| Hypotheses for the origin of agricultural crop pathogen species and their population structure have focused almost solely on the impact of current agricultural practices. Ironically, although much effort has been expended in understanding the evolution of crop species and the genetic consequences of domestication in crops, similar historical contingencies have been largely ignored in understanding the evolution of crop pathogens. I investigated the effects of the domestication and cultivation of maize on the origin and evolution of Ustilago maydis , a fungal pathogen of maize and the wild ancestors of maize, the teosintes. First, to determine whether agriculture played a role in the origin of U. maydis, I used phylogenetic methods to date the divergences of U. maydis from its allied taxa. The calculated dates for these divergences were millions of years prior to the domestication of maize, which occurred approximately 10,000 years ago, indicating U. maydis did not originate as a result of agriculture. Second, in order to evaluate the effects of domestication and agriculture of maize on the population structure on U. maydis, I developed simple sequence repeat markers and used these to genotype collections from maize and the teosintes in Mexico, and from maize in South America and the United States. Population genetic analyses of these data resulted in the identification of five genetically distinct populations: one in the United States, two in Mexico, and two in South America. One of the populations in Mexico was comprised solely of collections from one teosinte population, while the other population in Mexico was comprised of collections from maize and the teosintes, suggesting the latter population is a descendent of teosinte-infecting populations that were carried through the domestication event. The patterns of genetic diversity, linkage disequilibrium and genetic bottlenecks in these populations suggest that, like maize, U. maydis originated in Mexico, was later founded in South America, and was most recently founded in the United States. Together, my data indicate that U. maydis has coevolved with maize through the domestication and cultivation of maize. |
