Role for circadian clock genes in seasonal timing: testing the Bünning hypothesis.

Mirko Pegoraro,Joao S Gesto,Charalambos P Kyriacou,Eran Tauber,Patrick Emery

doi:10.1371/journal.pgen.1004603

Mirko Pegoraro, Joao S Gesto + Show 3 more

Open Access

https://doi.org/10.1371/journal.pgen.1004603

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Journal: PLoS genetics	Publication Date: Sep 4, 2014
Citations: 100	License type: CC BY 4.0

Affiliation: University of Leicester

Abstract

A major question in chronobiology focuses around the “Bünning hypothesis” which implicates the circadian clock in photoperiodic (day-length) measurement and is supported in some systems (e.g. plants) but disputed in others. Here, we used the seasonally-regulated thermotolerance of Drosophila melanogaster to test the role of various clock genes in day-length measurement. In Drosophila, freezing temperatures induce reversible chill coma, a narcosis-like state. We have corroborated previous observations that wild-type flies developing under short photoperiods (winter-like) exhibit significantly shorter chill-coma recovery times (CCRt) than flies that were raised under long (summer-like) photoperiods. Here, we show that arrhythmic mutant strains, per01, tim01 and ClkJrk, as well as variants that speed up or slow down the circadian period, disrupt the photoperiodic component of CCRt. Our results support an underlying circadian function mediating seasonal daylength measurement and indicate that clock genes are tightly involved in photo- and thermo-periodic measurements.

Highlights

Seasonal changes in day-length provide a reliable environmental cue used by many temperate species to adapt to their fluctuating environments
We address a major and long-standing question in chronobiology: whether the circadian clock is involved in photoperiodic timing, known as the Bunning hypothesis
The chill-coma recovery times (CCRt) of wild-type flies raised at different photoperiods is sexually dimorphic (Figure 1)

Summary

Introduction

Seasonal changes in day-length provide a reliable environmental cue used by many temperate species to adapt to their fluctuating environments. The Bunning hypothesis [7] invoked a link between the circadian and the photoperiodic mechanisms and suggested that circadian rhythmicity is required for day-length measurement. Bunning’s original model assumed that circadian oscillations consist of light (‘photophil’) and dark (‘scotophil’)-requiring phases. The relative size of the photophil and scotophil phases encodes the critical photoperiod (time of the year) that induces the seasonal response. The ‘internal coincidence model’, was proposed, where light plays only an indirect role, and the critical photoperiod is encoded by unique phase relationships between two internal oscillators. If the seasonal response peaks at 24 hr intervals (‘positive Nanda-Hamner’), a link, not necessarily causal, with the circadian system in photoperiodic timing is indicated [9,10]

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