Light-break experiments and photoperiodic time measurement in the spider mite Tetranychus urticae

To explain photoperiodic induction of diapause in the spider mite Tetranychus urticae (Acarina: Tetranychidae) a theoretical model was developed, consisting of two components, viz. a “clock” and a photoperiodic “counter” mechanism. The clock executes photoperiodic time measurement according to hourglass kinetics; the counter accumulates the photoperiodic information contained in a number of successive $\text{light}\text{dark}$ cycles by adding up the number of “long” and “short” nights experienced by the developmental stages of the mites sensitive to the photoperiod. The influence of the circadian system on photoperiodic induction is interpreted as an inhibitory effect exerted on the expression of the photoperiodic response; this effect is encountered only in certain photoperiodic regimes, where the circadian system and the photoperiod are out of “resonance” with each other. This “hourglass timer oscillator counter model”, devised to give a theoretical explanation of photoperiodic time measurement, the summation of photoperiodic information, and the influence of the circadian system on photoperiodic induction, proved to be consistent with experimental results obtained with T. urticae in both symmetrical and asymmetrical “skeleton” photoperiods, the latter based on diel as well as non-diel $\text{light}\text{dark}$ cycles.     

Photoperiodic time measurement in the spider mite Tetranychus urticae: A novel concept

To explain photoperiodic induction of diapause in the spider mite Tetranychus urticae a new theoretical model was developed which took into account both the hourglass and rhythmic elements shown to be present in the photoperiodic reaction of these mites. It is emphasized that photoperiodic induction is the result of time measurement as well as the summation and integration of a number of successive photoperiodic cycles: the model, therefore, consists of separate ‘clock’ and ‘counter’ mechanisms. In current views involvement of the circadian system in photoperiodism is interpreted in terms of the hypothesis that the photoperiodic clock itself is based on one or more circadian oscillators. Here a different approach has been chosen as regards the role of the circadian system in photoperiodism: the possibility, previously put forward by other authors, that some aspect of the photoperiodic induction mechanism other than the clock is controlled by the circadian system was investigated by assuming a circadian influence on the photoperiodic counter mechanism. The derivation of this ‘hourglass timer oscillator counter’ model of photoperiodic induction in T. urticae is described and its operation demonstrated on the basis of a number of diel and nondiel photoperiods, with and without light interruptions.     

Photoperiodic time measurement in insects: a review of clock models

Based on analyses of responses of insects and mites to a wide range of diel and nondiel experimental light-dark schedules, a variety of models have been developed for the photoperiodic clocks in these species by nearly as many investigators. According to some of these models, the photoperiodic clock is based on a mechanism separate from the circadian system, that is, a so-called "hourglass." According to other models, the clock is based on one or more circadian oscillators that may be coupled to each other and that may or may not show a certain degree of damping. In this context, a rapidly damping oscillator could be regarded as an hourglass. The present article gives an overview of the many different clock models and their philosophies, and it makes comparisons among them to provide a better understanding about how these models are related, if at all, and why the double circadian oscillator model is the most favored model at present.     

A ‘simple clock’ approach of circadian rhythms: An easy way to predict the clock's singularity

A graphical method is presented which allows the prediction of phase resetting curves of circadian rhythms for both type 1 and type 0 resetting, starting from one experimentally determined phase resetting curve. Calculations were based on literature data for the pupal eclosion rhythm of Drosophila pseudoobscura. The method is based on the assumption that for all practical purposes the rhythm may be approached as a “simple clock”, i.e. an oscillator with only one state variable, namely its phase or circadian time, CT. Besides predicting both “types” of phase resetting the method is capable to locate the “position” of the phase singularity and thus the transition from type 1 to type 0 resetting. This graphical method is an elaboration of the “transformation method”, developed in 1972 by A. Johnsson and H. G. Karlsson, which was effective in predicting phase resetting by “strong” stimuli, but failed in the case of “weak” stimuli. Predictions made using the extended transformation method are in good agreement with experimental results obtained with Drosophila. Also for the flesh fly, Sarcophaga argyrostoma, a prediction is made of the position of the phase singularity of the eclosion rhythm and compared with experimental results.     

