Strong genetic differentiation between North American and European populations of Phytophthora alni subsp. uniformis

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Abstract:

Alder decline caused by Phytophthora alni has been one of the most important diseases of natural ecosystems in Europe during the last 20 years. The emergence of Phytophthora alni subsp. alni (Paa)—the pathogen responsible for the epidemic—is linked to an interspecific hybridization event between two parental species: Phytophthora alni subsp. multiformis (Pam) and Phytophthora alni subsp. uniformis (Pau). One of the parental species, Pau, has been isolated in several European countries and recently in North America. The objective of this work was to assess the level of genetic diversity, the population genetic structure, and the putative reproduction mode and mating system of Pau. Five new polymorphic microsatellite markers were used to contrast both geographical populations. The study comprised 71 isolates of Pau collected from 8 European countries and 10 locations in North America. Our results revealed strong differences between continental populations (Fst=0.88; Rst=0.74), with no evidence for gene flow. European isolates showed extremely low genetic diversity compared to the North American collection. Selfing appears to be the predominant mating system in both continental collections. The results suggest that the European Pau population is most likely alien and derives from the introduction of a few individuals, while the North American population probably is an indigenous population.

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