1 and the pituitary GTH subunit genes. These results suggest that the lunar age-dependent expressions of the kisspeptin, GnIH, and their receptor genes may be primarily important in the control of the precisely timed semilunar spawning of the grass puffer.

Biological rhythms: Hormones under moon control

Grass puffers are fish that engage in mass spawning controlled by the phase of the moon. A new study shows that prostaglandins released by males and females fine tune these events. In addition, regulation of gnrh1 by a transcription factor expressed in a semilunar rhythm suggests a timing signal for the long-term coordination of gonadal maturation.

A Cryptochrome adopts distinct moon- and sunlight states and functions as sun- versus moonlight interpreter in monthly oscillator entrainment

The moon's monthly cycle synchronizes reproduction in countless marine organisms. The mass-spawning bristle worm Platynereis dumerilii uses an endogenous monthly oscillator set by full moon to phase reproduction to specific days. But how do organisms recognize specific moon phases? We uncover that the light receptor L-Cryptochrome (L-Cry) discriminates between different moonlight durations, as well as between sun- and moonlight. A biochemical characterization of purified L-Cry protein, exposed to naturalistic sun- or moonlight, reveals the formation of distinct sun- and moonlight states characterized by different photoreduction- and recovery kinetics of L-Cry's co-factor Flavin Adenine Dinucleotide. In Platynereis, L-Cry's sun- versus moonlight states correlate with distinct subcellular localizations, indicating different signaling. In contrast, r-Opsin1, the most abundant ocular opsin, is not required for monthly oscillator entrainment. Our work reveals a photo-ecological concept for natural light interpretation involving a "valence interpreter" that provides entraining photoreceptor(s) with light source and moon phase information.

Circalunar clocks-Old experiments for a new era

Circalunar clocks, which allow organisms to time reproduction to lunar phase, have been experimentally proven but are still not understood at the molecular level. Currently, a new generation of researchers with new tools is setting out to fill this gap. Our essay provides an overview of classic experiments on circalunar clocks. From the unpublished work of the late D. Neumann we also present a novel phase response curve for a circalunar clock. These experiments highlight avenues for molecular work and call for rigor in setting up and analyzing the logistically complex experiments on circalunar clocks. Re-evaluating classic experiments, we propose that (1) circalunar clocks in different organisms will have divergent mechanisms and physiological bases, (2) they may have properties very different from the well-studied circadian clocks and (3) they may have close mechanistic and molecular relations to seasonal rhythms and diapause.

Moonlight is a key entrainer of lunar clock in the brain of the tropical grouper with full moon preference

Background

Many animals in coral reefs exhibit lunar cycles in their reproduction, showing synchronous gametogenesis and spawning at a particular moon phase. How these lunar reproductive cycles are endogenously regulated remains unknown, although changes in moonlight between the new moon and full moon are likely involved in this rhythmic event.

Results

This study evaluated the possible role of cryptochrome (cry1, cry2, cry3) in phase shifting and setting in the honeycomb grouper Epinephelus merra, which is a typical lunar spawner with full moon preference. qPCR analysis revealed that when fish were reared under alternating light-dark conditions, the transcript levels of cry1 and cry2, but not of cry3, in the diencephalon and pituitary gland showed daily variations. Weekly collection at midnight showed increases in the transcript levels of cry1 and cry2 in the diencephalon, but not the pituitary gland, from the first quarter moon through the last quarter moon. In comparison to the new moon, these transcript levels were significantly lower at all other sampling times. The artificial full moon conditions for 1 month resulted in increased cry transcript levels in both tissues at 2 (cry1) or 2 and 4 (cry2) weeks after the initiation of full moon conditions.

Conclusions

These results indicate the importance of transient changes in “brightness at night” in the response to moonlight for the phase shift and of “darkness at night” during the new moon for the phase set to the determined moon phase. We concluded that the moon phase-dependent oscillation of clock genes plays a role in lunar cycle-dependent behaviors in fish.

