| Summary: | As a pilot study to identify candidate resistance ( R ) genes and examin'e how these are naturally deployed in a population, the geographic variation of R-genes in ninety-four UK Arabidopsis accessions was surveyed, screening for the presence of four known R genes functional in Arabidopsis: RPM], RPS4, RPS5 and RPS2. The functional presence / absence of the four R genes was ~etermined by each individual accessions ability to elicit a hypersensitive response (HR) following inoculation with a virulent bacterial isolate (carrier isolate) that had been modified to deliver each of the avirulence determinants to the host. The results of the survey are presented and show that although no statistical evidence for non-random gene association was found, individual accessions revealed a wide range of variation in their response to respective pathogens. Two novel results that were further researched due to findings in the database screen are presented in detail.The low frequency presence of functional RPM] in the 94 accessions screened led to the identification of novel RPAll alleles. The previously held belief that RPM] \vas only found as two haplotypes, as an example of long lived variation for resistance, where balanced polymorphism predicts old resistance and susceptibility alleles that vary substantially in sequence (Stahl et al., 1999) was shown to be a simplification of the evolutionary dynamics at the RPM] locus by the identification of the new RPMI alleles. As the new susceptible and resistant alleles have small sequence variations they are thought to have occurred recently.
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