MUTANTS OF DOWNY MILDEW RESISTANCE IN LACTUCA-SATIVA (LETTUCE)

As part of our investigation of disease resistance in lettuce, we gene rated mutants that have lost resistance to Bremia lactucae, the casual fungus of downy mildew. Using a rapid and reliable screen, we identif ied 16 distinct mutants of Latuca sativa that have lost activity of on e of four different downy mildew resistance genes (Dm). In all mutants , only a single Dm specificity was affected. Genetic analysis indicate d that the lesions segregated as single, recessive mutations at the Dm loci. Dm3 was inactivated in nine of the mutants. One of five Dm1 mut ants was selected from a population of untreated seeds and therefore c arried a spontaneous mutation. All other Dm1, Dm3, Dm5/8 and Dm7 mutan ts were derived from gamma- or fast neutron-irradiated seed. In two se parate Dm1 mutants and in each of the eight Dm3 mutants analyzed, at l east one closely linked molecular marker was absent. Also, high molecu lar weight genomic DNA fragments that hybridized to a tightly linked m olecular marker in wild type were either missing entirely or were trun cated in two of the Dm3 mutants, providing additional evidence that de letions had occurred in these mutants. Absence of mutations at loci ep istatic to the Dm genes suggested that such loci were either members o f multigene families, were critical for plant survival, or encoded com ponents of duplicated pathways for resistance; alternatively, the gene s determining downy mildew resistance might be limited to the Dm loci.