L

Figure 5.12 Correlation between phosphoglucose isomerase genotype and Hsp70 expression in Chrysomela aeneicollis (Coleoptera, Chrysomelidae).

Note: Genotypes with high Hsp70 expression are from cold environments.

Source: Dahlhoff and Rank. (2000). Proceedings of the National Academy of Sciences of the USA 97, 10056-10061. © 2000 National Academy of Sciences, U.S.A.

(PGI) allele that is more temperature sensitive (i.e. subject to unfolding at lower temperatures) than the southern, high temperature population. Within a given population, PGI genotypes that are more thermally labile also express greater levels of Hsp70 than those that are less so (Fig. 5.12). This study, therefore, provides one of the first demonstrations of the ways in which changes in the molecular chaperone system promote survival of populations that require different genotypes to function efficiently under local environmental conditions. Recent work on Drosophila is expanding knowledge of the relationship between relevant genetic markers and clinal variation in high and low temperature stress resistance, and body size (Weeks et al. 2002).

Alternatives to Hsps

Thermotolerance may involve homeoviscous adaptation, metabolic rate depression, enhanced stability of proteins as a consequence of their structure (i.e. allelic changes), constitutively expressed and induced heat shock proteins, and the synthesis of protective osmolytes (Zatsepina et al. 2001). In aphids and whiteflies, the latter is particularly important, and involves the accumulation of poly-hydric alcohols in response to high temperatures (Wolfe et al. 1998; Salvucci et al. 2000). Sorbitol accumulates to levels as high as 0.44 M within 3 h of exposure to high temperatures in Bemisia argentifolii (Hemiptera, Aleyrodidae), and appears to serve the same protective role as heat shock proteins in these insects (Wolfe et al. 1998; Salvucci 2000; Salvucci et al. 2000). That is, at physiological concentrations sorbitol increases the thermal stability of proteins by stabilizing their structure and preventing heat-induced aggregation, thus maintaining catalytic activity at high temperatures (Salvucci 2000). If the insects are deprived of nutrients, sorbitol production declines and heat shock proteins might assume greater importance in protecting proteins against thermal stress. Nonetheless, it appears that sorbitol is routinely produced as a rapid response to high temperature, via an unusual synthetic pathway involving fructose and an NADPH-dependent ketose reductase. Usually, sorbitol biosynthesis involves a substantially different pathway, is often slower than that found in whiteflies, and generally takes place as a longer-term response to cold (Storey and Storey 1991). However, in some insects, poly-hydric alcohols are also produced during rapid cold hardening (Lee et al. 1987).

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