Flies by NightEffects of Changing Day Length on Drosophila's Circadian Clock

Abstract

In Drosophila, two intersecting molecular loops constitute an autoregulatory mechanism that oscillates with a period close to 24 hr [1Dunlap J.C. Loros J.J. DeCousey P.J. Chronobiology. Sinauer Associates, Sunderland, MA2003Google Scholar, 2Dunlap J.C. Molecular bases for circadian clocks.Cell. 1999; 96: 271-290Abstract Full Text Full Text PDF PubMed Scopus (2363) Google Scholar, 3Hall J.C. Genetics and molecular biology of rhythms in Drosophila and other insects.Adv. Genet. 2003; 48: 1-280Crossref PubMed Google Scholar]. These loops touch when proteins from one loop, PERIOD (PER) and TIMELESS (TIM), repress the transcription of their parent genes, period (per) and timeless (tim), by blocking positive transcription factors from the other loop. The arrival of PER and TIM into the nucleus of a clock cell marks the timing of this interaction between the two loops [4Glossop N.R.J. Lyons L.C. Hardin P.E. Interlocked feedback loops within the Drosophila circadian oscillator.Science. 1999; 286: 766-768Crossref PubMed Scopus (315) Google Scholar]; thus, control of PER:TIM nuclear accumulation is a central component of the molecular model of clock function [1Dunlap J.C. Loros J.J. DeCousey P.J. Chronobiology. Sinauer Associates, Sunderland, MA2003Google Scholar, 2Dunlap J.C. Molecular bases for circadian clocks.Cell. 1999; 96: 271-290Abstract Full Text Full Text PDF PubMed Scopus (2363) Google Scholar, 3Hall J.C. Genetics and molecular biology of rhythms in Drosophila and other insects.Adv. Genet. 2003; 48: 1-280Crossref PubMed Google Scholar]. If a light pulse occurs early in the night as the heterodimer accumulates in the nucleus of clock cells, TIM is degraded, PER is destabilized, and clock time is delayed [1Dunlap J.C. Loros J.J. DeCousey P.J. Chronobiology. Sinauer Associates, Sunderland, MA2003Google Scholar, 2Dunlap J.C. Molecular bases for circadian clocks.Cell. 1999; 96: 271-290Abstract Full Text Full Text PDF PubMed Scopus (2363) Google Scholar, 3Hall J.C. Genetics and molecular biology of rhythms in Drosophila and other insects.Adv. Genet. 2003; 48: 1-280Crossref PubMed Google Scholar]. Alternatively, if TIM is degraded during the later part of the night, after peak accumulation, clock time advances. Current models state that the effect of a light pulse depends on the state of the PER:TIM oscillation, which turns on the changing levels of TIM. However, previous studies have shown that light:dark (LD) regimes mimicking seasonal changes cause behavioral adjustments while altering clock gene expression [5Majercak J. Sidote D. Hardin P.E. Edery I. How a circadian clock adapts to seasonal decreases in temperature and day length.Neuron. 1999; 24: 219-230Abstract Full Text Full Text PDF PubMed Scopus (304) Google Scholar, 6Qiu J. Hardin P.E. per mRNA cycling is locked to lights-off under photoperiodic conditions that support circadian feedback loop function.Mol. Cell. Biol. 1996; 16: 4182-4188PubMed Google Scholar]. This should be reflected in the adjustment of PER and TIM dynamics. We manipulated LD cycles to assess the effects of altered day length on PER and TIM dynamics in clock cells within the central brain as well as light-induced resetting of locomotor rhythms.