Diapause induction and photoperiodic clock in Chilo suppressalis (Walker) (Lepidoptera: Crambidae)

The mature larvae of the rice stem borer, Chilo suppressalis Walker (Lepidoptera: Crambidae) enters facultative diapause in response to short‐day conditions in the autumn (August–September). Diapause induction and photoperiodic clock mechanism were investigated in C. suppressalis larvae reared on an artificial diet in the present study. The critical night length for diapause induction was about 9 h 53 min to 10 h 39 min at 22 to 28°C. The third‐instar larvae were found to be relatively sensitive to diapause induction. Photoperiodic response under non‐24‐h light–dark cycles showed that scotophase length played an essential role in the induction of larval diapause in C. suppressalis, and consecutive exposure to long‐night cycles was necessary for a high diapause incidence. In the Nanda–Hamner experiment, diapause incidence peaked at scotophase of 12 h and dropped rapidly at scotophases > 24 h. In the Bünsow experiment, diapause incidence was clearly suppressed, especially at the light pulse located 8 h in the scotophase. Both the Nanda–Hamner and Bünsow experiments showed no rhythmic fluctuations with a period of about 24 h; thus the photoperiodic clock in C. suppressalis is a non‐oscillatory hourglass timer or a rapidly damping circadian oscillator.     

Time measurement in the photoperiodic induction of sexual rest in the terrestrial isopod Armadillidium vulgare (Latreille)

The photoperiodic control of sexual rest in Armadillidium vulgare was investigated using various experimental protocols. When reared in conditions of a Nanda-Hamner (i.e. resonance) protocol from their first parturial moult to their post experimental moult, females showed a weak resonance effect in sexual rest incidence. The transfer from a long day cycle to a symmetrical skeleton photoperiod--consisting of two equal light pulses per 24 h of continuous darkness--revealed the involvement of a circadian oscillatory system in the photoperiodic clock of this species. The data, obtained in the whole experiments, suggested that both oscillator and hourglass features are involved in the photoperiodic response controlling the sexual rest in Armadillidium vulgare. Moreover, when non-24-h light-dark cycles (with a long photophase) were applied, a mechanism responsible of arrest of reproduction also implied a photoperiodic counter which accumulated and added up the photoperiodic information within a sensitive period during post parturial intermoult.     

ELF3 modulates resetting of the circadian clock in Arabidopsis

The Arabidopsis early flowering 3 (elf3) mutation causes arrhythmic circadian output in continuous light, but there is some evidence of clock function in darkness. Here, we show conclusively that normal circadian function occurs with no alteration of period length in elf3 mutants in dark conditions and that the light-dependent arrhythmia observed in elf3 mutants is pleiotropic on multiple outputs normally expressed at different times of day. Plants overexpressing ELF3 have an increased period length in both constant blue and red light; furthermore, etiolated ELF3-overexpressing seedlings exhibit a decreased acute CAB2 response after a red light pulse, whereas the null mutant is hypersensitive to acute induction. This finding suggests that ELF3 negatively regulates light input to both the clock and its outputs. To determine whether ELF3's action is phase dependent, we examined clock resetting by using light pulses and constructed phase response curves. Absence of ELF3 activity causes a significant alteration of the phase response curve during the subjective night, and constitutive overexpression of ELF3 results in decreased sensitivity to the resetting stimulus, suggesting that ELF3 antagonizes light input to the clock during the night. The phase of ELF3 function correlates with its peak expression levels in the subjective night. ELF3 action, therefore, represents a mechanism by which the oscillator modulates light resetting.     

