Zhan*, Jiasui; Newton, Adrian. Race dynamics and invasion of complex races in Rhynchosporium secalis. In: International Meeting on "Population and Evolutionary Biology of Fungal Symbionts", Ascona, Switzerland, 2007. AB-15. Click here to download the presentation on pdf format.

Evolution of race complexity has been the main concern for sustainable disease management. Fitness cost associated with pathogens carrying unnecessary virulence factors is the fundamental assumption for preventing invasion of complex races into plant pathogen populations but this hypothesis has rarely been tested empirically on a temporal and spatial scale which is sufficient to distinguish truly evolutionary signals from random population fluctuation. We analyzed the virulences of ~1000 Rhynchosporium secalis strains collected from different parts of the United Kingdom over the last two decades (1984-2005) and we found clear patterns of boom-bust cycles for the majority of races and virulence factors. We also found a gradual increase in race complexity over time in the pathogen with a significant correlation between sampling time and race complexity of the pathogen (r20 = 0.71, p = 0.0002) and an average gaining of 0.1 virulence factors each year. A similar pattern of increasing race complexity was also found in Blumeria graminis, suggesting the invasion of complex races into pathogen populations may not be an independent event limited to a single pathogen species. The conditions (resistance pyramid and cultivar mixture) favoring complex races were not present in UK and we hypothesize the invasion of complex races is attributable to the combination of host resistance selecting for the corresponding virulence factors to fixation, or dominant frequency and genetic drift maintaining the frequency of the dominant virulence factors after the resistance alleles were withdrawn from the market due to low fitness penalty of carrying unnecessary virulence factors.

Keywords: race complexity, evolution, virulence

*Institution: Scottish Crop Research Institute

Email: jiasui.zhan@scri.ac.uk