Localization of a pathogenicity gene in Ophiostoma novo-ulmi and evidence that it may be introgressed from O-ulmi

Ophiostoma novo-ulmi, the principal agent of Dutch elm disease, has recentl y replaced another species of Dutch elm disease pathogen, O. ulmi, across m uch of the Northern Hemisphere, Field inoculations of the moderately resist ant elms Ulmus procera and Ulmus x Commelin were carried out with progeny o f a genetic cross between AST27, a Eurasian (EAN) O, novo-ulmi isolate with an unusually low level of pathogenicity, and H327, a highly aggressive EAN isolate. These confirmed the results of a previous study that indicated th at the difference in phenotype was controlled by a single nuclear gene. Thi s pathogenicity gene, designated here Pat1, is the first putative pathogeni city gene to be identified in O, novo-ulmi. In a bulked segregant analysis, involving 80 random primers, 10 RAPD (random amplified polymorphic DNA) ma rkers were identified linked to Pat1, Linkage distances between these marke rs and Pat1 were confirmed by genetic analysis of all individual progeny. F ive RAPD amplicons identified in AST27 were O. ulmi and not O. novo-ulmi sp ecific amplicons, and of these two were linked to Pat1, This suggests that the Pat1 allele conferring unusually low aggressiveness in AST27 may have b een acquired from O, ulmi via introgression, When RAPD marker OPK3(1050), l inked to Pat1, was used as a probe for Southern hybridization with electrop horetically separated chromosomes of O, novo-ulmi and O, ulmi, the results indicated that Pat1 is located on a 3.5 Mb chromosome previously designated chDNA II.