Relationships between genetic polymorphisms and fungicide resistance within Alternaria alternata

We cloned four kinds of nuclear, moderately repetitive DNA sequences from the Japanese pear pathotype of Alternaria alternata. Three of the four sequences were dispersed on several chromosomes of the fungus, The three dispersed repetitive sequences were used as DNA fingerprinting probes for comparison of the genetic structure between subpopulations of the Japanese pear pathotype that are sensitive and resistant to the fungicide polyoxin. Among 112 isolates collected from 15 sites in Tottori Prefecture, Japan, during July 1988, 4 and 29 isolates were identified as highly and moderately resistant, respectively; the remaining 79 were sensitive. DNA fingerprinting with the three repetitive sequences revealed a great deal of genetic variability in the pathogen population and could not differentiate polyoxin-resistant and -sensitive subpopulations. All 112 isolates were placed in 83 fingerprint types by pooling results of fingerprinting with three probes. A dendrogram of 83 types was constructed based on cluster analysis of similarity coefficients, using the unweighted pair group method with arithmetic average. The dendrogram clearly showed that subpopulations could not be differentiated in correlation with the level of polyoxin sensitivity. Similar variability also was observed in restriction fragment length polymorphisms of nuclear ribosomal RNA genes (rDNA). Polyoxin-resistant and -sensitive subpopulations carried multiple rDNA variants and shared similar variation. These results suggest that the two levels of polyoxin resistance evolved many times, resulting in a random distribution of resistance gene(s) within the genotypes.