Natural plasticity in circadian rhythms is mediated by reorganization in the molecular clockwork in honeybees

Various animals naturally switch to considerable periods of around-the-clock activity with no apparent ill effects. Such plasticity in overt circadian rhythms might be observed because the clock is masked by the influence of external factors, is uncoupled from behavioral outputs, or results from genuine plasticity in the clock machinery. We studied honeybees in which plasticity in circadian rhythms is socially modulated and associated with the division of labor. We confirm that "nurse" bees care for the brood around-the-clock even when experiencing a light:dark illumination regime. However, nurses transferred from the hive to individual cages in constant conditions have robust circadian rhythms in locomotor activity with an onset of activity at the subjective morning. These data indicate that circadian rhythmicity in nurses depends on their environment, and suggest that some clockwork components were entrained even in nurses active around the clock while in the hive. Brain oscillations in transcript abundance for the putative clock genes Period, Cryptochrome-m, Cycle, and Timeout were attenuated or totally suppressed in nurses as compared to behaviorally rhythmic foragers, irrespective of the illumination regime. These findings provide the first support for the hypothesis that natural plasticity in circadian rhythms is associated with reorganization of the internal clockwork.