Hourglass Timer Research Articles

Clocks, Hourglasses, and History‐dependent Clocks

Many organisms have free-running circadian clocks to anticipate the time of the day, including the Kai protein-based clock in the cyanobacterium Synechococcus elongatus. However, other species have an ‘hourglass’ timer, such as the cyanobacterium Prochlorococcus marinus. What conditions favor an hourglass timer or a free running clock over the other? We argue that this evolutionary choice is influenced by the noise levels in the external environment (e.g., light fluctuations due to weather) relative to noise levels in the internal environment of the cell (e.g., molecular fluctuations). Besides different clock dynamics in different species, we find that the clock of a single species, S. elongatus, exploits a clock-metabolic coupling to tune its clock dynamics towards an hourglass timer to deal with changing noise levels.

Clocks, Hourglasses, and History‐dependent Clocks

Many organisms have free‐running circadian clocks to anticipate the time of the day, including the Kai protein‐based clock in the cyanobacterium Synechococcus elongatus. However, other species have an ‘hourglass’ timer, such as the cyanobacterium Prochlorococcus marinus. What conditions favor an hourglass timer or a free running clock over the other? We argue that this evolutionary choice is influenced by the noise levels in the external environment (e.g., light fluctuations due to weather) relative to noise levels in the internal environment of the cell (e.g., molecular fluctuations). Besides different clock dynamics in different species, we find that the clock of a single species, S. elongatus, exploits a clock‐metabolic coupling to tune its clock dynamics towards an hourglass timer to deal with changing noise levels.Support or Funding InformationK.H. thanks the James S. McDonnell Foundation for support via a Postdoctoral Fellowship. A.M. thanks the Simons Foundation for support.

Unity and diversity in the insect photoperiodic mechanism*

AbstractThis 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.

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

AbstractThe 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.

Photoperiodic response of diapause induction in the pine caterpillar, Dendrolimus punctatus

AbstractThe pine caterpillar, Dendrolimus punctatus (Walker) (Lepidoptera: Lasiocampidae), is a multivoltine pest of pine trees in China, overwintering as larvae. Winter diapause was induced by short day length. The critical night length was about 10 h 40 min at 25, 28, and 31 °C in the field, showing a temperature‐compensated diapause induction. Transfer experiments from a short night (L16:D8) to a long night (L12:D12) or vice versa at different times after hatching showed that sensitivity to day length was restricted to the first 14 days; the required day number for a 50% response at 25 °C was about 3.5 days for short nights but 7.5 days for long nights, indicating that short nights are photoperiodically more effective. When four successive short nights (L16:D8) were used to interrupt the long‐night regime (L12:D12) at different development stages and vice versa, the results showed that the highest sensitivity to photoperiod occurred on the 4th−8th day, corresponding to the second larval instar. Experiments of alternating short‐night (L16:D8) and long‐night (L12:D12) cycles during the larval period showed that the information of short nights as well as long nights could be accumulated. By rearing the larvae under conditions other than 24‐h light–dark cycles, we clearly showed that the dark period (scotophase) played a major role in the determination of diapause. The Nanda‐Hamner and Bünsow experiments failed to reveal rhythmic fluctuations with a period of about 24 h in the occurrence of diapause. Therefore, the photoperiodic clock in D. punctatus is an hourglass timer or a damped circadian oscillator.

