Photoperiodic Clock Research Articles

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 °C revealed a critical night length between 10 and 12 h; night lengths ≥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.

Wavelength Dependency of Light-Induced Effects on Photoperiodic Clock in the Migratory Blackheaded Bunting (Emberiza melanocephala)

The effects of light wavelength on photoperiodic clock were determined in the migratory male blackheaded bunting (Emberiza melanocephala). We constructed an action spectrum for photoperiodic induction (body fattening, gain in body mass, and gonadal recrudescence) by exposing birds for 4.5 weeks to 13 h light per day (L:D = 13:11 h) of white (control), blue (450 nm), or red (640 nm) color at irradiances ranging from 0.028 to 1.4 W m−2. The threshold light irradiance for photoinduction was about 10-fold higher for blue, compared to red and white light. Phase-dependent effects of light wavelength on the photoperiodic clock were further examined in the next two sets of skeleton photoperiods (SKPs). In the first set of SKPs, birds were exposed for four weeks to asymmetrical light periods (L:D:L:D = 6:6:1:11 h) at 0.25 ± 0.01 W m−2; two light periods applied were of the same (450 nm: blue:blue, B:B; 640 nm, red:red, R:R) or different (blue:red, B:R or red:blue, R:B) wavelengths, or of white:white (W:W, controls). Photoperiodic induction occurred under R:R and B:R, but not under B:B and R:B light conditions; the W:W condition induced an intermediate response. The second set of SKPs used symmetrical light periods (L:D:L:D = 1:11:1:11 h), and measured effects also on the activity rhythm. Birds were first exposed to one of the four SKPs (R:R, B:B, R:B, or B:R) for three weeks, subsequently were released into dim constant light (LLdim; ˜0.01 W m−2, the night light used in an L:D cycle) for two weeks, and then were returned to respective SKPs for another three weeks. Activity was greater in the R:R compared to B:B, and in B:R compared to R:B light condition. Zugunruhe (intense nighttime activity, indicating migratory restlessness in a caged situation) developed under the R:R and B:R, but not the B:B and R:B, light condition. Under LLdim, all birds free-ran with a period >24 h, the Zugunruhe had a circadian period longer than the daytime activity, and the re-entrainment to SKPs was influenced by the position of light periods relative to circadian phase of the activity rhythm. Photoperiodic induction at the end of 8 weeks was found in the R:R and B:R, but not in B:B, light conditions; in the R:B condition only one bird had initiated testes. Taken together, these results suggest that in the blackheaded bunting, the circadian photoperiodic clock is differentially responsive to light wavelengths; this responsiveness is phase-dependent, and the development of Zugunruhe reflects a true circadian function. Wavelength-dependent response of the photoperiodic clock could be part of an adaptive strategy in evolution of the seasonality in reproduction and migration among photoperiodic species under wild conditions.

TIMELESS: A link between fly's circadian and photoperiodic clocks?

Potential involvement of circadian clock genes in so far unknown mechanism of photoperiodic time measurement is an important question of insect life-cycle regulation science. Here we report about the cloning of full-length cDNA of the structural homologue of the Drosophila's timeless gene in Chymomyza costata. Its expression was compared in two strains: a wild-type strain, responding to short days by entering larval diapause and a npd-mutant strain, showing no photoperiodic response. The timeless mRNA transcripts were not detectable by Northern blot analysis in the fly heads of npd-mutants, while they were detectable and showed typical daily oscillations in the wild-type strain. After disrupting the normal process of timeless transcription in the wild-type strain by injection of timeless double-strandRNA into early embryos of wild-type (RNAi method: Kennerdell & Carthew 1998, 2000), a certain proportion of the individuals adopted a npd-mutant phenotype, showing no-diapause in response to short-daylength. Cloning of genomic DNA fragments revealed that npd-mutants carry a different allele, timelessnpd, with a 13-bp insertion in an intron positioned within the 5'-leader sequence. Genetic linkage analysis showed that the 13-bp insertion (a marker for timelessnpd) and the absence of response to short days (a marker for npd-phenotype) are strictly co-inherited in the F2 progeny of the reciprocal crosses between wild-type and npd-mutant flies. Such results indicated that the locus npd could code for the timeless gene in C. costata and its product might thus represent a molecular link between circadian and photoperiodic clock systems in this fly.

