Patterns of Genetic Diversification in the Invasive Hybrid Plant Pathogen × and Its Parental Species .

In pathogenic fungi and oomycetes, interspecific hybridization may lead to the formation of new species having a greater impact on natural ecosystems than the parental species. From the early 1990s, a severe alder ( spp.) decline due to an unknown species was observed in several European countries. Genetic analyses revealed that the disease was caused by the triploid hybrid × , which originated in Europe from the hybridization of and × . Here, we investigated the population structure of × (158 isolates) and (85 isolates) in several European countries using microsatellite markers. Our analyses confirmed the genetic structure previously observed in other European populations, with populations consisting of at most two multilocus genotypes (MLGs) and × populations dominated by MLG Pxa-1. The genetic structure of × populations in the Czech Republic, Hungary and Sweden seemed to reflect the physical isolation of river systems. Most rare × MLGs showed a loss of heterozygosity (LOH) at one or a few microsatellite loci compared with other MLGs. This LOH may allow a stabilization within the × genome or a rapid adaptation to stress situations. Alternatively, alleles may be lost because of random genetic drift in small, isolated populations, with no effect on fitness of × . Additional studies would be necessary to confirm these patterns of population diversification and to better understand the factors driving it.