PHENOTYPIC AND GENOTYPIC VARIATION IN THE INTERACTION BETWEEN ARABIDOPSIS-THALIANA AND ALBUGO-CANDIDA

Two biotrophic parasites of the wild crucifer Arabidopsis thaliana (L. ) Heynh. are being used to explore the molecular basis and evolution o f genotype-specific recognition and host defense, Genes for recognitio n of Peronospora parasitica (downy mildew) are numerous in A. thaliana and located on four of the five chromosomes as described previously. Genes for recognition of the closely related parasite Albugo candida ( white blister) are described here, In contrast to the former parasite, less than 15% of the host accessions tested were capable of recognizi ng either of two isolates of A. candida. The geographic regions repres ented by these accessions included countries in eastern and western Eu rope, Asia, North America and Africa. Extensive collections from Engla nd and Germany were required to identify examples of incompatible inte ractions, Phenotypic variation among incompatible interactions include d reduced blister formation or complete lack of asexual reproduction b y the parasite, Variation in the extent of the host response was also observed, Three host genes for recognition of A. candida (RAC), each a ssociated with different interaction phenotypes, were identified throu gh inheritance studies with three accessions, One of these genes at lo cus RAC1 appeared to be completely dominant, whereas the other two gen es were only partially dominant or recessive under certain conditions, possibly including the effect of genetic background, One of the latte r two genes defined a second locus RAC2, RAC1 was mapped to the top ar m of chromosome 1 in the 1 cM interval between RFLP markers M254 and M 253.