Pulsed-field gel electrophoresis (PFGE) of the strains representing the 14 races of U. hordei, the causal agent of covered smut on barley, revealed extensive polymorphism in chromosome length and number. The purpose of this study was to determine by two approaches the exact chromosome number for each U. hordei race, and to ascertain whether some strains are aneuploid, using two approaches. A telomere-specific repeat from Fusarium oxysporum was used as a probe onto Southern blots of restriction digests of individual chromosome bands to determine the number of chromosomes contained in each band. Nineteen to twenty-three chromosomes were identified in the strains representing the 14 races of U. hordei. To ascertain the number of chromosomes identified by the telomere-specific probe, chromosome-specific libraries were constructed and linkage groups were established by using chromosome-specific fragments as probes. The homologous chromosomes identified by these probes were typically monosomic with a maximum of 15 percent of variability, but cases of disomy were also observed in some strains. The second objective of this study was to analyze a filamentous mutant of U. hordei, designated fil1-1, that was isolated following heat-shock treatment. The filamentous phenotype is of interest, because it is believed to be involved in pathogenicity. U. hordei is a dimorphic fungus which has yeast like cells that are non pathogenic, while dikaryons produced upon mating are filamentous and pathogenic. Molecular characterization of the fil1-1 mutant showed that it has suffered a 50 kb deletion in a 940 kb chromosome. Genetical and physical analysis placed the fill mutation near the terminus of one arm of the 940 kb chromosome. The filamentous phenotype reverted to the sporidial wild type in presence of cyclic AMP. Biochemical analyses revealed that the intracellular level of cyclic AMP is three-fold lower in the mutant phenotype than in the wild type. These results indicate that cyclic AMP is an important determinant in fungal morphogenesis. / Graduation date: 1998