Mating system and population structure of Tilletia controversa and allied taxa

Tilletia (Basidiomycota, Ustilaginomycetes) comprises approximately 140 species of obligate pathogens infecting members of the grass family Poaceae. Members of this genus include economically important pathogens of wheat and other cultivated grasses, but most species occur on wild or weedy hosts. The biology and economic importance of members of this genus are reviewed in chapter one.; Tilletia fusca infects two native annual fescues, Vulpia microstachys and V. octoflora that occur in central Washington. In chapter two, the mating system and genetic differentiation of sympatric T. fusca populations on the two Vulpia hosts from two sites were studied using seven microsatellite markers. All populations showed significant heterozygote deficiency with inbreeding coefficient FIS equals/close to 1, indicating that T. fusca has a high degree of inbreeding. Tilletia fusca populations from V. microstachys showed greater genetic diversity than populations from V. octoflora. Mean number of alleles/locus was 1.71 for T. fusca from V. octoflora and 4.14-4.71 from V. microstachys. Pairwise genetic distances between populations indicated that T. fusca is more differentiated by host than by geographic distance. These data suggest that T. fusca populations have become genetically isolated by host specialization and inbreeding.; In chapter three, the mating system and population structure of the dwarf bunt pathogen of wheat, T. controversa, and T. bromi , a closely related species infecting Bromus tectorum, were investigated using six microsatellite markers. Both T. controversa and T. bromi were inbreeding with FIS over all loci ranged from 0.497 to 0.678, and from 0.983 to 1, respectively. The populations of T. bromi showed higher genetic diversity than that of T. controversa. The gene diversity of T. controversa populations was 0.242 and 0.462 for T. bromi populations. No introgression was detected between the two species. Introgression therefore does not appear to be a source of genetic variation and new virulence in T. controversa and T. bromi populations. Recent mutation, high genetic variability and selection by host are most likely responsible for new pathotypes.