Inducing extra copies of the Hsp70 gene in Drosophila melanogaster increases energetic demand

Abstract

BackgroundMutations that increase gene expression are predicted to increase energy allocation to transcription, translation and protein function. Despite an appreciation that energetic tradeoffs may constrain adaptation, the energetic costs of increased gene expression are challenging to quantify and thus easily ignored when modeling the evolution of gene expression, particularly for multicellular organisms. Here we use the well-characterized, inducible heat-shock response to test whether expressing additional copies of the Hsp70 gene increases energetic demand in Drosophila melanogaster.ResultsWe measured metabolic rates of larvae with different copy numbers of the Hsp70 gene to quantify energy expenditure before, during, and after exposure to 36°C, a temperature known to induce robust expression of Hsp70. We observed a rise in metabolic rate within the first 30 minutes of 36°C exposure above and beyond the increase in routine metabolic rate at 36°C. The magnitude of this increase in metabolic rate was positively correlated with Hsp70 gene copy number and reflected an increase as great as 35% of the 22°C metabolic rate. Gene copy number also affected Hsp70 mRNA levels as early as 15 minutes after larvae were placed at 36°C, demonstrating that gene copy number affects transcript abundance on the same timescale as the metabolic effects that we observed. Inducing Hsp70 also had lasting physiological costs, as larvae had significantly depressed metabolic rate when returned to 22°C after induction.ConclusionsOur results demonstrate both immediate and persistent energetic consequences of gene copy number in a multicellular organism. We discuss these consequences in the context of existing literature on the pleiotropic effects of variation in Hsp70 copy number, and argue that the increased energetic demand of expressing extra copies of Hsp70 may contribute to known tradeoffs in physiological performance of extra-copy larvae. Physiological costs of mutations that greatly increase gene expression, such as these, may constrain their utility for adaptive evolution.

Cloning and differential expression of five heat shock protein genes associated with thermal stress and development in the polyphagous predatory mite Neoseiulus cucumeris (Acari: Phytoseiidae)

“…Perhaps this is the reason why expression of NcHsp40 differed so markedly from those of C. vestalis and P. puparum under heat stress. Interestingly, increased expression of Hsp70 in D. melanogaster increased thermal tolerance, and resulted in an increased energy requirement, reduced cell proliferation, decreased growth of larvae, depressed larval locomotion after induction and reduced adult fecundity (Hoekstra and Montooth 2013;Krebs and Feder 1997). In the present study, the expression levels of NcHsp60 and NcHsp40 were significantly down-regulated in mites exposed to 3 h at 35°C and in those treated at 35°C for 1 h and then allowed to recover for 3 h at 25°C (Figs.…”

Energy demand and the context-dependent effects of genetic interactions underlying metabolism

“…Male metabolic rate was unaffected by mito‐nuclear genotype ( and ). Significant effects of the nuclear genome at 25°C () and in our prior work (Hoekstra and Montooth ; Hoekstra et al. ; Greenlee et al.…”

Pleiotropic Effects of a Mitochondrial–Nuclear Incompatibility Depend upon the Accelerating Effect of Temperature inDrosophila

“…We measured larval CO 2 production (microliters per hour), using flow-through respirometry as a measure of metabolic rate as in Hoekstra and Montooth (2013). We measured larval metabolic rate for all genotypes developed at 16° and 25° and measured at 16° and 25°, to generate the thermal reaction norm for larval metabolic rate for each genotype.…”

“…Regulation of energy metabolism may also be important for physiological responses that maintain fitness across heterogeneous environments experienced within a lifetime. Induction of environmental responses can be energetically costly (Hoekstra and Montooth 2013), and low resting metabolic rates may be selectively favored to maximize the energy available for growth, reproduction, and physiological responses to the environment (Artacho and Nespolo 2009; Ketola and Kotiaho 2009). Thus, we expect the fate of mutations affecting energy metabolism to be governed by selection via their pleiotropic effects on traits associated with growth, reproduction, and the maintenance of homeostasis across heterogeneous environments.…”