The genetics of pathogenic aggressiveness in three dikaryons of Ustilago hordei

Abstract

The genetic determination of a quantitative component of pathogenicity, aggressiveness, was examined in progeny populations derived from three parent dikaryons of Ustilago hordei. Aggressiveness was assessed as the proportion of smutted plants produced from inoculated seed of a compatible barley cultivar. The parents differed in their origins and cultural histories and this was reflected in the variability of their progenies. A standard race-10 strain appeared to be homozygous for genes affecting aggressiveness, while a dikaryon produced by mating two unrelated sporidia was heterozygous and produced highly variable progeny populations. In these populations, aggressiveness was determined by a polygenic system which involved both additive and dominance gene effects. It was not possible to estimate the number of genes involved, but segregation was apparent in individual tetrads. Furthermore, a difference between opposite mating-type segregants suggested the presence of a factor which affected aggressiveness and was linked to mating type. The dominance effects were ambidirectional suggesting that genotypes giving an intermediate level of aggressiveness are the most fit. The aggressiveness of a pathogen strain is an important factor determining the severity of epidemics on compatible hosts. It is also a major component of fitness and may influence the frequency of virulence factors in pathogen populations and the evolution of new races.