Insect photoperiodism: effects of temperature on the induction of insect diapause and diverse roles for the circadian system in the photoperiodic response

This review considers the effects of temperature on insect diapause induction and the photoperiodic response, and includes constant temperature, temperature cycles, pulses and steps in daily light–dark cycles, constant darkness and in constant light, all with reference to various circadian‐based “clock” models. Although it is a comparative survey, it concentrates on two species, the flesh fly Sarcophaga argyrostoma and its pupal parasite Nasonia vitripennis, which possess radically different photoperiodic mechanisms, although both are based upon the circadian system. Particular attention is given to the effects of daily thermoperiod in darkness and to low and high temperature pulses in conjunction with a daily light–dark cycle, treatments that suggest that S. argyrostoma “measures” night length with a “clock” of the external coincidence type. However, N. vitripennis responds to seasonal changes in photoperiod with an internal coincidence device involving both “dawn” and “dusk” oscillators. Other species may show properties of both external and internal coincidence. Although the precepts of external coincidence have been well formulated and supported experimentally, those for internal coincidence remain obscure.     

A circadian system is involved in photoperiodic entrainment of the circannual rhythm of Anthrenus verbasci

In the circannual pupation rhythm of the varied carpet beetle, Anthrenus verbasci, entrainment to annual cycles is achieved by phase resetting of the circannual oscillator in response to photoperiodic changes. In order to examine whether a circadian system is involved in expression of the periodic pattern and phase resetting of the circannual rhythm as photoperiodic responses, we exposed larvae to light-dark cycles with a short photophase followed by a variable scotophase (the Nanda-Hamner protocol). When the cycle length (T) was a multiple of 24 h, i.e., 24, 48, or 72 h, short-day effects were clearer than when T was far from a multiple of 24 h, i.e., 36 or 60 h. Exposure to light-dark cycles of T = 36 h had effects similar to exposure to long-day cycles of T = 24 h. The magnitude of phase shifts depended on the duration and the phase of exposure to the cycles of T = 36 or 60 h. It was therefore concluded that a circadian system is involved in photoperiodic time measurement for phase resetting of the circannual oscillator of A. verbasci.     

Evidence for ‘dawn’ and ‘dusk’ hourglasses in Pimpla photoperiodic clock

Abstract

Resonance experiments (Nanda‐Hamner protocol) conducted at two temperatures for diapause termination in Pimpla instigator (Hymenoptera: Ichneumonidae) do not support the view that the photoperiodic clock has an oscillatory component, but suggest the presence of a non‐rhythmic timer or hourglass mechanism. These results are best explained by a two hourglasses model, one of which starts at light‐on and measures the photophase and the other is initiated by light‐off and measures the scotophase. The most likely hypothesis is that the ratio of photophase to scotophase lengths is the determining element. Good agreement is obtained between results predicted by two hourglasses model and results observed in Pimpla. The diurnal hourglass continues to run for long time (several months) in constant condition (LL) and does not require to be ‘turned over’ by D/L transition, in contrary to the classical model of hourglass which executes a single act of time measurement in extented phase and then stops. The most simple explanation is that some essential factor of diapause termination is synthesized during photophase and degraded during scotophase. Therefore an independent photoperiodic counter (for sommation of daily informations) is not necessary. The two hourglasses system serves as photoperiodic clock and accumulation of product as counter.     

Unity and diversity in the insect photoperiodic mechanism*

This review examines some of the models to account for time measurement in insect photoperiodism. It considers the supporting evidence for these models and the attempts to discriminate among them. Although hourglass timers may exist, it is suggested that most photoperiodic mechanisms, including many hourglass‐like timers, are circadian‐based, making Bünning's original hypothesis, that the circadian system somehow provides the essential “clockwork” for photoperiodic timing, the most persuasive unifying principle. The apparent diversity among modern species in their modes of time measurement is probably the result of differences between the underlying circadian systems that were adopted for seasonal night length measurement as the insects, or groups of insects, moved northwards into areas with a pronounced winter season. Photoperiodic time measurement, therefore, exhibits both unity (in their common circadian basis) and diversity in detail. Attention to this diversity may provide invaluable insights into the problem of photoperiodic time measurement at comparative, and molecular, levels.     

