Rare interspecific hybrids in natural populations of the Dutch elm disease pathogens Ophiostoma ulmi and O-novo-ulmi

Ophiostoma ulmi and O. novo-ulmi are partly reproductively isolated, morphologically, behaviourally and molecularly distinct species responsible for the first and current pandemics of Dutch elm disease, respectively. Among > 11000 isolates sampled from Dutch elm disease sites across Eurasia and North America since 1973, nine could not be assigned to O. ulmi or O. novo-ulmi. Of these isolates one (P129) was from Poland and eight (d10, d11, e12, e27, e28, e37, f30 and g3) were from a single bark sample in Portugal. These nine isolates were termed 'fast-waxy' isolates because of their unusual cultural characteristics. The possibility that they were interspecific hybrids was investigated. When compared with O. ulmi and O. novo-ulmi for colony pattern, growth-rate, optimum temperature for growth, vascular wilt ability, elm bark colonizing ability, cerato-ulmin toxin production ability to fertilize (as male) O. novo-ulmi, and ability to be fertilized (as female) by O. ulmi, they exhibited a combination of O. ulmi-like, O. novo-ulmi-like, intermediate or novel characters (female sterility) consistent with their being hybrids. P129 and representative Portuguese isolates d10 and e27 each exhibited a different combination of characters, indicating each was a different hybrid genotype. When d10 and e27 were independently crossed to the same O. novo-ulmi isolate, differences in their F-1 progeny sets for growth-rate and pathogenicity distributions were consistent with their being different recombinant genotypes. A molecular analysis of P129, d10 and e27 using RAPDs of genomic DNA, rDNA RFLPs and cerato-ulmin gene sequences confirmed that each was a unique interspecific hybrid. The mechanism of origin of these hybrids and their evolutionary significance are discussed. Combined experimental and circumstantial evidence indicates that they are relatively unfit, rare and probably transient, and that they arise when O. novo-ulmi invades territory occupied by O. ulmi and replaces it. Nonetheless, the possibility that the hybrids act as a genetic bridge, facilitating transfer of novel vegetative compatibility loci and other loci form O. ulmi to O. novo-ulmi at recent epidemic fronts, requires investigation.