Circadian rhythmicity participates in the photoperiodic determination of diapause in spider mites

Circadian and Neuroendocrine Basis of Photoperiodism Controlling Diapause in Insects and Mites: A Review

The photoperiodic system is concealed in the highly complex black-box, comprising four functional subunits: 1) a photo/thermo-sensitive input unit, 2) a photoperiodic clock based on a circadian system, 3) a condenser unit counting the number of inductive signals, and 4) a neuroendocrine switch that triggers a phenotypic shift. This review aims to summarize the research history and current reach of our understanding on this subject to connect it with the molecular mechanism of the circadian clock rapidly being unveiled. The review also focuses on the mode of intersubunit information transduction. It will scan the recent advancement in research on each functional subunit, but special attention will be given to the circadian clock-endocrine conjunct and the role of melatonin signaling in the regulation of insect photoperiodism. Prothoracicotropic hormone (PTTH) probably plays the most crucial role in the regulation of pupal diapause, which is the simplest model system of diapause regulation by hormones investigated so far, particularly in the Chinese oak silkmoth (Antheraea pernyi). A search for the trigger to release the PTTH found some candidates, that is, indoleamines. Indolamine metabolism is controlled by arylalkylamine N-acetyltransferase (aaNAT). Indolamine dynamics and aaNAT enzymatic activity changed according to photoperiods. aaNAT activity and melatonin content in the brain showed not only a photoperiodic response but also a circadian fluctuation. aaNAT had multiple E-boxes, suggesting that it is a clock-controlled gene (ccg), which implies that cycle (cyc, or brain-muscle Arnt-like 1 = Bmal1)/Clock (Clk) heterodimer binds to E-box and stimulates the transcription of aaNAT, which causes the synthesis of melatonin. RNAi against transcription modulators, cyc, or Clk downregulated aaNAT transcription, while RNAi against repressor of cyc/Clk, per upregulated aaNAT transcription. Immunohistochemical localization showed that the circadian neurons carry epitopes of melatonin-producing elements such as aaNAT, the precursor serotonin, HIOMT, and melatonin as well as clock gene products such as cyc-ir, Per-ir, and dbt-ir, while PTTH-producing neurons juxtaposed against the clock neurons showed hMT2-ir in A. pernyi brain. Melatonin probably binds to the putative melatonin receptor (MT) that stimulates Ca2+ influx, which in turn activates PKC. This induces Rab 8 phosphorylation and exocytosis of PTTH, leading to termination of diapause. All the PTTH-expressing neurons have PKC-ir, and Rab8-ir. When diapause is induced and maintained under short days, serotonin binding to 5HTR1B suppresses PTTH release in a yet unknown way. RNAi against this receptor knocked out photoperiodism; short day response is blocked and diapause was terminated even under the short day condition. The result showed that a relatively simple system controls both induction and termination in pupal diapause of A. pernyi: the circadian system regulates the transcription of aaNAT as a binary switch, the enzyme produces a melatonin rhythm that gates PTTH release, and 5HTR1B and MT are probably also under photoperiodic regulation. Finally, we listed the remaining riddles which need to be resolved, to fully understand this highly complex system in future studies.

Physiological and molecular mechanisms underlying photoperiodism in the spider mite: comparisons with insects

Photoperiodism is an adaptive, seasonal timing system that enables organisms to coordinate their development and physiology to annual changes in the environment using day length (photoperiod) as a cue. This review summarizes our knowledge of the physiological mechanisms underlying photoperiodism in spider mites. In particular, the two-spotted spider mite Tetranychus urticae is focussed, which has long been used as a model species for studying photoperiodism. Photoperiodism is established by several physiological modules, such as the photoreceptor, photoperiodic time measurement system, counter system, and endocrine effector. It is now clear that retinal photoreception through the ocelli is indispensable for the function of photoperiodism, at least in T. urticae. Visual pigment, which comprised opsin protein and a vitamin A-based pigment, is involved in photoreception. The physiological basis of the photoperiodic time measurement system is still under debate, and we have controversial evidence for the hourglass-based time measurement and the oscillator-based time measurement. Less attention has been centred on the counter system in insects and mites. Mite reproduction is possibly regulated by the ecdysteroid, ponasterone A. Prior physiological knowledge has laid the foundation for the next steps essential for the elucidation of the molecular mechanisms driving photoperiodism.

