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Effects of selection on populations of Ustilago hordei Christ, Barbara Jane


The barley - Ustilago hordei system was examined in population genetic studies. Three populations of Ustilago hordei were formed using teliospores of five different dikaryons for each population. Each dikaryon was heterozygous for a known virulence gene governing virulence to the barley cultivar Trebi. Each population was subdivided and simultaneously selected on Trebi and Odessa, a resistant and susceptible cultivar, respectively. Changes in the virulence allele frequency were monitored over the generations of selection. Increases in frequency of the virulent allele phenotype were observed for all populations on both cultivars. One population was observed to decline in percent smutted plants within the first generation of selection whereas the other two populations increased. It was observed that the teliospores of this population did not have the typical germination pattern for Ustilago hordei which was present in the other two populations. No apparent environmental factors such as temperature or exogenous nutrients were responsible for the abnormal germination. Abnormal germination was found to be passed on from the parents to the offspring and the next generation, suggesting that it was inherited. The results of the germination studies directly correlated abnormal germination with reduced percent smutted plants. Tetrad analysis of the individual dikaryons within the populations and the original F₁ dikaryons revealed that the changes in frequency of the virulence allele could have been produced by either the increase of the virulence allele or by increase of the appropriate combination of polygenes which modify virulence. The segregation patterns were similar on both Trebi and Odessa which may indicate that there is a resistance gene common to both cultivars or that the polygenes are nonspecific producing similar effects on both cultivars. This would explain the results from the selection experiments where there were increases in frequency of the virulent phenotype regardless of cultivar.

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