The capacity of Drosophila to heat harden associates with low rates of heat-shocked protein synthesis
Introduction
Thermal tolerance is an important adaptive trait that often determines the habitat range and abundance of species (Parmesan, 1996; Bowler and Terblanche, 2008). A species’ capacity to adapt to thermal stress in the face of encroaching global warming, over and above its capacity to acclimate, will contribute to extinction risk (Walther et al., 2002; Deutsch et al., 2008; Williams et al., 2008). An understanding of the physiological mechanisms and genetics of heat tolerance in a species will improve our efforts to predict adaptation and to forecast changes in population dynamics as habitat temperatures increase. Drosophila species are particularly suitable ectotherms to elucidate such mechanisms because large differences in thermal tolerance occur among related species (Kimura, 2004; Kellett et al., 2005). In addition, heritable intra-specific tolerance differences occur among strains of wide-ranging species like D. melanogaster that occupy diverse climatic regions (Zatsepina et al., 2001; Hoffmann et al., 2002).
Substantial effort has focused on dissecting and understanding the mechanisms that underlie thermotolerance variation among species and strains of Drosophila (Feder and Hofmann, 1999; Hoffmann et al., 2003). Much of the focus has been on the role of the heat-shock protein (Hsp) genes (Parsell and Lindquist, 1993), particularly Hsp70 (Bettencourt et al., 1999; Sørensen et al., 2003), and other genes that are part of the cellular heat-stress response (McColl et al., 1996). The cellular response to a non-lethal heat shock has been characterised thoroughly (Lindquist, 1986; Morimoto et al., 1997; Korsloot et al., 2004) and involves complex interactions to switch on synthesis of protective heat-shock proteins and curtail synthesis of normal ‘housekeeping’ proteins. Just how fast or efficiently this protein synthesis control is implemented has been a relatively neglected aspect of investigations into thermotolerance variation. Recently, genomic technologies have uncovered a host of new thermotolerance candidate genes (Leemans et al., 2000; Sørensen et al., 2005; Laayouni et al., 2007) and the ‘GO’-grouping of many of these into functional categories such as translation and regulation of transcription support the idea that protein synthesis changes are important for thermotolerance.
In the early 1980s, Alahiotis and Stephanou (1982) and Stephanou et al. (1983) found an association between high levels of heat tolerance in selected lines and the control of protein synthesis. The kinetics of protein synthesis, assessed in ovarian tissues following a heat shock, was associated with changes in the timing and extent of heat-shock protein production, with timing and extent of housekeeping protein shutdown, and with heat-stress survival differences between the lines. While the relationship between protein synthesis and thermotolerance has subsequently been considered in other organisms (for review see Craig, 1985), only a few Drosophila studies have further addressed this issue (Mitchell et al., 1979; Dingley and Maynard-Smith, 1968).
Here we characterise two sets of D. melanogaster lines for variation in rates of protein synthesis and ask if protein synthesis levels relate to levels of heat tolerance variation. We use single-pair mating lines derived from mass-bred populations that were recently collected from locations central in the Australian eastern coast cline (Hoffmann and Weeks, 2007). The use of single-pair lines serves to limit variation within lines and maximises variation among lines. We explored associations between protein synthesis capacity, both before and following a mild heat-shock, and the heat tolerance of the lines characterized with and without prior heat hardening using the small vial knockdown assay. Our data suggest that natural variation in the capacity to synthesise proteins after a mild heat shock in this species influences variation in hardening ability of the adults.
Section snippets
Line derivation and maintenance
The Coffs Harbour set of 30 single-pair mating lines was derived from a field collection of D. melanogaster in March 2005 from Coffs Harbour on the central eastern coast of Australia, as previously described (Johnson et al., 2009). All cultures were maintained at 25 °C under a 12:12 light:dark regime in 250 mL bottles containing a treacle-semolina medium and assays were done on adults raised at controlled non-crowded densities. Tolerance tests were performed at generation F2 after the lines were
Results and discussion
Isofemale lines and single-pair matings using flies derived from natural populations have been widely used to examine levels of genetic variation, patterns of associations among traits and the molecular basis of genetic variation (Hoffmann and Parsons, 1988; David et al., 2005; Krebs and Feder, 1997; Rako et al., 2007). This method minimises problems associated with laboratory adaptation and inbreeding as well as unnatural levels of linkage disequilibrium generated when populations are recently
Acknowledgements
We thank Rebecca Hallas, Jennifer Shirriffs and Rhonda Rawlinson for assistance with the experimental work and the Australian Research Council for financial support through their Special Research Centre Program.
References (42)
- et al.
Temperature adaptation of Drosophila populations. The heat-shock protein system
Comparative Biochemistry and Physiology B
(1982) - et al.
Temperature acclimatization in the absence of protein synthesis in Drosophila subobscura
Journal Insect Physiology
(1968) - et al.
Comparing different measures of heat resistance in selected lines of Drosophila melanogaster
Journal of Insect Physiology
(1997) - et al.
Adaptation of Drosophila to temperature extremes: bringing together quantitative and molecular approaches
Journal of Thermal Biology
(2003) - et al.
Thermal tolerance trade-offs associated with the right arm of chromosome 3 and marked by the hsr-omega gene in Drosophila melanogaster
Heredity
(2003) - et al.
Experimental evolution of Hsp70 expression and thermotolerance in Drosophila melanogaster
Evolution
(1999) - et al.
Insect thermal tolerance: what is the role of ontogeny, aging and senescence?
Biological Reviews
(2008) - et al.
Selection for knockdown resistance to heat in Drosophila melanogaster at high and low larval densities
Evolution
(1998) The heat shock response
CRC Critical Reviews in Biochemistry
(1985)- et al.
Isofemale lines in Drosophila: an empirical approach to quantitative trait analysis in natural populations
Heredity
(2005)