Flies by NightEffects of Changing Day Length on Drosophila's Circadian Clock (original) (raw)

morning and evening peaks, the locomotor activity has a strong tendancy to be bimodal [3, 8]. In the current Seattle, Washington 98195 2 Department of Biology and "seasonal-change" experiments, however, secondary peaks were also evident (Table 1). Under long night National Science Foundation Center for Biological Timing lengths (Ն12 hr), morning locomotor activity began late in the night, and peaks typically coincided with lights-Brandeis University Waltham, Massachusetts 02454 on. However, the presence of a peak about 2 hr before lights-on is evident in addition to the stronger peak associated with lights-on that predominates in the mean (Table 1). We were especially interested in the evening peak Summary because this locomotor maximum persists when flies are transferred from an LD cycle into DD, whereas the In Drosophila, two intersecting molecular loops constimorning peak typically disappears (reviewed in [3]). The tute an autoregulatory mechanism that oscillates with evening peak coincided with lights-off when day length a period close to 24 hr [1, 2, 3]. These loops touch when was less than 12 hr, and it anticipated lights-off as dayproteins from one loop, PERIOD (PER) and TIMELESS length increased. Thus, in LD 14:10, the peak occurred (TIM), repress the transcription of their parent genes, about 1 hr earlier than lights-off; in LD 16:8, it occurred period (per) and timeless (tim), by blocking positive 1.0-2.0 hr earlier, and in LD 18:6 it occurred about 3.5 transcription factors from the other loop. The arrival hr earlier (Figures 1E, 1F, and 1G). Under the shortest of PER and TIM into the nucleus of a clock cell marks nights, the evening peak appeared to stabilize at about the timing of this interaction between the two loops 14.0-14.5 hr after lights-on. [4]; thus, control of PER:TIM nuclear accumulation is a central component of the molecular model of clock PER and TIM Levels Adjust to Photoperiod function [1, 2, 3]. If a light pulse occurs early in the We analyzed the effects of day-length on temporal pronight as the heterodimer accumulates in the nucleus files of PER and TIM nuclear accumulation within key of clock cells, TIM is degraded, PER is destabilized, pacemaker neurons, the LN v 's, which express per and and clock time is delayed [1, 2, 3]. Alternatively, if TIM tim along with the Pigment dispersing factor (Pdf) gene is degraded during the later part of the night, after peak and the neuropeptide it encodes ([9]; LN v 's are reviewed accumulation, clock time advances. Current models in [3, 10, 11]). There are two classes of cells included in state that the effect of a light pulse depends on the the LN v 's: small and large. With one exception (see Figstate of the PER:TIM oscillation, which turns on the ure 4), we saw no differences between the relative prochanging levels of TIM. However, previous studies files of PER and TIM accumulation in these cell types have shown that light:dark (LD) regimes mimicking (data not shown). Therefore, we will emphasize the small seasonal changes cause behavioral adjustments while cells, in part because of their known relevance to behavaltering clock gene expression [5, 6]. This should be ioral rhythmicity [3]. reflected in the adjustment of PER and TIM dynamics. To evaluate possible differences between the nuclear We manipulated LD cycles to assess the effects of accumulation of PER and TIM in the LN v 's, we conaltered day length on PER and TIM dynamics in clock structed detailed time courses of their expression under cells within the central brain as well as light-induced LD 8:16 and LD 16:8. Although a weak nuclear TIM signal resetting of locomotor rhythms. was detectable under LD 8:16 4-6 hr after lights-off, the cytoplasmic signal was far greater (Figures 1A and 1B). Clear nuclear accumulation of TIM in LD 8:16 was evi-Results dent 8 hr after lights-off (Figures 2A and 2B). Peak levels of nuclear TIM accumulation occurred 10 hr after lights-Locomotor Activity Adjusts to Photoperiod As day length varies within a 24 hr LD cycle, mammals off and decreased to undetectable baseline levels by 2 hr after lights-on (Figures 1A and 1B). This profile of TIM and insects display a correlated adjustment in peak nuclear accumulation under LD 8:16 is similar to that observed under LD 12:12 conditions [12].