Cyclic Presence and Absence of Conspecifics Alters Circadian Clock Phase But Does Not Entrain the Locomotor Activity Rhythm of the Fruit Fly Drosophila melanogaster

Circadian clocks use a wide range of environmental cues, including cycles of light, temperature, food, and social interactions, to fine-tune rhythms in behavior and physiology. Although social cues have been shown to influence circadian clocks of a variety of organisms including the fruit fly Drosophila melanogaster, their mechanism of action is still unclear. Here, the authors report the results of their study aimed at investigating if daily cycles of presence and absence (PA) of conspecific male visitors are able to entrain the circadian locomotor activity rhythm of male hosts living under constant darkness (DD). The results suggest that PA cycles may not be able to entrain circadian locomotor activity rhythms of Drosophila. The outcome does not change when male hosts are presented with female visitors, suggesting that PA cycles of either sex may not be effective in bringing about stable entrainment of circadian clocks in D. melanogaster. However, in hosts whose clock phase has already been set by light/dark (LD) cycles, daily PA cycles of visitors can cause measurable change in the phase of subsequent free-running rhythms, provided that their circadian clocks are labile. Thus, the findings of this study suggest that D. melanogaster males may not be using cyclic social cues as their primary zeitgeber (time cue) for entrainment of circadian clocks, although social cues are capable of altering the phase of their circadian rhythms. (Author correspondence: vsharma@jncasr.ac.in, vksharmas@gmail.com)     

Early- and Late-Emerging Drosophila melanogaster Fruit Flies Differ in Their Sensitivity to Light During Morning and Evening

The authors derived early and late populations of fruit flies showing increased incidence of emergence during morning or evening hours by imposing selection for timing of emergence under 12:12?h light/dark (LD) cycles. From previous studies, it was clear that the increased incidence of adult emergence during morning and evening hours in early and late populations was a result of evolution of divergent and characteristic emergence waveforms in these populations. Such characteristic waveforms are henceforth referred to as “evolved emergence waveforms” (EEWs). The early and late populations also evolved different circadian clocks, which is evident from the divergence in their clock period (τ) and photic phase response curve (PRC). Although correlation between emergence waveforms and clock properties suggests functional significance of circadian clocks, τ and PRCs do not satisfactorily explain the early and late emergence phenotypes. In order to understand the functional significance of the PRC for early and late emergence phenotypes, the authors investigated whether circadian clocks of these flies exhibit any difference in photosensitivity under entrained conditions. Such differences would suggest that the light requirement for circadian entrainment of the emergence rhythm in early and late populations is different. To test this, they examined if early and late flies differ in their light utilization behavior, first by assaying their emergence rhythm under complete photoperiod and then in three different skeleton photoperiods. The results showed that early and late populations require different durations of light during the morning and evening to achieve their EEWs, suggesting that for the circadian entrainment of the emergence rhythm, early and late flies utilize light from different parts of the day. (Author correspondence: vsharma@jncasr.ac.in or vksharmas@gmail.com)     

On the Adaptive Significance of Circadian Clocks for Their Owners

Circadian rhythms are believed to be an evolutionary adaptation to daily environmental cycles resulting from Earth's rotation about its axis. A trait evolved through a process of natural selection is considered as adaptation; therefore, rigorous demonstration of adaptation requires evidence suggesting evolution of a trait by natural selection. Like any other adaptive trait, circadian rhythms are believed to be advantageous to living beings through some perceived function. Circadian rhythms are thought to confer advantage to their owners through scheduling of biological functions at appropriate time of daily environmental cycle (extrinsic advantage), coordination of internal physiology (intrinsic advantage), and through their role in responses to seasonal changes. So far, the adaptive value of circadian rhythms has been tested in several studies and evidence indeed suggests that they confer advantage to their owners. In this review, we have discussed the background for development of the framework currently used to test the hypothesis of adaptive significance of circadian rhythms. Critical examination of evidence reveals that there are several lacunae in our understanding of circadian rhythms as adaptation. Although it is well known that demonstrating a given trait as adaptation (or setting the necessary criteria) is not a trivial task, here we recommend some of the basic criteria and suggest the nature of evidence required to comprehensively understand circadian rhythms as adaptation. Thus, we hope to create some awareness that may benefit future studies in this direction. (Author correspondence: vsharma@jncasr.ac.in or vksharmas@gmail.com)     

Short- and Long-day Responses in the Pre-adult Developmental Duration of Two Species of Camponotus Ants

