ANALYSIS OF DIVERSITY IN POPULATIONS OF PLANT-PATHOGENS - THE BARLEY POWDERY MILDEW PATHOGEN ACROSS EUROPE

Understanding population genetics and evolution within species require s recognition of variation within and between populations and the abil ity to distinguish between the potential causes of an observed distrib ution of variation. For this aim several established indices of divers ity, and a novel one, were applied to population samples of the barley powdery mildew pathogen, Erysiphe graminis f. sp, hordei. Random spor e samples were obtained from the air along transects through regions o f interest across large parts of Europe in 1990. Significant geographi cal differences in diversity of virulence genotypes occurred among reg ional sub-samples. Diversity was highest in the samples from eastern G ermany, Denmark and Austria, whereas the lowest values were found in t he samples from Italy, southern France and parts of western Germany. D iversity in the pathogen population was generally related to the degre e of diversification of host resistance in time and space, although th ere was considerable variation in ranking among different measures of diversity. Sensitivity to sample size proved to be the major problem w ith the use of several established indices of diversity. Working with very large sample sizes we used multiple random subsamples of various smaller sizes to determine how the mean index values changed with chan ging sample size. The Shannon index proved to be considerably affected by sample size, in contrast to the Simpson index that was therefore u sed as a global measure of diversity. Limits of confidence were estima ted for the Simpson index using the bootstrap method of numerical resa mpling. The two aspects contributing to global diversity, richness and evenness, were considered separately to allow meaningful interpretati on of the Simpson index. Random sub-sampling was used to reduce the in fluence of sample size differences for these measures. Dissimilarity, a novel measure of diversity for use in plant pathology, indicates the average number of major genes for host resistance against which patho types in a sample respond differently. It is thus able to account for the genetic relationship among pathotypes, which is not considered by any other index. The approaches developed in this study help to compar e major forces driving evolution of large-scale populations of the bar ley mildew pathogen.