Physiological mechanisms of seasonal and rapid cold-hardening in insects

Insects have evolved a number of physiological mechanisms for coping with the detrimental effects of low temperature. As autumn progresses, insects use environmental signals such as shortening day lengths and gradually decreasing temperatures to trigger seasonal cold-hardening adaptations. These mechanisms include dramatic changes in biochemistry, cell function and gene expression that permit improved cell function and viability at low temperature. Insects are also capable of enhancing cold tolerance on a much shorter time scale, in a process called rapid cold-hardening (RCH). Rapid cold-hardening allows insects to improve cold tolerance almost instantaneously (i.e. within minutes to hours) to cope with sudden cold snaps and regularly-occurring diurnal drops in temperature. Initially, it was assumed that RCH would share many of the same basic mechanisms as seasonal cold-hardening, albeit on a shorter time scale. Although there is some evidence supporting this, recent work has called into question some of the original hypotheses concerning the mechanisms of RCH. Also, some mechanisms important for seasonal cold-hardening, such as up-regulation of stress proteins, are unlikely to function at the temperatures and time scales at which RCH occurs. In the present review, the current understanding of the physiological mechanisms governing both seasonal cold-hardening and RCH are summarized. A synthesis of the current literature suggests that these two forms of cold-hardening may be more mechanistically distinct than originally anticipated.