The daily rhythms of genes, cells and organs

All living organisms have time‐measuring devices that affect their development, generation time, lifespan and lifestyle. These biological timers can be categorized into hourglasses and oscillators; the latter can be further classified as ultradian, circadian and infradian. Most physiological processes in mammals are influenced by a complex circadian timing system in which the master pacemaker in the brain synchronizes numerous subsidiary oscillators in peripheral cells. The phase of the master clock—located in the suprachiasmatic nucleus (SCN) of the ventral hypothalamus—must be readjusted every day by light cues to stay tuned to the actual geophysical time. In both SCN neurons and peripheral cells, the circadian clockwork is constructed from interconnected feedback loops in gene expression that function in a cell‐autonomous fashion. A variety of output pathways translate these gene‐expression cycles into physiological and behavioural rhythms. Experiments on a variety of organisms have shown that their maximal lifespan is determined by an hourglass timer (Fig 1, left panel), which measures different timespans in each species and is further modulated by the genetic makeup and lifestyle of the individual. Species‐specific hourglass clocks also determine the duration of embryonic development, the timespan required to reach sexual maturity and the time at which a woman reaches menopause. Hourglass mechanisms thus determine the timing of biological events that happen only once in a lifetime—unless the hourglass is flipped around by a mystical force, a process that adherents of the Hindu religion believe occurs after death. Figure 1. Two types of biological clock. Biological clocks that determine the duration of unique events for an organism or populations of organisms are termed hourglass timers. Examples of such processes are indicated below the hourglass. Oscillators are designed to drive reiterative processes, of which a few examples are listed below the picture. Yeast respiration is controlled by an oscillator that generates 40‐min …

Egg timers: how is developmental time measured in the early vertebrate embryo?

Eggs and early embryos appear to be programmed to undertake particular developmental decisions at characteristic times, although precisely how these decisions are timed is unknown. We discuss the possible roles and interactions during early vertebrate development of two broad categories of timers: 1) those that involve cyclic or sequential mechanisms, referred to as clocks; and 2) those that require an increase or decrease in some factor to a threshold level for progression of time, referred to as hourglass timers. It is concluded that both clock-like timers linked to various features of the cell cycle and hourglass timers are involved in early developmental timing. The possible involvement of elements of circadian clock timers is also considered. BioEssays 22:57-63, 2000.

Photoperiodic time measurement and related physiological mechanisms in insects and mites.

For over 60 years, many species of insects and mites have been known to respond in an adaptive way to seasonal changes in daylength. Responses to photoperiod allow them to survive periods of unfavorable environmental conditions. Since the early 1950s, light-sensitive and temperature-compensated circadian clocks have also been known to regulate the timing of many events (behavioral, physiological, biochemical) in all eukaryotic organisms, including insects and mites. Over the past 30 years, considerable effort has been directed at elucidation of the nature of the clock mechanism that underlies photoperiodism. Mathematical models using a number of insect systems have been developed that attempt to causally relate the circadian clock to photoperiodic time measurement. Although some experimental evidence supports these circadian-based models, some insects and mites appear to utilize a non-periodic hourglass timer as the photoperiodic clock. Future work in this field would benefit by following the approach that has proven to be very successful in identifying genes and gene products that regulate circadian rhythmicity in Drosophila melanogaster.

Photoperiodic time measurement in a Russian strain of the spider mite, Tetranychus urticae

Photoperiodic time measurement was studied in a Russian strain of the spider mite Tetranychus uriticae. Apart from the normal photoperiodic response curve responses were determined for a series of symmetrical ‘skeleton’ photoperiods, a series of light-break experiments (asymmetrical ‘skeleton’ photoperiods), and a resonance experiment. In all experiments the responses were found to differ from those previously obtained in identical experiments with a Dutch strain of T. urticae. In spite of the many differences observed between both geographic strains, the data obtained for the Russian strain of mites could be fully explained by the ‘hourglass timer-oscillator counter’ model, a theoretical model developed to explain photoperiodic induction of diapause in the Dutch strain of the spider mite. According to the model the differences in the photoperiodic responses of both strains of mites were entirely attributable to differences in the light-kinetics of the hourglass timer which is assumed to measure photoperiodic time in these mites; dark-kinetics appeared to be very similar in both mite strains. The results demonstrate the great intraspecific variability in the photoperiodic mechanism of T. urticae; the model interpretation shows, however, that even marked differences in response curves do not necessarily indicate the existence of essential differences in the underlying timing mechanism.

Insect photoperiodism: diversity of results in night-break experiments, including nonresponsiveness to light.

