Common features in diverse insect clocks

Abstract

This review describes common features among diverse biological clocks in insects, including circadian, circatidal, circalunar/circasemilunar, and circannual clocks. These clocks control various behaviors, physiological functions, and developmental events, enabling adaptation to periodic environmental changes. Circadian clocks also function in time-compensation for celestial navigation and in the measurement of day or night length for photoperiodism. Phase response curves for such clocks reported thus far exhibit close similarities; specifically, the circannual clock in Anthrenus verbasci shows striking similarity to circadian clocks in its phase response. It is suggested that diverse biological clocks share physiological properties in their phase responses irrespective of period length. Molecular and physiological mechanisms are best understood for the optic-lobe and mid-brain circadian clocks, although there is no direct evidence that these clocks are involved in rhythmic phenomena other than circadian rhythms in daily events. Circadian clocks have also been localized in peripheral tissues, and research on their role in various rhythmic phenomena has been started. Although clock genes have been identified as controllers of circadian rhythms in daily events, some of these genes have also been shown to be involved in photoperiodism and possibly in time-compensated celestial navigation. In contrast, there is no experimental evidence indicating that any known clock gene is involved in biological clocks other than circadian clocks.

Keywords: Anatomical location; Celestial navigation; Circadian; Circalunar; Circannual; Circasemilunar; Circatidal; Clock gene; Phase response curve; Photoperiodism.

Figure 1

Comparison of phase response curves (PRCs) in diverse insect clocks. (A) Type 0 PRC in circadian clocks in response to light pulses. (B) Type 1 PRC in circadian clocks in response to light pulses. (C) PRC in the circadian clock for photoperiodism in Sarcophaga argyrostoma in response to 15-min light pulses (redrawn from [43]). (D) PRC in the circatidal clock of Apteronemobius asahinai in response to periodic inundations (0.5-h inundation pulses provided four times at intervals of 12.4 h) (reproduced from [45] with kind permission from Elsevier). (E) PRC in the circannual clock of Anthrenus verbasci in response to 4-week long-day pulses (reproduced from [46] with kind permission from Springer Science+Business Media). (F) PRC in the circannual clock of A. verbasci in response to 2-week long-day pulses (reproduced from [47] with kind permission from Springer Science+Business Media). The periods of clocks are shown in terms of angle degrees (0–360°). In circannual PRCs (E, F), open and closed circles represent the phase shifts in the first and second pupation group after pulse perturbation, respectively, and broken lines in (E) show the split into advanced and delayed groups.

Figure 2

Induction of arrhythmicity by a specific stimulus in circadian and circannual clocks. (A) Circadian eclosion rhythm in Drosophila pseudoobscura. Arrhythmicity was induced with a blue-light pulse at an intensity of 0.1 W/m² near conditions of a pulse length of 50 s and of a time of pulse onset of 6.8 h after transfer to constant darkness (lower left), whereas without pulses, circadian rhythmicity was shown in constant darkness (upper left) [42]. The right panel shows the arrhythmicity induced by a blue-light pulse (0–120 s). R values were calculated from hourly emergence counts in three or four circadian cycles and are plotted as a function of the time of pulse onset. The larger circles represent the experiments using stimuli particularly close to conditions of a pulse length of 50 s and of a time of pulse onset of 6.8 h after transfer to constant darkness (reproduced from [42] with kind permission from Elsevier). (B) Circannual pupation rhythm in Anthrenus verbasci. Arrhythmicity was induced with a 4-week long-day pulse of LD 16:8 applied nine weeks after exposing larvae to LD 12:12 (lower left), whereas without pulses, circannual rhythmicity was shown with a period of approximately 40 weeks under constant short-day conditions of LD 12:12 (upper left) (reproduced from [47] with kind permission from Springer Science+Business Media). The right panel shows the arrhythmicity induced by a 4-week long-day pulse. R values were calculated from weekly pupal counts in one or two circannual cycles (closed circles [46]), or three circannual cycles (open circles [47]) and are plotted as a function of the time of pulse onset. Statistically complete arrhythmicity is shown by R values approaching approximately 150 [42]. In practice, R values of 30 or less are considered highly rhythmic and those greater than 90 arrhythmic [2,42,47].

