Quantitative genetics and metabolomics of aerobic metabolism

The quantitative genetic, biochemical, and physiological bases of variation in maximal aerobic metabolic rate (MMR) are important for understanding exercise physiology and the evolution of aerobic performance, but they still are not well understood. To this end, I studied three aspects of MMR. First, I estimated the genetic variances and covariances of MMR and basal metabolic rate (BMR). Second, I identified how membrane fatty acid (FA) composition changed in response to selection for increased MMR. Third, I measured metabolite expressions in organs primarily responsible for MMR and BMR. Such an approach allowed me to better understand the mechanistic connection (e.g., shared organs) between MMR and BMR and the evolution of aerobic energy metabolism.In my first chapter, I determined the genetic variances and covariances of MMR and BMR. The genetic variances and covariances of metabolic traits must be known to predict how they respond to selection and how covariances among them might affect their evolutionary trajectories. To this end, I used the animal model to estimate the genetic variances and covariances of MMR and BMR in a genetically heterogeneous stock of laboratory mice. Narrow-sense heritability (h2) was approximately 0.38+0.08 for body mass, 0.24+0.07 for whole-animal MMR, 0.26+0.08 for whole-animal BMR, 0.16+0.06 for mass-independent MMR, and 0.19+0.07 for mass-independent BMR. All h2 estimates were significantly different from zero. The phenotypic correlation of whole animal MMR and BMR was 0.56+0.02, and the corresponding genetic correlation was 0.79+0.12. The phenotypic correlation of mass-independent MMR and BMR was 0.13+0.03, and the corresponding genetic correlation was 0.72+0.03. The genetic correlations of metabolic rates were significantly different from zero, but not significantly different from one. The genetic correlation is not so high as to preclude independent evolution of MMR and BMR.For second chapter of my dissertation, I tested how selection for increased MMR changes membrane fatty acid (FA) composition in a genetically heterogeneous stock of laboratory mice. The membrane pacemaker hypothesis predicts that the unsaturation index (UI) of membrane FAs is positively linked to the high BMR in endotherms. To test this hypothesis, I examined the membrane FA composition of liver and gastrocnemius muscle in mice after 7 generations of selection for increased MMR (high-MMR). Although mass-independent BMR was 3.5% higher in high-MMR mice, the liver UI was not higher than in control mice. Concentration of 16:0 and 18:0 FAs were lower in the liver of high-MMR mice, whereas a greater concentration of 18:1 n-7 FA was found in the gastrocnmeius muscle of high-MMR mice. Moreover, individual variation in UI had no influence on either BMR or MMR. However, concentration of 16:1 n-7, 18:1 n-9, and 22:5 n-3 FAs in the gastrocnemius were significant predictors of BMR, but none of the liver FAs were significant predictors of BMR. In both muscle and liver 20:4 n-6 FA was a significant predictor of MMR and in liver 20:3 n-6 FA was another significant predictor of MMR. The findings did not support the prediction that UI is positively correlated with BMR, but more broadly MMR and BMR were linked to membrane FA composition changes in the skeletal muscle and liver.For third chapter of my dissertation, I examined how 7 generations of selection for high MMR changes metabolite expression of the organs primarily responsible for resting metabolic rate (i.e., the liver) and of organs primarily responsible for MMR (i.e., skeletal muscle as represented by the gastrocnemius and plantaris muscles). One of the pivotal challenges in evolutionary physiology is elucidating the functional connection between MMR and BMR because the main contributors to MMR are skeletal muscles wheras the main contributors to BMR are visceral organs. To this end, I used an untargeted global metabolomic analysis of the gastrocnemius and plantaris muscles and of the liver during resting metabolism to reveal adaptive metabolic responses to selection for increased MMR in a genetically heterogeneous stock of laboratory mice. In the plantaris muscle, metabolic profiles of high-MMR and control mice did not differ. In the liver, amino acid and tricarboxylic acid cycle (TCA cycle) metabolite amounts were lower in high-MMR mice than in controls. For the gastrocnemius muscle, amino acid and TCA cycle metabolite amounts were higher in high-MMR mice than in controls, indicating elevated amino acid and energy metabolism. Moreover, amounts of free fatty acids and triacylglycerol fatty acids in gastrocnemius muscle were lower in high-MMR mice than in controls, indicating elevated energy metabolism. Selection for increased MMR resulted in elevated amino acid and energy metabolism in the gastrocnemius muscle of high-MMR mice. These mice also exhibited a 3.5% correlated increase in mass-independent BMR. Because the untargeted metabolomic profiles were at resting metabolic rate and not at MMR, the elevated amino acid and energy metabolism in the gastrocnemius muscle of high-MMR mice may account for their correlated increase in mass-independent BMR. This dissertation provided quantitative genetic parameter estimates on MMR and BMR, tested the membrane pacemaker hypothesis of metabolism with a manipulative experiment using whole animals, and examined the biochemical variation between resting metabolism and increased MMR. Overall, the estimated genetic correlation between MMR and BMR is consistent with the assumption of the aerobic capacity model. In addition, the metabolic and fatty acid profiles suggest that increased MMR and BMR in high-MMR mice might be mechanistically linked via elevated amino acid and energy metabolism in the musculature. Lastly, my results add a genetic component to the already demonstrated roles of diet and exercise in determining membrane and intra-muscle fatty acid compositio