The Still Dark Side of the Moon: Molecular Mechanisms of Lunar-Controlled Rhythms and Clocks

Starting with the beginning of the last century, a multitude of scientific studies has documented that the lunar cycle times behaviors and physiology in many organisms. It is plausible that even the first life forms adapted to the different rhythms controlled by the moon. Consistently, many marine species exhibit lunar rhythms, and also the number of documented "lunar-rhythmic" terrestrial species is increasing. Organisms follow diverse lunar geophysical/astronomical rhythms, which differ significantly in terms of period length: from hours (circalunidian and circatidal rhythms) to days (circasemilunar and circalunar cycles). Evidence for internal circatital and circalunar oscillators exists for a range of species based on past behavioral studies, but those species with well-documented behaviorally free-running lunar rhythms are not typically used for molecular studies. Thus, the underlying molecular mechanisms are largely obscure: the dark side of the moon. Here we review findings that start to connect molecular pathways with moon-controlled physiology and behaviors. The present data indicate connections between metabolic/endocrine pathways and moon-controlled rhythms, as well as interactions between circadian and circatidal/circalunar rhythms. Moreover, recent high-throughput analyses provide useful leads toward pathways, as well as molecular markers. However, for each interpretation, it is important to carefully consider the, partly substantially differing, conditions used in each experimental paradigm. In the future, it will be important to use lab experiments to delineate the specific mechanisms of the different solar- and lunar-controlled rhythms, but to also start integrating them together, as life has evolved equally long under rhythms of both sun and moon.

A photoperiodic time measurement served by the biphasic expression of Cryptochrome1ab in the zebrafish eye

The zebrafish (Danio rerio) is a model species that is used to study the circadian clock. It possesses light-entrainable circadian clocks in both central and peripheral tissues, and its core circadian factor cryptochromes (CRYs) have diverged significantly during evolution. In order to elucidate the functional diversity and involvement of CRYs in photoperiodic mechanisms, we investigated the daily expression profiles of six Cry transcripts in central (brain and eye) and peripheral (fin, skin and muscle) tissues. The zCry genes exhibited gene-specific diurnal conserved variations, and were divided into morning and evening groups. Notably, zCry1ab exhibited biphasic expression profiles in the eye, with peaks in the morning and evening. Comparing ocular zCry1ab expression in different photoperiods (18L:6D, 14L:10D, 10L:14D and 6L:18D) revealed that zCry1ab expression duration changed depending on the photoperiod: it increased at midnight and peaked before lights off. zCry1ab expression in constant light or dark after entrainment under long- or short-day conditions suggested that the evening clock and photic input pathway are involved in photoperiod-dependent zCry1ab expression. Laser microdissection followed by qRT-PCR analysis showed that the evening peak of zCry1ab was likely ascribed to visual photoreceptors. These results suggest the presence of an eye-specific photoperiodic time measurement served by zCry1ab.

Demystifying Circalunar and Diel Rhythmicity in Acropora digitifera under Constant Dim Light

Life on earth has evolved under constant environmental changes; in response to these changes, most organisms have developed an endogenous clock that allows them to anticipate daily and seasonal changes and adapt their biology accordingly. Light cycles synchronize biological rhythms and are controlled by an endogenous clock that is entrained by environmental cues. Light is known to play a key role in the biology of symbiotic corals as they exhibit many biological processes entrained by daily light patterns. In this study, we aimed at determining the effect of constant dim light on coral's perception of diel and monthly cycles. Our results show that under constant dim light corals display a loss of rhythmic processes and constant stimuli by light, which initiates signal transduction that results in an abnormal cell cycle, cell proliferation, and protein synthesis. The results emphasize how constant dim light can mask the biological clock of Acropora digitifera.

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Light entrains many biological processes governed by the endogenous clock

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Constant dim light overrides the biological clock of A. digitifera corals

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Artificial light impacts the processes that allow corals to thrive in our oceans

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The increase of artificial light in coastal areas is a growing threat to coral reefs

