Mosquito circadian and circa-bi-dian flight rhythms: a two-oscillator model

A damping circadian clock drives weak oscillations in metabolism and locomotor activity of aphids (Acyrthosiphon pisum)

Timing seasonal events, like reproduction or diapause, is crucial for the survival of many species. Global change causes phenologies worldwide to shift, which requires a mechanistic explanation of seasonal time measurement. Day length (photoperiod) is a reliable indicator of winter arrival, but it remains unclear how exactly species measure day length. A reference for time of day could be provided by a circadian clock, by an hourglass clock, or, as some newer models suggest, by a damped circadian clock. However, damping of clock outputs has so far been rarely observed. To study putative clock outputs of Acyrthosiphon pisum aphids, we raised individual nymphs on coloured artificial diet, and measured rhythms in metabolic activity in light-dark illumination cycles of 16:08 hours (LD) and constant conditions (DD). In addition, we kept individuals in a novel monitoring setup and measured locomotor activity. We found that A. pisum is day-active in LD, potentially with a bimodal distribution. In constant darkness rhythmicity of locomotor behaviour persisted in some individuals, but patterns were mostly complex with several predominant periods. Metabolic activity, on the other hand, damped quickly. A damped circadian clock, potentially driven by multiple oscillator populations, is the most likely explanation of our results.

Plausible link between circa'bi'dian activity rhythms and circadian clock systems in the large black chafer Holotrichia parallela

Two-day rhythms, referred to as circa'bi'dian rhythms, have been reported in humans and mosquitos. However, these rhythms only appear under constant conditions, and the functional mechanisms of 2-day rhythms were unknown. Here, we report clear circabidian rhythms of large black chafers (Holotrichia parallela, Coleoptera: Scarabaeidae) in both the laboratory and field. Under 12 h:12 h light:dark (L:D) conditions at 25°C, H. parallela appeared on the ground at the beginning of the dark phase every 2 days. Under constant darkness, H. parallela exhibited free-running with a period of 47.9±0.3 h, suggesting the existence of a clear circabidian rhythm entrained to two 12 h:12 h L:D cycles. Phase responses of the circabidian rhythm to light pulses occurred under constant darkness in a phase-dependent manner. Phase responses suggest that there are two circadian cycles, each consisting of a less-responsive and more-responsive period, in a circabidian oscillation, and the circabidian rhythm is driven by the circadian clock. A mark-recapture study showed that beetles repeatedly appeared on the same tree approximately every 2 days. However, the periodicity was not as rigid as that observed under laboratory conditions in that individuals often switched appearance days. For instance, a large precipitation made the 2-day rhythm shift phase by half a cycle of the rhythm at a time. We propose a novel function of the circadian clock characterized by the release of an output signal every two cycles to produce the 2-day rhythm.

Rhythms and synchronization patterns in gene expression in the Aedes aegypti mosquito

Background

Aedes aegypti is arguably the most studied of all mosquito species in the laboratory and is the primary vector of both Dengue and Yellow Fever flaviviruses in the field. A large number of transcriptional studies have been made in the species and these usually report transcript quantities observed at a certain age or stage of development. However, circadian oscillation is an important characteristic of gene expression in many animals and plants, modulating both their physiology and behavior. Circadian gene expression in mosquito species has been previously reported but for only a few genes directly involved in the function of the molecular clock.

Results

Herein we analyze the transcription profiles of 21,494 messenger RNAs using an Ae. aegypti Agilent^® microarray. Transcripts were quantified in adult female heads at 24 hours and then again at 72 hours and eight subsequent time points spaced four hours apart. We document circadian rhythms in multiple molecular pathways essential for growth, development, immune response, detoxification/pesticide resistance. Circadian rhythms were also noted in ribosomal protein genes used for normalization in reverse transcribed PCR (RT-PCR) to determine transcript abundance. We report pervasive oscillations and intricate synchronization patterns relevant to all known biological pathways.

Conclusion

These results argue strongly that transcriptional analyses either need to be made over time periods rather than confining analyses to a single time point or development stage or exceptional care needs to be made to synchronize all mosquitoes to be analyzed and compared among treatment groups.

Possible evidence for morning and evening oscillators in Drosophila melanogaster populations selected for early and late adult emergence

In this paper, we report the results of our study aimed at a systematic analysis of the circadian phenotypes of fruit flies Drosophila melanogaster selected for early and late adult emergence, in light of the "morning and evening oscillator" (M and E) model for circadian clocks. We monitored adult emergence and activity/rest rhythms in these flies under light/dark (LD) cycles with short (8:16 h), normal (12:12 h) and long (16:8 h) photoperiods, as well as under constant darkness (DD). Across all the three LD cycles, the early populations displayed a morning phenotype with peak of emergence and activity occurring earlier than the controls and greater anticipation to "lights-on" and weak anticipation to "lights-off", while the late populations showed an evening phenotype with peak of emergence and activity occurring later than the controls and greater anticipation to lights-off and weak anticipation to lights-on. The gate of adult emergence and duration of activity in the early populations was narrower than the controls, while those of the late populations were wider than the controls. In addition, the circadian periodicities of adult emergence and activity/rest rhythms of the early flies were significantly shorter than the controls, while those of the late flies were significantly longer than the controls. In summary, the circadian phenotypes indicate that the early populations have evolved a dominant M oscillator, while the late populations have evolved a dominant E oscillator, thus providing an empirical support for the M and E model in Drosophila.

