The genomic dynamics and evolutionary mechanism of the Pi2/9 locus in rice

The Pi2/9 locus contains at least four resistance specificities to Magnaporthe grisea and belongs to a gene complex comprised of multiple genes that encode highly homologous nucleotide binding site (NBS) and leucine rich repeat (LRR) proteins. To investigate the genetic events involved in the evolution of the Pi2/9 locus, we analyzed the Pi2/9 locus at the inter- and intralocus levels in five rice cultivars. The NBS-LRR genes in the five cultivars belong to the same phylogenetic clade among rice NBS-LRR genes, and all have a phase-2 intron at the N-terminus. However, the paralogs within each haplotype show a significant sequence divergence and their N-terminal intron and 5' regulatory regions are very different. On the contrary, the orthologs from different haplotypes are highly similar, indicating an obvious orthologous relationship has been maintained during the evolution of the Pi2/9 locus. These results suggest that sequence diversification in the 5' regulatory regions and N-terminal introns of the paralogs may have led to suppression of meiotic recombination between the paralogs within each haplotype, facilitating the maintenance of the orthologous relationship among rice cultivars. Our observations provide valuable insight into the genomic dynamics and evolutionary mechanism of an NBS-LRR resistance-gene complex in rice.