Abstract
BackgroundMost organisms have evolved a circadian clock in order to anticipate daily environmental changes and many of these organisms are also capable of sophisticated measurement of daylength (photoperiodism) that is used to regulate seasonal events such as diapause, migration and polymorphism. It has been generally accepted that the same elements are involved in both circadian (daily) and seasonal (annual) rhythms because both rely upon daily light-dark cycles. However, as reasonable as this sounds, there remains no conclusive evidence of such a molecular machinery in insects. We have approached this issue by using RNA interference (RNAi) in Riptortus pedestris.ResultsThe cuticle deposition rhythm exhibited the major properties of circadian rhythms, indicating that the rhythm is regulated by a circadian clock. RNAi directed against the circadian clock genes of period and cycle, which are negative and positive regulators in the circadian clock, respectively, disrupted the cuticle deposition rhythm and distinct cuticle layers were produced by these RNAi. Simultaneously, period RNAi caused the insect to avert diapause under a diapause-inducing photoperiod whereas cycle RNAi induced diapause under a diapause-averting photoperiod. The expression patterns of juvenile hormone-regulated genes and the application of juvenile hormone analogue suggested that neither ovarian development itself nor a downstream cascade of juvenile hormone secretion, were disturbed by period and cycle RNAi.ConclusionsThis study revealed that the circadian clock genes are crucial not only for daily rhythms but also for photoperiodic diapause. RNAi directed against period and cycle had opposite effects not only in the circadian cuticle deposition rhythm but also in the photoperiodic diapause. These RNAi also had opposite effects on juvenile hormone-regulated gene expression. It is still possible that the circadian clock genes pleiotropically affect ovarian development but, based on these results, we suggest that the circadian clock operated by the circadian clock genes, period and cycle, governs seasonal timing as well as the daily rhythms.See Commentary: http://www.biomedcentral.com/1741-7007/8/115
Highlights
Most organisms have evolved a circadian clock in order to anticipate daily environmental changes and many of these organisms are capable of sophisticated measurement of daylength that is used to regulate seasonal events such as diapause, migration and polymorphism
Cuticle deposition rhythm Alternating bright and dark layers were observed in the endocuticle of the hind leg tibia of R. pedestris (Figure 1a)
We confirmed that in R. pedestris the cuticle deposition rhythm free-ran under constant conditions, that the number of deposited cuticle layers varied with the given number of temperature cycles and that the Q10 value of the rhythm was close to 1.0 between 22.5°C and 27.5°C
Summary
Most organisms have evolved a circadian clock in order to anticipate daily environmental changes and many of these organisms are capable of sophisticated measurement of daylength (photoperiodism) that is used to regulate seasonal events such as diapause, migration and polymorphism. In the central clock residing in the brain of Drosophila melanogaster, the CYCLE (CYC)/CLOCK (CLK) heterodimer acts as a positive regulator of the transcription of period (per), timeless (tim) and other output genes, whereas the PER/TIM heterodimer acts as a negative regulator of CYC/CLK activity. Entraining this oscillator to light depends on CRYPTOCHROME, which is a blue-light photopigment that promotes TIM degradation when stimulated by light. Some diversity in the molecular clockwork has been presented between the central and the peripheral clocks [13] and among insects [14], the functional roles of PER and CYC are considered to be identical - PER acts as a negative regulator and CYC acts as a positive regulator [9]
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