Abstract
This review starts by comparing photoperiodic diapause with non-photoperiodic quiescence in four representative species, paying particular attention to overwintering in Drosophila melanogaster. In the second part it describes dormancy strategies of insects from the equator to the polar regions, addressing topics such as the role of the circadian system in photoperiodic time measurement, latitudinal clines in diapause-related traits, hourglass-like photoperiodic clocks based on dampening circadian oscillators, and the dormancy strategies of insects close to the equator or at high latitudes where seasonal changes in photoperiod are unreliable or absent.
Highlights
Insects have evolved on a planet that revolves on its axis every 24 h and completes its orbit around the Sun in a little more than 365 days
In a wider context the review considers possible historical changes in the evolution of insect dormancy programmes. It addresses the following questions: How have insects adapted their seasonal development to local climatic factors? How have they adapted to seasonal changes as their geographical distributions extended into higher latitudes? What is the nature of the photoperiodic clock, its relationship to the circadian system? And how do diapause and quiescence regulate insect seasonality close to the equator or at latitudinal extremes where photoperiod becomes unreliable?
Diapause and quiescence as separate components of the insect dormancy programme The first section of this review shows that both diapause and quiescence contribute to the seasonal regulation of insect development and reproduction
Summary
Insects have evolved on a planet that revolves on its axis every 24 h and completes its orbit around the Sun in a little more than 365 days. During the northern winter (and southern summer) the opposite latitudinal effects are evident These seasonal and latitudinal changes in photoperiod are important regulators of insect development and reproduction and form the subject matter of this review. Quiescence and diapause often involve similar ‘downstream’ reactivation pathways, either the brain (PTTH)prothoracic gland (ecdysteroid) axis in the regulation of larval and pupal (i.e. developmental) dormancies, or the brain-corpus allatum (juvenile hormone) axis in adult (i.e. reproductive) dormancies (Denlinger, 1985) They differ, in their ‘upstream’ control: diapause is regulated by a brain-centred photoperiodic clock measuring day- or nightlength and considered by many authors to be based. It addresses the following questions: How have insects adapted their seasonal development to local climatic factors? How have they adapted to seasonal changes as their geographical distributions extended into higher latitudes? What is the nature of the photoperiodic clock, its relationship to the circadian system? And how do diapause and quiescence regulate insect seasonality close to the equator or at latitudinal extremes where photoperiod becomes unreliable?
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