Rapid emergence of hybrids between the two subspecies ofOphiostoma novo‐ulmiwith a high level of pathogenic fitness

Abstract

During the 1970s Europe was invaded by two subspecies of the Dutch elm disease pathogenOphiostoma novo‐ulmi: subsp. americanafrom the west and subsp.novo‐ulmifrom the east. As a result their geographic ranges began to overlap in several areas. Only a weak prezygotic barrier to hybridization exists between the subspecies and in 1980 two hybrids were detected in the Netherlands. A subset of 107O. novo‐ulmiisolates collected in a subspecies overlap zone in Limburg, Netherlands in 1983 was characterized for three phenotypic markers and seven RAPD PCR markers. By phenotype, 33% were shown to be hybrid whereas by RAPD markers 69% were shown to be hybrid. Some isolates shown to be hybrid by phenotype were not revealed to be hybrid by PCR and vice versa. Combining the phenotype and RAPD data the estimated hybrid frequency was ∼78%. The mean growth rate of Limburg hybrid isolates was significantly faster than that of the Limburg subsp.novo‐ulmiisolates but not significantly different from Limburg subsp.americanaisolates. The Limburg hybrid isolates were just as pathogenic as the parent subspecies on both clonalUlmus proceraand onU. × Commelin. A subset of 100 isolates collected in another subspecies overlap zone at Orvieto, Italy in 1986 was also assessed with RAPD markers and∼72% were shown to be hybrids. When 20 isolates of a ‘pure’ subsp.novo‐ulmipopulation in the Baltic Ports area of Poland collected in 1980 were assessed by RAPD markers three isolates exhibited early introgression of subsp.americanaDNA. This study therefore demonstrates very rapid emergence ofO. novo‐ulmisubspecies hybrids and introgressants in Europe in the early 1980s. In terms of two major fitness characters, growth rate and pathogenicity, these early hybrids were as fit as their parent subspecies. It is likely that complex hybrid swarms are now expanding across the continent.