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

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

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

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

Impacts of "supermoon" events on the physiology of a wild bird

The position of the Moon in relation to the Earth and the Sun gives rise to several predictable cycles, and natural changes in nighttime light intensity are known to cause alterations to physiological processes and behaviors in many animals. The limited research undertaken to date on the physiological responses of animals to the lunar illumination has exclusively focused on the synodic lunar cycle (full moon to full moon, or moon phase) but the moon's orbit-its distance from the Earth-may also be relevant. Every month, the moon moves from apogee, its most distant point from Earth-and then to perigee, its closest point to Earth. Here, we studied wild barnacle geese (Branta leucopsis) to investigate the influence of multiple interacting lunar cycles on the physiology of diurnally active animals. Our study, which uses biologging technology to continually monitor body temperature and heart rate for an entire annual cycle, asks whether there is evidence for a physiological response to natural cycles in lunar brightness in wild birds, particularly "supermoon" phenomena, where perigee coincides with a full moon. There was a three-way interaction between lunar phase, lunar distance, and cloud cover as predictors of nighttime mean body temperature, such that body temperature was highest on clear nights when the full moon coincided with perigee moon. Our study is the first to report the physiological responses of wild birds to "supermoon" events; the wild geese responded to the combination of two independent lunar cycles, by significantly increasing their body temperature at night. That wild birds respond to natural fluctuations in nighttime ambient light levels support the documented responses of many species to anthropogenic sources of artificial light, that birds seem unable to override. As most biological systems are arguably organized foremost by light, this suggests that any interactions between lunar cycles and local weather conditions could have significant impacts on the energy budgets of birds.

Effect of Circadian Clock and Light-Dark Cycles in Onchidium reevesii: Possible Implications for Long-Term Memory

The sea slug Onchidium reevesii inhabits the intertidal zone, which is characterized by a changeable environment. Although the circadian modulation of long-term memory (LTM) is well documented, the interaction of the circadian clock with light-dark masking in LTM of intertidal animals is not well understood. We characterized the LTM of Onchidium and tested the expression levels of related genes under a light-dark (LD) cycle and constant darkness (i.e., dark-dark, or DD) cycle. Results indicated that both learning behavior and LTM show differences between circadian time (CT) 10 and zeitgeber time (ZT) 10. In LD, the cry1 gene expressed irregularly, and per2 expression displayed a daily pattern and a peak expression level at ZT 18. OnCREB1 (only in LD conditions) and per2 transcripts cycled in phase with each other. In DD, the cry1 gene had its peak expression at CT 10, and per2 expressed its peak level at CT 18. OnCREB1 had two peak expression levels at ZT 10 or ZT 18 which correspond to the time node of peaks in cry1 and per2, respectively. The obtained results provide an LTM pattern that is different from other model species of the intertidal zone. We conclude that the daily transcriptional oscillations of Onchidium for LTM were affected by circadian rhythms and LD cycle masking.

Moonlight controls lunar-phase-dependency and regular oscillation of clock gene expressions in a lunar-synchronized spawner fish, Goldlined spinefoot

Goldlined spinefoot, Siganus guttatus, inhabits tropical and subtropical waters and synchronizes its spawning around the first quarter moon likely using an hourglass-like lunar timer. In previous studies, we have found that clock genes (Cryptochrome3 and Period1) could play the role of state variable in the diencephalon when determining the lunar phase for spawning. Here, we identified three Cry, two Per, two Clock, and two Bmal genes in S. guttatus and investigated their expression patterns in the diencephalon and pituitary gland. We further evaluated the effect on their expression patterns by daily interruptions of moonlight stimuli for 1 lunar cycle beginning at the new moon. It significantly modified the expression patterns in many of the examined clock(-related) genes including Cry3 in the diencephalon and/or pituitary gland. Acute interruptions of moonlight around the waxing gibbous moon upregulated nocturnal expressions of Cry1b and Cry2 in the diencephalon and pituitary gland, respectively, but did not affect expression levels of the other clock genes. These results highlighted the importance of repetitive moonlight illumination for stable or lunar-phase-specific daily expression of clock genes in the next lunar cycle that may be important for the lunar-phase-synchronized spawning on the next first quarter moon.

