Diapause: delaying the developmental clock in response to a changing environment.

Seasonal changes can induce organisms to modify their developmental growth. Many holometabolous insects, especially Lepidoptera, trigger diapause, an "actively induced" dormancy, for overwintering. Diapause is an alternative developmental pathway that reversibly blocks developmental growth during specific transitions and enhances the hibernating potential of the organism. Changes in environmental cues, such as light and temperature, trigger modifications in the levels, or in the timing, of developmental hormones. These in turn switch the developmental trajectory (diapause or direct development), strongly altering larval/pupal growth and inducing the appearance of diapause-bound seasonal morphs (polyphenism). We also discuss an example of vertebrate diapause using the killifish embryo as an example where diapause is an environmentally determined developmental switch analogous to that observed in lepidopteran dormancy. Based on the examples discussed here, we propose that the complex physiological responses leading to diapause might evolve quickly by relatively limited genetic changes in the regulation of hormonal signals that program normal developmental transitions.