XVI International Botanical Congess

Agricultural ecosystems produce strong evolutionary forces that lead to rapid evolution in plant pathogenic fungi. We utilized RFLP markers and DNA fingerprints to quantify the effects of selection, immigration, gene flow, drift, and sexual vs. asexual reproduction in field populations of the wheat pathogen Mycosphaerella graminicola. We found that gene flow was significant on a global and a local scale. The nuclear genome had a high level of genetic diversity while the mitochondrial genome had a low level of diversity. This may have resulted from a global selective sweep facilitated by the international commerce in wheat. Selection on individual clones was high during an epidemic. Genetic drift was insignificant over a six-year period. The importance of immigration and sexual shifted over the course of an epidemic cycle..