Quantifying the contribution of spontaneous mutation to genetic variation in Arabidopsis thaliana

Spontaneous mutation results from random genetic modifications that evade mechanisms promoting genomic stability and reproductive fidelity to upset the maintenance and transmission of identical genetic material. It thus continually generates variation that may reduce organismal fitness by disrupting well-adapted genetic pathways or augment it by providing a selective advantage. Because selection favors some of these unique forms while eliminating others, it is challenging to quantify the rates at which new alleles are produced and the ranges of their phenotypic effects. However, the mutation accumulation approach, whereby mutations are allowed to accrue in distinct lines under conditions of relaxed selection, provides a direct method of estimating these properties.; This dissertation describes two experiments conducted upon independently-derived sets of Arabidopsis thaliana mutation accumulation lines. First, I present the results of an assay of lines derived from the 'Columbia' accession. After 24 generations of mutation accumulation, I detected divergence among the lines for six of the seven life-history and morphological traits I measured. I found a mutational bias toward earlier flowering but no statistically significant trend toward reduced reproductive biomass of the plants. This is consistent with previous studies conducted upon these same lines that suggest mutations in aggregate have only a negligible effect on mean fitness.; I also describe a similar assay conducted upon another set of mutation accumulation lines derived from an accession designated 'A6.' Recent studies of Drosophila melanogaster have suggested substantial variation in the mutational process among different populations, and as intraspecific heterogeneity in the mutation rate influences their evolutionary trajectories, the focus of this experiment was to investigate whether similar variation exists among conspecific plants. Conservative estimates of the mutational variance, or the rate at which new variation was generated, are highly significant for two traits, namely days to bolting and leaf number at flowering, and marginally significant for days to flowering and reproductive biomass. Conclusions regarding the directionality and magnitude of the average effects of mutation are ambiguous due to the effects of unanticipated variation in the potting medium used in this assay.