Selection and mutation of the avirulence gene AVR-Pii of the rice blast fungus Magnaporthe oryzae

Magnaporthe oryzae avirulence (AVR) genes are predicted to be involved in pathogen invasion and their virulence functions are restricted by the presence of the cognate resistance (R) genes. In this study, the distribution and variation of the avirulence (AVR-Pii) gene of M. oryzae in Yunnan province, China were analysed to understand haplotype diversity of AVR-Pii under field conditions. The presence of AVR-Pii in 454 field isolates of M. oryzae collected in Yunnan province was examined using gene-specific PCR markers. The results showed that 82 M. oryzae isolates carried AVR-Pii. Among them, 39 (35.5%), 5 (15.2%), 4 (14.3%), 2 (13.3%), 25 (12.8%) and 7 (9.7%) of the M. oryzae isolates carried AVR-Pii from central, southeastern, southwestern, northwestern, western and northeastern Yunnan province, respectively. Of these isolates, 55 were sequenced, while the remaining 27 isolates were not suitable for PCR-based sequencing and were not used for further analysis. Moreover, three AVR-Pii haplotypes were identified among the 55 isolates, in which H1 was identical with a previous sequence in GenBank (accession no. ) and H2 and H3 were novel variants. All DNA sequence variations were found to occur in the protein-coding region resulting in amino acid substitutions. One virulent haplotype of AVR-Pii to Pii was identified among 55 field isolates, suggesting that the AVR-Pii gene has lost avirulence function through base substitution. These findings suggest that AVR-Pii is under positive selection and AVR-Pii mutations are responsible for overcoming race-specific resistance in nature.