Timing is a critical adaptive feature of diapause, the neuroendocrine-regulated developmental program that enables organisms to adapt to seasonally varying environments. It is long recognized that the timing of diapause termination is not exclusively under environmental control but also involves an intrinsic physiological mechanism. The nature of this intrinsic control, and the relationship between intrinsic physiological and extrinsic environmental control mechanisms, are not well known. We sought to characterize the physiological and environmental controls of adult reproductive diapause in the monarch butterfly Danaus plexippus (Western North American population) using artificial growth chamber experiments.
Results/Conclusions
We uncover a long-term, intrinsic timer that terminates overwintering diapause in monarchs. Head transcriptome profiling indicates that brain signaling changes and juvenile hormone signaling activity, a known hormonal control of monarch diapause, increases spontaneously under diapause-inducing artificial conditions. This mechanism is distinct from diapause termination resulting from natural environmental cues, which follows different temporal dynamics. Nevertheless, artificial and natural termination eventually converge onto indistinguishable physiological responses and nearly identical molecular (transcriptomic) signatures. These results suggest that in nature, monarch overwintering diapause is under control of an intrinsic physiological timer that is modulated by environmental cues. Photoperiod is unlikely to control the internal timer or be the natural modulatory environmental cue. Understanding the environmental and physiological mechanisms of diapause sheds light on the fundamental problem of biological timing, and also helps inform expectations for how monarch populations may respond to impending climate change.