Abstract
BackgroundRice blast fungus Magnaporthe oryzae is one of the most devastating pathogens in rice. Avirulence genes in this fungus share a gene-for-gene relationship with the resistance genes in its host rice. Although numerous studies have shown that rice blast R-genes are extremely diverse and evolve rapidly in their host populations, little is known about the evolutionary patterns of the Avr-genes in the pathogens.ResultsHere, six well-characterized Avr-genes and seven randomly selected non-Avr control genes were used to investigate the genetic variations in 62 rice blast strains from different parts of China. Frequent presence/absence polymorphisms, high levels of nucleotide variation (~10-fold higher than non-Avr genes), high non-synonymous to synonymous substitution ratios, and frequent shared non-synonymous substitution were observed in the Avr-genes of these diversified blast strains. In addition, most Avr-genes are closely associated with diverse repeated sequences, which may partially explain the frequent presence/absence polymorphisms in Avr-genes.ConclusionThe frequent deletion and gain of Avr-genes and rapid non-synonymous variations might be the primary mechanisms underlying rapid adaptive evolution of pathogens toward virulence to their host plants, and these features can be used as the indicators for identifying additional Avr-genes. The high number of nucleotide polymorphisms among Avr-gene alleles could also be used to distinguish genetic groups among different strains.
Highlights
Rice blast fungus Magnaporthe oryzae is one of the most devastating pathogens in rice
The results of this study suggested that presence/absence polymorphisms might be the major mechanism underlying the evolution of rice blast Avr-genes
Presence/absence (P/A) polymorphisms in Avr genes far, no more than 10 Avr genes have been cloned from M. oryzae
Summary
Rice blast fungus Magnaporthe oryzae is one of the most devastating pathogens in rice. Rice blast, caused by infection with the ascomycete fungus Magnaporthe oryzae, is one of the most devastating known rice diseases. It threatens the stability of rice production worldwide [1]. Nine Avr-genes, all of which encode proteins of unknown function, have been cloned. These include Avr-Pita, Avr-CO39, PWL1, PWL2, ACE1, Avr-Pizt, Avr-Pia, Avr-Pii, and Avr-Pik/km/kp [5,9,10,15,16,17]
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