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Background: Rice blast is a very serious disease caused by Magnaporthe oryzae. The avirulence (AVR) genes of rice blast are perceived by the corresponding rice-blast resistance (R) genes and prompt specific resistance. A mutantion in AVR is a major force for new virulence. Exploring mutations in AVR among M. oryzae isolates from rice-production fields could aid assessment of the efficacy and durability of R genes. We studied the probable molecular-evolutionary patterns of AVR-Pib alleles by assaying their DNA-sequence diversification and examining their avirulence to the corresponding Pib resistance gene under natural conditions in China.Results: PCRs detected results from M. oryzae genomic DNA revealed that 162 out of 366 isolates collected from Yunnan Province contained AVR-Pib alleles. Among them, 36.1–73.3% isolates from six different rice-production areas of Yunnan contained AVR-Pib alleles. Furthermore, 36 (28.6%) out of 126 isolates had a transposable element (TE) insertion in AVR-Pib, which resulted in altered virulence. The TE insertion was identified in isolates from rice rather than from Musa nana Lour. Twelve AVR-Pib haplotypes encoding three novel AVR-Pib variants were identified among the remaining 90 isolates. AVR-Pib alleles evolved to virulent forms from avirulent forms by base substitution and TE insertion of Pot2 and Pot3 in the 5′ untranslated region of AVR-Pib. Conclusions: Our results revealed that AVR-Pib alleles suffered neutral evolution and divergences that led to overcome specific resistant Pib alleles under field conditions. The TE insertion in AVR-Pib was selected and adapted to rice and other Gramineae species.

The complete genome of the dsRNA virus isolated from Rhizoctonia solani AG-1 IA 9–11 (designated as Rhizoctonia solani dsRNA virus 11, RsRV11 ) were determined. The RsRV11 genome was 9,555 bp in length, contained three conserved domains, SMC, PRK and RT-like super family, and encoded two non-overlapping open reading frames (ORFs). ORF1 potentially coded for a 204.12 kDa predicted protein, which shared low but significant amino acid sequence identities with the putative protein encoded by Rhizoctonia solani RNA virus HN008 (RsRV-HN008) ORF1. ORF2 potentially coded for a 132.41 kDa protein which contained the conserved motifs of the RNA-dependent RNA polymerase (RdRp). Phylogenetic analysis indicated that RsRV11 was clustered with RsRV-HN008 in a separate clade independent of other virus families. It implies that RsRV11, along with RsRV-HN008 possibly a new fungal virus taxa closed to the family Megabirnaviridae, and RsRV11 is a new member of mycoviruses.