UBC Theses and Dissertations
A genetic study of host : parasite relationship involving Hordeum vulgare and Ustilago hordei Sidhu, Gurmel Singh
The present study was undertaken to learn whether the kind of host-parasite relationships described by H. H. Flor also exist in Hordeum vulgare:Ustilago hordei system. This system provides additional advantages of genetic analysis over most other known parasitic systems, therefore was more suitable for this type of study. Moreover, a complete genetic analysis of a parasitic system involving a smut fungus has never been attempted before, making the system all the more important. To understand gene-for-gene relationships it becomes of fundamental importance to elucidate the genetics of one member of a parasitic system in order to exploit the genetics of the other. This can be efficiently achieved if the genetic purity of interacting members of a parasitic system is ascertained. In cases where sexual reproduction regularly occurs (such as in heterothallic smuts) a greater magnitude of genetic variability is expected as compared to asexually reproducing fungi or fungi with homothallic mating systems. Since one member of the parasitic system with which the present study deals is a heterothallic smut fungus (Ustilago hordei) the study was initiated by establishing which specific virulence genes were borne by the test cultures. The host, barley (Hordeum vulgare), is naturally inbreeding thus creating no serious problem of variability. Two recessive genes for virulence (Uhv₁ and Uhv₂) were discovered in the two test cultures of U. hordei through backcross method. (The back-cross method is more useful than the F₂ ratios obtainable from a tetrad). The culture having virulence gene Uhv₁ was derived from teliospore E through selfing and was effective on both the varieties Hannchen and Vantage. The second culture, possessing virulence gene Uhv₂, however, originated from teliospore D through selfing and was effective on variety Excelsior. Thus the genotype of the two test cultures possessing virulence genes Uhv₁ and Uhv₂ become Uhv₁Uhv₁UhV₂UhV₂ and UhV₁UhV₁Uhv₂Uhv₂ respectively. On the basis of the two virulence genes (Uhv₁ and Uhv₂) the genetics of resistance and of susceptibility of three barley varieties, Excelsior, Hannchen and Vantage was investigated. It was not practical to test single F₂ barley plants to the two test cultures, therefore the study was extended to F₃ progeny. Segregations obtained from F₃ progenies derived from crosses EXH and EXV revealed that resistance and susceptibility to the two test cultures were inherited independently of one another; the resistance was expressed as a dominant characteristic at both loci. The host alleles discovered through the interactions with culture UhV₁UhV₁Uhv₂Uhv₂ were designated as UhR₁ and Uhr₁; those revealed by interactions with culture Uhv₁Uhv₁UhV₂UhV₂ were designated as UhR₂ and Uhr₂ - The genotype UhR₁UhR₁Uhr₂Uhr₂ was thus assigned to varieties Hannchen and Vantage and Uhr₁Uhr₁UhR₂UhR₂ to variety Excelsior. The interrelationships between the two loci for resistance in the host and the two corresponding loci for virulence in the pathogen thus lead to the conclusion that a gene-for-gene relationship also exists in the Hordeum vulgare:Ustilago hordei system.
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