INTRAGENIC RECOMBINATION AND DIVERSIFYING SELECTION CONTRIBUTE TO THEEVOLUTION OF DOWNY MILDEW RESISTANCE AT THE RPP8 LOCUS OF ARABIDOPSIS

Pathogen resistance (R) genes of the NBS-LRR class (for nucleotide bin ding 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-pa thogen arms race. We cloned the RPP8 gene (for resistance to Peronospo ra parasitica) and compared the structure of alleles at this locus in resistant Landsberg erecta (Ler-0) and susceptible Columbia (Col-0) ac cessions. RPP8-Ler encodes an NBS-LRR protein with a putative N-termin al leucine zipper and is more closely related to previously cloned R g enes that confer resistance to bacterial pathogens than it is to other known RPP genes. The RPP8 haplotype in Ler-0 contains the functional RPP8-Ler gene and a nonfunctional homolog, RPH8A. In contrast, the rpp 8 locus in Col-0 contains a single chimeric gene, which was likely der ived from unequal crossing over between RPP8-Ler and RPH8A ancestors w ithin a Ler-like haplotype. Sequence divergence among RPP8 family memb ers has been accelerated by positive selection on the putative ligand binding region in the LRRs. These observations indicate that NBS-LRR m olecular evolution is driven by the same mechanisms that promote rapid sequence diversification among other genes involved in non-self-recog nition.