An Overview of Monthly Rhythms and Clocks

Organisms have evolved to cope with geophysical cycles of different period lengths. In this review, we focus on the adaptations of animals to the lunar cycle, specifically, on the occurrence of biological rhythms with monthly (circalunar) or semi-monthly (circasemilunar) period lengths. Systematic experimental investigation, starting in the early twentieth century, has allowed scientists to distinguish between mythological belief and scientific facts concerning the influence of the lunar cycle on animals. These studies revealed that marine animals of various taxa exhibit circalunar or circasemilunar reproductive rhythms. Some of these rely on endogenous oscillators (circalunar or circasemilunar clocks), whereas others are directly driven by external cues, such as the changes in nocturnal illuminance. We review current insight in the molecular and cellular mechanisms involved in circalunar rhythms, focusing on recent work in corals, annelid worms, midges, and fishes. In several of these model systems, the transcript levels of some core circadian clock genes are affected by both light and endogenous circalunar oscillations. How these and other molecular changes relate to the changes in physiology or behavior over the lunar cycle remains to be determined. We further review the possible relevance of circalunar rhythms for terrestrial species, with a particular focus on mammalian reproduction. Studies on circalunar rhythms of conception or birth rates extend to humans, where the lunar cycle was suggested to also affect sleep and mental health. While these reports remain controversial, factors like the increase in "light pollution" by artificial light might contribute to discrepancies between studies. We finally discuss the existence of circalunar oscillations in mammalian physiology. We speculate that these oscillations could be the remnant of ancient circalunar oscillators that were secondarily uncoupled from a natural entrainment mechanism, but still maintained relevance for structuring the timing of reproduction or physiology. The analysis and comparison of circalunar rhythms and clocks are currently challenging due to the heterogeneity of samples concerning species diversity, environmental conditions, and chronobiological conditions. We suggest that future research will benefit from the development of standardized experimental paradigms, and common principles for recording and reporting environmental conditions, especially light spectra and intensities.

Light- and circadian-controlled genes respond to a broad light spectrum in Puffer Fish-derived Fugu eye cells

Some cell lines retain intrinsic phototransduction pathways to control the expression of light-regulated genes such as the circadian clock gene. Here we investigated the photosensitivity of a Fugu eye, a cell line established from the eye of Takifugu rubripes, to examine whether such a photosensitive nature is present. Microarray analysis identified 15 genes that showed blue light-dependent change at the transcript level. We investigated temporal profiles of the light-induced genes, as well as Cry and Per, under light-dark, constant light (LL), and constant dark (DD) conditions by quantitative RT-PCR. Transcript levels of Per1a and Per3 genes showed circadian rhythmic changes under both LL and DD conditions, while those of Cry genes were controlled by light. All genes examined, including DNA-damage response genes and photolyase genes, were upregulated not only by blue light but also green and red light, implying the contribution of multiple photopigments. The present study is the first to identify a photosensitive clock cell line originating from a marine fish. These findings may help to characterize the molecular mechanisms underlying photic synchronization of the physiological states of fishes to not only daily light-dark cycles but also to various marine environmental cycles such as the lunar or semi-lunar cycle.

Canonical and cellular pathways timing gamete release in Acropora digitifera, Okinawa, Japan

Natural light cycles are important for synchronizing behavioural and physiological rhythms over varying time periods in both plants and animals. An endogenous clock, regulated by positive and negative elements, interacting in feedback loops controls these rhythms. Many corals exhibit diel cycles of polyp expansion and contraction entrained by solar light patterns and monthly cycles of spawning or planulation that correspond to nocturnal lunar light cycles. However, despite considerable interest in studies of coral reproduction, there is currently not enough molecular information about the cellular pathways involved with synchronizing spawning/planulation in broadcast spawners and brooders. To determine whether the endogenous clock is implicated in the regulation of reproductive behaviour in corals, we characterized the transcriptome of Acropora digitifera colonies at twelve time points over a 2-month period of full and new moons, starting with the day of spawning in June 2014. We identified 608 transcripts with differential expression only on the spawning night during the coral setting phase and gamete release. Our data revealed an upregulation of light-sensing molecules and rhodopsin-like receptors that initiate signalling cascades, including the glutamate, SMAD signalling and WNT signalling pathways, neuroactive ligand-receptor interactions and calcium signalling. These are all involved in cell cycling, cell movement, tissue polarity, focal adhesion and cytoskeleton reorganization and together lead to gamete release. These findings can improve the understanding of many time-based cycles and extend our knowledge of the interplay between exogenous signals and the endogenous clock in cnidarians.

