HOST-PATHOGEN INTERACTIONS IN NATURAL POPULATIONS OF LINUM MARGINALE AND MELAMPSORA LINI: I. PATTERNS OF RESISTANCE AND RACIAL VARIATION IN A LARGE HOST POPULATION.

Populations of wild flax, Linum marginale and its associated rust fungus Melampsora lini growing at Kiandra, New South Wales, Australia, were sampled during the 1986-1987 growing season. Thirteen different races of M. lini were detected in a sample of 96 isolates. The distribution of isolates was uneven: race A comprised 73% of the samples; race N, 8%; and race H, 5%; while the remaining races were represented by only one or two samples. The dominance of race A increased over the course of the growing season, comprising 67% of the early season samples and increasing to 78% for those collected late in the season. The overall diversity of the pathogen population decreased late in the growing season, but this trend was not statistically significant. The average virulence of individual isolates of the pathogen population increased during the growing season. This trend was most pronounced among the minor races, where the mean number of differential hosts infected increased from 4.58 for early season samples to 5.12 and 5.08 for mid and late season samples, respectively. In contrast to the virulence pattern in the pathogen, the L. marginale population displayed a more even distribution of resistance. In a sample of 67 plants 10 resistance phenotypes were detected from their pattern of resistance/susceptibility to seven pathogen isolates. No phenotype had a frequency that exceeded 30%. Resistance phenotypes were randomly distributed on both a population level and on a fine scale. © 1991 The Society for the Study of Evolution.