A circadian clock measures photoperiodic time in the male lizard Anolis carolinensis

Summary Photoperiod plays an important role in controlling the annual reproductive cycle of the male lizard Anolis carolinensis. The nature of photoperiodic time measurement in Anolis was investigated by exposing anoles to 3 different kinds of lighting paradigms (resonance, T cycles, and night breaks) to determine if photoperiodic time measurement involves the circadian system. Both the reproductive response and the patterns of entrainment of the activity rhythm were assessed. The results show that the circadian system is involved in photoperiodic time measurement in this species and that a discrete photoinducible phase resides in the latter half of the animals' subjective night. Significantly, the ability of the circadian system to execute photoperiodic time measurement is crucially dependent on the length of the photoperiod. Resonance, T cycle and night break cycles utilizing a photoperiod 10–11 h in duration reveal circadian involvement whereas these same cycles utilizing 6 or 8 h photoperiods do not.Abbreviation CRPP circadian rhythm of photoperiodic sensitivity     

Further studies on the involvement of the circadian system in photoperiodic control of antifreeze protein production in the beetle Dendroides canadensis

The role of circadian rhythmicity in the photoperiodic time measuring processes regulating antifreeze protein production in the beetle Dendroides canadensis was further investigated. Using “T” experiments larvae were exposed to environmental light cycle periods close to the period length of the endogenous circadian oscillator. The following light cycles were employed: light/dark 8/13, 8/14, 8/16, 8/18 and 8/19 corresponding to period lengths of 21, 22, 24, 26 and 27 h. Larvae maintained in cycles equal to or less than 24 h displayed a characteristic short-day response, showing significantly (P < 0.01) greater antifreeze protein activity than did those measured on the day of collection in late summer. In contrast, a long-day response was observed in larvae maintained under a 26- or 27-h light cycle in that antifreeze protein activity did not differ from that measured on the initial collection date.

The role of photoperiod and temperature in influencing the photoperiodic timing processes were examined with a series of resonance experiments. The first group consisted of a 24, 36, 48, 60 or 72-h light cycle, each with an 8-h photophase at temperatures of 20 or 17°C. Rhythmic increases in antifreeze protein levels at intervals of 24 h occurred under both temperatures. However, the lower temperature displaced the resonance curve in the vertical direction (i.e. increasing % population response) and reduced the difference between peaks and troughs on the resonance curve. Resonance experiments incorporating a 14-h photophase resulted in low antifreeze protein activity under all conditions except a 36-h light cycle in which a 67% induction was observed.

Eight hour resonance experiments were also conducted with D. canadensis collected in early spring to determine whether the circadian system participates in the photoperiodic timing processes influencing the spring termination of antifreeze protein production. Positive resonance results were obtained in that only larvae maintained in cycles of 36 and 60 h displayed significantly (P < 0.01) lower antifreeze activity when compared to animals on the initial collection date.

The combined results emphasize the involvement of the circadian system in the photoperiodic control of antifreeze protein production by D. canadensis during the fall and spring. Furthermore, the induction of antifreeze protein production is a function of light cycle and its waveform (photoperiod). Temperature appears to modify the photoperiodic response in some manner involving the photoperiodic time measuring processes. It is concluded that the photoperiodic response of antifreeze protein production by D. canadensis is dependent upon the entrainment of the circadian system by the light cycle.     

Experimental evidence for a non-clock role of the circadian system in spider mite photoperiodism

In the spider mite Tetranychus urticae photoperiodic time measurement proceeds accurately in orange-red light of 580 nm and above in light/dark cycles with a period length of 20 h but not in 'natural' cycles with a period length of 24 h. To explain these results it is hypothesized that the photoperiodic clock in the spider mite is sensitive to orange-red light, but the Nanda-Hamner rhythm (a circadian rhythm with a free-running period tau of 20 h involved in the photoperiodic response) is not and consequently free runs in orange-red light. To test this hypothesis a zeitgeber was sought that could entrain the Nanda-Hamner rhythm to a 24-h cycle without inducing diapause itself, in order to manipulate the rhythm independently from the orange-red sensitive photoperiodic clock. A suitable zeitgeber was found to be a thermoperiod with a 12-h warm phase and a 12-h cold phase. Combining the thermoperiod with the long-night orange-red light/dark regime, both with a cycle length of 24 h, resulted in a high diapause incidence, although neither regime was capable of inducing diapause on its own. The conclusion is that the Nanda-Hamner rhythm is necessary for the realization of the photoperiodic response, but is not part of the photoperiodic clock, because photoperiodic time measurement takes place in orange-red light whereas the rhythm is not able to 'see' the orange-red light. It is speculated that the Nanda-Hamner rhythm is involved in the timely synthesis of a substrate for the photoperiodic clock in the spider mite.     

