Loss of functional reduces robustness of 24-hour behavioral rhythms in monarch butterflies.

Light is one of the strongest cues for entrainment of circadian clocks. While some insect species rely only on visual input, others like use both the visual system and the deep-brain blue-light photoreceptor cryptochrome for entraining circadian rhythms. Here, we used the monarch butterfly (), which possesses a light-sensitive (), to test the conservation of mechanisms of clock entrainment. We showed that loss of functional reduced the amplitude and altered the phase of adult eclosion rhythms, and disrupted brain molecular circadian rhythms. Robust rhythms could be restored by entrainment to temperature cycles, indicating a likely functional core circadian clock in mutants. We also showed that rhythmic flight activity was less robust in mutants, and that visual impairment in mutants impacted flight suppression at night. Our data suggest that dpCRY1 is a major photoreceptor for light-entrainment of the monarch circadian clock.