Abstract
Wild females of Pyrrhocoris apterus exhibit seasonal changes in neuroendocrine activity and, consequently, reproduction. Long days (18 h light/6 h dark) (LD) stimulate reproduction, whereas short days (12 h light/12 h dark) (SD) induce reproductive arrest (diapause). This study reveals how photoperiod influences the expression of the circadian clock gene, period (per) in the insect's head. There is only a weak diurnal rhythm in per mRNA expression under LD and SD. However, levels of per mRNA are consistently higher (up to 10-fold) under SD than under LD. The influence of photoperiod on per gene expression is linked to a developmental output (diapause vs. reproduction); mutant females, reproducing under both LD and SD, show low per mRNA levels under both photoperiodic conditions. Thus, the magnitude of per gene expression may be important to the translation of photoperiodic signals into a hormonal message. Levels of per mRNA are related to properties of locomotor activity rhythms. Low per mRNA levels (displayed by wild females in LD and mutant females in both LD and SD) are associated with long free-running periods (τ~26-27 h) and late peaks of activity (ψR,L~10-12 h), whereas high per mRNA levels coincide with short free-running periods (τ~24 h) and early peaks of activity (ψR,L~4-6 h). Overall, the data provide a background for a molecular approach to the long-standing question about the role of the circadian system in insect photoperiodism.
Highlights
It is generally accepted that biological timing depends on the generation of circadian rhythms, and on the measurement of time intervals
The conventional view that the circadian system is involved in the transduction of photoperiodic signals into a developmental output implies that molecular components of the circadian system somehow respond to alterations in photoperiod
Because per mRNA levels are linked to both photoperiod and developmental output, the magnitude of per gene expression may be important to the transduction of photoperiodic signals into a hormonal message
Summary
It is generally accepted that biological timing depends on the generation of circadian rhythms, and on the measurement of time intervals (day length/length of night). While the expression of circadian output deter mines the circadian activities of the organism, the interval timing results in seasonal changes in the developmental pathways. The relationship between these two phenomena is still largely unresolved. While our understanding of molecular mechanisms of circadian rhythms has greatly advanced, due to the enormous power of Drosophila melanogaster genetics (Schotland & Sehgal, 2001), molecular events, from photoreception through time measurement and accumulation of photoperiodic “infor mation” to neuroendocrine activity, controlling the onset of diapause or continuation of development/reproduction are still unknown
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