Diapause is a deep resting stage that facilitates temporal avoidance of unfavourable environmental conditions, and is used by many insects to adapt their life cycle to seasonal variation. Although considerable work has been invested in trying to understand each of the major diapause stages, we know very little about the extrinsic and intrinsic drivers of transitions between stages, especially diapause termination. Here I will present three studies using pupae of the butterfly Pieris napi. (1) In this species, diapause is associated with both temperature-dependent and -independent processes. While the general diapause phenotype is established in a temperature-independent fashion, diapause termination is temperature-dependent and requires a cold signal. (2) The shape of cold-accumulation follows a unimodal nonlinear thermal reaction norm, with optimal rates at winter temperatures. We model this reaction norm as a mirrored version of a typical thermal performance curve and use it to successfully predict diapause termination in multiple fluctuating laboratory temperatures. (3) A putative underlying physiological mechanism of thermal accumulation involves the key developmental hormones prothoracicotropic hormone (PTTH) and ecdysone, whose temperature-dependence we have studied and manipulated during diapause. While PTTH is present throughout diapause in two pairs of neurosecretory cells in the brain, it is absent in the axons and PTTH concentration in the haemolymph is significantly lower during diapause than during post diapause development, indicating that the PTTH pathway is silenced during diapause. At low temperatures, reflecting natural overwintering conditions, diapause termination propensity after ecdysone injection is precocious compared to controls. The link between ecdysone sensitivity and low -temperature dependence reveals a putative mechanism of how diapause termination operates in insects that is in line with adaptive expectations for diapause.
More information here