Dynamics of disease resistance polymorphism at the Rpm1 locus of Arabidopsis

The co-evolutionary ‘arms race’1 is a widely accepted model for the evolution of host–pathogen interactions. This model predicts that variation for disease resistance will be transient, and that host populations generally will be monomorphic at disease-resistance (R -gene) loci. However, plant populations show considerable polymorphism at R -gene loci involved in pathogen recognition2. Here we have tested the arms-race model in Arabidopsis thaliana by analysing sequences flanking Rpm1, a gene conferring the ability to recognize Pseudomonas pathogens carrying AvrRpm1 orAvrB (ref. 3). We reject the arms-race hypothesis: resistance andsusceptibility alleles at this locus have co-existed for millions of years. To account for the age of alleles and the relative levels ofpolymorphism within allelic classes, we use coalescence theory to model the long-term accumulation of nucleotide polymorphism in the context of the short-term ecological dynamics of disease resistance. This analysis supports a ‘trench warfare’ hypothesis, inwhich advances and retreats of resistance-allele frequency maintain variation for disease resistance as a dynamic polymorphism4,5.