Intragenic Recombination and Diversifying Selection Contribute to the Evolution of Downy Mildew Resistance at the RPP8 Locus of Arabidopsis

The Plant Cell, Vol. 10, 1861–1874, November 1998

John M. McDowell, Murali Dhandaydham, Terri A. Long, Mark G. M. Aarts, Stephen Goff, Eric B. Holub, and Jeffery L. Dangl

Summary Pathogen resistance ( R ) genes of the NBS-LRR class (for nucleotide binding site and leucine-rich repeat) are found in many plant species and confer resistance to a diverse spectrum of pathogens. Little is known about the mechanisms that drive NBS-LRR gene evolution in the host-pathogen arms race. We cloned the RPP8 gene (for resistance to Peronospora parasitica ) and compared the structure of alleles at this locus in resistant Landsberg erecta (L er -0) and susceptible Columbia (Col-0) accessions. RPP8-L er encodes an NBS-LRR protein with a putative N-terminal leucine zipper and is more closely related to previously cloned R genes that confer resistance to bacterial pathogens than it is to other known RPP genes. The RPP8 haplotype in L er -0 contains the functional RPP8-L er gene and a nonfunctional homolog, RPH8A. In contrast, the rpp8 locus in Col-0 contains a single chimeric gene, which was likely derived from unequal crossing over between RPP8-L er and RPH8A ancestors within a L er -like haplotype. Sequence divergence among RPP8 family members has been accelerated by positive selection on the putative ligand binding region in the LRRs. These observations indicate that NBS-LRR molecular evolution is driven by the same mechanisms that promote rapid sequence diversification among other genes involved in non-self-recognition.