Patterns of domestication in the Compositae and beyond

Domestication is the process of evolutionary change that results in the phenotypic and genetic differences between a crop species and its ‘wild’ progenitor. Domesticated species vary widely in their phylogenetic diversity, diversity of uses, and degree of domestication. Here, we attempt to better understand the traits and processes that govern this diversity of domesticated species by taking a comparative view of domestication. First, we compare patterns of domestication in the Compositae family (chapter 2) and propose that the prevalence of secondary defense compounds, the lack of carbohydrates that can be digested by the human gut, and the predominantly mechanical or wind-dependent seed dispersal syndrome of the family are key reasons for the apparent paucity of crops in the Compositae family. We then report on the establishment of genomic tools and resources in the form of a library of expressed sequence tags, a set of microsatellite loci, and the full sequence of the chloroplast genome for one particular domesticated species in the Compositae, the oil-seed crop Noug (Guizotia abyssinica) (chapter 3). A combination of genotypic, phenotypic and eco-geographic analyses is then used to test whether high levels of crop-wild gene flow and/or unfavorable phenotypic correlations are the reason why noug appears to be only semi-domesticated (chapter 4). Even though we did not find evidence for either of these hypotheses, our data revealed evidence of local adaptation of noug cultivars to different precipitation regimes, as well as high levels of phenotypic plasticity, which may permit reasonable yields under diverse environmental conditions. We then suggest that domestication may also have been slowed by noug’s outcrossing mating system. The idea that transitions in mating systems and other reproductive barriers between crops and their wild progenitors play a role in domestication is then further explored in a systematic comparison of several crops of major economic importance within and beyond the Compositae family (chapter 5). The majority of such crops appear to indeed be isolated from their progenitors by one or more reproductive barriers, even in the absence of geographical isolation during domestication.Science, Faculty ofBotany, Department ofGraduat