A GENETIC SURVEY OF THE PATHOGENIC FUNGUS OPHIOSTOMA-ULMI ACROSS A DUTCH ELM DISEASE FRONT IN WESTERN CANADA

The natural population structure of the Dutch elm pathogen Ophiostoma ulmi was determined for isolates collected from across a Western Canad ian disease front through an analysis of restriction-site polymorphism s in the ribosomal DNA repeat, length mutations in the mitochondrial g enomes, and through DNA fingerprinting of the nuclear genomes using a minisatellite DNA probe. The 8.8-kbp rDNA repeat was selected from a g enomic library, and restriction-site and genic maps were constructed f or the nonaggressive and aggressive subgroups of O. ulmi. There were o nly three restriction-site differences that distinguished these two su bgroups and no intrasubgroup variation was detected. All of the isolat es collected from the disease front were of the aggressive subgroup an d were represented by two distinct nuclear and four mitochondrial geno types. The majority of the isolates were of a single genotype (type A nuclear DNA; type I mtDNA), indicating the presence of a single very l arge clone extending across much of Manitoba and into Saskatchewan.