Map Positions of Three Loci in Arabidopsis thaliana Associated with Isolate-Specific Recognition of Peronospora parasitica (Downy Mildew). Mahmut Tor. Department of Biological Sciences, Wye College, University of London, Wye, Ashford, Kent, TN25 5AH, U.K. Eric B. Holub(2), Edemar Brose(1), Rob Musker(1), Nick Gunn(2), Canan Can(1), Ian R. Crute(2), and Jim L. Beynon(1). (1) Department of Biological Sciences, Wye College, University of London, Wye, Ashford, Kent, TN25 5AH, U.K.; (2) department of Plant Pathology and Weed Science, Horticulture Research International (East Mailing), West Mailing, Kent, ME19 6BJ, U.K. MPMI 7:214-222. Accepted 10 December 1993. Copyright 1994 The American Phytopathological Society.

Our research is aimed al understanding the molecular basis for gene-for-gene interactions between plant parasites and their hosts. A.s a prelude to cloning, the positions in the Arabidopsis thaliana genome were investigated for four of the 10 loci (RPPI, RPP2, RPP4, and RPP7) that have been identified as associated with the genotype-specific recognition of the biotrophic Oomycete Peronospora parasitica (downy mildew). A single cross between accessions Col-5 and Nd-1 was primarily used to map their chromosomal locations. RPPI from Nd-1 was characterized by the absence of asexual sporulation and the occurrence of necrotic pits, visible macroscopically on cotyledons 3 days after inoculation with isolates Emoy2 or Hiksl; this locus was mapped to chromosome 3 in the interval between gll and m 249 (6.6 cM above m249 and 3.9 cM below a RAPD marker OPC12₁₂₅₀). RPP2 and RPP7 from accession Col-5(g//) were characterized by the absence of asexual sporulation and the occurrence of necrotic flecks visible 7 days after inoculation with isolates Cala2 and Hiksl, respectively. RPP2 was located between ag and B9 on chromosome 4. RPP7 maps within 13 cM of m422, but linkage with other markers on chromosome 5 was not confirmed. RPP4 from accession Col-5 was characterized by the occurrence of necrotic flecks and delayed, light sporulation 7 days after inoculation with isolates Emoy2 and Emwal; this locus also maps to chromosome 4, 14.8 cM above RPP2.