Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The anther smut fungus, Ustilago violacea, grows either saprophytically (yeastlike cells) or as a parasite of the Caryophyllaceae (mycelial cells). Factors affecting two types of development controlled by the mating type gene, sporulation and myceliation, were examined.;Sporulation. Freshly isolated diploid cells, heterozygous for the mating type alleles, a(,1) and a(,2), develop into sexual phase precursor cells (SPP) on complete medium at temperatures below 20(DEGREES)C. The a(,1)/a(,2) diploid produces at least four different cell types, termed opaques, spontaneously at high frequencies (> 3 x 10('-3)). These types are: (1) neutral strains (op-N) which do not mate and still initiate sporulation but under altered conditions. (2) Strains which mate as a(,1) types (op-a(,1)) and (3) cells which mate as a(,2) types (op-a(,2)). (4) Constitutively self mating (op-C) strains. Types 2 to 4 have lost the ability to develop into SPP cells. These four strains were shown to remain diploid and to be altered only at the mating type locus or chromosome. Genetic analyses of tetraploid (op-a(,1) x op-a(,2)) and triploid (op-a(,1) x haploid a(,2)) crosses indicated that the mating opaques probably arise after mitotic crossing-over yielding a(,1)/a(,1) (op-a(,1)) and a(,2)/a(,2) (op-a(,2)) types. Experiments showing similar increases in opaque frequency and mitotic recombination near marker alleles following UV irradiation support this conclusion.;Myceliation. Aqueous extracts from plants that host Ustilago species were found to induce the mycelial stage of U. violacea and some other smut species in cells that were potentially pathogenic i.e. expressed both mating type alleles. Aqueous extracts from most non-host species were inactive. However, efficient extraction of all tested plant species with organic solvents indicated that the stimulatory compound was universal in distribution. (alpha)-Tocopherol was identified as the major active compound in plant extracts. All tested host species contained amounts of tocopherol above the threshold level (5 x 10('-8) M) while non-hosts had either (1) above threshold levels of tocopherol; (2) above threshold levels of tocopherol plus toxins or inhibitors; or (3) sub-threshold levels of tocopherol. These results suggest that the availability of tocopherol may form one basis for the restriction of the host range of Ustilago species.



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