Circadian rhythms of locomotor activity and temperature selection in sleepy lizards, <Emphasis Type="Italic">Tiliqua rugosa</Emphasis>

This study examined whether the daily rhythms of locomotor activity and behavioural thermoregulation that have previously been observed in Australian sleepy lizards (Tiliqua rugosa) under field conditions are true circadian rhythms that persist in constant darkness (DD) and whether these rhythms show similar characteristics. Lizards held on laboratory thermal gradients in the Australian spring under the prevailing 12-hour light : dark (LD) cycle for 14 days displayed robust daily rhythms of behavioural thermoregulation and locomotor activity. In the 13-day period of DD that followed LD, most lizards exhibited free-running circadian rhythms of locomotor activity and behavioural thermoregulation. The predominant activity pattern displayed in LD was unimodal and this was retained in DD. While mean levels of skin temperature and locomotor activity were found to decrease from LD to DD, activity duration remained unchanged. The present results demonstrate for the first time that this species’ daily rhythm of locomotor activity is an endogenous circadian rhythm. Our results also demonstrate a close correlation between the circadian activity and thermoregulatory rhythms in this species indicating that the two rhythms are controlled by the same master oscillator(s). Future examination of seasonal aspects of these rhythms, may, however, cause this hypothesis to be modified.     

Photoperiodic ovarian response in yellow-throated sparrow, Gymnorhis xanthocollis: involvement of circadian rhythm

The experiments aim to investigate the mechanism of photoperiodic time measurement during photoperiodic ovarian response of subtropical yellow-throated sparrow. Groups of the photosensitive female birds were exposed to various night-interruption cycles for a period of 35 days. These light-dark cycles consisted of a basic photophase of 6h and 1h photointerruption of the 18h dark phase in 24h cycle at different points. A control group was also placed under 7L/17D. Ovarian response was observed in the night-interruption cycles in which the photointerruption of dark phase was made 12h after the onset of basic photophase. The results are consistent with the Bünning hypothesis and indicate that an endogenous circadian rhythm is involved in photoperiodic time measurement during initiation of ovarian growth in this species.     

The field energetics and water fluxes of free-living wombats (Marsupialia: Vombatidae)

Wombats are large, fossorial, herbivorous marsupials exhibiting physical and behavioural characteristics indicative of extreme energy conservation. Previous energetics studies have been limited to their basal metabolism under laboratory conditions; little is known of the energetics of free-living wombats. We measured seasonal field metabolic rates (FMR) and water fluxes in the three species of free-living wombat using the doubly labelled water technique, to further investigate the extent of energy conservation in the Vombatidae. Measurements were taken during the wet and dry annual extremes of their characteristically harsh environments, which corresponded to seasonal extremes of food and water availability. Seasonal FMRs for all wombat species were lower than that recorded for other marsupials and well below that predicted for herbivorous mammals. Dry-season FMR of Lasiorhinus kreftii was 40% of that predicted for a mammal. Wombats maintained energy balance during the poor season by reducing FMR to about half that of the good season. Water flux rates during the dry season for the arid-adapted Lasiorhinus are amongst the lowest recorded for mammals, being only 25% of that predicted for a similarly sized herbivorous mammal. These low water flux rates enable wombats in semi-arid areas to maintain water balance without drinking. Estimated food and nitrogen intake rates were also low. We conclude that the energetically frugal lifestyle of the Vombatidae is amongst the most extreme for mammals.     

Sleep EEG spectral analysis in a diurnal rodent:Eutamias sibiricus

1. Sleep was studied in the diurnal rodent Eutamias sibiricus, chronically implanted with EEG and EMG electrodes. Analysis of the distribution of wakefulness, nonrapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep over the 24 h period (LD 12:12) showed that total sleep time was 27.5% of recording time during the 12 h light period and 74.4% during the 12 h dark period. Spectral analysis of the sleep EEG revealed a progressive decay in delta power density in NREM sleep during darkness. Power density of the higher frequencies increased at the end of darkness. Power density of the higher frequencies decreased and that of the lower frequencies increased during light. 2. Analysis of the distribution of vigilance states under three different photoperiods (LD 18:6; 12:12; 6:18) revealed that changes in daylength mainly resulted in a redistribution of sleep and wakefulness over light and darkness. Under long days the percentage of sleep during light was enhanced. The time course of delta power density in NREM sleep was characterized by a long rising part and a short falling part under long days, while a reversed picture emerged under short days. As a consequence, the power density during days. As a consequence, the power density during light was relatively high under long days. 3. After 24 h sleep deprivation by forced activity, no significant changes in the percentages of wakefulness and NREM were observed, whereas REM sleep was slightly enhanced. EEG power density, however, was significantly increased by ca. 50% in the 1.25-10.0 Hz range in the first 3 h of recovery sleep. This increase gradually decayed over the recovery night. 4. The same 24 h sleep deprivation technique led to a ca. 25% increase in oxygen consumption during recovery nights. While the results of the EEG spectral analysis are compatible with the hypothesis that delta power density reflects the 'intensity' of NREM sleep as enhanced by prior wakefulness and reduced by prior sleep, such enhanced sleep depth after sleep deprivation is not associated with reduced energy expenditure as might be anticipated by some energy conservation hypotheses on sleep function.     

