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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.
Highlights
Mutations that increase gene expression are predicted to increase energy allocation to transcription, translation and protein function
We use dynamic and sensitive measures of larval metabolic rate during the heat shock response to demonstrate that increased gene copy number increases the energetic cost of this inducible response in D. melanogaster. We hypothesize that this increase in energy expenditure represents replenishing of the adenosine triphosphate (ATP) pool in response to the cost of rapid, increased gene expression and protein accumulation. We discuss these results in the context of known pleiotropic effects of the Hsp70 locus, and we argue that the energetic costs associated with increased gene expression may sets upper bounds on the persistence of genetic material available for adaptive evolution
Hsp70 gene copy number affects metabolic rate during 36°C exposure To quantify the effects of Hsp70 induction on wholeorganism energetics, we used flow-through respirometry to continuously measure larval metabolic rate via CO2 production before, during, and after a 60 min exposure to the non-lethal temperature of 36°C (Figure 1)
Summary
Mutations that increase gene expression are predicted to increase energy allocation to transcription, translation and protein function. We use the well-characterized, inducible heat-shock response to test whether expressing additional copies of the Hsp gene increases energetic demand in Drosophila melanogaster. Fluctuating biotic, abiotic and cellular environments challenge metabolic homeostasis; and yet, in the face of this, many organisms have evolved physiological responses that require energy investment to routinely resist environmental stress [3]. Gene duplication of the Hsp locus is responsible for divergent evolution of this response among Drosophila species, and the subsequent conservation and retention of extra genomic copies of Hsp within Drosophila melanogaster is taken as evidence for adaptive evolution of increased stress tolerance via increased Hsp inducible expression [8,9,10]
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