Figure 3

Neurons or regions important for the circadian rhythm, solar compass, photoperiodism, and circatidal rhythm. In cockroaches, the accessory medulla containing PDF-expressing neurons is the location of the circadian clock regulating activity rhythms [71,73]. In crickets, the circadian clock is located in the lamina and medulla regions [74]. *It is likely that the circatidal clock is not located in the optic lobe, but is probably located in the central brain [28], although the possibility that the clock is located in an extra-brain region cannot be excluded. In Rhodnius prolixus (Heteroptera), lateral neurons, which co-express clock proteins and PDF, are considered to be the circadian clock that regulates the activity and hormone-release rhythms [75,76]. The region containing these neurons is involved in the photoperiodic diapause in another heteropteran, Riptortus pedestris [77]. The PI and PL regions are also important for photoperiodic diapause in heteropterans [78,79]. In flies, among neurons expressing clock proteins in the brain, several groups of lateral neurons control the circadian activity rhythm. One group of neurons that co-express clock proteins and PDF is important for photoperiodism [80]. Neurosecretory cells in the PI and PL regulate photoperiodic diapause [81]. In butterflies, the PL is the main location of the circadian clock in the brain [82,83]. Neurons necessary for the solar compass are located in the central complex, and the circadian clock in the antennae is necessary for time compensation of the solar compass system [84,85]. Note that the neuronal location may differ among species. La, lamina; Me, medulla; AMe, accessory medulla; Lo, lobula; PI, pars intercerebralis; PL, pars lateralis; CC, central complex; An, antenna.

Figure 4

Molecular mechanisms of the circadian clock and effects of clock gene RNAi on the circadian clock in Riptortus pedestris . (A) A general model of the insect circadian clock mechanism. Positive elements of CYC and CLK form a heterodimer and activate transcription of clock genes per, tim, and cry-m and many other genes called clock-controlled genes (ccg). Their protein products are synthesized in the cytoplasm and PER, TIM and CRY-m form a complex. In Drosophila melanogaster, only PER and TIM form a heterodimer. The complex enters the nucleus and acts as a negative element that represses CYC/CLK transcriptional activity. Reduction of ccg transcript levels and consequent reduction of CCG protein levels lead to a decrease of repressive regulation of CYC/CLK by PER/TIM/CRY-m, and therefore CYC/CLK-mediated transcription increases again. These phases in which ccg transcription is activated or repressed are repeated in approximately 24 h. CRY-d causes degradation of TIM in a light-dependent manner. (B) When negative elements PER and CRY-m are eliminated by RNAi in R. pedestris, the circadian clock remains at the phase in which ccg transcription is activated [129]. (C) When positive elements CYC and CLK are eliminated, the circadian clock remains at the phase in which ccg transcription does not occur [130]. Note that the absence of transcriptional activity would also decrease the protein levels of negative elements.

Figure 5

A hypothetical explanation of clock gene RNAi in Riptortus pedestris . The same molecular machinery is required for the circadian rhythm of cuticle deposition and the photoperiodic response [129,130,147-149]. In the intact group, the circadian clock generates rhythmic transcription of ccg, regulating the circadian rhythm with alternating deposition of polarized cuticle layers and nonpolarized cuticle layers, which are observed as bright (indicated by small arrows) and dark layers, respectively, under a light microscope with crossed polarizers. The intact group shows a clear photoperiodic response: Female adults develop their ovaries under long-day conditions but enter diapause under short-day conditions. In the per or cry-m RNAi group, constant high levels of ccg transcripts abolish the circadian rhythm of cuticle deposition, and only nonpolarized layers are deposited. Photoperiodism is also disrupted and ovarian development is induced irrespective of the photoperiod. In the cyc or Clk RNAi group, constant low levels of ccg transcripts abolish the circadian rhythm and only polarized layers are deposited. In this case as well, photoperiodism is disrupted and diapause is induced irrespective of the photoperiod.