Potential diversity in vegetative compatibility types of Ophiostoma novo-ulmi in North America

The diversity of vegetative compatibility (vc) types of the Dutch elm disease fungus, Ophiostoma novo-ulmi, in North America was recently shown to be very low compared to populations in Europe (Brasier, 1996, Mycologia 88:951-964). To determine if low diversity was caused by a lack of polymorphism at vegetative incompatibility (vie) loci, we made crosses between isolates in the three common vc types and estimated the number of vic genes segregating. The most common vc type (vc type 1) differed from the next two most common vc types (vc types 2 and 10) by at least five and seven vic loci, respectively. Therefore, mating between vc types 1 and 2 or between 1 and 10 could potentially result in at least 32 or 128 vc types, respectively, which is much greater than the 16 vc types previously observed among 112 field isolates from North America. Because vc types 2 and 10 were almost always found in the same population as vc type 1, lack of polymorphism at vic loci can be eliminated as a cause of the low diversity observed. Two vc types produced in crosses were the same as vc types present in the North American sample, suggesting that recombination occurs in nature. Restricted recombination and/or selection against recombinant vc types are the simplest hypotheses to explain the limited number of vc types observed in nature.