Weak and strong phase response curves of the onion fly circadian clock at temperature changes of 1 °C and 4 °C

With increasing soil depth, the amplitude and phase of the daily temperature cycle decreases and is delayed, respectively. The onion fly, Delia antiqua, which pupates at a soil depth of 2–20 cm, advances the eclosion phase of its circadian clock as the temperature amplitude decreases. This “temperature-amplitude response” compensates for the depth-dependent phase delay of the temperature change and ensures eclosion in the early morning. To clarify the physiological mechanisms that induce a temperature-amplitude response, we performed phase-resetting experiments using a 12-h high- or low-temperature pulse with an amplitude of 1 °C or 4 °C. Based on the results obtained, four phase transition curves and four phase response curves were constructed. These curves show that the phase of the eclosion clock shifted more as the magnitude of the temperature change increased. The 24-h temperature cycle delayed, rather than advanced, the phase of the D. antiqua circadian eclosion rhythm. Therefore, we propose that a small phase delay is caused by a small temperature amplitude at a deep site in the soil and a large phase delay is caused by a large temperature amplitude at a shallow site, leading to the temperature-amplitude response exhibited by D. antiqua.