Identification of an avirulence gene island in Pyriculazia chromosomes and analyses of its transposition and disorganization

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The significance of AVR1-CO39, an avirulence gene of the blast fungus corresponding to Pi-CO39(t) in rice cultivars, during the evolution and differentiation of the blast fungus was evaluated by studying its function and distribution in Pyricularia spp. When the presence/absence of AVR1-CO39 was plotted on a dendrogram constructed from rDNA sequences, a perfect parallelism was observed between its distribution and the phylogeny of Pyricularia isolates. AVR1-CO39 homologs were exclusively present in one species, P.oryzae, suggesting that AVR1-CO39 appeared during the early stage of evolution of P.oryzae. Transformation assays showed that all the cloned homologs tested are functional as an avirulence gene, indicating that selection has maintained their function. Nevertheless, Oryza isolates (isolates virulent on Oryza spp.) in P. oryzae were exceptionally non-carriers of AVR1-CO39. All Oryza isolates suffered from one of the two types of known rearrangements at the Avr1-CO39 locus, i.e., G type and J type. These types were congruous to the two major lineages of Oryza isolates from Japan determined by MGR586 and MAGGY. These results indicate that AVR1-CO39 was lost during the early stage of evolution of the Oryza-specific subgroup of P.oryzae. Interestingly, its corresponding resistance gene, Pi-CO39(t) is not widely distributed in Oryza spp.Homologs of AVR-Pita, an avirulence gene corresponding to Pita, were widely distributed not only in P.oryzae, but also in P.grisea and P.spp. Interestingly, the chromosomes with the homologs were different from isolates to isolates. Analyses of flanking regions of these homologs suggested that the insertion of transposable elements into their flanking regions gave them mobility.