Date of Award
Doctor of Philosophy
Ecology, Evolution, and Organismal Biology
Anne M. Bronikowski
Jo Anne Powell-Coffman
How an individual responds to environmental stress (temperature changes, toxins) affects how that individual behaves, its ability to reproduce, and its lifespan. We have limited understanding of how stress response is integrated across single genes, proteins, and metabolites; to whole genomes and transcriptomes; and ultimately its effect on whole organism lifetime fitness. Expanding our understanding of the stress response across generations, and placing it in an evolutionary context, is an even more daunting challenge. Yet without an in-depth understanding of the plasticity and evolution of stress responses, we cannot predict how environmental changes will affect species' ability to respond and survive in our dramatically changing world.
The complex molecular network that underlies physiological stress response is comprised of nodes (proteins, metabolites, mRNAs, etc.) whose connections span cells, tissues, and organs. Variable nodes are points in the network upon which natural selection may act. The aim of my dissertation research was to identify variable nodes that will reveal how the molecular stress network may evolve among populations in different habitats, and how the evolution of these stress networks might impact life-history evolution. To address this, I utilize closely related natural populations of garter snakes (Thamnophis elegans) that occur in discrete habitats that have enhanced their divergence along the pace-of-life continuum; the slow-living phenotype has slower growth, smaller reproductive effort per bout, and extended median lifespan relative to the fast-living phenotype. We take a multifaceted molecular approach -cellular physiology, transcriptomics, and genomics - to test whether lab-born juveniles of these divergent phenotypes vary concomitantly at candidate nodes of the stress response network under unstressed and induced-stress conditions. As molecular resources for this species was lacking, I used next-generation sequencing techniques to generate a large-scale multi-organ transcriptome that provided genetic resources necessary for the rest of this research.
I found, in response to heat stress, some measures increased in both life-history phenotypes: plasma corticosterone; gene expression of heat shock proteins (HSPs); expression of environmental sensing pathways; and gene expression of mitochondrial rRNAs; and State III mitochondrial respiration. These results supported predicted relationships among these traits. As well, the phenotypes diverged at multiple nodes in both unstressed conditions and in their response to stress. Under unstressed conditions the slow-living phenotype had higher expression of the mitochondrial genome, higher State IV mitochondrial respiration, higher circulating levels of reactive oxygen species, lower liver gene expression of a key antioxidant, and higher erythrocyte DNA damage relative to the fast-living phenotype. In response to heat stress the fast-lived phenotype increased its expression of the mitochondrial genome, increased its circulating levels of reactive oxygen species and its DNA damage relative to the slow-living phenotype. Additionally, mitochondrial haplotypes - defined by nonsynonymous changes - were unique to each phenotype suggesting diversifying selection between the phenotypes. These results support the hypothesis that these evolutionarily divergent life-history phenotypes have also diverged in their molecular stress response networks. In addition I identified specific nodes involved in oxidative stress and mitochondrial function at which selection appears to be acting in these divergent life-history phenotypes. More broadly, these results lend further support to the prediction of tightly integrated molecular interactions between stress networks and life-history traits. Finally, this research furthers our understanding of how changing environmental stresses may drive the evolution of molecular networks.
Tonia Sue Schwartz
Schwartz, Tonia Sue, "The evolution of stress response and complex life history traits in natural populations of garter snakes" (2012). Graduate Theses and Dissertations. 12916.