On the adaptive significance of circadian clocks for their owners

Circadian rhythms are believed to be an evolutionary adaptation to daily environmental cycles resulting from Earth's rotation about its axis. A trait evolved through a process of natural selection is considered as adaptation; therefore, rigorous demonstration of adaptation requires evidence suggesting evolution of a trait by natural selection. Like any other adaptive trait, circadian rhythms are believed to be advantageous to living beings through some perceived function. Circadian rhythms are thought to confer advantage to their owners through scheduling of biological functions at appropriate time of daily environmental cycle (extrinsic advantage), coordination of internal physiology (intrinsic advantage), and through their role in responses to seasonal changes. So far, the adaptive value of circadian rhythms has been tested in several studies and evidence indeed suggests that they confer advantage to their owners. In this review, we have discussed the background for development of the framework currently used to test the hypothesis of adaptive significance of circadian rhythms. Critical examination of evidence reveals that there are several lacunae in our understanding of circadian rhythms as adaptation. Although it is well known that demonstrating a given trait as adaptation (or setting the necessary criteria) is not a trivial task, here we recommend some of the basic criteria and suggest the nature of evidence required to comprehensively understand circadian rhythms as adaptation. Thus, we hope to create some awareness that may benefit future studies in this direction.

Characterization of arylalkylamine N-acetyltransferase from silkmoth (Antheraea pernyi) and pesticidal drug design based on the baculovirus-expressed enzyme

Arylalkylamine N-acetyltransferase (AANAT; EC 2.3.1.87) catalyzes the N-acetylation of arylalkylamines. A cDNA encoding AANAT (ApAANAT) was cloned from Antheraea pernyi by PCR. The cDNA of 1966 bp encodes a 261 amino acid protein. The amino acid sequence was found to have a high homology with Bombyx mori AANAT (BmNAT) but had very low homology with vertebrate AANATs. Amino acid sequence analysis revealed that four insect AANATs cloned from three species including ApAANAT formed a distinct cluster from the vertebrate group. A recombinant ApAANAT protein was expressed in Sf9 cells using a baculovirus expression system, having AANAT activity. The transformed cell extract acetylated tryptamine, serotonin, dopamine, tyramine, octopamine and norepinephrine. The AANAT activity was inhibited at over 0.03 mM tryptamine. Although insect AANATs have been considered as a target of insecticide, this type of insecticide has never been developed. Screening a chemical library of Otsuka Chemical Co., Ltd., we found a novel compound and its derivatives that inhibited the AANAT activity of ApAANAT. This may facilitate investigation of the monoamine metabolic pathway in insects and the development of new types of insecticides and inhibitors of AANATs.

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.

Controversial aspects of photoperiodism in insects and mites

This review examines several controversial aspects of photoperiodism in insects and mites including the role of the circadian system in night length measurement, the nature of apparent hourglass-like responses, and whether or not the circadian component in photoperiodism is the same as that in overt behavioural rhythms. These aspects of the phenomenon are discussed in terms of the entrainment of circadian oscillations by cycles of light and temperature. There is considerable variety of photoperiodic response within the insects (and other arthropods) to show, inter alia, circannual rhythms, internal and external coincidence night length timers, and in some species, non-circadian hourglass-like devices. Many apparent hourglass-like responses, however, could be circadian 'clocks' of the external coincidence type involving oscillations that dampen below threshold in extended periods of darkness. The review also concludes that there is little evidence in favour of the "Hourglass clock-oscillator counter" model proposed for the mite Tetranychus urticae by Vaz Nunes and Veerman (1982a). The responses of this species to complex light and temperature cycles may also be interpreted in terms of a damped oscillator version of external coincidence.