We assessed the effect of different day/night lengths on the pre-adult developmental time of two species of Camponotus ants that normally develop in dark underground nests. We assayed larval (egg-to-pupal formation), pupal (pupal formation-to-adult emergence), and pre-adult (egg-to-adult emergence) durations in these ants under three different light/dark (LD) cycles of 12:12?h, 10:14?h, and 14:10?h. We observed that the pre-adult development time of ants under these day lengths was significantly different. Although both species developed fastest under 12:12?h LD, when asymmetric LD cycles were compared, night-active species (Camponotus compressus) developed faster under short days (10:14?h) and day-active species (C. paria) developed faster under long days (14:10?h). This day/night-length-mediated difference in pre-adult developmental duration was mostly due to modulation of larval duration; however, in day-active species it was also via altered pupal duration. These results thus indicate that the two species of Camponotus ants respond differently to short and long days, suggesting that seasonal timers regulate pre-adult development time in tropical ant species living in dark underground nests. (Author correspondence: vsharma@jncasr.ac.in/vksharmas/gmail.com)     

Seasonal Effect on Infants' Sleep Regulation: A Preliminary Study in a Mediterranean Climate

Infants' sleep-wake rhythms are influenced by multiple factors, including developmental and contextual aspects, as well as circadian cycles. Empirical studies that address the seasonal impact on infants' sleep are scarce. The present study examined aspects of sleep schedule and quality, comparing summer and winter months in a Mediterranean climate. This report is based on a convenience sample of 34 healthy 7-mo-olds, an age in which sleep is well consolidated and regulated compared with the first few months of life. Sleep was measured with actigraphy, in the home context. It was found that compared with winter, in the summer months, sleep onset occurred at a later hour, and more motor activity during sleep was detected. Although the overall sleep quality, as defined by sleep efficiency score, was similar in the two seasons, in the summer, more active sleep was observed. The authors discuss the finding in terms of circadian rhythms, developmental characteristics, as well as possible environmental factors and family routines, and call for more studies, in different climates and geographical zones, and in different developmental periods. (Author correspondence: cohen.dini@gmail.com or anats@edu.haifa.ac.il)     

Timekeeping Through Social Contacts: Social Synchronization of Circadian Locomotor Activity Rhythm in the Carpenter Ant Camponotus paria

In ant colonies a large proportion of individuals remain inside nests for most of their lives and come out only when necessary. It is not clear how, in a nest of several thousand individuals, information about local time is communicated among members of the colony. Central to this seem to be circadian clocks, which have an intrinsic ability to keep track of local time by entraining to environmental light-dark, temperature, and social cycles. Here, the authors report the results of their study aimed at understanding the role of cyclic social interactions in circadian timekeeping of a day-active species of carpenter ant Camponotus paria. The authors found that daily social interactions with visitors (worker ants) was able to synchronize the circadian locomotor activity rhythm of host worker ants and queens, in one-on-one (pair-wise) and multi-individual (group-wise) interactions. Interestingly, the outcome of cyclic social interactions was context specific; when visitor workers socially interacted with host workers one-on-one, host workers considered the time of interaction as subjective day, but when visitor workers interacted with a group of workers and queens, the hosts considered the time of interaction as subjective night. These results can be taken to suggest that members of the ant species C. paria keep track of local time by socially interacting with workers (foragers) who shuttle in and out of the colony in search of food. (Author correspondence: vsharma@jncasr.ac.in)     

Daily Changes in Ultraviolet Light Levels Can Synchronize the Circadian Clock of Bumblebees (Bombus terrestris)

Endogenous circadian clocks are synchronized to the 24-h day by external zeitgebers such as daily light and temperature cycles. Bumblebee foragers show diurnal rhythms under daily light:dark cycles and short-period free-running circadian rhythms in constant light conditions in the laboratory. In contrast, during the continuous light conditions of the arctic summer, they show robust 24-h rhythms in their foraging patterns, meaning that some external zeitgeber must entrain their circadian clocks in the presence of constant light. Although the sun stays above the horizon for weeks during the arctic summer, the light quality, especially in the ultraviolet (UV) range, exhibits pronounced daily changes. Since the photoreceptors and photopigments that synchronize the circadian system of bees are not known, we tested if the circadian clocks of bumblebees (Bombus terrestris) can be entrained by daily cycles in UV light levels. Bumblebee colonies were set up in the laboratory and exposed to 12?h:12?h UV?+?:UV? cycles in otherwise continuous lighting conditions by placing UV filters on their foraging arenas for 12?h each day. The activity patterns of individual bees were recorded using fully automatic radiofrequency identification (RFID). We found that colonies manipulated in such a way showed synchronized 24-h rhythms, whereas simultaneously tested control colonies with no variation in UV light levels showed free-running rhythms instead. The results of our study show that bumblebee circadian rhythms can indeed be synchronized by daily cycles in ambient light spectral composition. (Author correspondence: r.stanewsky@qmul.ac.uk)     