Hypothalamic expression and moonlight-independent changes of Cry3 and Per4 implicate their roles in lunar clock oscillators of the lunar-responsive Goldlined spinefoot

Lunar cycle-associated physiology has been found in a wide variety of organisms. Studies suggest the presence of a circalunar clock in some animals, but the location of the lunar clock is unclear. We previously found lunar-associated expression of transcripts for Cryptochrome3 gene (SgCry3) in the brain of a lunar phase-responsive fish, the Goldlined spinefoot (Siganus guttatus). Then we proposed a photoperiodic model for the lunar phase response, in which SgCry3 might function as a phase-specific light response gene and/or an oscillatory factor in unidentified circalunar clock. In this study, we have developed an anti-SgCRY3 antibody to identify SgCRY3-immunoreactive cells in the brain. We found immunoreactions in the subependymal cells located in the mediobasal region of the diencephalon, a crucial site for photoperiodic seasonal responses in birds. For further assessment of the lunar-responding mechanism and the circalunar clock, we investigated mRNA levels of Cry3 as well as those of the other clock(-related) genes, Period (Per2 and Per4), in S. guttatus reared under nocturnal moonlight interruption or natural conditions. Not only SgCry3 but SgPer4 mRNA levels showed lunar phase-dependent variations in the diencephalon without depending on light condition during the night. These results suggest that the expressions of SgCry3 and SgPer4 are not directly regulated by moonlight stimulation but endogenously mediated in the brain, and implicate that circadian clock(-related) genes may be involved in the circalunar clock locating within the mediobasal region of the diencephalon.

Identification and characterization of cryptochrome4 in the ovary of western clawed frog Xenopus tropicalis

CRY proteins can be classified into several groups based on their phylogenetic relationships, and they function as a photoreceptor, a photolyase, and/or a transcriptional repressor of the circadian clock. In order to elucidate the expression profile and functional diversity of CRYs in vertebrates, we focused on XtCRY4, a member of the uncharacterized cryptochrome family CRY4 in Xenopus tropicalis. XtCRY4 cDNA was isolated by RT-PCR, and a phylogenetic analysis of deduced sequence of XtCRY4 suggested that the vertebrate Cry4 genes evolved at much higher evolutionary rates than mammalian-type Cry genes, such as the CRY1 and CRY2 circadian clock molecules. A transcriptional assay was performed to examine the transcriptional regulatory function as circadian repressor, and XtCRY4 had marginal effects on the transactivation of XtCLOCK/XtBMAL1 via E-box element. In situ hybridization and quantitative RT-PCR was performed to detect mRNA expression in native tissues. Quantitative RT-PCR revealed that XtCry4 mRNA was highly transcribed in the ovary. In situ hybridization showed the presence of XtCry4 transcripts in the oocytes, testis, renal tubules, the visual photoreceptors, and the retinal ganglion cells. A specific antiserum to XtCRY4 was developed to detect endogeneous expression of XtCRY4 protein in the ovary. The expression level was estimated by immunoblot analysis, and this is the first detection and estimation of endogenous expression of CRY protein in the ovary. These results suggest that X. tropicalis ovary may respond to blue-light by using XtCRY4.

Separation of photo-induced radical pair in cryptochrome to a functionally critical distance

Cryptochrome is a blue light receptor that acts as a sensor for the geomagnetic field and assists many animals in long-range navigation. The magnetoreceptor function arises from light-induced formation of a radical pair through electron transfer between a flavin cofactor (FAD) and a triad of tryptophan residues. Here, this electron transfer is investigated by quantum chemical and classical molecular dynamics calculations. The results reveal how sequential electron transfer, assisted by rearrangement of polar side groups in the cryptochrome interior, can yield a FAD-Trp radical pair state with the FAD and Trp partners separated beyond a critical distance. The large radical pair separation reached establishes cryptochrome's sensitivity to the geomagnetic field through weakening of distance-dependent exchange and dipole-dipole interactions. It is estimated that the key secondary electron transfer step can overcome in speed both recombination (electron back-transfer) and proton transfer involving the radical pair reached after primary electron transfer.

Impacts of moonlight on fish reproduction

The waxing and waning cycle of the moon is repeated at approximately 1-month intervals, and concomitant changes occur in the levels of moonlight and cueing signals detected by organisms on the earth. In the goldlined spinefoot Siganus guttatus, a spawner lunar-synchronized around the first quarter moon, periodic changes in moonlight are used to cue gonadal development and gamete release. Rearing of mature fish under artificial constant full moon and new moon conditions during the spawning season leads to disruption or delay of synchronous spawning around the predicted moon phase. Melatonin, an endogenous transducer of the environmental light/dark cycle, increases in the blood and in the pineal gland around the new moon period and decreases around the full moon period. In synchrony with melatonin fluctuation, melatonin receptor(s) mRNA abundance is higher during the new moon period than during the full moon. The melatonin/melatonin receptor system is likely affected by moonlight. Measurements of the expression patterns of clock genes in neural tissues demonstrate that Cryptochrome (Cry1 and Cry3) and Period (Per2) fluctuate with lunar periodicity, the former peaking in the medial part of the brain around the first quarter moon period, and the latter peaking in the pineal gland around the full moon. Some clock genes may respond to periodic changes in moon phase and appear to be involved in the generation of lunar-related rhythmicity in lunar spawners. Thus, some fish use moonlight-related periodicities as reliable information for synchronizing the timing of reproductive events.