Three night-break experiment protocols were utilized in an attempt to help clarify the role of the circadian system in photoperiodic time measurement in the European corn borer, Ostrinia nubilalis. Larvae raised in a light-dark (LD) cycle consisting of 12 hr of light alternating with 12 hr of darkness (LD 12:12), at a constant temperature of 30 degrees C, enter a state of arrested growth and development known as diapause (Takeda and Skopik, 1985). In the present research (Experiment 1), the induction of diapause was prevented by 1-hr light pulses that systematically scanned the dark phase of LD 12:12. Thus, the importance of 12 hr of uninterrupted darkness for maximal induction of diapause is stressed. The same experimental protocol applied to larvae already in diapause (Experiment 2), however, resulted in a bimodal curve of diapause termination. Although this result is consistent with the proposition that a nonperiodic hourglass timer underlies this event (Skopik and Takeda, 1986), it does not rule out the circadian system. Like LD 12:12, a thermoperiod in constant darkness (12 hr at 4 degrees C alternating with 12 hr at 25 degrees C) also induces diapause. Scanning such a thermoperiod with 1-hr light pulses, however, resulted in only a small effect (reduction of diapause) when light fell in the early to middle part of the warm phase (Experiment 3). Thus, the time-measuring system, under these experimental conditions, showed only a weak response to light. This unexpected result is discussed with respect to Experiment 1 and two general models that have been proposed to account for photoperiodic time measurement in insects.

Similarities between the photoperiodic clocks of Megoura and Tetranychus: Experiments with spider mites

According to the “hourglass timer-oscillator counter” model, photoperiodic time measurement in the spider mite Tetranychus urticae is performed by a non-oscillatory or hourglass clock. The main characteristics of the spider mite's clock are the same as those of the hourglass timer developed by Lees (1973) for the aphid, Megoura viciae; the hourglass clock, applied in the spider mite's model, may be regarded as a redescription in mathematical terms of Lees's word model. To investigate the similarity between the photoperiodic clocks of Tetranychus and Megoura, some experiments which had been specifically designed by Lees to test the hourglass concept in Megoura have been repeated with Tetranychus. Experimental results were compared, both with the results of similar experiments with M. viciae, and with predictions based on the “hourglass timer-oscillator counter” model. A close resemblance was found between the results obtained with both species, which shows that photoperiodic time measurement in the spider mite and the aphid is accomplished by photoperiodic clocks the kinetics of which have much in common and may, apart from velocity parameters, even be principally the same. In the instances where differences were observed, these appeared to be attributable for the most part (according to the “hourglass timer-oscillator counter” model) to the influence of the circadian system which, in certain regimes, may strongly affect photoperiodic induction in T. urticae, but which has no detectable influence in M. viciae.

Brood Deprivation and the Control of Mouthbrooding in Pseudocrenilabrus multicolor (Cichlidae)

and Summary The female of the mouthbrooding cichlid Pseudocrenilabrus multicolor from East Africa takes up her eggs during spawning and carries them continuously in her mouth for approximately 10 days at 27 °C. Then the young — now able to swim — are released from her mouth. A former study showed that the point of release is under the control of an internal timing program in the mother which can be only slightly modified by age-specific stimuli from the brood. The present investigation is concerned with how brood deprivation influences mouthbrooding in first spawners. If the brood is removed from the mother's mouth for longer periods of time (several days), the internal timing program can be split into two components that control different behavioural parameters: 1 Brood care motivation, i.e. the readiness to take up brood into the mouth and keep it there but not eat it, is maintained up to 10 to 11 days after spawning (at 27 °C), regardless of the duration of previous brood deprivation. 2 In contrast, the point of release of brood from the mouth is retarded as a linear function of the duration of previous brood deprivation. Additionally, brood older than 10 days at the point of release is released one day earlier than younger brood. The mechanisms of these different time-measuring systems are discussed with the aid of a model which assumes internal hour-glass timers. The adaptive significance of the mechanisms remains unclear.

