Thermal tolerance variation in natural populations of Drosophila melanogaster: the role of two genes and their mode of action

Abstract

Since heat stress affects most organisms it is important that we understand how adaptation occurs to increasingly warm environment, especially the underlying changes in physiology, biochemistry and genetics. Few studies have shown links between physiological mechanisms and heat tolerance phenotypes in an adaptive context. Therefore the overall aim of this thesis was to use the model organism Drosophila melanogaster to elucidate the role of two heat-tolerance candidate genes hsr-omega and hsp90 in thermal adaptation, and to look at this in a physiological context which included examining rates of protein synthesis, a postulated underlying process. Using geographically diverse populations of D. melanogaster from eastern Australia I found that heat tolerance is a plastic trait that depends on rearing temperature and heat-stimulus conditions, and that the adaptive latitudinal clines in heat tolerance depend on these rearing conditions. Protein synthesis rate showed latitudinal clines that also depend on both the temperature at which flies are reared (18 or 25 °C) and heat-stress conditions (either unstressed (basal) or following a 37 °C heat stimulus), and these clines ran in parallel to clines in heat knockdown tolerance, although no evidence that the clines are connected was obtained. Consistent negative correlations between variation in protein synthesis rate and heat knockdown tolerance in a derived North/South hybrid population confirmed the importance of protein synthesis rate as a factor underlying heat tolerance variation within populations. However the latitudinal cline in protein synthesis did not help explain the latitudinal heat tolerance variation as this would require a positive association between the two variables. A gene thought to help control rates of general protein synthesis following heat stimulus, hsr-omega, was investigated for changes in expression across latitude. Clines in basal and heat-stimulated omega-n transcript level suggest that there is adaptive genetic differentiation in hsr-omega expression between populations from different climatic regions. I show for the first time evidence for a link between expression of another heat shock gene, hsp90, and adult heat knockdown tolerance. Tissue levels of hsp90 transcript and protein were negatively associated with tolerance in several independent data sets. Further, this negative association extended to a set of populations from different thermal niches and revealed a positive linear latitudinal cline for both basal hsp90 transcript and protein level. These data suggest that heritable variation in hsp90 expression contributes to traits that facilitate adaptation to different climatic regions, including the clinal variation in thermal tolerance. I also discuss a plausible causal role for hsp90 as a negative regulator of the cellular heat shock response that predicts the above negative hsp90-tolerance association, particularly the interaction between Hsp90 protein and Heat shock factor. Overall these data make a significant contribution to understanding the process of adaption to divergent thermal habitats and to the cellular processes and genes that facilitate thermal adaptation.