Genetic structure of Magnaporthe oryzae populations in three island groups in the Philippines

Abstract

Genetic structure of the rice blast pathogen, Magnaporthe oryzae was characterized using 453 monoconidial isolates collected from different rice agroecosystems from Luzon, Visayas and Mindanao, the three island groups in the Philippines in experimental plots and in farmer fields. Using 12 microsatellite markers, a total of 93 new multi-locus genotypes (MLGs) were determined when compared to the reference isolates representing the genotypes previously published. Analysis revealed a weak geographic structuring of pathogen genotypes, with some of those collected from Luzon clustering with those from Visayas or Mindanao. Nevertheless, none of the strains from Mindanao clustered with those from the Visayas and none from Visayas clustered with those from Mindanao. This structure likely results from limited natural migration and from some events of long distance migration, probably through the transport of infected seeds. Significant genetic differences were noted on isolates collected from distinct rice agroecosystems. A subset of 30 isolates, representative of MLGs, were tested for fertility status. Of these, eight induced the production of perithecia, of which six were male-fertile and two were also female-fertile. In addition, MAT1.1 isolates represented 36% of our sample whereas it represented only 5% in the collection of reference strains. Pathogenicity test of a subset of 24 isolates inoculated to 16 differential rice varieties demonstrated that most isolates were avirulent to varieties which carry the resistance genes Pi1, Pi33 Pi9, Pikm, and the combination of Piz and Pish.