The Circadian Clock Machinery During Early Development of Senegalese sole (Solea senegalensis): Effects of Constant Light and Dark Conditions

Circadian rhythms are established very early during vertebrate development. In fish, environmental cues can influence the initiation and synchronization of different rhythmic processes. Previous studies in zebrafish and rainbow trout have shown that circadian oscillation of clock genes represents one of the earliest detectable rhythms in the developing embryo, suggesting their significance in regulating the coordination of developmental processes. In this study, we analyzed the daily expression of the core clock components Per1, Per2, Per3, and Clock during the first several days of Senegalese sole development (0–4 d post fertilization or dpf) under different lighting regimes, with the aim of addressing when the molecular clock first emerges in this species and how it is affected by different photoperiods. Rhythmic expression of the above genes was detected from 0 to 1 dpf, being markedly affected in the next few days by both constant light (LL) and dark (DD) conditions. A gradual entrainment of the clock machinery was observed only under light-dark (LD) cycles, and robust rhythms with increased amplitudes were established by 4 dpf for all clock genes currently studied. Our results show the existence of an embryonic molecular clock from the 1st d of development in Senegalese sole and emphasize the significance of cycling LD conditions when raising embryos and early larvae. (Author correspondence: munoz.cueto@uca.es; carlos.pendon@uca.es)     

Circadian Modulation of Acute Alcohol Sensitivity But Not Acute Tolerance in Drosophila

An increased understanding of the factors affecting behavioral and neurological responses to alcohol and alcohol physiology is necessary given the tremendous toll alcohol abuse and alcoholism exert on individuals and society. At the behavioral and molecular levels, the response to alcohol appears remarkably conserved from Drosophila to humans, suggesting that investigations across model species can provide insight into the identification of common modulatory factors. We investigated the interaction between the circadian clock and alcohol sensitivity, alcohol tolerance, and alcohol absorbance in Drosophila melanogaster. Using a loss-of-righting reflex (LoRR) assay, we found that flies exhibit a circadian rhythm in the LoRR, with the greatest sensitivity to alcohol occurring from mid to late night, corresponding to the flies' inactive phase. As predicted, a circadian rhythm in the LoRR was absent in circadian mutant flies and under conditions in which the circadian clock was nonfunctional. Circadian modulation of the response to alcohol was not due to circadian regulation of alcohol absorbance. Similar to other animals, Drosophila develop acute and chronic tolerance to alcohol upon repeat exposures. We found that the circadian clock did not modulate the development of acute alcohol tolerance measured as the difference in sensitivity to alcohol between naïve and pre-exposed flies. Thus, the circadian clock modulates some, but not all, of the behavioral responses to alcohol exposure, suggesting that specific mechanisms underlie the observed circadian modulation of LoRR rather than global cellular circadian regulation. This study provides valuable new insights in our understanding of the circadian modulation of alcohol-induced behaviors that ultimately could facilitate preventative measures in combating alcohol abuse and alcoholism. (Author correspondence: lyons@bio.fsu.edu)     

AGE-DEPENDENT ALTERATIONS IN HUMAN PER2 LEVELS AFTER EARLY MORNING BLUE LIGHT EXPOSURE

In our modern society, we are exposed to different artificial light sources that could potentially lead to disturbances of circadian rhythms and, hence, represent a risk for health and welfare. Investigating the acute impact of light on clock-gene expression may thus help us to better understand the mechanisms underlying disorders rooted in the circadian system. Here, we show an overall significant reduction in PER2 expression in oral mucosa with aging in the morning, noon, and afternoon. In the afternoon, 10?h after exposure to early morning blue light, PER2 was significantly elevated in the young compared to green light exposure and to older participants. Our findings demonstrate that human buccal samples are a valuable tool for studying clock-gene rhythms and the response of PER2 to light. Additionally, our results indicate that the influence of light on clock-gene expression in humans is altered with age. (Author correspondence: urs.albrecht@unifr.ch)     

