GENETICS OF CRONARTIUM-RIBICOLA - IV - POPULATION-STRUCTURE IN WESTERN NORTH-AMERICA

Population genetic parameters were estimated for six populations of Cr onartium ribicola in western North America from British Columbia to th e southern Sierra Nevada, and two outgroups from eastern North America , using isozyme, random amplified polymorphic DNA (RAPD) and restricti on fragment length polymorphism (RFLP) markers on cultured haploid clo nes. Diversity was low, with only 8% polymorphism in the 212 markers i dentified. Each polymorphic locus had only two alleles, except for an RFLP marker in the ribosomal DNA complex with multiple alleles, that r esulted from variable numbers of tandem repeats. Expected heterozygosi ty within populations, estimated from diploid teliospores, was only 0. 025. The three types of markers were highly consistent with each other for these parameters. Yet, populations were highly differentiated; th e proportion of the total variation attributable to differences among populations was 0.205. Multivariate statistical analysis as well as di fferent clustering algorithms based on contrasting evolutionary assump tions (drift, mutation) all showed similar relationships and differenc es among populations. Genetic distances were not associated with geogr aphic distances; western populations within a few kilometres of each o ther were often more distant from each other genetically than they wer e from eastern populations across the continent. The lack of pattern o ver the landscape of this metapopulation is consistent with aspects of the life cycle and epidemiological behavior of the pathogen, in which genetic drift appears to play a major role.