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Circadian clocks are endogenous time keeping mechanisms that drive near 24-h behavioural, physiological and metabolic rhythms in organisms. It is thought that organisms possess circadian clocks to facilitate coordination of essential biological events to the external day and night (extrinsic advantage) so as to enhance Darwinian fitness. However, on Earth, there are a number of habitats that are not subject to such robust daily cycling of geo-physical factors. Do organisms living under such conditions exhibit rhythmic behaviours that are driven by endogenous circadian clocks? We attempt to critically survey studies of rhythms (or the lack of them) in organisms living in a range of constant environments. Many such organisms do show rhythms in behaviour and/or physiological variables. We suggest that such presence of rhythms may be indicative of an underlying clock that facilitates, (a) internal synchrony among rhythms, and (b) temporal partitioning of incompatible cellular processes (intrinsic advantage). We then highlight reasons that limit our interpretations about the presence (or absence) of clocks in such organisms living under constant conditions, and suggest possible methods to conclusively test whether or not rhythms in these organisms are driven by endogenous circadian clocks with the hope that it may enhance our understanding of circadian clocks in organisms under constant environments. [ABSTRACT FROM AUTHOR]

Organisms are believed to have evolved circadian clocks as adaptations to deal with cyclic environmental changes, and therefore it has been hypothesized that evolution in constant environments would lead to regression of such clocks. However, previous studies have yielded mixed results, and evolution of circadian clocks under constant conditions has remained an unsettled topic of debate in circadian biology. In continuation of our previous studies, which reported persistence of circadian rhythms in Drosophila melanogaster populations evolving under constant light, here we intended to examine whether circadian clocks and the associated properties evolve differently under constant light and constant darkness. In this regard, we assayed activity-rest, adult emergence and oviposition rhythms of D. melanogaster populations which have been maintained for over 19 years (~330 generations) under three different light regimes – constant light (LL), light–dark cycles of 12:12 h (LD) and constant darkness (DD). We observed that while circadian rhythms in all the three behaviors persist in both LL and DD stocks with no differences in circadian period, they differed in certain aspects of the entrained rhythms when compared to controls reared in rhythmic environment (LD). Interestingly, we also observed that DD stocks have evolved significantly higher robustness or power of free-running activity-rest and adult emergence rhythms compared to LL stocks. Thus, our study, in addition to corroborating previous results of circadian clock evolution in constant light, also highlights that, contrary to the expected regression of circadian clocks, rearing in constant darkness leads to the evolution of more robust circadian clocks which may be attributed to an intrinsic adaptive advantage of circadian clocks and/or pleiotropic functions of clock genes in other traits. [ABSTRACT FROM AUTHOR]

Following decades of research under controlled laboratory conditions, there has been growing interest in the recent past to study circadian rhythms in nature. Recent studies conducted under natural conditions have been fruitful in exploring several characteristics of circadian rhythms that remained cryptic and previously masked under standard laboratory conditions, reemphasizing that the complexity of circadian rhythms in nature increases multifold under the influence of multiple zeitgebers. However, our understanding of the contributions of different zeitgebers in shaping various rhythm characteristics still remains elusive. Previously, Vaze et al. reported that chronotype differences between the morning emerging (early) and evening emerging (late) populations of Drosophila melanogaster are considerably enhanced under natural conditions compared to standard laboratory conditions. In the present study, we assess the role of 2 primary zeitgebers in nature—light and temperature—individually and in unison in driving chronotype differences. We report that when provided independently, temperature cycles enhance divergence between the early and late chronotypes more strongly than light, but when together, light and temperature appear to act antagonistically and that appropriate phase difference between light and temperature cycles is essential to promote chronotype divergence. Thus, our study highlights the importance of light and temperature, as well as their interaction with circadian clocks in mediating early and late chronotypes in fruit flies D. melanogaster. [ABSTRACT FROM PUBLISHER]

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: /) [ABSTRACT FROM AUTHOR]

Extensive research has been carried out to understand how circadian clocks regulate various physiological processes in organisms. The discovery of clock genes and the molecular clockwork has helped researchers to understand the possible role of these genes in regulating various metabolic processes. In Drosophila melanogaster, many studies have shown that the basic architecture of circadian clocks is multi-oscillatory. In nature, different neuronal subgroups in the brain of D. melanogaster have been demonstrated to control different circadian behavioural rhythms or different aspects of the same circadian rhythm. Among the circadian phenomena that have been studied so far in Drosophila, the egg-laying rhythm is unique, and relatively less explored. Unlike most other circadian rhythms, the egg-laying rhythm is rhythmic under constant light conditions, and the endogenous or free-running period of the rhythm is greater than those of most other rhythms. Although the clock genes and neurons required for the persistence of adult emergence and activity/rest rhythms have been studied extensively, those underlying the circadian egg-laying rhythm still remain largely unknown. In this review, we discuss our current understanding of the circadian egg-laying rhythm in D. melanogaster, and the possible molecular and physiological mechanisms that control the rhythmic output of the egg-laying process. [ABSTRACT FROM AUTHOR]

