Doctor of Philosophy / Department of Biology / Theodore J. Morgan / Natural populations are highly complex and consist of genetically variable individuals that belong to continuously varying age classes. Genotype and age interact to determine how individuals respond to environmental stress, which ultimately determines the evolutionary trajectories and persistence of populations in variable environments. For small ectothermic species, seasonal and diurnal variation in temperature is an important source of environmental stress that impacts activity patterns and suites of phenotypes directly related to whole organism fitness. I used the genetic and ecological model Drosophila melanogaster to investigate the influence of seasonal and diurnal thermal variability on survival and reproduction in genetically diverse populations. First, I characterized changes in cold tolerance and phenotypic plasticity within a natural population as it responded to seasonal shifts in developmental and short-term acclimation and thermal selection. I found that seasonal variation in cold tolerance was significantly influenced by developmental acclimation that occurred in the field as well as in the lab, where flies that developed under warmer conditions had reduced cold tolerance relative to flies that developed under cooler conditions. Second, I characterized the effect of variation in age on stress response phenotypes in a genetically variable population. I measured genotype- and age-specific responses to multiple environmental stressors, and identified regions of the genome that were associated with age-specific stress tolerance. Genome-wide association mapping revealed that age-specific phenotypes were influenced by distinct sets of polymorphisms and genes, suggesting that the evolution of age-related decline in phenotypes is driven by mutation accumulation within phenotypes, but both mutation accumulation and antagonistic pleiotropy between phenotypes. Next, I characterized the costs and benefits of acclimation for survival and reproduction to understand how physiological and behavioral plasticity interact to determine fitness. I found that phenotypic plasticity and the capacity for acclimation significantly influenced behavioral reproductive success, but the thermal cues that led to adaptive acclimation response in survival also led to decreased reproductive success. However, genotypes with the capacity to acclimate were more likely to survive thermal variation and more likely to reproduce, suggesting that genetic capacity for phenotypic plasticity has important implications for whole organism fitness. Finally, I measured the effect of acclimation on the induction of diapause and ability to survive cold stress in the recently introduced invasive species Drosophila suzukii. D. suzukii is endemic to Asia and was first detected in California in 2008 and in Topeka, KS in 2013. Its recent invasion history thus provides an interesting model to understand the role of plasiticy in the response to a novel and variable environment. I found that diapause was induced through a plastic response to acclimation and short photoperiod, though diapause was more drastically induced by acclimation. Overall, my research provides critical insights into how organisms respond to thermal variation by intergrating quantitative genetics, ecology, evolution, and life history tradeoffs. Collectively, my research demonstrates that the ability of organisms to survive thermal stress is a function of genetic capacity to tolerate stress, genetic capacity for phenotypic plasticity, prior exposure to thermal variation, and the age of the individual.

When investigating the developmental roles of genes on phenotypic expression it may seem reasonable to assume that a mutation would result in consistent phenotypic change. However, increasing evidence has shown this is not often the case, and the “wild-type” genetic background of an individual plays a large role in phenotypic expression of mutations and severity of genetic mediated diseases. Previous work has demonstrated that degree of genetic background effects shows a non-linear relationship with severity of mutational effects. This relationship is characterized by alleles of moderate phenotypic expressivity showing the relatively greatest degree of background dependence and between genotype variability in comparison with alleles of severe and modest phenotypic expressivity. Our previous work has shown this relationship for Drosophila melanogaster wing size through a scalloped (sd) allelic series crossed to naturally derived strains from the Drosophila Genetics Reference Panel (DGRP). I explored these effects with a miniature (m) allelic series where the results from our experiment suggest a vastly different response. m when compared to sd is characterized by a more linear relationship, whereby alleles of moderate phenotypic effect do not show increased background dependence nor increased variability within and between strains. Furthermore, our results suggest a strong correlation across DGRP strains with respect to m mutational severity and that the effect m has on wing shape is not largely due to wing size. Our working hypotheses for why this might be occurring is due to the increased interaction of sd with other aspects of wing development relative to that of m, the differences in when the genes are playing active roles in wing development, or the effects the mutations have on the wing to affect size. To add to our previous results employingutilizing sd, I am beginnings to elucidate the non-linear relationship of genetic background effects with severity of mutational effects at a gene expression level. I am accomplishing this through crossing autilizing a sd allelic series crossed to six naturally derived DGRP strains used in previous experiments involving wing size. Preliminary results agree with previous work on genetic background effects, displaying a non-linear relationship with the severity of mutational effect. I aim to continue to explore this relationship including more genotypes and investigating more genes to better elucidate the mechanistic causes of genetic background effects. / Thesis / Master of Science (MSc) / When investigating the roles of genes on phenotype it may seem intuitive that a mutation affecting gene function would display a consistent change in phenotype. Increasing evidence has asserted that this may not always be the case and genetic background effects may affect the genotype-phenotype relationship affecting experimental design, disease treatment and evolutionary trajectories. Here, we investigate the mechanisms involved in these genetic background effects utilizing Drosophila melanogaster wing tissue. We outline a change from the typically observed non-linear relationship between genotype and phenotype and for the first time quantify shape change effects by the miniature mutation.