Intragenic
Recombination and Diversifying Selection Contribute to the Evolution of
Downy Mildew Resistance at the RPP8 Locus of Arabidopsis |
The Plant Cell, Vol. 10, 18611874, 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. |