GENETIC DIFFERENTIATION AMONG ISOLATES OF PYRICULARIA INFECTING RICE AND WEED HOSTS

DNA polymorphisms among Philippine isolates of the blast fungus from r ice and 16 weed species were examined using restriction analysis of mi tochondrial DNA (mtDNA) and DNA blot hybridization using ribosomal, si ngle-copy, and repetitive DNA probes. Four rDNA hybridization patterns and six mtDNA restriction profiles were observed. The most frequent r DNA and mtDNA types were shared by isolates infecting rice and a group of weed species. Cluster analysis of the data obtained using probes f or single-copy nuclear loci confirmed the phylogenetic relationships s uggested by the mtDNA analysis, but differentiated the rice and non-ri ce-infecting isolates at the 50% similarity level. High levels of poly morphism between weed- and rice-infecting isolates were detected by si x repetitive DNA probes. Probe MGR586 showed a consistently high numbe r of hybridizing bands for rice-infecting isolates and low number of h ybridizing bands for non-rice-infecting isolates. Subpopulations of th e fungus attacking rice and adjacent weeds in two rice fields were fou nd to be genetically differentiated. The results presented strongly su ggest that although Pyricularia populations infecting rice and many ri ce-field weeds share a common ancestry, populations of the pathogen-in fecting weed hosts do not provide inoculum for the rice crop in the Ph ilippines.