MODELING ACTIVITY RHYTHMS IN FIDDLER CRABS

Burrowing crabs of the genus Uca inhabit tidal mudflats and beaches. They feed actively during low tide and remain in their burrows when the tide is high. The timing of this activity has been shown to persist in the absence of external light and tidal cues, indicating the presence of an internal timing mechanism. Researchers report the persistence of several variations in locomotor activity under laboratory conditions that cannot be explained by a single circatidal clock. Previous studies supported two alternative hypotheses: the presence of either two circalunidian clocks, or a circadian and circatidal clock to regulate these activity rhythms. In this paper, we formulate mathematical models to describe and test these hypotheses. The models suggested by the literature contain some important differences beyond the frequency of proposed clocks, and these are reflected in the mathematical formulations and simulation results. One hypothesis suggests independent phase oscillators, while the other hypothesis suggests that they are coupled in anti-phase. Neither model is able to recover all of the variations in locomotor acitivity observed under laboratory conditions. However, we propose a new model that incorporates aspects of both existing hypotheses and is able to reproduce all laboratory observations. (Author correspondence: verzi@math.sdsu.edu)     

Seasonal and Lunar Variation in the Emergence Time of a Population of Uca lactea annulipes (Milne-Edwards, 1837) at a Shore in Kuwait

This study monitored the endogenous emergence time of the fiddler crab Uca lactea annulipes (Milne-Edwards, 1837) in the field, for the first time, at an intertidal shore in Kuwait, from 1997 to 2001. The results revealed a significant cyclic change in the median emergence time as the season progressed from winter, through spring and summer, to autumn (.44, 1.29, 3.12, and 1.1?h prior to the dead-low tide, respectively). The data also revealed a significant shift in the median emergence time according to moon phase (2.27?h at new moon versus 2.56?h at full moon prior to the dead-low tide). (Author correspondence: l.almusawi@ropme.org)     

BLOCKING ENDOCYTOSIS IN DROSOPHILA'S CIRCADIAN PACEMAKER NEURONS INTERFERES WITH THE ENDOGENOUS CLOCK IN A PDF-DEPENDENT WAY

The neuropeptide pigment-dispersing factor (PDF) plays an essential role in the circadian clock of the fruit fly Drosophila melanogaster, but many details of PDF signaling in the clock network are still unknown. We tried to interfere with PDF signaling by blocking the GTPase Shibire in PDF neurons. Shibire is an ortholog of the mammalian Dynamins and is essential for endocytosis of clathrin-coated vesicles at the plasma membrane. Such endocytosis is used for neurotransmitter reuptake by presynaptic neurons, which is a prerequisite of synaptic vesicle recycling, and receptor-mediated endocytosis in the postsynaptic neuron, which leads to signal termination. By blocking Shibire function via overexpression of a dominant negative mutant form of Shibire in PDF neurons, we slowed down the behavioral rhythm by 3?h. This effect was absent in PDF receptor null mutants, indicating that we interfered with PDF receptor-mediated endocytosis. Because we obtained similar behavioral phenotypes by increasing the PDF level in regions close to PDF neurons, we conclude that blocking Shibire did prolong PDF signaling in the neurons that respond to PDF. Obviously, terminating the PDF signaling via receptor-mediated endocytosis is a crucial step in determining the period of behavioral rhythms. (Author correspondence: charlotte.foerster@biologie.uni-regensburg.de)     

Circadian Typology,Age, and the Alternative Five-Factor Personality Model in an Adult Women Sample

Research on personality and circadian typology indicates evening-type women are more impulsive and novelty seeking, neither types are more anxious, and morning types tend to be more active, conscientious, and persistent. The purpose of this study is to examine the differences between circadian typologies in the light of the Zuckerman's Alternative Five-Factor Model (AFFM) of personality, which has a strong biological basis, in an adult sample of 412 women 18 to 55 yrs of age. The authors found morning-type women had significant higher scores than evening-type and neither-type women on Activity, and its subscales General Activity and Work Activity. In contrast, evening-type women scored significantly higher than morning-type women on Aggression-Hostility, Impulsive Sensation Seeking, and its subscale Sensation Seeking. In all groups, results were independent of age. These findings are in accordance with those previously obtained in female student samples and add new data on the AFFM. The need of using personality models that are biologically based in the study of circadian rhythms is discussed. (Author correspondence: anna.muro@uab.cat)     