Circadian clocks regulate physiological and behavioral processes in a wide variety of organisms, and any malfunction in these clocks can cause significant health problems. In this paper, we report the results of our study on the physiological consequences of circadian dysfunction (malfunctioning of circadian clocks) in two wild‐type populations of fruit flies ( Drosophila melanogaster ). We assayed locomotor activity behavior and lifespan among adult flies kept under constant dark (DD) conditions of the laboratory, wherein they were categorized as rhythmic if their activity/rest schedules followed circadian (approximately 24 h) patterns, and as arrhythmic if their activity/rest schedules did not display any pattern. The rhythmic flies from both populations lived significantly longer than the arrhythmic ones. Based on these results, we conclude that circadian dysfunction is deleterious, and proper functioning of circadian clocks is essential for the physiological well being of D. melanogaster. [ABSTRACT FROM AUTHOR]

Circadian clocks are believed to have evolved in parallel with the geological history of the earth, and have since been fine-tuned under selection pressures imposed by cyclic factors in the environment. These clocks regulate a wide variety of behavioral and metabolic processes in many life forms. They enhance the fitness of organisms by improving their ability to efficiently anticipate periodic events in their external environments, especially periodic changes in light, temperature and humidity. Circadian clocks provide fitness advantage even to organisms living under constant conditions, such as those prevailing in the depth of oceans or in subterranean caves, perhaps by coordinating several metabolic processes in the internal milieu. Although the issue of adaptive significance of circadian rhythms has always remained central to circadian biology research, it has never been subjected to systematic and rigorous empirical validation. A few studies carried out on free-living animals under field conditions and simulated periodic and aperiodic conditions of the laboratory suggest that circadian rhythms are of adaptive value to their owners. However, most of these studies suffer from a number of drawbacks such as lack of population-level replication, lack of true controls and lack of adequate control on the genetic composition of the populations, which in many ways limits the potential insights gained from the studies. The present review is an effort to critically discuss studies that directly or indirectly touch upon the issue of adaptive significance of circadian rhythms and highlight some shortcomings that should be avoided while designing future experiments. [ABSTRACT FROM AUTHOR]

In this paper we report the results of an experiment to assess how closely repeated brief light pulses (LPs) mimic the effects of 12:12 h light/dark (LD) cycles (PPc). The locomotor activity rhythm of individual fruit flies from a laboratory population of Drosophila melanogaster was monitored under four different photoperiodic regimens, created using 12 h of light and 12 h of darkness or brief light pulses (LPs). The phase relationship (Ψ) and the stability (precision) of the locomotor activity rhythm during entrainment were estimated in order to compare the state of the circadian clocks under the four different photoperiodic regimens. The flies ( n = 72) were subjected to four different LD cycles: (i) 12 h of light and 12 h of darkness (complete photoperiod, PPc); (ii) a single brief LP of 15 min duration presented close to the onset of activity (SLP-1); (iii) a single brief LP of 15 min duration administered close to the offset of activity (SLP-2); and (iv) two brief LPs administered 12 h apart (skeleton photoperiod, PPs). The locomotor activity rhythm of the flies was first monitored under constant darkness (DD) for about 10 days and then under the four different photoperiodic regimens for about 10 days, and finally in DD for the remainder of the experiment. The Ψ of the locomotor activity rhythm and its precision under PPc and PPs did not differ significantly, but they were significantly different from the SLP-1 and SLP-2 conditions. The results provide interesting insights into photoentrainment mechanisms of circadian clocks in D. melanogaster , and suggest that skeleton photoperiods, but not single brief LPs, mimic the actions of complete photoperiods. [ABSTRACT FROM AUTHOR]

