THE MOLECULAR EVOLUTION OF THE RICE BLAST RESISTANCE GENE Pi36

The single-locus rice blast resistance gene Pi36 is of the coiled-coil NBS-LRR (nucleotide binding site-leucine-rich repeat) type. We have investigated its evolution by resequencing a set of haplotypes (six ssp. indica and four ssp. japonica cultivars and three wild rice accessions) both within and outside the gene's coding region. It was found that the average nucleotide diversity was similar to 4.5%, characteristic of an intermediately diversified pattern. The K-a/K-s ratio and Tajima's D test were used to show that the gene's evolution reflects a mixture of purifying and balancing selection. In all, 186 significant gene conversion events were identified across the full coding region, suggesting that gene conversion has also been an important driver of sequence diversification.