The Effect of Sexual and Asexual Reproduction on the Isozyme Structure of Populations ofPuccinia graminis

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Abstract
Isozyme markers were used to compare the genetic structure of a sexually and an asexually reproducing population of the wheat stem rust pathogen occurring in the United States in 1972, 1975, and 1976. The structure of the sexual population was determined directly by electrophoretic analysis of individual isolates collected in the appropriate year and stored since in liquid nitrogen. The structure of the asexual population was estimated from a known association of particular isozyme and virulence phenotypes. In all 3 yr, the sexual population was always more diverse than the asexual one whether this was measured by allelic richness. Nei''s gene diversity index, or by the frequency distribution of the number of heterozygous loci per individual. Moreover, the genetic structure of the sexual population approximated to that expected under panmixis (random mating) whereas than of the asexual population showed marked deviations. These deviations were associated with the clonal nature of the asexual population and the apparently complete association of isozyme and virulence phenotype. By contrast, no association was detected between individual isozyme alleles or between individual isozyme alleles and virulence genes in the sexual population.