DNA Restriction Fragment Length Polymorphism and Somatic Variation in the Lettuce Downy Mildew Fungus, Bremia lactucae. S. H. Hulbert. Department of Vegetable Crops, University of California, Davis, CA 95616. R. W. Michelmore. Department of Vegetable Crops, University of California, Davis, CA 95616.. MPMI 1:17-24. Accepted 4 August 1987. Copyright 1987 The American Phytopathological Society.

Twenty-five isolates of Bremia lactucae, representing diverse geographic origins and virulence phenotypes, were assayed for restriction fragment length polymorphisms (RFLPs) at 35 nuclear loci. Most isolates from Europe were clearly diploid. In contrast, many of the isolates from Australia, Japan, Wisconsin, and California had more than two alleles at several loci, indicating they were either polyploids or stable heterokaryons. One of these isolates (California pathotype IV) seemed to be a somatic hybrid between two California diploid isolates of the same mating type (pathotypes II and III). This is the first good evidence for natural somatic fusion in the Oomycetes. The RFLP genotypes of other polyploid or heterokaryotic populations indicated that they had also arisen by somatic hybridization. Variation between similar nondiploid isolates seemed to have been due to the somatic loss of alleles. Stable somatic changes in virulence could not be induced in a heterozygous diploid isolate. On the average, the diploid field isolates were heterozygous for 44% of their RFLP loci. In Europe, isolates exhibited great diversity, which was consistent with the frequent occurrence of the sexual cycle.