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UBC Theses and Dissertations

Genetic studies of the host-parasite relationship between Ustilago hordei and Hordeum vulgare Ebba, Tadessa

Abstract

Genetic studies were carried out on the fungal parasite Ustilago hordei (Pers.) Lagerh. and on its host, Hordeum vulgave L. (cultivated barley). In these studies of the host-parasite relationship, special emphasis was placed on the genetic investigation of the pathogenicity. The thesis is divided into four parts. Part I deals with multial1 elism of genes for virulence (v-genes) in the parasite, and demonstrated that four different levels of virulence (obtained on the barley cultivar Trebi) are controlled by alternative alleles at a single genetic locus in the parasite. This is the first demonstrated example of a series of multiple alleles determining different levels of virulence. Part II concerns the identification and characterization of v-genes in U. hordei and of resistance genes (R-genes) in H. vulgave. Three v-genes (two of them new, one of them previously known) were identified. It was shown that the previously identified gene was expressed either as a dominant or a recessive, dependingoon the conditions under which it was tested, and that the newly-identified genes were both recessive. Cultures possessing the newly-discovered v-genes were used in identifying two new R-genes in the barley host. A study of interactions involving the newly discovered v- and R-genes led to the conclusion that these interactions have their basis in gene-for-gene relationships. Part III deals with the synthesis of a complex biotype of u. hordei possessing v-genes at two genetic loci. Disease reaction obtained with this complex biotype were compared both qualitatively and quantitatively with those obtained with the simpler, parental biotypes. In tests on certain cultivars the complex biotype produced either the same or higher levels of disease reaction. Because the new biotype has ah extended host range it is considered that under certain conditions it would be comparatively more fit than either of the parental biotypes from which it was derived. Part IV of the thesis concerns the effects of nutritional deficiency on the action of v-genes. Dikaryons which were homozygous for arg, ad or met were in all cases non-pathogenic; for those which were homozygous for pdx, pathogenicity was unaffected. For dikaryons which were heterozygous for one or more nutritional deficiences, pathogenicity was either unimpaired or reduced, depending on the combination (deficiency: virulence gene: host cultivar) which was tested. It was concluded that the specificity of pathogen biotypes was not determined by the availability or non-availability of specific nutritional factors. However, the effects were not entirely non-specific, since changes in levels of virulence were shown only in certain tests.

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