THE TIMING OF DIVISION IN CHLAMYDOMONAS

SummaryThe duration of the cell cycle of Chlamydomonas is determined by two timed periods which each follow an initiating stimulus. A commitment timer, initiated by the beginning of growth in autonomous daughter cells, is dependent upon concurrent growth and controls the time of first commitment to divide. Division processes, which are then initiated, occupy a second timer period of 6 h duration at 25°C.Absence of division control by an endogenous oscillator was indicated by an unfluctuating ability to resume cycles of consistent duration when daughter cells were held in darkness or when growing cells in light were subjected to darkness. A consistent time from the beginning of daughter growth to commitment was also observed when daughter formation was delayed by an earlier temperature reduction. Ethanol, which commonly affects circadian oscillators, did not disturb synchrony.The division cycle is controlled by growth and not by direct response to light, since commitments ceased when growth was prevented by lack of CO2 in the light. Growth in daughter cells initiated the timer leading to commitment since commitment was reached at similar times after resupply of CO2 or reillumination. Division synchrony in diurnal cycles of periodic illumination is not caused by entrainment of an oscillator but by the growth requirement of the hourglass timer leading to commitment. A model of division control is presented.

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 light 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 light 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.

Photoactivation of urocanase in Pseudomonas putida. Temperature-compensated in vitro model of an hourglass timer.

Photoactivation of urocanase in Pseudomonas putida. Temperature-compensated in vitro model of an hourglass timer.

Differences in measurement of day-length and photoperiodism in two stocks from subarctic and temperate climates in the Carabid beetle Pterostichus nigrita F.

Experiments on the measurement of day-length as a basis for photoperiodism were made with a stock of the carabid Pterostichus nigrita from the Subarctic in Swedish Lapland (64-66° N). Results were compared with those of earlier investigations on a Central European stock (51° N). P. nigrita from Lapland reacted differently from the Central European ones in experiments with abnormal photoperiods (cycle durations from 12 to 72 h; light period in all cases 8 h). Whereas, in the Central European stock, short-day induced reactions (previtellogenesis in the females, maturation of the male gonads) could only be observed if those cycles represented multiples of 24 h in whole numbers, 'short-day" reactions in the Lapland stock were inducible with all cycles in nearly 100% of the specimens investigated (only in LD 8:4 was this percentage slightly diminished). Experiments with "dark breaks" of 2 h in extreme long-day (LD 20:4) revealed nearly 100% short-day maturation, irrespective of the temporal location of these dark breaks in beetles from the Lapland stock, whereas in Central European beetles only dark breaks during certain scotophile phases had such an effect. In the Central European P. nigrita, two hour light breaks during the night phase of an LD 14:10 had a destructive effect on short-day maturation processes; in the Lapland stock, these were possible in spite of 1 h light breaks during the night of an LD 19:5. In the Lapland stock, developmental processes which in the Central European stock are confined to short-day conditions could occur in all types of experiments, provided that about 3 to 4 h of darkness were given during each 24 h cycle, whether uninterrupted or not.Light break experiments revealed that the long-day process for ovarian development (vitellogenesis) is induced in the same way as with the Central European P. nigrita. It occurs if light falls into a photosensitive phase during the second half of the 24 h cycle.Thus, for the Subarctic population, a model is proposed in which short-day is measured by means of an hour-glass timer, whereas long-day is perceived by means of a circadian oscillator.

Light-driven periodic changes in urocanase activity of a heterotrophic bacterium.

Urocanase activity in Pseudomonas putida cells showed periodic changes corresponding to a light-dark cycle. Apparent interconversion of the active and inactive forms of urocanase was accomplished by photoactivation by nearultraviolet light and by dark thermal inactivation. The purified enzyme exhibited similar behavior. This system suggests a possible molecular basis for an hour-glass timer. Photoinactivation by light of 290 nm alternated with photoactivation at 340 nm also generated regular changes in urocanase activity.