Polyploidy, inbreeding depression, and the evolution of mating systems in flowering plants

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Mating systems control the movement of genes through time and space, making the evolution of mating systems a central question in evolutionary biology. Some of the most persistent questions surrounding the evolution of mating systems in plants are those that address the evolution of self-fertilization. A selfing individual passes on three copies of its genome to offspring for every two copies passed on by an outcrossing individual. This ?cost of outcrossing? provides a 50% fitness advantage to selfing variants, and, unless counteracted by some other selective force, results in increased population selfing rates. Several non-mutually exclusive phenomena can select against the evolution of selfing, including pollen discounting, temporal and spatial variation in environmental conditions, gender specialization, and inbreeding depression. An interesting pattern noted by early plant biologists is the association between genome duplication (polyploidy) and self-fertilization, with polyploids exhibiting higher levels of self-fertilization than diploids. Although several phenomena might influence the evolution of this pattern, reduced inbreeding depression among polyploids relative to diploids could play a pivotal role. In this dissertation I (1) evaluate the validity and strength of the association between polyploidy and self-fertilization in flowering plants, (2) develop simulation-based models to explore the relationship between polyploidy and inbreeding depression, and (3) conduct experiments to compare levels of inbreeding depression in four species of annual plants that vary in both mating system and ploidy. I demonstrate that, on average, polyploid angiosperms exhibit higher levels of self-fertilization than their diploid relatives. I then show that polyploid and diploid populations differ in their response to selection and levels of inbreeding depression. Although younger polyploids should exhibit less inbreeding depression than diploids, older polyploids might exhibit the opposite pattern. Finally, I demonstrate that both mating system and ploidy influence levels of inbreeding depression in the genus Clarkia (Onagraceae). Selfing taxa exhibit less inbreeding depression than outcrossing taxa, and polyploid taxa exhibit less inbreeding depression than diploid taxa.

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