Photoperiodic clock of diapause induction in Pseudopidorus fasciata (Lepidoptera: Zygaenidae)

Pseudopidorus fasciata enters diapause as fourth instar larvae at short day lengths. Using 24-h light-dark cycles, the photoperiodic response curves in this species appeared to be similar with a critical night length of 10.5h at temperatures below 30 degrees C. At an average temperature of 30.5 degrees C, the critical night length had shifted to between 15 and 17h. In experiments using non-24-h light-dark cycles, it was clearly demonstrated that the dark period (scotophase) was the decisive phase for a diapause determination. In night interruption experiments using 24-h light-dark cycles, a 1-h light pulse at LD12:12 completely reversed the long night effect and averted diapause in all treatments. At LD 9:15 light pulses of 1-h, 30- or 15-min also averted diapause effectively when both the pre-interruption (D(1)) or the post-interruption scotophases (D(2)) did not exceed the critical night length. If D(1) or D(2) exceeded the critical night length diapause was induced. The most crucial event for the photoperiodic time measurement in this species is the length of the scotophase. A 10-min light pulse placed in the most photosensitive phase reversed diapause in over 50% of the individuals. Night interruption experiments under non-24-h light-dark cycles indicated that the photoperiodic clock measured only D(1) regardless of the length of D(2), suggesting that the most inductive cycles are often those in which L+D are close to 24h. In resonance experiments, this species showed a circadian periodicity at temperatures of 24.5 or 26 degrees C, but not at 30.5 and 23.3 degrees C. On the other hand, Bünsow and skeleton photoperiod experiments failed to reveal the involvement of a circadian system in this photoperiodic clock. These results suggest the photoperiodic clock in this species is a long-night measuring hourglass and the circadian effect found in the final expression of the photoperiodic response in the resonance experiments may be caused by a disturbing effect of the circadian system in unnatural regimes.     

Tidal,Daily, and Lunar‐Day Activity Cycles in the Marine Polychaete Nereis virens

The burrow emergence activity of the wild caught ragworm Nereis virens Sars associated with food prospecting was investigated under various photoperiodic (LD) and simulated tidal cycles (STC) using a laboratory based actograph. Just over half (57%) of the animals under LD with STC displayed significant tidal (～12.4 h) and/or lunar‐day (～24.8 h) activity patterns. Under constant light (LL) plus a STC, 25% of all animals were tidal, while one animal responded with a circadian (24.2 h) activity rhythm suggestive of cross‐modal entrainment where the environmental stimulus of one period entrains rhythmic behavior of a different period. All peaks of activity under a STC, apart from that of the individual cross‐modal entrainment case, coincided with the period of tank flooding. Under only LD without a STC, 49% of the animals showed nocturnal (～24 h) activity. When animals were maintained under free‐running LL conditions, 15% displayed significant rhythmicity with circatidal and circadian/circalunidian periodicities. Although activity cycles in N. virens at the population level are robust, at the individual level they are particularly labile, suggesting complex biological clock‐control with multiple clock output pathways.     

A case of abnormal size in eggs and larvae of the Taiga tick,Ixodes persulcatus (Acari: Ixodidae)

Egg batches laid by females of Ixodes persulcatus (maintained under a photoperiod of L:D 20:4 before feeding) contained eggs of two sizes: numerous normal eggs of 0.578 × 0.421 mm and some giant eggs of 0.776×0.515 mm. Giant eggs were 32–34% greater in length and 22–24% greater in width than normal eggs. Females maintained under a short-day photoperiod (L:D 12:12) laid only normal eggs (0.588×0.417 mm). There were no differences between these eggs in viability and developmental time, but larvae hatched from giant eggs were larger (length of body 23–28% greater and of gnathosoma 12–15% greater) than larvae from normal eggs. This dimorphism of eggs and larvae was not sex determined, but may be a maternal photoperiodic effect.     

