Loss of a 1.6 Mb chromosome in Pyricularia oryzae harboring two alleles of AvrPik leads to acquisition of virulence to rice cultivars containing resistance alleles at the Pik locus.

A small and extra chromosome of 1.6 Mb was previously identified in a Pyricularia oryzae strain, 84R-62B. To understand a role of the 1.6 Mb chromosome in the pathogenic changeability of P. oryzae, we performed experiments designed to characterize the 1.6 Mb chromosome in the present study. A gene family encoding secreted protein Pex31s in P. oryzae consists of five homologs, Pex31- A to - E. Among them, Pex31- A and - D are known to be recognized by Pik- m and Pik/Pik- m/ Pik- p, respectively. In the present study, we identified Pex31- A and - D in the genome of 84R-62B. Segregation analyses using an F population between 84R-62B and another rice blast strain, Y93-245c-2, revealed a strong linkage between the two homologs and the 1.6 Mb chromosome of 84R-62B. A CHEF-Southern analysis revealed an association between the 1.6 Mb chromosome and the homologs, indicating that both homologs are located on the 1.6 Mb chromosome of 84R-62B. The loss of the 1.6 Mb chromosome was observed in subcultures of a F progeny, F1-327. These subcultures concomitantly acquired virulence on Pik, Pik- m, and Pik- p. The present study is the first report showing that loss of a small and extra chromosome leads to pathogenic mutation of P. oryzae and may provide a new insight into the mechanisms generating pathogenic variation of this fungus. [ABSTRACT FROM AUTHOR]