Analysis of diversity in populations of plant pathogens: the barley powdery mildew pathogen across Europe

Abstract

Understanding population genetics and evolution within species requires recognition of variation within and between populations and the ability to distinguish between the potential causes of an observed distribution of variation. For this aim several established indices of diversity, and a novel one, were applied to population samples of the barley powdery mildew pathogen, Erysiphe graminis f. sp. hordei. Random spore samples were obtained from the air along transects through regions of interest across large parts of Europe in 1990. Significant geographical differences in diversity of virulence genotypes occurred among regional sub-samples. Diversity was highest in the samples from eastern Germany, Denmark and Austria, whereas the lowest values were found in the samples from Italy, southern France and parts of western Germany. Diversity in the pathogen population was generally related to the degree of diversification of host resistance in time and space, although there was considerable variation in ranking among different measures of diversity. Sensitivity to sample size proved to be the major problem with 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 changing sample size. The Shannon index proved to be considerably affected by sample size, in contrast to the Simpson index that was therefore used as a global measure of diversity. Limits of confidence were estimated for the Simpson index using the bootstrap method of numerical resampling. The two aspects contributing to global diversity, richness and evenness, were considered separately to allow meaningful interpretation of the Simpson index. Random sub-sampling was used to reduce the influence 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 pathotypes 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 compare major forces driving evolution of large-scale populations of the barley mildew pathogen.