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.     

Circadian calling behavior of the adult female brown-banded cockroach,Supella longipalpa (F.) (Dictyoptera: Blattellidae)

The diel pattern of pheromone-releasing behavior (calling) of the adult female brown-banded cockroach, Supella longipalpa (F.),was examined. Calling occurs discontinuously throughout most of the scotophase in a 12L: 12D photoperiodic regime. Females exhibited a free-running calling rhythm after transfer to continuous light or dark conditions. Shifts in the temporal pattern of calling following changes in the timing of the photoperiodic cues indicated that lights-off is the Zeitgeber. Studies of insects under long- and short-day conditions suggest that, although insects within a population may call synchronously or asynchronously, respectively, the duration of an individual's calling bout is innately limited.     

Inheritance patterns of photoperiodic diapause induction in Leptinotarsa decemlineata

Photoperiod is a reliable indicator of season and an important cue that many insects use for phenological synchronization. Undergoing range expansion insects can face a change in the local photoperiod to which they need to resynchronize. Rapid range expansion can be associated with rapid photoperiodic adaptation, which can be associated with intense selection on strongly heritable polygenic traits. Alternatively, it is proposed that, in insects with an XO sex‐determination system, genes with large effect residing on the sex chromosome could drive photoperiodic adaptation because the gene or genes are exposed to selection in the sex carrying only a single X‐chromosome. The present study seeks to understand which of these alternatives more likely explains the rapid photoperiodic adaptation in European Colorado potato beetles Leptinotarsa decemlineata Say. Diapause induction is assessed in beetles from a northern and a southern population, as well as from reciprocal hybrid crosses between the northern and southern population, when reared at an intermediate length photoperiod. The crosses within population display the expected responses, with the northern and southern populations showing high and low diapause propensity, respectively. The hybrids show intermediate responses in all studied traits. No clear difference in the responses in hybrids depending on the latitudinal origin of their father or mother is detected, even though partial paternal line dominance is seen in the responses of male beetles in one hybrid cross. These results therefore indicate that, in L. decemlineata, photoperiodic diapause induction is strongly heritable, and has an additive polygenic autosomal background.     

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.     

Investigation of the endogenous rhythm of flowering inChenopodium rubrum L.

Under the conditions applied in our laboratory 4 1/2 days old plants ofChenopodium rubrum require 2–3 photoperiodic cycles for maximal flowering response, whereas 2 1/2 days old plants are able to flower after having obtained a single inductive cycle. The period length of the free-running rhythm of flowering observed in 2 1/2 days old plants after a single transfer from light to darkness is 30h and the first peak of flowering occurs at about hour 12 in darkness. When a cycle consisting of 16h darkness and 8h light or of 8h darkness and 8h light precedes the long dark period the rhythm is rephased. Rephasing is greater when the light commenced to act on the positive slope of the first peak of the free running rhythm than when it impinged on the negative slope. With an 8h interruption of darkness by light rhythm phase is controlled by the light-on, as well as by the light-off signal. Feeding 0.4 M glucose during the long period of darkness enhanced the amplitude of the flowering response and, moreover, substituted for one photoperiodic cycle.     

Photoperiodic induction of ovarian growth and plasma estradiol secretion in a migratory finch, Emberiza melanocephala: involvement of circadian rhythm

To demonstrate the involvement of circadian rhythm in photoperiodic time measurement, photosensitive female blackheaded buntings were kept under different photoperiodic schedules consisting of 6 h of main photophase coupled with scotophases of various durations. Ovarian mass and circulating plasma estradiol concentration were found to be elevated in cycles of 6L:6D, 6L:36D, 6L:54D and in control 15L:9D groups. But cycles of 6L:18D, 6L:42D and 6L:66D did not stimulate ovarian growth or elevate circulating plasma estradiol concentration. These results are consistent with the Bünning hypothesis according to which a photoperiodic response is elicited as a result of the coincidence of light with the photoinducible phase of an endogenous circadian rhythm. The results thus indicate the involvement of a circadian rhythm of photoinducibility in ovarian growth and estradiol secretion.     