Differential control of morning and evening components in the activity rhythm of Drosophila melanogaster--sex-specific differences suggest a different quality of activity

The rhythms of locomotor activity of male and virgin or mated female flies were compared in the Drosophila melanogaster wild-type strains CantonS, Berlin, and OregonR. Under light-dark conditions, most flies showed a bimodal activity pattern with a morning peak around lights-on and an evening peak before lights-off. For all strains, a distinct sexual dimorphism was observed in the phase of the morning peak. Males had a significantly earlier morning peak than females and consequently a larger phase angle between morning and evening peak (psi(m, e)). Under constant dark conditions, the morning component merged with the evening component to a unimodal activity band in about half of the flies. In those flies who maintained bimodality, the sex-specific difference in psi(m, e) disappeared. Other sex-specific differences were now apparent: Males showed a shorter free-running period than females, and in two of the three strains, females were more active than males. Morning and evening components seem to contribute to the free-running period. Spontaneous or externally provoked change in psi(m, e) were correlated with period changes. In some flies, the morning and the evening components showed splitting, indicating that they are the output of two different oscillators. The sexual dimorphism in the phase of the morning peak under LD-conditions suggests that the function of activity during morning and evening peak might be different, for example, during the morning peak, males are active to find females. Overall, the results underline the multioscillatory nature of Drosophila's circadian system.

Effects of photoperiod and aging on locomotor activity rhythms in the onion fly, Delia antiqua

At photoperiods longer than 8h per 24h, adults of the day-active onion fly Delia antiqua showed a major peak of locomotor activity in the late photophase and also bursts of activity induced by lights-on or lights-off. At shorter photoperiods the activity peaks fused. After transfer from long photoperiods to constant darkness (DD), the rhythm free-ran, but only the major peak persisted. This suggests that only the major peak is controlled by the circadian pacemaker. At long photoperiods, the daily phase of the major peak occurred progressively later with age. As a result, the activity at short photoperiods often shifted from photophase to scotophase in old flies. The free-running period (tau) also changed with age; tau was shorter than 24h until 14-20 days after eclosion and thereafter became longer, but a few individuals repeated changes in tau. The phase delay of locomotor activity with age in D. antiqua would be attributable to the increase in tau.

Entrainability of circadian activity of the mosquito Culex pipiens pallens to 24-hr temperature cycles, with special reference to involvement of multiple oscillators

Entrainability of the circadian rhythm of mosquito flight and locomotor activity was investigated by giving a 24-hr temperature cycle (28 degrees C for 16 hr, 23 degrees C for 8 hr) to the free-running rhythm or by shifting the temperature cycle. The phase angle difference between the entrained rhythm and the temperature cycle depended on the free-running period, but not on the circadian phase at which the temperature cycle started. The rhythm transients (advancing or delaying) to entrainment depended on phase, but did not depend on either the phase or the free-running period for their average speed. We postulate that flight and locomotor activity in the mosquito may be controlled by two circadian oscillators moving in different directions in response to the temperature cycle. It is likely that at least one of the oscillators is diphasic.

Role of the optic lobes in the regulation of the locomotor activity rhythm of Drosophila melanogaster: behavioral analysis of neural mutants

The locomotor activity patterns of the Drosophila melanogaster brain mutants optomotor blind (omb), lobula plateless (lop), minibrain (mnb), small optic lobes (sol), sine oculis (so), and the double mutants mnb;so and sol;so, all of which show reductions in the optic lobes, were investigated and compared with those of the wild-type. In none of the mutants was the number of arrhythmic flies significantly higher than in the wild-type, indicating that the optic lobes are not the sole site of a pacemaker controlling the locomotor activity rhythm. However, these mutations greatly influence the stability of the circadian system, in that the number of flies simultaneously showing two or more circadian components increased as the optic lobe defects became more severe. In flies with the strongest reduction of the optic lobes, two free-running circadian components were found almost exclusively. This suggests a two-oscillator control of the locomotor activity. Eyeless mutants also expressing a neural mutation were entrained by light:dark (LD) cycles, but their activity pattern in LD was changed compared to the wild-type and the eyeless mutant so.

Colliding of activity onset and offset: evidence for multiple circadian oscillators

When hamsters, Mesocricetus auratus, are kept in dim light, wheel running at the onset and the offset of their active phase have different circadian periods. As a result, the active phase expands and eventually the two activity components collide. There is then a temporary explosion of activity at a time that was previously in the rest period. This is followed by disorganization of the rhythm or by phase jumps. Such phase jumps probably stem from the interaction of two oscillators with different natural periods rather than from threshold changes for the expression of activity.