The eclosion and oviposition rhythms of flies from a population of Drosophila melanogaster maintained under constant conditions of the laboratory were assayed under constant light (LL), constant darkness (DD), and light/dark (LD) cycles of 10:10 h (T20), 12:12 h (T24), and 14:14 h (T28). The mean (±95% confidence interval; CI) free-running period (τ) of the oviposition rhythm was 26.34 ± 1.04 h and 24.50 ± 1.77 h in DD and LL, respectively. The eclosion rhythm showed a τ of 23.33 ± 0.63 h (mean ± 95% CI) in DD, and eclosion was not rhythmic in LL. The τ of the oviposition rhythm in DD was significantly greater than that of the eclosion rhythm. The eclosion rhythm of all 10 replicate vials entrained to the three periodic light regimes, T20, T24, and T28, whereas the oviposition rhythm of only about 24 and 41% of the individuals entrained to T20 and T24 regimes, respectively, while about 74% of the individuals assayed in T28 regimes showed entrainment. Our results thus clearly indicate that the τ and the limits of entrainment of eclosion rhythm are different from those of the oviposition rhythm, and hence this reinforces the view that separate oscillators may regulate these two rhythms in D. melanogaster. [ABSTRACT FROM AUTHOR]

The effect of 'novel running wheels' on circadian clocks of the nocturnal field mouse Mus booduga was investigated during free-running and entrained conditions. In order to find out whether daily access to novel running wheels can entrain the locomotor activity rhythms experimental animals ( n = 6) were provided with 'novel running wheels' at a fixed time of the day. The control animals ( n = 5) were handled similar to the experimental animals but were not given access to novel running wheels. The results show that daily access to novel running wheels entrained the free-running locomotor activity rhythm of these mice. The post-entrainment free-running period (τ) of the experimental animals was significantly shorter than the pre-entrainment τ, whereas the pre- and post-treatment τ of the control animals did not differ significantly. In separate set of experiments, the effect of access to novel running wheels on the rate of re-entrainment was studied after a 6 h phase advance/delay in 24 h (12:12 h) light/dark (LD) cycles. Experimental animals were given access to novel running wheels for 3-h, 1 h after the 'lights-off' only on the first day of the 'new LD cycles'. Experimental animals took fewer cycles to re-entrain to 6-h phase advanced LD cycles compared to the control animals. After a phase delay in the LD cycles by 6h, the experimental animals took more number of cycles to re-entrain compared to the control animals. These results thus suggest that access to novel running wheel can act as a Zeitgeber for the circadian clocks of the nocturnal mouse M. booduga , and can also modify the rates of re-entrainment to phase shifted LD cycles, in a time-dependent manner. [ABSTRACT FROM AUTHOR]

Although lithium is one of the most commonly used drugs in the prophylaxis and treatment of bipolar disorder, the mechanisms underlying its therapeutic action are still unclear. Together with its mood-stabilizing effects, lithium is also known to influence the circadian clocks of several organisms including man. Circadian rhythms are altered in patients with bipolar disorder, and it is believed that these rhythms may play an important role in disease mechanisms. It is therefore possible that some of the therapeutic actions of lithium may be related to its effect on circadian clocks. Identifying the targets for lithium's action on circadian clocks would therefore be important both for understanding the mechanisms of its therapeutic effect and also in further understanding disease mechanisms in bipolar disorders. Using Drosophila melanogaster as a model system, we show that long-term administration of lithium results in lengthening of the free-running period (τ) of circadian locomotor activity rhythm of flies in constant darkness (DD). This effect occurs at concentrations similar to the plasma levels of lithium used in the treatment of bipolar disorder. The lithium-treated flies also show reduced activity of one of the previously reported targets of lithium action, Glycogen Synthase Kinase 3β (GSK 3β). GSK 3β has been shown to be involved in the regulation of circadian clocks as the down regulation of this protein results in an elongation of τ. The τ elongation resembles the effect seen with lithium administration in a number of organisms including man, and taken together with the earlier observations our results suggest that lithium inhibits the activity of GSK 3β to produce its effect on circadian clocks. [ABSTRACT FROM AUTHOR]

Circadian clocks are ubiquitous and are found in organisms ranging from bacteria to mammals. This ubiquity of occurrence implies adaptive significance, but to date there has been no rigorous empirical evidence to support this. It is believed that an organism possessing circadian clocks gains fitness advantage in two ways: (i) by synchronizing its behavioral and physiological processes to cyclic environmental factors (extrinsic adaptive value); (ii) by coordinating its internal metabolic processes (intrinsic adaptive value). There is preliminary circumstantial evidence to support both. Several studies using organisms living in constant environments have shown that these organisms possess functional circadian clocks, suggesting that circadian clocks may have some intrinsic adaptive value. Studies to assess the adaptive value of circadian clocks in periodic environments suggest that organisms may have a fitness advantage in those periodic environments, which closely match their own intrinsic periodicity. Furthermore, evidence from organisms living in the wild, selection studies, and studies on latitudinal clines suggest that circadian clocks may have an extrinsic adaptive value as well. In this paper, I have presented several hypotheses for the emergence of circadian clocks and have reviewed some major empirical studies suggesting adaptive significance of circadian clocks. [ABSTRACT FROM AUTHOR]

