Behavioral adaptation to warm conditions via Lim1-mediated acceleration of neuronal clocks.

Temperature compensation stabilizes the speed of circadian clocks. Uncompensated molecular clock cycles would accelerate severalfold with each 10 °C increase, precluding reliable timekeeping. Despite such thermal buffering, some clock-controlled behavioral cycles complete by up to two hours earlier or later depending on environmental temperatures. We show that temperature-dependent changes in the speed of behavioral cycles can be explained by changes in the speed of the clock itself. Although the speed of all clocks is insensitive to thermal energy, we found that in neurons the clock speed is regulated by temperature information. When the threshold of ~26 °C is exceeded for ~24 h, a pathway mediated by the LIM-homeodomain transcription factor Lim1 instructs the clocks in the Drosophila brain to accelerate. Clock acceleration enables earlier morning awakening. This work suggests that modestly altering the clock speed enables behavioral thermoadaptation, via regulated steps that do not compromise the reliability of circadian timekeeping.