20 40 60
Figure 5.15 The induction of tolerance to a —10°C cold shock after short chilling pre-treatments at 0°C in Sarcophaga crassipalpis.
Source: Reprinted with permission from Lee et al. Science 238, 1415-1417. © 1987, with permission from AAAS.
clear that a short pretreatment at a high temperature can also confer resistance to cold shock (Chen et al. 1991; Sinclair and Chown 2003). In consequence, it appears that the mechanisms providing protection against heat and cold are similar, although there is only partial overlap of these mechanisms.
At least in pharate adults of S. crassipalpis, rapid cold hardening is associated with a threefold increase in glycerol levels to 81.4 mM. Although this change is insufficient to have a colligative effect on cold hardiness (see Section 5.3.2), glycerol probably plays an important role in protecting membranes against low temperature damage associated with phase transitions (Lee et al. 1987). However, glycerol is not produced in response to brief pretreatment at a high temperature in this species (Chen et al. 1991), nor is it produced in response to cold shock in D. melanogaster and Lymatria dispar (Lepidoptera, Lymantriidae) (Yocum et al. 1991; Denlinger et al. 1992; Kelty and Lee 1999). Rather, in response to cold shock, these species, S. crassipalpis, several Drosophila species, and Leptinotarsa decemlineata (Coleoptera, Cocci-nellidae) upregulate heat shock protein synthesis, including the 92, 78, 75, 72, 70, 45, and 23 kDa proteins (Burton et al. 1988; Denlinger et al. 1992;
Denlinger and Lee 1998; Goto and Kimura 1998; Yocum et al. 1998; Yocum 2001). It is likely that at low temperatures these molecular chaperones fulfil a role similar to the one they assume at high temperatures, by providing chaperoning functions, and removing proteins denatured by low temperature stress. Their protective role must also extend further because both thermotolerance and rapid cold hardening provide protection from the negative effects of high and low temperature shock on fecundity in S. crassipalpis (Rinehart et al. 2000b).
The relevance of rapid cold hardening to the field situation is only now being explored. Kelty and Lee (2001) demonstrated that during thermoperiodic cycles identical to those likely to be experienced in the field, D. melanogaster demonstrates rapid cold hardening during the cooling phase of the cycles that is only partly lost during the subsequent warming phase. Moreover, during subsequent cooling phases tolerance of low temperatures is further improved. However, it is not only survival of a cold shock that improves following rapid cold hardening, but also the ability of flies to tolerate low temperatures that normally cause knockdown, although this improvement ceases after the first thermal cycle. Kelty and Lee (2001) argued that rapid cold hardening provides subtle benefits in the field, allowing adult flies to remain active for longer than otherwise would be possible. It has also been suggested that rapid cold hardening in Antarctic springtails and mites contributes to survival of what would otherwise be lethal temperatures (Worland and Convey 2001; Sinclair et al. 2003a). However, in this case it is the supercooling point (SCP) that is altered, rather than a lethal temperature above the SCP, as is found in all of the freezing intolerant insects examined to date.
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