Evidence for a Plastic Dual Circadian Rhythm in the Oyster Crassostrea gigas

Although a significant body of literature has been devoted to the chronobiology of aquatic animals, how biological rhythms function in molluscan bivalves has been poorly studied. The first objective of this study was to determine whether an endogenous circadian rhythm does exist in the oyster, Crassostrea gigas. The second objective was to characterize it in terms of robustness. To answer these questions, the valve activity of 15 oysters was continuously recorded for 2 mo in the laboratory under different entrainment and free-running regimes using a high-frequency noninvasive valvometer. The present work demonstrates the presence of a circadian rhythm in the oyster Crassostrea gigas. First, oysters were entrained by 12?L:12 D conditions. Then, free-running conditions (D:D and L:L) indicated that the most frequently observed period ranged from 20 to 28?h, the circadian range. That endogenous circadian rhythm was characterized as weak. Indeed, the period (τ) of the individual animals exhibited high plasticity in D:D and L:L, and the animals immediately followed a 4-h phase advance or delay. Additionally, C. gigas appeared as a dual organism: all oysters were nocturnal at the beginning of the laboratory experiment (January), whereas they were diurnal at the end (March). That shift was progressive. Comparison with a full-year in situ record showed the same behavioral duality as observed in the laboratory: the animals were nocturnal in autumn–winter and diurnal in spring–summer. The significant advantage of a plastic and dual circadian rhythm in terms of adaptability in a highly changing environment is discussed. (Author correspondence: d.tran@epoc.u-bordeaux1.fr)     

CIRCADIAN MODULATION OF CRUSTACEAN HYPERGLYCEMIC HORMONE IN CRAYFISH EYESTALK AND RETINA

Previous studies suggested the retina could be a putative locus of daily crustacean hyperglycemic hormone (CHH) secretion, as it possesses its own metabolic machinery and is independent of the well-known CHH eyestalk locus responsible for the circadian secretion of this peptide. However, it has been proposed that hemolymph glucose and lactate concentrations play a dual role in the regulation of CHH in crayfish. To elucidate the temporal relationship between these two different CHH production loci and to examine their relationship with glucose regulation, we investigated the expression of CHH daily and circadian rhythms in the eyestalk and retina of crayfish using biochemical methods and time series analysis. We wanted to determine whether (1) putative retina and eyestalk CHH rhythmic expressions are correlated and if the oscillations of the two metabolic products of lactate and glucose in the blood due to CHH action on the target tissue correlate, and (2) retina CHH (RCHH) and the possible retinal substrate glycogen and its product glucose are temporally correlated. We found a negative correlation between daily and circadian changes of relative CHH abundance in the retina and eyestalk. This correlation and the cross-correlation values found between eyestalk CHH and hemolymph and glucose confirm that CHH produced by the X-organ sinus gland complex is under the previously proposed dual feedback control system over the 24?h time period. However, the presence of both glycogen and glucose in the retina, the cross-correlation values found between these parameters and hemolymph lactate and glucose, as well as RCHH and hemolymph and retina metabolic markers suggest RCHH is not under the same temporal metabolic control as eyestalk CHH. Nonetheless, their expression may be linked to common rhythms-generating processes. (Author correspondence: mlfm@hp.fciencias.unam.mx; mfajul@gmail.com)     

WINTERTIME LOSS OF ULTRADIAN AND CIRCADIAN RHYTHMS OF BODY TEMPERATURE IN THE SUBTERRANEAN EUTHERMIC MOLE VOLE,ELLOBIUS TALPINUS

Subterranean common mole voles, Ellobius talpinus, were implanted with long-term recording electronic thermometers to obtain hourly body temperature (T_b) data during either the wintertime or summertime. The two individuals tested during the summertime had significant circadian and ultradian rhythms in their T_b. Four of the five mole voles tested during the wintertime lacked rhythmicity in their T_b. The fifth individual lacked circadian rhythms but had ultradian rhythms in its T_b. A loss of circadian rhythms in T_b during deep torpor or hibernation has been reported for a few species of mammals. Inasmuch as the mole voles' wintertime T_b remained at euthermic levels, our results show that a loss of circadian body temperature rhythms in mole voles does not require the low T_b of deep torpor or hibernation. A tentative conclusion, based on these few animals, is that in common mole voles the T_b rhythms may disappear during the wintertime even though their T_b remains high. (Author correspondence: eug_nov@ngs.ru)