The genetics of sunflower domestication
Wills, David M.
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The chapters of this dissertation investigate the early stages of the evolution of cultivated sunflower. Previous archeological and genetic investigations of domesticated sunflower have argued for either: (1) a single origin of domestication in the east central United States, or (2) one origin in the United States and a second origin in Mexico, potentially represented by the primitive Hopi and Havasupai landraces. The first chapter reports an analysis of chloroplast haplotype diversity in wild and domesticated sunflower, the results of which support a single origin of domestication of the extant cultivated lineages, most likely somewhere outside of Mexico. These results, combined with previous analyses based on nuclear markers, confirmed that the Hopi and Havasupai landraces represent the most basal domesticated sunflower lineages available. Having confirmed the Hopi sunflower as a primitive domesticate, the second chapter utilizes a wild × Hopi mapping population to investigate the minimum genetic changes required to transform wild sunflower into a useful crop plant. The genetic architecture of sunflower domestication was found to involve a large number of loci, most of which had small to moderate phenotypic effects. This is a unique genetic architecture, which previously has been undocumented in other crop species. Despite being one of the two most basal domesticated lineages, the Hopi sunflower exhibits several unique traits - primarily achene shape, anthocyanin pigmentation, and exceptionally late flowering time - which appear to be byproducts of post-domestication selection. The third chapter thus utilizes the same wild × Hopi mapping population investigate the genetic architecture of these traits. Unlike typical domestication-related traits, these traits exhibited a relatively simple genetic basis, with two genomic regions being largely responsible for the divergence of the Hopi sunflower from other cultivated sunflower lineages. The simple genetic control of these traits may be the product of constraint imparted by the complex genetic architecture of domestication.