Dynamics of disease resistance polymorphism at the Rpm1 locus of Arabidopsis

The co-evolutionary 'arms race'(1) is a widely accepted model for the evolu tion of host-pathogen interactions. This model predicts that variation for disease resistance will be transient, and that host populations generally w ill be monomorphic at disease-resistance (R-gene) loci. However, plant popu lations show considerable polymorphism at R-gene loci involved in pathogen recognition(2). Here we have tested the arms-race model in Arabidopsis thal iana by analysing sequences flanking Rpm1, a gene conferring the ability to recognize Pseudomonas pathogens carrying AvrRpm1 or AvrB (ref. 3). We reje ct the arms-race hypothesis: resistance and susceptibility alleles at this locus have co-existed for millions of years. To account for the age of alle les and the relative levels of polymorphism within allelic classes, we use coalescence theory to model the long-term accumulation of nucleotide-polymo rphism in the context of the short-term ecological dynamics of disease resi stance. This analysis supports a 'trench warfare' hypothesis, in which adva nces and retreats of resistance-allele frequency maintain variation for dis ease resistance as a dynamic polymorphism(4,5).