Integration of Light and Temperature in the Regulation of Circadian Gene Expression in Drosophila

Abstract

Circadian clocks are aligned to the environment via synchronizing signals, or Zeitgebers, such as daily light and temperature cycles, food availability, and social behavior. In this study, we found that genome-wide expression profiles from temperature-entrained flies show a dramatic difference in the presence or absence of a thermocycle. Whereas transcript levels appear to be modified broadly by changes in temperature, there is a specific set of temperature-entrained circadian mRNA profiles that continue to oscillate in constant conditions. There are marked differences in the biological functions represented by temperature-driven or circadian regulation. The set of temperature-entrained circadian transcripts overlaps significantly with a previously defined set of transcripts oscillating in response to a photocycle. In follow-up studies, all thermocycle-entrained circadian transcript rhythms also responded to light/dark entrainment, whereas some photocycle-entrained rhythms did not respond to temperature entrainment. Transcripts encoding the clock components Period, Timeless, Clock, Vrille, PAR-domain protein 1, and Cryptochrome were all confirmed to be rhythmic after entrainment to a daily thermocycle, although the presence of a thermocycle resulted in an unexpected phase difference between period and timeless expression rhythms at the transcript but not the protein level. Generally, transcripts that exhibit circadian rhythms both in response to thermocycles and photocycles maintained the same mutual phase relationships after entrainment by temperature or light. Comparison of the collective temperature- and light-entrained circadian phases of these transcripts indicates that natural environmental light and temperature cycles cooperatively entrain the circadian clock. This interpretation is further supported by comparative analysis of the circadian phases observed for temperature-entrained and light-entrained circadian locomotor behavior. Taken together, these findings suggest that information from both light and temperature is integrated by the transcriptional clock mechanism in the adult fly head.

Author Summary

A key adaptation to life on Earth is provided by internal daily time-keeping mechanisms that allow anticipation of the alternations between night and day. To act as reliable time-keeping mechanisms, circadian clocks have to be able to synchronize to environmental time cues, maintain ∼24-h rhythms under constant conditions, run at approximately the same pace over a range of environmental temperatures, and efficiently communicate time-of-day information to other biological systems. Clock-controlled oscillations in gene expression play an essential role in producing overt circadian rhythms. For most organisms, light/dark cycles appear to constitute the most powerful entrainment cue, but daily temperature cycles have also been demonstrated to efficiently synchronize circadian rhythms. This study uses the fruit fly Drosophila melanogaster as a model to compare the clock-dependent and clock-independent daily gene expression rhythms produced in response to light/dark cycles versus temperature cycles. A broad temperature-driven expression program was found in the heads of both wild-type and arrhythmic mutant flies, but wild-type flies also exhibited a more specific temperature-entrained circadian expression response that resembled the circadian response following light entrainment. The phase relationship between the temperature- and light-entrained circadian rhythms suggests that in nature light and temperature act cooperatively to synchronize the circadian clock.