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 induction of ovarian maturation in crayfish Procambarus clarkii is mediated by extraretinal photoreception

The current study was carried out to investigate whether the photoperiodic induction of ovarian maturation in crayfish is based on a photosensitive rhythm related to extraretinal photoreceptors. To test this, two batches of 61 juvenile crayfish Procambarus clarkii consisting of [1] intact organisms and [2] animals lacking retina and lamina were exposed to 24h light-dark cycles of different photoperiodic schedules based on a night-break protocol for 3 months. Both batches of crayfish showed the greatest ovarian maturation (size, color, degree and size of oocytes) when the light pulse interrupted the scotophase at 21:00 and 05:00, showing a bimodal photoinducible rhythm. Results of the current study indicate that crayfish ovarian maturation depends on a photoinducible rhythm with two possible states that is related to the circadian clock of crayfish. This phenomenon is mediated by extraretinal photoreceptors. Results are interpreted in the light of models of external coincidence. (Chronobiology International, 18(3), 423-434, 2001)     

Entrainment Properties of the Locomotor Activity Rhythm of Drosophila melanogaster Under Different Photoperiodic Regimens

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

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.     

Invertebrate models for biomedical research, testing, and education

Invertebrate animals have been used as medicinals for 4,000 years and have served as models for research and teaching since the late 1800s. Interest in invertebrate models has increased over the past several decades as the research community has responded to public concerns about the use of vertebrate animals in research. As a result, invertebrates are being evaluated and recognized as models for many diseases and conditions. Their use has led to discoveries in almost every area of biology and medicine--from embryonic development to aging processes. Species range from terrestrial invertebrates such as nematodes and insects to freshwater and marine life including planarians, crustaceans, molluscs, and many others. The most often used models are the fruit fly Drosophila melanogaster and the minuscule nematode Caenorhabditis elegans. Topics in this article are categorized by biologic system, process, or disease with discussion of associated invertebrate models. Sections on bioactive products discovered from invertebrates follow the models section, and the article concludes with uses of invertebrates in teaching. The models reviewed can serve as references for scientists, researchers, veterinarians, institutional animal care and use committees (IACUCs), and others interested in alternatives to vertebrate animals.     

Circadian clock genes period and cycle regulate photoperiodic diapause in the bean bug Riptortus pedestris males

The photoperiodic response is crucial for many insects to adapt to seasonal changes in temperate regions. It was recently shown that the circadian clock genes period (per) and cycle (cyc) are involved in the photoperiodic regulation of reproductive diapause in the bean bug Riptortus pedestris females. Here, we investigated the involvement of per and cyc both in the circadian rhythm of cuticle deposition and in the photoperiodic diapause of R. pedestris males using RNA interference (RNAi). RNAi of per and cyc disrupted the cuticle deposition rhythm and resulted in distinct cuticle layers. RNAi of per induced development of the male reproductive organs even under diapause-inducing short-day conditions, whereas RNAi of cyc suppressed development of the reproductive organs even under diapause-averting long-day conditions. Thus, the present study suggests that the circadian clock operated by per and cyc governs photoperiodism of males as that of females.     

Genetic correlation between circadian eclosion rhythm and photoperiodic diapause in Drosophila littoralis

Populations of Drosophila littoralis are known to be latitudinally highly variable in photoperiodic adult diapause and pupal eclosion rhythm. Phenotypic correlations between the two time-measuring systems among the strains from different latitudes are, however, weak. In the present study, two differing strains were crossed reciprocally in order to search for causal (genetic) correlations between the two traits in the strains. Segregation in the F2 generations showed that variation in each trait was based on a few variable loci only. In the F2, flies having different eclosion times also differed in their diapause. This association was not complete and could have been due to genetic linkage between the traits. For that reason, the hybrid generations were raised for eight generations more to allow recombination between the traits. In F8, selection against diapause was started in the lines by raising them in a light-dark cycle of 15:9, where only females of the southern type reproduce. After eight selected generations, the lines were studied for the traits. Diapause was completely of the southern type, and the eclosion rhythm had also changed in parallel. The change in the phase of the free-running rhythm was not complete. From the present experiment, and from earlier knowledge of the geographical variation in D. littoralis, I conclude that the same pacemaker that is seen in the eclosion rhythm could also participate in daylength measurement for diapause. However, there are also noncorrelated variable parts in the measuring systems of both traits, which may mask the correlated variation.     