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

Photoperiodic control of diapause termination was systematically investigated in Pseudopidorus fasciata. In 24 h light-dark cycles, the rate of diapause termination in this species depended on photoperiod. The critical night length (CNL) for diapause termination was 10 h, 0.5 h shorter than that for diapause induction. Night-interruption experiments with T = 24 showed that diapause was effectively terminated when the scotophases separated by light pulse were shorter than the critical night length (10 h); no developing individuals were found if the duration of the pre-interruption scotophase or the post-interruption scotophase exceeded the CNL. A 15-min light pulse was sufficient to reverse the effect of long night when it was placed 8 h after lights-off. Resonance experiments with a constant photophase of 12 h or 16 h and various scotophases of 4-80 h showed an hourglass-type photoperiodic response, where no rhythmicity was found. In another resonance experiment with constant scotophase of 8 h and various photophases of 4-72 h, all individuals developed into cocoons. In the BĂźnsow experiment, the response curve showed two apparent peaks for diapause termination, one being 8 h after lights-off, and another 8 h before lights-on. However, there was no periodic rhythmicity, which again indicates an hourglass principle. The results lead to the conclusion that the same photoperiodic clock mechanism (a long-night measuring hourglass) is involved in both diapause induction and termination.

Selection for high diapause incidence in blow flies ( Calliphoravicina) maintained under long days increases the maternal critical daylength: some consequences for the photoperiodic clock

Using a population of Calliphora vicina from southern Scotland (55 °N), showing a critical day length for maternal induction of diapause of about 14.5 h per day, strains of flies were selected for a high incidence of larval diapause under long day length (LD 16:8 h). Diapause incidence was raised from under 10% to almost 100% within five or six generations. The selected flies showed an increase in their critical day length to over 16 h per day, and a high incidence of larval diapause under very long photophases. Selected flies, however, showed mean circadian periods for locomotor activity little different from the original stock, or from flies selected for high diapause under LD 12:12 h, and a Nanda–Hamner profile lacking peaks and troughs of diapause incidence at about 24 h intervals. These results are interpreted to show that the photoperiodic regulation of diapause and the control of overt behavioural rhythmicity are ‘separate’ physiological systems.

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 τ 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.

The plant photoperiodic clock - what is it?

The plant photoperiodic clock - what is it?

Short-day and long-day expression patterns of genes involved in the flesh fly clock mechanism: period, timeless,cycle and cryptochrome

Though our knowledge of the molecular details of the circadian clock has advanced rapidly, the functional elements of the photoperiodic clock remain largely unknown. As a first step to approach this issue, we report here the sequences and expression patterns of period ( per), timeless ( tim), cycle ( cyc) and cryptochrome ( cry) mRNAs in the flesh fly Sarcophaga crassipalpis. Nucleotide and deduced amino acid sequences of the genes in S. crassipalpis show high similarity to homologous genes in other insects that have been investigated. S. crassipalpis TIM has a unique C-terminus that contains a poly Q region. A diel rhythmicity of per and tim mRNA abundance was detected in the adult heads (peak during scotophase), while cry and cyc mRNA abundance remained fairly constant throughout. The abundance of cyc mRNA was quite low when compared to per, tim and cry mRNA. Rearing temperature affected the amount of per and tim mRNAs: abundance of per mRNA increased at 20 °C when compared to 25 °C, but that of tim mRNA decreased. Photoperiod influenced the expression patterns of per and tim mRNA: the peak of per mRNA expression shifted in concert with onset of the scotophase, while a shift in tim mRNA expression was less pronounced. The amplitude of tim mRNA was severely dampened under long daylength, but that of per mRNA was not affected. These distinct patterns of expression suggest that this information could be used to determine photoperiodic responses such as diapause.

The Influence of Light Wavelength on Phase-Dependent Responsiveness of the Photoperiodic Clock in Migratory Blackheaded Bunting

We investigated the phase-dependent effects of light wavelength on photoperiodic clock in the migratory blackheaded bunting. Two experiments were performed, employing a skeleton paradigm (6 hours light : 6 hours darkness : 1 hour light : 11 hours darkness; 6L : 6D : 1L : 11D) at 37 ± 2 lux intensity. In the experiment 1, both 6 and 1 h light pulses were given at the same wavelength, 500 nm (green) or 650 nm (red). A group exposed to both pulses of white light served as control. In the experi-ment 2, the two light pulses were given at two different wavelengths, 6 h at 500 nm (green) and 1 h at 640 nm (red) in one group or vice-versa in the other. There was almost no photoinduction when both light pulses in experiment 1, or 1 h light pulse in experiment 2, were green. On the other hand, birds fattened and testes recrudesced when both the light pulses in experiment 1, or 1 h light pulse in experiment 2, were red. Birds receiving both pulses of white light in experiment 1 showed an intermediate response. Taken together, these results indicate that the photoperiodic clock in buntings is differentially responsive at its various circadian phases to different light wavelengths.