Crassulacean Acid Metabolism Abiotic Stress-Responsive Transcription Factors: a Potential Genetic Engineering Approach for Improving Crop Tolerance to Abiotic Stress

This perspective paper explores the utilization of abiotic stress-responsive transcription factors (TFs) from crassulacean acid metabolism (CAM) plants to improve abiotic stress tolerance in crop plants. CAM is a specialized type of photosynthetic adaptation that enhances water-use efficiency (WUE) by shifting CO2 uptake to all or part of the nighttime when evaporative water losses are minimal. Recent studies have shown that TF-based genetic engineering could be a useful approach for improving plant abiotic stress tolerance because of the role of TFs as master regulators of clusters of stress-responsive genes. Here, we explore the use of abiotic stress-responsive TFs from CAM plants to improve abiotic stress tolerance and WUE in crops by controlling the expression of gene cohorts that mediate drought-responsive adaptations. Recent research has revealed several TF families including AP2/ERF, MYB, WRKY, NAC, NF-Y, and bZIP that might regulate water-deficit stress responses and CAM in the inducible CAM plant Mesembryanthemum crystallinum under water-deficit stress-induced CAM and in the obligate CAM plant Kalanchoe fedtschenkoi. Overexpression of genes from these families in Arabidopsis thaliana can improve abiotic stress tolerance in A. thaliana in some instances. Therefore, we propose that TF-based genetic engineering with a small number of CAM abiotic stress-responsive TFs will be a promising strategy for improving abiotic stress tolerance and WUE in crop plants in a projected hotter and drier landscape in the 21st-century and beyond

Sporobolus stapfianus: Insights into desiccation tolerance in the resurrection grasses from linking transcriptomics to metabolomics

Predominant clusters of SDATs that share distinct patterns of abundance during dehydration: A. Predominant patterns of abundance for transcripts in clusters that exhibited increased abundance during dehydration. B. Predominant patterns of abundance for transcripts in clusters that exhibited a decreased abundance during dehydration. (PDF 226 kb

Mitochondrial haplotypes are not associated with mice selectively bred for high voluntary wheel running

Mitochondrial haplotypes have been associated with human and rodent phenotypes, including nonshiveringthermogenesis capacity, learning capability, and disease risk. Although the mammalian mitochondrial D-loop ishighly polymorphic, D-loops in laboratory mice are identical, and variation occurs elsewhere mainly betweennucleotides 9820 and 9830. Part of this region codes for thetRNAArggene and is associated with mitochondrialdensities and number of mtDNA copies. We hypothesized that the capacity for high levels of voluntary wheel-running behavior would be associated with mitochondrial haplotype. Here, we analyzed the mtDNA poly-morphic region in mice from each of four replicate lines selectively bred for 54 generations for high voluntarywheel running (HR) and from four control lines (Control) randomly bred for 54 generations. Sequencing thepolymorphic region revealed a variable number of adenine repeats. Single nucleotide polymorphisms (SNPs)varied from 2 to 3 adenine insertions, resulting in three haplotypes. We found significant genetic differentiationsbetween the HR and Control groups (Fst= 0.779,p?0.0001), as well as among the replicate lines of micewithin groups (Fsc= 0.757,p?0.0001). Haplotypes, however, were not strongly associated with voluntarywheel running (revolutions run per day), nor with either body mass or litter size. This system provides a usefulexperimental model to dissect the physiological processes linking mitochondrial, genomic SNPs, epigenetics, ornuclear-mitochondrial cross-talk to exercise activity

Example of annotated R code of model selection process

File contains an annotated example of the procedure for model selection we used to develop our statistical models. The example uses the data from the experiment to quantify growth rate. For a text summary of the model selection procedure and a description of the final, “best fit” model for each response variable please see Electronic Appendix B of "Speeding up growth: selection for mass-independent maximal metabolic rate alters growth rates" by Downs, Cynthia J., Brown, Jessi L., Wone, Bernard W. M., Donovan, Eward R., Hayes, Jack P

Data from: Speeding up growth: selection for mass-independent maximal metabolic rate alters growth rates

Investigations into relationships between life-history traits, such as growth rate and energy metabolism, typically focus on basal metabolic rate (BMR). In contrast, investigators rarely examine maximal metabolic rate (MMR) as a relevant metric of energy metabolism, even though it indicates the maximal capacity to metabolize energy aerobically, and hence it might also be important in trade-offs. We studied the relationship between energy metabolism and growth in mice (Mus musculus domesticus Linnaeus) selected for high mass-independent metabolic rates. Selection for high mass-independent MMR increased maximal growth rate, increased body mass at 20 weeks of age, and generally altered growth patterns in both male and female mice. In contrast, there was little evidence that the correlated response in mass-adjusted BMR altered growth patterns. The relationship between mass-adjusted MMR and growth rate indicates that MMR is an important mediator of life histories. Studies investigating associations between energy metabolism and life histories should consider MMR, as it is potentially as important in understanding life history as basal metabolic rate