Circadian nature of the photoperiodic clock in Japanese quail

Summary The photoperiodic clock in quail (Coturnix colurnix japonica) is based upon a rhythm of photoinducibility (Øi) but the extent to which this rhythm is circadian remains unclear. Two types of experiment investigated this situation. In the first, gonadectomized quail were adapted to live in periods of darkness by training them on a schedule containing one short day and 3 days of darkness (SD/DD/DD/DD). They were then exposed to a single pulse of 6 or 10 h of light at different times across 3 days of darkness. The photoperiodic response, measured by the increase in LH secretion, showed clear rhythmicity, demonstrating unequivocally the circadian nature of Øi. The second set of experiments employed Nanda-Hamner cycles and varied the length of the photoperiod from 6 to 11 h. Responsiveness in a 36 h or a 60 h cycle was highly dependent upon the length of the photoperiod, something not predicted from theory. For instance, LD 6:30 was not photoperiodically inductive but LD 10:26 was clearly inductive. Close analysis of patterns of LH secretion indicated an unexpected delay before induction occurred and then a rapid rise to a stable level of induction. When LH was measured in every pulse under LD 10:26 there was no evidence that LH levels alternately increased and decreased. This is not consistent with the simplest interpretation of Nanda-Hamner experiments where alternate pulses of light are thought to entrain the rhythm or induce a photoperiodic response by coinciding with Øi. It is concluded that the quail's photoinducible rhythm is indeed based on a circadian rhythm but one that is only weakly self-sustaining. Possibly as a consequence of this, the rhythm's behaviour under abnormal photoperiodic cycles may be rather different from that found in other species and from other circadian rhythms in quail.Abbreviations Øi photoinducible phase - LH luteinizing hormone     

Microfibrils are a major component of the mesangial matrix in the glomerulus of the rat kidney

Summary Certain secretory cells in the hypophysial pars tuberalis of the Djungarian hamster display marked circannual structural alterations. The present investigation deals with the immunohistochemical properties of this cell group. A distinct TSH-like immunoreactivity was found in secretory cells of this type in the pars tuberalis of animals exposed to long photoperiods, whereas under short photoperiods the TSH-like immunoreactivity was nearly absent. In the pars distalis, the number and distribution of TSH-positive cells did not differ significantly between animals maintained under long and under short photoperiods. LH-and FSH-positive cells could not be detected in the pars tuberalis, but they are clearly present in the pars distalis of both groups of hamsters. Our immunocytochemical results suggest that photoperiodic stimuli influence the secretory activity of TSH-like immunoreactive cells in the pars tuberalis. A connection with the neuroendrocrine-thyroid axis is discussed.The study was supported by the Deutsche Forschungsgemeinschaft (Wi 558/3-1, Pe 134/2-4)     

Diapause induction and clock mechanism in the cabbage beetle, Colaphellus bowringi (Coleoptera: Chrysomelidae)

Photoperiodic control of diapause induction was investigated in the short-day species, Colaphellus bowringi, which enters summer and winter diapause as adult in the soil. Photoperiodic responses at 25 and 28 degrees C revealed a critical night length between 10 and 12 h; night lengths > or =12 h prevented diapause, whereas night lengths <12 h induced summer diapause in different degree. Experiments using non-24-h light-dark cycles showed that the duration of scotophase played an essential role in the determination of diapause. Night-interruption experiments with T=24 h showed that diapause was effectively induced by a 2-h light pulse in most scotophases; whereas day-interruption experiments by a 2-h dark break had a little effect on the incidence of diapause. The experiments of alternating short-night cycles (LD 16:8) and long-night cycles (LD 12:12) during the sensitive larval period showed that the information of short nights as well as long nights could be accumulated. Nanda-Hamner experiments showed three declining peaks of diapause at 24 h circadian intervals. Bünsow experiments showed two very weak peaks for diapause induction, one being 8 h after lights-off, and another 8 h before lights-on, but it did not show peaks of diapause at a 24 h interval. These results suggest that the circadian oscillatory system constitutes a part of the photoperiodic clock of this beetle but plays a limited role in its photoperiodic time measurement.     

Photoperiodic time measurement in insects and mites: a critical evaluation of the oscillator-clock hypothesis

The validity of the oscillator-clock hypothesis for photoperiodic time measurement in insects and mites is questioned on the basis of a re-interpretation of available experimental evidence. The possible role of the circadian system in photoperiodism in arthropods is critically reviewed. Apart from the outcome of kinetic experiments, based on diel and non-diel light/dark cycles, evidence from various genetic and physiological experiments is discussed in relation to the oscillator-clock hypothesis. The conclusion is that photoperiodic time measurement in insects and mites is performed by a non-circadian 'hourglass' clock. Experimental evidence suggests a non-clock role for the circadian system in the photoperiodic mechanism of insects and mites.     

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.     

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.