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 10.5 h at temperatures below 30°C. At an average temperature of 30.5°C, the critical night length had shifted to between 15 and 17 h. 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 24 h. In resonance experiments, this species showed a circadian periodicity at temperatures of 24.5 or 26°C, but not at 30.5 and 23.3°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.

Encoding le quattro stagioni within the mammalian brain: photoperiodic orchestration through the suprachiasmatic nucleus.

Within the suprachiasmatic nucleus (SCN) is a pacemaker that not only drives circadian rhythmicity but also directs the circadian organization of photoperiodic (seasonal) timekeeping. Recent evidence using electrophysiological, molecular, and genetic tools now strongly supports this conclusion. Important questions remain regarding the SCN's precise role(s) in the brain's photoperiodic circuits, especially among different species, and the cellular and molecular mechanisms for its photoperiodic "memory." New data suggesting that SCN "clock" genes may also function as "calendar" genes are a first step toward understanding how a photoperiodic clock is built from cycling molecules.

Insect photoperiodism and circadian clocks: models and mechanisms.

Photoperiodic clocks allow organisms to predict the coming season. In insects, the seasonal adaptive response mainly takes the form of diapause. The extensively studied photoperiodic clock in insects was primarily characterized by a "black-box" approach, resulting in numerous cybernetic models. This is in contrast with the circadian clock, which has been dissected pragmatically at the molecular level, particularly in Drosophila. Unfortunately, Drosophila melanogaster, the favorite model organism for circadian studies, does not demonstrate a pronounced seasonal response, and consequently molecular analysis has not progressed in this area. In the current article, the authors explore different ways in which identified molecular components of the circadian pacemaker may play a role in photoperiodism. Future progress in understanding the Drosophila circadian pacemaker, particularly as further output components are identified, may provide a direct link between the clock and photoperiodism. In addition, with improved molecular tools, it is now possible to turn to other insects that have a more dramatic photoperiodic response.

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.

Phenotypic evolution and the genetic architecture underlying photoperiodic time measurement

A wide variety of higher plants, vertebrates, and arthropods use the length of day to synchronize growth, development, reproduction, dormancy, and migration with the changing seasons. In the pitcher-plant mosquito, Wyeomyia smithii, critical photoperiod mediating the onset and maintenance of larval diapause has evolved about 10 standard deviations in mean critical photoperiod between the ancestral, Gulf Coast populations, and the derived, Canadian populations. We are seeking to understand how this evolution has been accomplished at both the genetic and the physiological levels. At the genetic level, average heterozygosity at protein-coding loci decreases with latitude of origin, while genetic variation for photoperiodic response increases with latitude of origin, particularly within the formerly glaciated regions of North America. Hybridization experiments reveal widespread genetic differences in critical photoperiod due to epistasis. We ascribe the increase in genetic variation in photoperiodic response, despite directional and stabilizing selection to the contrary, to the release of additive from epistatic variance during successive founder events in W. smithii's northward dispersal following recession of the Laurentide Ice Sheet, and to the resulting genetic drift and reorganization of genetic architectures in descendent populations. At the physiological level, northern populations of W. smithii, as well as northern populations of spider mites, flies, moths, and beetles in both North America and Europe show a declining expression of the rhythmic component of photoperiodic response. In Drosophila melanogaster, when the epistatic coupling between the period locus and photoperiodic response is disrupted, the critical photoperiod is shifted towards shorter daylengths; analogously in W. smithii, when epistatic interactions are disrupted in the recombining generations of hybrid populations, the critical photoperiod is shifted towards shorter daylengths. The implications here are (1) that in W. smithii, it is the epistatic modification of the photoperiodic response curve by the circadian clock that is being disrupted in the recombining generations and (2) that post-glacial range expansion into the North-Temperate Zone by arthropods in general may involve uncoupling of the circadian and photoperiodic clocks. Despite the tremendous advances that have been made in understanding circadian rhythmicity at the molecular level, virtually nothing is known about how or whether any of the downstream ‘clock-controlled genes’ connect with photoperiodic time measurement. Except in W. smithii, little is known about how this connection changes with seasonal adaptation of photoperiodic response through evolutionary time. It is our goal and desire that our top–down approach to the evolution of photoperiodic time measurement will meet and mesh with the bottom–up approach that is being developed so fruitfully for circadian clocks.