Deciphering time measurement: the role of circadian 'clock' genes and formal experimentation in insect photoperiodism

This review examines possible role(s) of circadian ‘clock’ genes in insect photoperiodism against a background of many decades of formal experimentation and model building. Since ovarian diapause in the genetic model organism Drosophila melanogaster has proved to be weak and variable, recent attention has been directed to species with more robust photoperiodic responses. However, no obvious consensus on the problem of time measurement in insect photoperiodism has yet to emerge and a variety of mechanisms are indicated. In some species, expression patterns of clock genes and formal experiments based on the canonical properties of the circadian system have suggested that a damped oscillator version of Pittendrigh's external coincidence model is appropriate to explain the measurement of seasonal changes in night length. In other species extreme dampening of constituent oscillators may give rise to apparently hourglass-like photoperiodic responses, and in still others there is evidence for dual oscillator (dawn and dusk) photoperiodic mechanisms of the internal coincidence type. Although the exact role of circadian rhythmicity and of clock genes in photoperiodism is yet to be settled, Bünning's general hypothesis (Bünning, 1936) remains the most persuasive unifying principle. Observed differences between photoperiodic clocks may be reflections of underlying differences in the clock genes in their circadian feedback loops.     

MPer1 and mper2 are essential for normal resetting of the circadian clock

Mammalian Per1 and Per2 genes are involved in the mechanism of the circadian clock and are inducible by light. A light pulse can evoke a change in the onset of wheel-running activity in mice by shifting the onset of activity to earlier times (phase advance) or later times (phase delays) thereby advancing or delaying the clock (clock resetting). To assess the role of mouse Per (mPer) genes in circadian clock resetting, mice carrying mutant mPer1 or mPer2 genes were tested for responses to a light pulse at ZT 14 and ZT 22, respectively. The authors found that mPer1 mutants did not advance and mPer2 mutants did not delay the clock. They conclude that the mammalian Per genes are not only light-responsive components of the circadian oscillator but also are involved in resetting of the circadian clock.     

The circadian clocks of plants and cyanobacteria

Classical research on the circadian rhythms of plants helped to demonstrate that all living organisms utilize circadian clocks to adapt their day–night cycles and that the clock is the basis for photoperiodic time measurements. Molecular models for the circadian oscillator have now been elucidated in Drosophila, Neurospora, mice and cyanobacteria. All share a similar feedback structure, but key proteins in each of the oscillators are different. A plant clock model has yet to be proposed, but clock mutants of Arabidopsis are expected to reveal key proteins in the mechanism. Here we discuss how a self-sustained oscillation is established in eukaryotic and prokaryotic models, and the polyphyletic evolution of these clock systems.     

BIOLOGICAL TIMING AND THE CLOCK METAPHOR: OSCILLATORY AND HOURGLASS MECHANISMS

Living organisms have developed a multitude of timing mechanisms— “biological clocks.” Their mechanisms are based on either oscillations (oscillatory clocks) or unidirectional processes (hourglass clocks). Oscillatory clocks comprise circatidal, circalunidian, circadian, circalunar, and circannual oscillations—which keep time with environmental periodicities—as well as ultradian oscillations, ovarian cycles, and oscillations in development and in the brain, which keep time with biological timescales. These clocks mainly determine time points at specific phases of their oscillations. Hourglass clocks are predominantly found in development and aging and also in the brain. They determine time intervals (duration). More complex timing systems combine oscillatory and hourglass mechanisms, such as the case for cell cycle, sleep initiation, or brain clocks, whereas others combine external and internal periodicities (photoperiodism, seasonal reproduction). A definition of a biological clock may be derived from its control of functions external to its own processes and its use in determining temporal order (sequences of events) or durations. Biological and chemical oscillators are characterized by positive and negative feedback (or feedforward) mechanisms. During evolution, living organisms made use of the many existing oscillations for signal transmission, movement, and pump mechanisms, as well as for clocks. Some clocks, such as the circadian clock, that time with environmental periodicities are usually compensated (stabilized) against temperature, whereas other clocks, such as the cell cycle, that keep time with an organismic timescale are not compensated. This difference may be related to the predominance of negative feedback in the first class of clocks and a predominance of positive feedback (autocatalytic amplification) in the second class. The present knowledge of a compensated clock (the circadian oscillator) and an uncompensated clock (the cell cycle), as well as relevant models, are briefly reviewed. Hourglass clocks are based on linear or exponential unidirectional processes that trigger events mainly in the course of development and aging. An important hourglass mechanism within the aging process is the limitation of cell division capacity by the length of telomeres. The mechanism of this clock is briefly reviewed. In all clock mechanisms, thresholds at which “dependent variables” are triggered play an important role. (Chronobiology International, 18(3), 329–369, 2001)     