Action spectrum for the suppression of arylalkylamine N-acetyltransferase activity in the two-spotted spider mite Tetranychus urticae

An action spectrum was obtained for the suppression of arylalkylamine N-acetyltransferase (NAT) activity in the two-spotted spider mite Tetranychus urticae by irradiating the mite with monochromatic lights of various wavelengths using the Okazaki Large Spectrograph at the National Institute for Basic Biology, Okazaki, Japan. Fluence-response curves were obtained for wavelengths between 300 and 650 nm by irradiating the mite for 4 h day(-1). The samples were frozen after the third exposure. A negative correlation between the logarithmic fluence rate and NAT activity was detected in the range of 0.01-1 micromol m(-2) s(-1) for wavelengths between 300 and 500 nm and in the range of 0.1-10 micromol m(-2) s(-1) for wavelengths between 550 and 650 nm. The constructed action spectrum indicated that the photoreceptors mediating the circadian and/or photoperiodic systems might be UV-A- and blue-type photoreceptors with absorption peaks at 350 and 450 nm.

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

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

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.

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.

The circadian basis of ovarian diapause regulation in Drosophila melanogaster: is the period gene causally involved in photoperiodic time measurement?

Females of a wild-type strain of Drosophila melanogaster (Canton-S), and of several clock mutants (period), were able to discriminate between diapause-inducing short days and diapause-averting long days with a well-defined critical daylength. The critical daylengths of a short-period mutant (pers) and a long-period mutant (perL2) were almost identical, both to each other and to that of Canton-S. The critical daylength of an arrhythmic mutant (perol), however, was about 3 hr shorter than that of Canton-S, and that of per- was about 5 hr shorter. Exposure of Canton-S females to Nanda-Hamner experiments, consisting of a 10-hr photophase coupled to a dark phase varying between 4 and 74 hr, showed (1) that the photoperiodic clock in D. melanogaster measures nightlength rather than daylength, and (2) that photoperiodic time measurement is somehow based on (or affected by) constituent oscillators in the circadian system. Nanda-Hamner results for the period mutants all showed similar profiles regardless of genotype, or the presence or absence of per locus DNA. These results suggest that photoperiodic induction and locomotor activity do not share a common pacemaker in D. melanogaster, and that the per gene is not causally involved in nightlength measurement by the photoperiodic clock, although flies in which the per locus is missing (per-) or defective (perol) show an altered critical value.

Geographical variation in photoperiodic induction of diapause in the spider mite (Tetranychus urticae): a causal relation between critical nightlength and circadian period?

The photoperiodic response of 10 strains of the two-spotted spider mite (Tetranychus urticae), originating between 40.5 degrees and 60 degrees N in Western and Central Europe, was found to be highly variable. The critical nightlength for photoperiodic induction of diapause was strongly correlated with latitude for the lowland populations and varied from 7.75 hr in the north to 13.25 hr in the south. The length of the circadian period, taken as the peak-to-peak interval in response curves of resonance experiments done with T. urticae, varied between 17.75 and 21.5 hr and appeared weakly correlated with latitude. Only a very weak correlation was observed between critical nightlength and circadian period. These results do not provide evidence in favor of a circadian-based photoperiodic clock in T. urticae. On the other hand, they also do not refute this possibility, as there may be other circadian or noncircadian factors affecting the critical nightlength, which could mask the influence of circadian period.

Diet-dependent switch from circadian to hourglass-like operation of an insect photoperiodic clock

The results of resonance experiments gave evidence that circadian organization was involved in photoperiodic diapause induction of Pieris brassicae. However, giving the larvae an artificial diet instead of the host plant deeply altered the resonance effect and led to a response that could be accounted for by the operation of a noncircadian timer (hourglass). This important change, however, had no effect on the insect's ability to discriminate between short- and long-day 24-hr cycles. Thus, it is possible via the metabolism to switch the photoperiodic mechanism to a circadian or an hourglass-like mode of operation without impairing the photoperiodic time measurement.