EU2, a fourth evolutionary lineage of Phytophthora ramorum

Studies in North America and Europe over the past decade have demonstrated the occurrence of three lineages of Phytophthora ramorum informally designated the NA1, NA2 and EU1 lineages. Each lineage appears to represent a reproductively isolated population, but whether they have come from different
geographic regions is unknown. Only the EU1 lineage had been found in Europe until recently. EU1 is believed to have been introduced into Europe around 1990. Since then it has spread widely and rapidly across the continent, including the UK and Ireland, via the plant trade. In 2011 P. ramorum isolates from Northern Ireland and a closely adjacent area of western Scotland, mostly from Larix but also from Quercus, Rhododendron and Vaccinium, were found to have molecular profiles not matching those of any known lineage. Following a phylogenetic study based on eleven polymorphic loci and an SSR analysis they were assigned to a new lineage, informally designated EU2. This analysis indicates the EU2 lineage
may be ancestral to the other lineages. No SSR-based intra-EU2 lineage genotypic diversity was detected. All EU2 isolates examined to date have all been of A1 mating type. As this is the same mating type as that of EU1 in Europe, sexual recombination with EU1 lineage genotypes already resident in the
UK is unlikely. The earliest isolation dates to 2007. Present evidence points to a recent introduction of EU2 in the context of ongoing phytosanitary emergency measures. The arrival of EU2 highlights an urgent need to identify the geographic origins of P. ramorum in order to understand the organism’s natural ecology, the processes that have produced the lineages, and whether further lineages exist.
Presently, studying the organism in the context of introduction and invasion, we may only be looking at half the picture.