Sex‐linked inheritance of diapause induction in the butterfly Pieris napi

Many temperate insects survive harsh environmental conditions, such as winter, by entering a state of developmental arrest. This diapause state is predominantly induced by photoperiod. The photoperiod varies with latitude and has led to local adaptation in the photoperiodic induction of diapause in many insects. To understand the rapid evolution of the photoperiodic threshold, it is important to investigate and understand the underlying genetic mechanisms. In the present study, the genetic basis of photoperiodic diapause induction is investigated in the green‐veined white butterfly Pieris napi (Lepidoptera, Pieridae) by assaying diapause induction in a range of conditions for a Swedish and Spanish population. Furthermore, the inheritance of diapause induction is assessed in reciprocal F1 hybrids and backcrosses between the two populations. The southern population shows a clear photoperiodic threshold determining diapause or direct development, whereas the northern populations show a high incidence of diapause, regardless of photoperiod. The hybrid crosses reveal that the inheritance of diapause induction is strongly sex‐linked, and that diapause incidence in the genetic crosses is highly dependent on photoperiod. This emphasizes the importance of assaying a range of conditions in diapause inheritance studies. The results indicate a strongly heritable diapause induction with a major component on the Z‐chromosome, as well as a minor effect of the autosomal background.     

Photoperiodism and enzyme rhythms: Kinetic characteristics of the photoperiodic induction of Crassulacean acid metabolism

Summary The effect of photoperiod on Crassulacean acid metabolism (CAM) in Kalanchoe blossfeldiana Poellniz, cv. Tom Thumb, has characteristics similar to its effect on flowering in this plant (although these two phenomena are not causally related). The photoperiodic control of CAM is based on (a) dependance on phytochrome, (b) an endogenous circadian rhythm of sensitivity to photoperiodic signals, (c) a balance between specific positive (increase in enzyme capacity) and negative (inhibitory substances) effects of the photoperiod. Variations in malate content, capacity of phosphoenolpyruvate (PEP) carboxylase, and capacity of CAM inhibitors in young leaves were measured under photoperiodic conditions noninductive for CAM and after transfer into photoperiodic conditions inductive for CAM. Essential characteristics of the photoperiodic induction of CAM are: 1) lag time for malate accumulation; 2) after-effect of the inductive photoperiod on the malate accumulation, on the increase in PEP carboxylase capacity, and on the decrease in the level of long-day produced inhibitors; final levels of malate, enzyme capacity and inhibitor are proportional to the number of inductive day-night cycles; 3) cireadian rhythm in PEP carboxylase capacity with a fixed phase under noninductive photoperiods and a continuously shifting phase under inductive photoperiods, after complex advancing and delaying transients. Kinetic similarities indicate that photoperiodic control of different physiological functions, namely, CAM and flowering, may be achieved through similar mechanisms. Preliminary results with species of Bryophyllum and Sedum support this hypothesis. Phase relationships suggest different degrees of coupling between endogenous enzymic rhythm and photoperiod, depending on whether the plants are under long days or short days.     

Variation in the timing of the reproductive season among breeds of sheep in relation to differences in photoperiodic synchronization of an endogenous rhythm.

