Temperature cycles drive Drosophila circadian oscillation in constant light that otherwise induces behavioural arrhythmicity.

The fruit fly, Drosophila melanogaster, shows a clear circadian locomotor rhythm in light cycles and constant darkness. Although the rhythm disappears in constant light, we found that temperature cycles drive the circadian rhythm both in locomotor activity and molecular abundance of PERIOD (PER) and TIMELESS (TIM). The thermoperiodically induced locomotor rhythm entailed an anticipatory activity at the late thermophase, which required several transient cycles to establish a steady-state entrainment, suggesting that the rhythm is endogenous and driven by a circadian clock. Western blot analysis revealed that PER and TIM increased during the cryophase, peaking at the middle to late cryophase. PER was also cyclically expressed under the temperature cycle in the known per-expressing neurons, i.e. so-called lateral (LNs) and dorsal neurons (DNs), and two pairs of cells (LPNs) that were located in the lateral posterior protocerebrum. It is thus suggested that the temperature cycle induces the cycling of PER and TIM either by blocking somewhere in the photic entrainment pathway during the cryophase or temporally activating their translation to sufficient protein levels to drive a circadian oscillation. In flies lacking pigment-dispersing factor (PDF) or PDF-expressing cells, the anticipatory activity was relatively dispersed. disco(2) mutant flies lacking the lateral neurons still showed an anticipatory activity, but with dispersed activity. These behavioural results suggest that not only LNs but also DNs and LPNs can, at least, partially participate in regulating the thermoperiodically induced rhythm.