Biology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2749
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Item MOVING TOWARD AN OPTIMUM: THE ADAPTATION GENETICS OF ARABIDOPSIS THALIANA.(2015) Stearns, Frank Warren; Fenster, Charles B; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Adaptation accounts for many of the interesting characteristics of biodiversity. Despite this, the genetic mechanisms underlying the process of adaptation in nature are largely unknown. While general principles are emerging, important questions remain. Although experimental evidence has corroborated theoretical predictions, very few studies have tested macroorganisms in nature, where adaptation is most relevant. My dissertation addresses several important questions in adaptation genetics in the context of fitness landscapes, primarily using the model plant Arabidopsis thaliana. Fitness landscapes are used to visualize the relationship between genetics and fitness (evolutionary success of an individual). Although fitness landscapes have been considered metaphorical, recent work (and this dissertation) suggests they may approximate reality, providing testable predictions. I first assess pleiotropy (when one gene has multiple effects), an important component of fitness landscape models. I examine this concept in historical context and suggest future directions for research. Next I evaluate how well genetic relatedness corresponds to climate adaptation across the native range of A. thaliana and find support for parallel evolution (identical but independent genetic changes), suggesting that fitness landscapes are complex. In my next chapter, using a combination of natural and artificial conditions, I examine how induced mutations impact traits that are fitness indicators as compared to general traits. I find that new mutations tend to reduce fitness, whereas their effect on general traits is bidirectional. This result is more pronounced under stressful field conditions. Finally, I evaluate a mathematical model of adaptation in the field using induced mutations in A. thaliana. I find support for a previous result from laboratory studies - that lineages that are less well adapted to an environment are more likely to benefit from new mutations whereas lineages that are well adapted are more likely to be disrupted by new mutations - and extend that to the wild. Throughout I explore the importance of contingency in evolution, sometimes underscoring how it leads to unpredictable adaptation (chapters one and two), yet also demonstrating that the actions of mutations can be fit to simplifying assumptions (chapters three and four). These studies therefore significantly contribute to the emerging scholarship on adaptation genetics.Item LEUNIG, LEUNIG HOMOLOG, AND SEUSS ARE TRANSCRIPTIONAL CO-REPRESSORS THAT REGULATE FLOWER DEVELOPMENT, MUCILAGE SECRETION, AND PATHOGEN RESISTANCE(2009) Bui, Minh; Higgins, William J; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Transcriptional repression is an important regulatory mechanism for development. My thesis focuses on dissecting the function of Groucho (Gro)/Transducin-Like Enhancer of split (TLE) family of transcriptional repressors in plant development. My work characterizes two Arabidopsis thaliana genes, LEUNIG (LUG), first discovered to repress transcription of the floral homeotic gene AGAMOUS (AG), and LEUNIG_HOMOLOG (LUH), a gene with the highest sequence similarity to LUG. To investigate the functional redundancy between LUG and LUH, I constructed and analyzed lug; luh double mutants, and concluded that both LUG and LUH repress AG expression in the flower, with LUG playing a more prominent role than LUH. The double mutant also revealed a previously unknown function of LUG and LUH in embryogenesis because lug-3; luh-1 double mutants are embryo lethal, while the single mutants develop normal embryos. During the course of this study, I developed a new genotyping method called Simple Allele-discriminating PCR (SAP), which is cost-effective, quick, and easy to perform. This method has greatly facilitated my research as well as others in the lab. A second part of my thesis addresses the role of LUG and LUH in other developmental processes besides flower development. My data indicate that these two genes, like their counter parts in fungi and animals, act as "global co-repressors" in various developmental and physiological processes. My thesis work revealed that both co-repressors, together with its interacting protein SEUSS (SEU), repress the Salicylic Acid (SA) pathogen defense pathway. Although lug-3, luh-1, and seu-1> mutants induced PR1 expression at higher levels than wild-type, only lug-3 and seu-1 mutants were pathogen resistant. Furthermore, LUH functions as a positive regulator in seed mucilage secretion, a process important for proper seed germination, hydration, and dispersal. I propose a possible connection between the defect in mucilage secretion and pathogen defense in luh-1 mutant plants and seeds, which places the foundation for further investigation and may uncover mucilage secretion as a major defense mechanism. My thesis has provided important insights into how transcriptional co-repressors regulate diverse developmental and physiological pathways in higher plants.