Figure 5.7 (a) Drosophila melanogaster adult survival (mean ± SD) of a 38°C treatment for 30 min, following laboratory natural selection at three different temperatures. (b) Responses of knockdown resistance to artificial selection in low rearing density (open circles) and high rearing density (closed circles) lines of Drosophila melanogaster.
Source: (a) is from Gilchrist et al. 1997 table 1 and (b) is redrawn from Bubli et al. 1998.
improved knockdown resistance (McColl et al. 1996; Gilchrist et al. 1997). Induction of Hsps following thermal stress might also aid in repair of damage (Krebs and Feder 1997). Alternatively, alterations in cell membrane composition (Gracey et al. 1996) or changes in allozymes or their concentrations might also be involved (Cossins and Bowler 1987; Somero et al. 1996). Because development at higher temperatures often results in an increase in basal thermotolerance but a decline in the response to hardening (Cavicchi et al. 1995; Bettencourt et al. 1999; Sorensen et al. 1999, 2001), one explanation for increased basal thermotolerance is the cost of a low-level, induced stress response (see below and Krebs and Feder 1997,1998a; Zatsepina et al. 2001).
Continuous expression of heat shock proteins reduces survival and fecundity, inhibits growth, and thus affects development time (Krebs and Loeschcke 1994; Feder and Krebs 1998; Feder 1999). It also acts as a substrate sink, and interferes with cellular functioning (Zatsepina et al. 2001). In consequence, at high temperatures there would be a considerable premium for reduction of this response, and probably an increase in basal ther-motolerance allowing the organisms to cope with what are otherwise potentially injurious temperatures. This basal thermotolerance may be a consequence of constitutively expressed Hsps (Lansing et al. 2000), the presence of osmolytes (Wolfe et al.
1998) or alterations in membranes and allozymes (Zatsepina et al. 2001).
On the other hand, constitutive resistance to high temperatures may not be maintained in individuals that do not routinely experience such temperatures because there are costs associated with supporting a biochemistry associated with these conditions. For example, Watt (1977, 1983) has shown that in Colias butterflies (Lepidoptera, Pieridae), adaptive adjustment of enzyme functional characteristics involves a trade-off of kinetic flexibility and efficacy at low temperatures against stability at high temperatures (see Section 6.4.1).
Despite variation in the temperature at which heat induces injury, the response to heat shock is similar in the majority of the insect species that have been examined. High temperature injury generally results from disruption of the function of membranes, especially synaptic membranes (Cossins and Bowler 1987), alterations in the cell microenvironment (e.g. pH), perturbation of protein structure, and DNA lesions (Somero 1995; Feder
1999). In turn, these changes affect development, muscular contraction, and several other processes at higher organizational levels (see Denlinger and Yocum 1998 for comprehensive discussion). The responses of membrane systems to temperature generally involve alterations in the composition of cellular lipids (Gracey et al. 1996; Somero et al. 1996), while expression of heat shock proteins, which act as molecular chaperones to proteins (Hendrick and Hartl 1993), is now recognized as one of the most widespread and conserved responses to stress, including thermal stress (Lindquist 1986; Feder and Hofmann 1999).
Intense thermal stress can perturb the structure of an organism's proteins. During normal cellular functioning proteins are generally folded, but may be unfolded during transport, synthesis of poly-peptides, and assembly of multimeric proteins. Stress may also result in unfolding. In this unfolded state, exposed amino acid side groups, especially hydrophobic residues, can lead to interactions between these 'non-native' proteins and folded proteins, inducing the latter to unfold. The result is irreversible aggregations of unfolded proteins. These unfolded proteins reduce the cellular pool of functional proteins and may also be cytotoxic (Feder 1996, 1999; Feder and Hofmann 1999). Molecular chaperones interact with the unfolded proteins to minimize their harmful effects by binding to the exposed side groups, preventing unfolded proteins from interacting. In an ATP-dependent manner they also release the proteins so that they can fold properly, and may also target proteins for degradation or removal from the cell (Parsell and Lindquist 1993; Feder 1996). Stress proteins, or heat shock proteins, therefore, function as molecular chaperones and have been found in virtually all prokaryotes and eukaryotes (but see Hofmann et al. 2000). These heat shock proteins comprise several families that are recognized by their molecular weight, and include Hsp100, Hsp90, Hsp70, Hsp60, and a family of smaller proteins (Denlinger et al. 2001).
In insects, the best known of these families is Hsp70, especially because of its dramatic increase in Drosophila in response to high temperature stress. Indeed, the history of physiological and biochemical investigations of the role of Hsps in insects is essentially a history of the investigation of the role of Hsps in Drosophila and Sarcophaga crassipalpis (Diptera, Sarcophagidae) (Denlinger and Lee 1998; Denlinger and Yocum 1998; Feder and Krebs 1998). In the 1960s it was recognized that heat induces puffing of Drosophila chromosomes. Subsequently, much of the work demonstrating the importance of Hsps for thermal tolerance, both in the field and in the laboratory, has involved work on Drosophila species, and especially
D. melanogaster (Alahiotis and Stephanou 1982; Alahiotis 1983). In the main, conclusive demonstrations of the association between Hsp70 expression and thermotolerance have come from investigations of isofemale lines and genetically engineered strains of D. melanogaster (Fig. 5.8) (Krebs and Feder 1997; Feder 1999). Moreover, investigations of several other Drosophila species have demonstrated that the temperature at which heat shock protein expression is induced varies considerably, both naturally and in response to laboratory selection (Bettencourt et al. 1999; Feder and Hofmann 1999).
Heat shock is also known to induce expression of Hsp70 in several other insect species such as moths, ants, and parasitic wasps (Denlinger et al. 1991,1992; Gehring and Wehner 1995; Maisonhaute et al. 1999). Therefore, it seems likely that Hsp70 will be identified as a common component of the heat shock response in most taxa, although the nature and complexity of the response is likely to vary (Joplin and Denlinger 1990; Yocum and Denlinger 1992). Undoubtedly, much of the future work on the role of Hsps in thermotolerance will involve investigation of induced thermotolerance
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