Photoperiod may regulate seasonal reproduction either by providing the primary driving force for the reproductive transitions or by synchronizing an endogenous reproductive rhythm. This study evaluated whether breed differences in timing of the reproductive seasons of Finnish Landrace (Finn) and Galway ewes are due to differences in photoperiodic drive of the reproductive transitions or to differences in photoperiodic synchronization of the endogenous rhythm of reproductive activity. The importance of decreasing photoperiod after the summer solstice in determining the onset and duration of the breeding season was tested by housing ewes from the summer solstice in either a simulated natural photoperiod or a fixed summer-solstice photoperiod (18 h light:6 h dark; summer-solstice hold). Onset of the breeding season within each breed did not differ between these photoperiodic treatments, but Galway ewes began and ended their breeding season earlier than Finn ewes. The duration of the breeding season was shorter in Galway ewes on summer-solstice hold than on simulated natural photoperiod; duration did not differ between photoperiodic treatments in Finn ewes. The requirement for increasing photoperiod after the winter solstice for initiation of anoestrus was tested by exposing ewes from the winter solstice to either a simulated natural photoperiod or a winter-solstice hold photoperiod (8.5 h light:15.5 h dark). Onset of anoestrus within each breed did not differ between these photoperiodic treatments, but the time of this transition differed between breeds. These observations suggest that genetic differences in timing of the breeding season in Galway and Finn ewes do not reflect differences in the extent to which photoperiod drives the reproductive transitions, because neither breed requires shortening days to enter the breeding season or lengthening days to end it at appropriate times. These findings are consistent with the hypothesis that photoperiod synchronizes an endogenous rhythm of reproductive activity in both breeds and that genetic differences in timing of the breeding season reflect differences in photoperiodic synchronization of this rhythm.     

Insect photoperiodic calendar and circadian clock: independence, cooperation, or unity?

The photoperiodic calendar is a seasonal time measurement system which allows insects to cope with annual cycles of environmental conditions. Seasonal timing of entry into diapause is the most often studied photoperiodic response of insects. Research on insect photoperiodism has an approximately 80-year-old tradition. Despite that long history, the physiological mechanisms underlying functionality of the photoperiodic calendar remain poorly understood. Thus far, a consensus has not been reached on the role of another time measurement system, the biological circadian clock, in the photoperiodic calendar. Are the two systems physically separated and functionally independent, or do they cooperate, or is it a single system with dual output? The relationship between calendar and clock functions are the focus of this review, with particular emphasis on the potential roles of circadian clock genes, and the circadian clock system as a whole, in the transduction pathway for photoperiodic token stimulus to the overt expression of facultative diapause.     

Growth effects of 2-thiouracil and possibility of selective inhibition of floral differentiation inChenopodium rubrum L.

The effect of 2-thiouracil on vegetative growth and floral differentiation was investigated inChenopodium rubrum plants grown in water cultures. Between the low concentrations of the agent, stimulating vegetative growth and floral differentiation, and those inhibiting both these processes, a narrow concentration range was found (1.10^?5 m to 2.10^?5 m), where growth was inhibited selectively. At a concentration of 1.10^?4 m a selective inhibition of development was found when 2-thiouracil was applied at the beginning of photoperiodic induction. Inhibition of development was strong regardless of whether 2-thiouracil was applied before, during or closely after 4 days of photoperiodic induction; the degree of growth inhibition, however, changed in dependence on photoperiodic induction. The strongest relative inhibition of development, calculated as a ratio between development and growth, was observed always at the beginning of photoperiodic induction. Investigation of plant growth as well as the anatomical and autoradiographic study after the application of 2-thiouracil indicate that the inhibition becomes evident at the end of 4 days of application by an overall growth inhibition and a decrease of mitotic activity. Reversal by uracil was possible after simultaneous application of 2-thiouracil. The nature of the selective inhibition is discussed and two possible interpretations of the data obtained are analyzed: a) different response of growth processes in apices and young vegetative organs respectively with regard to different participation of cell division and elongation, b) specific inhibition of floral differentiation.     

Reduced body mass, food intake, and testis size in response to short photoperiod in adult F344 rats

Background  

Although laboratory rats are often considered classic nonseasonal breeders, peripubertal rats of two inbred strains, F344 and BN, have both reproductive and nonreproductive responses to short photoperiods. Unmanipulated adult rats have not been reported to have robust responses to short photoperiod alone, although several treatments can induce photoperiodic responses in adults. In this study, we tested the hypotheses that unmanipulated F344 rats retain responses to short photoperiod as adults and that they have the necessary elements for an endogenous circannual rhythm of sensitivity to short photoperiod.