A simple computer-aided device was developed to record the activity of small animals. The device comprises of aktographs, opto-couplers, opto-amplifier units, a data acquisition unit ('Activity Monitor') and a Pentium processor based personal computer (PC) attached to an addition parallel printer port. The aktographs are devices which facilitate organisms to exhibit its natural activity. The aktographs for fruit flies, ants and mice are tubes with food supply and moisture, petri dishes adequately ventilated with arrangements for constant supply of watch sugar or honey solution, and cages attached with running wheels, respectively. Two mutually perpendicular beams of infra-red (IR) emission from a cross-hair detection system across the aktographs and an interruption to these beams creates an electrical signal ('transition') which is amplified using the opto-amplifier unit and sent t the activity monitor which consists of shift registers. The number of moments the IR beams are interrupted is taken as a measure of the amount of activity, based on the assumption that a more active animal would interrupt the IR beams more often compared to a less active one. Programs were written in C++, which facilitated data acquisition and graphical representation in terms of a daily plot of amount of activity as a function of time of the day, and actograms of daily activity were arranged chronologically one below the other the activity patterns of individual animals (fruit fly, ants, and mice) were monitored using this device. Based on the performance and features of this device, we feel that with some improvement in the software this can substitute some of the existing activity recording devices, which are costly and not flexible. [ABSTRACT FROM AUTHOR]

We have described a simple approach for the analysis and isolation of multiple periodicities from a biological time series. For the estimation of the periodicities, we used simulated data and data from ongoing experiments in our laboratory. Two time series were simulated one which consisted of only white noise and the other consisted while noise along with periodicities of 6, 11, 17 and 23 h, to demonstrate that our method can successfully isolate multiple patterns in a time series. Our method of analysis is objective, simple, flexible and adaptive since it distinctly delineates the individual contribution from an overlap of multiple periodicities. The key features of our method are: (i) identification of a reliable phase reference point, (ii) scanning the time series using a moving window m increments, (iii) use of Siegel's modification of Fisher's method to detect significant periodicit(y)ies in the time series. The use of window sizes of increasing length to examine the time series elegantly reduces noise while identifying periodicities that are otherwise not apparent. Finally, the periodogram can be smoothed in order to normalize the contribution by attendant frequency components within the waveform. A minimum critical value for relative contribution of various frequencies was calculated to delineate the periodicities that contributed significantly to the time series. We executed this method of time series analysis using MS Excel and C. [ABSTRACT FROM AUTHOR]

Background: Previous studies have implicated a role for circadian clocks in regulating pre-adult development of organisms. Among them two approaches are most notable: 1) use of insects whose clocks have different free-running periods and 2) imposition of artificial selection on either rate of development, timing of emergence or circadian period in laboratory populations. Using these two approaches, influence of clock on rate of development has been elucidated. However, the contribution of circadian clocks in determining time taken for pre-adult development has remained unclear. Here we present results of our studies aimed to understand this influence by examining populations of fruit flies carrying three different alleles of the period gene and hence having different free-running periods. We tried to achieve similarity of genetic background among the three strains while also ensuring that they harbored sufficient variation on loci other than period gene. Results: We find that under constant conditions, flies with long period have slower development whereas in presence of light-dark cycles (LD) of various lengths, the speed of development for each genotype is influenced by whether their eclosion rhythms can entrain to them. Under LD 12:12 (T24), where all three strains entrain, they do not show any difference in time taken for emergence, whereas under LD 10:10 (T20) where long period flies do not entrain and LD 14:14 (T28) where short period flies do not entrain, they have slower and faster pre-adult development, respectively, compared to the controls. We also show that a prior stage in development namely pupation is not rhythmic though time taken for pupation is determined by both the environmental cycle and period allele. Conclusion: We discuss how in presence of daily time cues, interaction of the cyclic environmental factors with the clock determines the position and width of the gate available for a fly to emerge (duration of time within a cycle when adult emergence can occur) resulting in an altered developmental duration from that observed under constant conditions. We also discuss the relevance of genetic background influencing this regulation. [ABSTRACT FROM AUTHOR]