Erwin Bünning and Tony Lees, two giants of chronobiology, and the problem of time measurement in insect photoperiodism

This paper examines the views of Erwin Bünning and Tony Lees on the mechanism of photoperiodic time measurement, the former advocating a circadian basis for the phenomenon and the latter a non-circadian hourglass-like timer. This difference in opinion led to a protracted split among workers on photoperiodism, some supporting an oscillatory clock and others an "hourglass", and gave rise to the often stated opinion that the two forms of time measurement were mutually exclusive. This paper, however, suggests that both oscillatory and hourglass-like properties are to be seen in insect photoperiodism. Furthermore, the differences between the two apparently conflicting models may be resolved if, following Bünning, "hourglasses" are regarded as damping circadian oscillators, with the more self-sustained (clearly oscillatory) and more highly damped (hourglass-like) responses being parts of a continuous series. Since circadian rhythmicity is an all-pervading and fundamental aspect of insect biology, currently opening up to genetic and molecular analysis, recognition of the basic similarity of a wide range of insect photoperiodic timers may help to unravel the biochemical nature of the mechanism(s) involved.     

Modeling Temperature Compensation in Chemical and Biological Oscillators

All physicochemical and biological oscillators maintain a balance between destabilizing reactions (as, for example, intrinsic autocatalytic or amplifying reactions) and stabilizing processes. These two groups of processes tend to influence the period in opposite directions and may lead to temperature compensation whenever their overall influence balances. This principle of “antagonistic balance” has been tested for several chemical and biological oscillators. The Goodwin negative feedback oscillator appears of particular interest for modeling the circadian clocks in Neurospora and Drosophila and their temperature compensation. Remarkably, the Goodwin oscillator not only gives qualitative, correct phase response curves for temperature steps and temperature pulses, but also simulates the temperature behavior of Neurospora frq and Drosophila per mutants almost quantitatively. The Goodwin oscillator predicts that circadian periods are strongly dependent on the turnover of the clock mRNA or clock protein. A more rapid turnover of clock mRNA or clock protein results, in short, a slower turnover in longer period lengths. (Chronobiology International, 14(5), 499–510, 1997)     

Action Spectrum for Resetting the Circadian Phototaxis Rhythm in the CW15 Strain of Chlamydomonas: II. Illuminated Cells

The action spectrum for resetting the phase of the circadian clock in Chlamydomonas reinhardtii is different depending upon whether the light stimuli are presented to cells that were in darkness versus dim illumination before stimulation. In this report, we show that phase resetting of illuminated cells appears to be mediated by components of the photosynthetic apparatus. This conclusion is based upon the action spectrum for phase-shifting illuminated cells (which looks like that for photosynthesis) and upon the fact that inhibitors of photosynthetic electron transport also inhibit the light-induced phase shift of illuminated cells. Both of these characteristics differ from that of cells taken from darkness. We, therefore, believe that at least two resetting pathways for this circadian clock exist and that both of these pathways are ecologically significant.