Phenotypic and Genotypic Characterization of Interactions Between Isolates of Peronospora parasitica and Accessions of Arabidopsis thaliana. Eric B. Holub. Department of Plant Pathology and Weed Science, Horticulture Research International-East Mailing, West Mailing, Kent, ME19 6BJ. Jim L. Beynon(2), and Ian R. Crute(1). (1) Department of Plant Pathology and Weed Science, Horticulture Research International-East Mailing, West Mailing, Kent, ME19 6BJ, (2) Department of Biological Sciences, University of London-Wye College, Wye, Kent, TN25 5AH, U.K;. MPMI 7:223-239. Accepted 10 December 1993. Copyright 1994 The American Phytopathological Society.

The parasitic symbiosis between the wild crucifer Arabidopsis thaliana and the biotrophic Oomycete Peronospora parasitica (downy mildew) provides an excellent opportunity to explore (he evolution mid molecular basis of plant defense. By describing the phenolypic and genotypic variation observed in this symbiosis, a foundation has been laid for investigating the process of genotype-specific recognition. Interaction phenotype?, incorporating aspects of both host response and parasite development, are described in this paper for combinations of ll host accessions and seven parasite isolates. The timing and degree of asexual reproduction by the parasite varied among the combinations and provided an indirect assessment of colonization. Necrosis of host cells ranged from minute flecks visible macroscopically 7 days after inoculation (dai) to more extensive pits clearly visible 3 dai and often expanding until the entire cotyledon was necrotic 7 dai. Twelve host loci associated with different interaction phenotypes were postulated in part by studying segregation among F2 progeny from crosses of a half-diallel among nine A. thaliana accessions; advanced generations including rccom-binant inbred lines were used to confirm the identity of loci. A single cross between two host accessions can be used to characterize and map several loci associated with isolate-specific recognition of P. parasitica and distinct interaction phenotypes. Partial dominance and genetic epistasis are postulated as being common features of this parasitic symbiosis.