Doctor of Philosophy (PhD)
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In nature, organisms have evolved to survive in stressful environments. This has driven organisms to adopt a wide range of unique adaptations. Investigating the mechanistic basis of these adaptations is an important tool for discovery that has led to major advances in science and medicine.
We study how organisms survive life without food, or starvational stress. Environmental stressors have shaped the quantity and quality of food sources across the globe. This has led to vast differences in the ability of some organisms to tolerate starvation over others. Many researchers have used Drosophila melanogaster as a model to study global patterns of variation in starvation resistance. We complement this approach by experimentally evolving populations of D. melanogaster to become starvation resistant in the laboratory.
Starvation-selected D. melanogaster are ~3-4 times more resistant to starvation than their unselected controls. They do this primarily through an increase in stored lipids. Excess fats provide an abundant source of energy that can be metabolized to fuel all aspects of life when nutrients are scarce. However, the enhanced ability to survive starvation and store high levels of body fat comes at a cost to many other aspects of fitness. These evolutionary tradeoffs include a developmental delay, decreases in fecundity, metabolic rate, activity levels and flight performance and disrupted sleeping patterns. Because some of these phenotypes are characteristic of metabolic disorder in mammals, we use starvation-selected D. melanogaster as a model to study the physiological and genetic basis of obesity.
Obesity is one of the leading risk factors for heart disease. Here, we tested the hypothesis that heart function is an evolutionary tradeoff with starvation resistance. We found that the hearts of starvation-selected D. melanogaster were dilated and poorly contractile- a condition known as dilated cardiomyopathy. We observed that heart dysfunction was highly correlated with increased fat tissue located between the heart and dorsal cuticle. The excess tissue physically altered the gross anatomical placement of the heart. Further experiments led us to suspect that this physical interference was the primary source of dysfunction. During these analyses we also discovered that the starvation-selected flies avoided lipotoxicity in the heart. Lipotoxicity is a common mechanism of obesity-associated heart dysfunction where excess lipids are ectopically stored within the myocardium. Instead, we found that the fat body of the starvation-selected flies contained very large lipid droplets whose estimated increase in volume could account for the ~2-fold whole body increase in total lipids. We hypothesize that the increased lipid droplet volume in the fat body of the selected populations is an adaptive response to increase the amount of stored lipids for starvation resistance, while limiting the harmful effects of lipotoxicity in peripheral tissues.
We were next interested in exploring the genetic basis of adaptation to starvation selection. To do this, we resequenced genomic DNA from the starvation-selected populations and their fed controls after 83 generations of starvation-selection. In total we found 1,046,373 polymorphic sites, with many loci diverging between selection treatments. We found evidence for genetic heterogeneity between the replicates of the selected populations. The relative genetic differences between the starvation-selected replicates were physiologically relevant, as they correlated with differences in starvation resistance. We surveyed the genome for signatures of selection and found many large troughs in heterozygosity that approached fixation within the selected populations. These regions were variable across selected replicates, consistent with hard selective sweeps. In addition to these extreme regions of reduced heterozygosity, we found many locations across the genome that differed in allele frequency between selective treatments. To identify which regions were divergent due selection, we generated a novel algorithm to filter the effects of genetic drift from the dataset. After applying our drift filter, we mapped SNPs within each population to sets of genes. We focused on the most conserved mechanisms across replicates and found 1,453 genes, which were associated with a SNP under selection in all three replicate populations. These genes were enriched for many biological processes, including many associated with the evolved physiologies of the starvation-selected populations, including regulation of metabolism. Many of the candidate genes were found within the insulin and adipokinetic hormone-signaling pathways. 542 of the candidate genes are previously uncharacterized in D. melanogaster. These genes can be functionally validated for their potential role in starvation resistance and lipid metabolism in future studies.
Finally, we performed experiments to investigate the mechanistic basis of the ~24 hour delay in larval development in the starvation-selection populations. This delay provides starvation-selected larva more time to feed and accumulate lipids that are then used for adult starvation resistance. In D. melanogaster a large pulse of the steroid hormone 20-hydroxyecdysone (20E) occurs near the end of larval development. This late-larval 20E pulse initiates a transcriptional cascade that commits the animal to pupariation. Our preliminary data suggest that the developmental delay in the starvation-selected populations is the result of a delay in the timing of the 20E pulse. If this hypothesis is correct, 20E-induced gene expression should be similar in the selected populations and their unselected controls at the onset of pupariation. To test this we resequenced the transcriptome of the larval fat body in the selected populations and their unselected controls during and after the onset of pupariation. We found that a large fraction of differentially expressed genes had similar patterns of expression between samples of the selection treatments. These data suggest that 20E-signaling functions similarly in both groups, just at a different time point in response to selection. However, there was a smaller, yet significant number of genes which were differentially expressed in response to selection. Furthermore we found that expression profiles differed among replicate populations of the starvation-selected line. Taken together, these results suggest that selection may have altered the timing of the late-larval 20E pulse, as well as the tissue-specific response to the 20E signal. These findings provide further insight into the mechanistic basis of the ~24 hour delay in larval development, which will be explored in future studies.
Drosophila melanogaster; Experimental Evolution; Genomics; Heart Disease; Obesity; Starvation Resistance
Biology | Cell Biology | Ecology and Evolutionary Biology | Evolution
Hardy, Christopher Michael, "Physiology and Genetics of Starvation-Selected Drosophila melanogaster" (2016). UNLV Theses, Dissertations, Professional Papers, and Capstones. 2679.