Heat shock cold shock and rapid hardening

Cold and heat shock are the stresses inflicted on insects by brief exposures to either low (but non-freezing), or high temperatures, respectively (Lee et al. 1987; Lee 1989; Denlinger et al. 1991). Injury is generally positively related to the duration and magnitude of the stress, and may eventually culminate in death. Although variation in insect thermal tolerances has long been appreciated, and often ascribed to adaptation to local environments (Shelford 1911; Mellanby 1932; Andrewartha and Birch 1954; Messenger 1959), the complexity of these responses and the nature of their underlying physiological mechanisms have been enjoying renewed attention. There are undoubtedly many reasons for renewed vigour in this field. Among them is certainly the realization that investigations of model organisms can convincingly confirm (or reject) previously held, but often untested, adaptive hypotheses (Huey and Kingsolver 1993; Feder and Krebs 1998; Feder et al. 2000a). Furthermore, it is clear that a comprehensive understanding of the effects of global climate change will necessarily mean a thorough comprehension of the responses of insects to changing temperature (Hoffmann and Blows 1993; Cavicchi et al. 1995; Coleman et al. 1995, see also Chapter 7).

Recent investigations have not only confirmed geographic (and seasonal) variation in basal ther-motolerance (Stanley et al. 1980; Krebs and Loeschcke 1995a; van der Merwe et al. 1997; Gibert and Huey 2001) (Fig. 5.5), but have also revealed considerable responses to selection in the laboratory of both basal and inducible thermotolerance (Hoffmann and Watson 1993; Cavicchi et al. 1995; Krebs and Loeschcke 1996; Loeschcke and Krebs 1996). Inducible thermotolerance, or hardening, is the increase in tolerance to potentially lethal temperatures that results from a brief exposure of organisms to moderately stressful temperatures

Standardized contrast in latitude

Figure 5.5 Geographic variation in chill coma temperature in 15 species of the Drosophila obscura-group of flies, corrected for phylogenetic non-independence.

Standardized contrast in latitude

Figure 5.5 Geographic variation in chill coma temperature in 15 species of the Drosophila obscura-group of flies, corrected for phylogenetic non-independence.

Source: Physiological and Biochemical Zoology, Gibert and Huey, 74, 429-434. © 2001 by The University of Chicago. All rights reserved. 1522-2152/2001/7403-00129$03.00

(Lee et al. 1987; Chen et al. 1990; Dahlgaard et al. 1998). In the context of cold, hardening has traditionally been regarded as a slow process that gradually increases an insect's tolerance of low temperatures (Denlinger and Lee 1998). However, in the context of high temperatures, hardening has come to be associated with a more rapid response to short-term, moderately stressful exposures (Denlinger et al. 1991; Loeschcke et al. 1997; Dahlgaard et al. 1998). Over the past several decades a similar, rapid response to cold has been demonstrated in a wide variety of species (Chen et al. 1987; Lee et al. 1987; Czajka and Lee 1990; Coulson and Bale 1992; Larsen and Lee 1994; McDonald et al. 1997). In consequence, here we restrict the term 'hardening' to rapid responses to either moderately high or low temperature exposures, preferring to consider longer-term cold hardiness a 'programmed response to cold' (Section 5.3). This distinction is not just a matter of convenience. Rather, it reflects the fact that hardening is generally a short-term change in thermotolerance (which is often lost within a matter of hours or days), as a consequence of a given, short-term temperature treatment. In contrast, longer-term cold hardening is a programmed set of responses to an often predictable environmental cue such as slowly declining temperatures or changes in photoperiod (Baust and Rojas 1985). While this distinction is obviously not always clear-cut, and both processes are likely to contribute to the survival of insects at low temperatures, we consider the two responses sufficiently distinct to treat them separately. In addition, the hardening responses to cold and heat are similar in several ways (Lee et al. 1987; Denlinger et al. 1991; Denlinger and Yocum 1998; Wolfe et al. 1998; Rinehart et al. 2000a; Yocum 2001), whereas 'programmed responses to cold' have no obvious equivalent high temperature response (Chown 2001). Where they occasionally do, such as in the case of aestivation, these latter responses seem closer to each other than either is to the rapid hardening response.

Notwithstanding this distinction, it is evident that both hardening and programmed responses to cold can be considered forms of acclimation (in the laboratory) or acclimatization (in the field). While the mechanisms underlying acclimation have long fascinated physiologists (Kingsolver and Huey 1998), the evolutionary benefits (or lack) thereof have only recently been carefully examined with the advent of evolutionary physiology (Feder et al. 2000a). In consequence, the terminology associated with acclimation must now be interpreted under the broader evolutionary rubric of phenotypic plasticity (Huey and Berrigan 1996). Although there is considerable potential for confusion given the wide variety of uses that characterize the term 'acclimation' (Spicer and Gaston 1999), recent reviews have considerably clarified the terminology.

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