Circadian component influences the photoperiodic induction of diapause in a drosophilid fly, Chymomyza costata

The last-instar larvae of a drosophilid fly, Chymomyza costata enter diapause in response to the dark-phases longer than 9 h (Yoshida, T., Kimura, M.T., 1995. The photoperiodic clock in Chymomyza costata. Journal of Insect Physiology 41, 217-222). In order to switch the developmental programming of the sensitive larvae from continuous development to diapause, after they were transferred from the short (8 h) to the long (14 h) dark-phase, significantly less time (1–2 days) was required when the dark-phase was abruptly and asymmetrically extended into the evening, than when it was extended symmetrically into both morning and evening (2–3 days), or asymmetrically into the morning hours (4–6 days). Diapause was also induced in 40–70% of sensitive larvae that were reared under the gradually shortening light-phase (from 16 h to 2 h, by 1 h in each cycle), despite that the dark-phase remained constant and short (8 h). Larvae developed continuously, however, when reared under the gradually extending light-phase (from 16 h to 24 h) and a constantly short dark-phase. We interpret such results, with the help of the two-oscillator model of circadian rhythmicity (Pittendrigh, C.S., Daan, S., 1976. A functional analysis of circadian pacemakers in nocturnal rodents. V. Pacemaker structure: A clock for all seasons. Journal of Comparative Physiology A 106, 333–355), as indicating that two mutually coupled oscillators (evening and morning) differing in their entrainability may participate in measuring of the dark-phase duration. The levels of dopamine (DA) and serotonin (5-HT) in the larval CNS transiently increased (by up to 20%) after the dusk, while no apparent change was observed during the dawn. The dusk-related increase was observed also after the asymmetric extension of the dark-phase into evening, while the asymmetric extension into morning had no effect on the levels of the DA and 5-HT.

Phasic response of the photoperiodic clock to wavelength and intensity of light in the redheaded bunting, Emberiza bruniceps

We have investigated phasic response of the photoperiodic clock to wavelength (color) and intensity of light in the male redheaded bunting ( Emberiza bruniceps). Two experiments were performed. Experiment I examined whether varying the wavelength and intensity of first (entraining, E) and/or second (inducing, I) light pulse will alter the effects of a skeleton photoperiod. Birds were subjected for a period of 6 weeks to skeleton photoschedules (6L:5D:1L:12D; 6 h E-light pulse, 1 h I-light pulse) containing either an E-pulse at 5 or 20 lx coupled with I-pulse at 100 lx, or an E-pulse at 100 lx, coupled with I-pulse at 5- or 20-lx intensity. Additional two groups that received both E- and I-pulses at 100 lx served as controls. All photoschedules were employed in two colors—white and red (654 nm). There was the wavelength- and intensity-dependent stimulation of the testis growth and development. Long wavelengths of light (red light) induced faster and greater gonadal response, but the effects were also intensity dependent. Experiment II tested whether in the photostimulated birds held on long photoperiods the change in wavelength and intensity of light hours in the morning (entraining period) or the evening (inducing period) will influence the maintenance of the photoperiodic sensitivity. Birds were subjected initially to a long photoperiod of 14L:10D (L = ∼500 lx; D = 0 lx) and then after 3 weeks, a 4-h light period in the morning (zeitgeber time, zt, 0–4) or in the evening (zt 10–14) of 14 L was substituted with white, green (528 nm), or red (654 nm) light at ∼20-lx intensity. One group maintained on 14L:10D and other exposed to 10L:14D served as controls. After another 7 weeks, all birds were subjected to 16L:8D for a further 4 weeks to test for their responsivity to long-day photostimulation as a consequence of exposure to different experimental photoperiods. Testes regrew under 16L:8D only in birds that were exposed to 10L:14D or to 14L:10D with a green light pulse. However, there was no effect of the timing (morning, evening) of the light pulse. Taken together, the results from both the experiments indicate that in the redheaded bunting (1) the photoperiodic clock responds differentially to different wavelengths (colors, spectra) and intensities of light, and (2) the effects of wavelength and intensity of light on the clock are phase dependent, and such phasic effects can be seen in skeleton photoperiods in which light is applied discretely at different circadian phases.