Influence of moonlight on mRNA expression patterns of melatonin receptor subtypes in the pineal organ of a tropical fish

The goldlined spinefoot, Siganus guttatus, is a lunar-synchronized spawner, which repeatedly releases gametes around the first quarter moon during the reproductive season. A previous study reported that manipulating moonlight brightness at night disrupted synchronized spawning, suggesting involvement of this natural light source in lunar synchronization. The present study examined whether the mRNA expression pattern of melatonin receptor subtypes MT1 and Mel1c in the pineal organ of the goldlined spinefoot is related to moonlight. Real-time quantitative polymerase chain reaction analysis revealed that the abundance of MT1 and Mel1c mRNA at midnight increased during the new moon phase and decreased during the full moon phase. Exposing fish to moonlight intensity during the full moon period resulted in a decrease in Mel1c mRNA abundance within 1h. Fluctuations in the melatonin receptor genes according to changes in the moon phase agreed with those of melatonin levels in the blood. These results indicate that periodic changes in cues from the moon influence melatonin receptor mRNA expression levels. The melatonin-melatonin receptor system may play a role in predicting the moon phase through changes in night brightness.

Circadian and circalunar clock interactions in a marine annelid

Life is controlled by multiple rhythms. Although the interaction of the daily (circadian) clock with environmental stimuli, such as light, is well documented, its relationship to endogenous clocks with other periods is little understood. We establish that the marine worm Platynereis dumerilii possesses endogenous circadian and circalunar (monthly) clocks and characterize their interactions. The RNAs of likely core circadian oscillator genes localize to a distinct nucleus of the worm’s forebrain. The worm’s forebrain also harbors a circalunar clock entrained by nocturnal light. This monthly clock regulates maturation and persists even when circadian clock oscillations are disrupted by the inhibition of casein kinase 1δ/ε. Both circadian and circalunar clocks converge on the regulation of transcript levels. Furthermore, the circalunar clock changes the period and power of circadian behavior, although the period length of the daily transcriptional oscillations remains unaltered. We conclude that a second endogenous noncircadian clock can influence circadian clock function.

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Marine worms coexpress circadian clock gene orthologs in the forebrain and eye

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The circalunar clock is functional in the absence of circadian clock oscillations

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The circalunar clock changes the period length of circadian-clock-controlled behavior

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The circalunar clock does not change the period of circadian molecular oscillations

Photoreception and signal transduction in corals: proteomic and behavioral evidence for cytoplasmic calcium as a mediator of light responsivity

Little is known about how corals sense and respond to light. In this report the proteome of coral is explored using 2D protein electrophoresis in two species, Montastraea cavernosa and Acropora millepora. Multiple protein species have major shifts in abundance in both species when sampled in daylight compared to corals sampled late in the night. These changes were observed both in larvae lacking zooxanthellae and in adult tissue containing zooxanthellae, including both Pacific and Caribbean corals. When larvae kept in the dark were treated with either thapsigargin or ionomycin, compounds that raise the level of cytoplasmic calcium, the night pattern of proteins shifted to the day pattern. This implies that photoreceptors responding to light elevate calcium levels and that calcium acts as the second messenger relaying light responses in corals. Corals spawn at night, and spawning can be delayed by exposure to light or pushed forward by early artificial sunsets. In a series of behavioral experiments, treatment of corals with ionomycin or thapsigargin was found to delay broadcast spawning in M. franksi, demonstrating that pharmacologically altering cytoplasmic calcium levels generates the same response as light exposure. Together these results show that the photo-responsive cells of corals detect and respond to light by altering cytoplasmic calcium levels, similarly to the transduction pathways in complex invertebrate eyes. The primacy of cytoplasmic calcium levels in light responsivity has broad implications for coral reproduction, including predicting how different species spawn at different times after sunset and how reproductive isolation is achieved during coral speciation.