MOLECULAR RELATIONSHIPS BETWEEN OPHIOSTOMA-ULMI AND THE NAN AND EAN RACES OF O-NOVO-ULMI DETERMINED BY RESTRICTION-FRAGMENT-LENGTH-POLYMORPHISMS OF NUCLEAR-DNA

Southern blots of restriction endonuclease-digested nuclear DNA from 37 isolates of Ophiostoma ulmi and O. novo-ulmi (EAN and NAN races) were hybridised with probes from a library of cloned fragments of 0. ulmi nuclear DNA to search for restriction fragment length polymorphisms (RFLPs). About 40% of enzyme-probe combinations tested were unable to distinguish between 0. ulmi and 0. novo-ulmi isolates. Another 40% were able to distinguish all 0. ulmi isolates from all 0. novo-ulmi isolates, though not between the EAN and NAN races. 2.3 % of enzyme-clone combinations were able to distinguish all EAN from all NAN isolates, and these were also able to distinguish all the 0. ulmi isolates. A number of clones detected variability within 0. ulmi and within the EAN and NAN races. Overall, 16 distinct nuclear RFLP types were identified, five in 0. ulmi, three in the EAN race and six in the NAN race. Isolates within a vegetative compatibility group had the same, or a closely similar, nuclear RFLP type. The RFLP profiles were consistent with an evolutionary direction 0. ulmi --> 0. novo-ulmi EAN --> 0. novo-ulmi NAN.