The effect of temperature on the photoperiodic ‘clock’ and ‘counter’ of a Scottish clone of the vetch aphid, Megoura viciae

Photoperiodic response curves were determined for a Scottish clone of the vetch aphid, Megoura viciae Buckton, at three temperatures: 12.5, 15, and 17.5°C. Critical night lengths (CNLs) for ovipara (sexual female) induction were 6 h, 7 h and 8 h, respectively. High incidences of ovipara production were observed in all night lengths longer than the CNL including continuous darkness (DD), as well as in continuous light (LL) at 12.5 and 15°C. At the same three temperatures, the number of long- or short-night cycles required for half of the experimental aphids to be ovipara producers (i.e. the required day number, RDN) was determined. The RDN for long-night cycles (LD12:12) could not be determined at 12.5°C, but was temperature compensated between 15 and 17.5°C. The RDN for short-night cycles (LD20:4) could not be determined at any temperature. However, as induction of oviparae was always 100% in 12.5°C, 94–100% in 15°C and dropped from 100% to between 47 and 71% in 17.5°C, it seems that short-night accumulation was temperature dependent. When fourth-stadium larvae were transferred from LD20:4 at 20°C to the same light–dark cycle at 15°C, the aphids, when adult, switched to the production of oviparae after about 4 weeks. First-born progeny kept in LD20:4 and 15°C switched to the production of oviparae about 7 days after the moult to adult. Thus, the photoperiodic response can be directly affected by temperature, irrespective of photoperiod. Model-generated response curves using the ‘double circadian oscillator model’ for photoperiodic time measurement (Vaz Nunes, M., 1998. A double circadian oscillator model for quantitative photoperiodic time measurement in insects and mites. Journal of Theoretical Biology 194, 299–311) closely resembled the observations. Differences between these data and the results of previous experiments with an English clone of M. viciae could be accounted for by differences in the photoperiodic clocks (damping rate and period) as well as the photoperiodic counters.

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.

Thermoperiodic responses in insects and mites simulated with the double circadian oscillator clock.

The "double circadian oscillator model" for the photoperiodic clock has been used to simulate thermoperiodic responses in insects and mites. Two assumptions have been made: (1) the clock measures cryophase in a similar way to scotophase, and (2) temperature cycles are able to entrain the clock in a similar way to LD cycles. Simulations showed that Assumption 1 causes the "critical cryophase" to be of about equal duration as the critical night length. Assumption 2 is not always needed if diapause incidences in DD are high at low temperatures but low or zero at high temperatures. The latter assumption is needed, however, if high diapause occurs in thermoperiodic cycles in DD, whereas nondiapause occurs in DD with both high and low constant temperatures. The model accounts for the observation that the amplitude of the temperature cycle is important in some insects, whereas the temperature of the cryophase is crucial in others.

Photoperiodic induction of diapause in the spider mite Tetranychus urticae: qualitative or quantitative time measurement?

Abstract. Insects and mites may measure photoperiods eitfier by classifying them as long or short relative to a critical value (qualitative time measurement) or by using the absolute value (quantitative time measurement). The spider mite Tetranychus urticae is thought to use a qualitative mechanism of time measurement. In this paper we present the results of experiments with an inbred line of the spider mite (to keep genetic variation in photoperiodic responses small), to test whether quantitative aspects also play a role. Differences in diapause incidence in different long‐night photoperiods at different temperatures may be an indication of quantitative responses to photoperiod.The effect of temperature on the photoperiodic response curve was studied at 16oC, 19oC and 22oC. The response curves appeared to be similar at 16oC and 19oC, with a critical nightlength between 10 and 11 h. At 22oC, diapause induction was less than 100% in all long‐night regimens and die critical nightlength had shifted to 12 h. Maximum diapause induction (93%) occurred in a light‐dark cycle with a 16 h dark phase (LD 8:16 h). Diapause induction was lowest in long‐night photoperiods with dark phases of 20 h and longer.The number of light‐dark cycles needed for 50% diapause induction at 19oC varied. between 12.1 and 14.7 for LD 6:18 h, between 10.9 and 12.5 for LD 8:16 h, between 10.6 and 11.6 for LD 10:14 h, and between 10.1 and 10.7 for LD 12:12 h. Independent of die light‐dark regimen, diapause induction took place in some individuals after receiving 8 cycles and virtually all individuals entered diapause after 16 cycles.No effect was found of the photoperiodic treatment during prediapause development (LD 6:18 h, LD 8:16 h, LD 10:14 h, LD 12:12 h) on diapause duration. The average diapause duration at LD 10:14 h and 19oC was 61 days over all four treatments.We explained the results by hypothesising that nightlengths are assessed qualitatively and mat the photoperiodic clock operates more accurately near the critical nightlength.