Abstract
Rice blast, caused by the fungal pathogen
Magnaporthe oryzae
, is a devastating disease of rice worldwide. Among the 85 mapped resistance (R) genes against blast, 13 have been cloned and characterized. However, how these genes originated and how they evolved in the
Oryza
genus remains unclear. We previously cloned the rice blast R-genes
Pi2, Pi9
, and
Piz
-
t,
and analyzed their genomic structure and evolution in cultivated rice. In this study, we determined the genomic sequences of the
Pi2/9
locus in four wild
Oryza
species representing three genomes (AA, BB and CC). The number of
Pi2/9
family members in the four wild species ranges from two copies to 12 copies. Although these genes are conserved in structure and categorized into the same subfamily, sequence duplications and subsequent inversions or uneven crossing overs were observed, suggesting that the locus in different wild species has undergone dynamic changes. Positive selection was found in the leucine-rich repeat region of most members, especially in the largest clade where
Pi9
is included. We also provide evidence that the
Pi9
gene is more related to its homologues in the recurrent line and other rice cultivars than to those in its alleged donor species
O. minuta
, indicating a possible origin of the
Pi9
gene from
O. sativa.
Comparative sequence analysis between the four wild
Oryza
species and the previously established reference sequences in cultivated rice species at the
Pi2/9
locus has provided extensive and unique information on the genomic structure and evolution of a complex R-gene cluster in the
Oryza
genus.