Strong Genetic Differentiation Between North American and European Populations of Phytophthora alni subsp. uniformis

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 P. alni subsp. alni -the pathogen responsible for the epidemic-is linked to an interspecific hybridization event between two parental species: P. alni subsp. multiformis and P. alni subsp. uniformis. One of the parental species, P. alni subsp. uniformis, 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 P. alni subsp. uniformis. Five new polymorphic microsatellite markers were used to contrast both geographical populations. The study comprised 71 isolates of P. alni subsp. uniformis collected from eight 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 with the North American collection. Selfing appears to be the predominant mating system in both continental collections. The results suggest that the European P. alni subsp. uniformis population is most likely alien and derives from the introduction of a few individuals, whereas the North American population probably is an indigenous population.

Genetic Diversity and Origins of the Homoploid-Type Hybrid Phytophthora ×alni

“…1. For P. uniformis, the European and North American isolates described by Aguayo et al (19) were added as reference isolates to track the contribution of this invasive species to the emergence of P. ϫalni. DNA extractions were performed from pure fresh or lyophilized mycelium using the BioSprint 96 DNA plant kit in combination with the BioSprint 96 automated workstation (Qiagen, Courtaboeuf, France), following the BS-96DNA-plant protocol, or using the Qiagen DNeasy Plant minikit according to supplier instructions.…”

“…A set of microsatellite markers was generated independently from the two parental species. These markers were developed from microsatellite-enriched libraries sequenced with a Roche GS-FLX Titanium pyrosequencing platform following the procedure described by Malausa et al (24) and Aguayo et al (19). Markers obtained from the P. ϫmulti-formis microsatellite-enriched library did not show polymorphism within any species of the P. alni complex; they were therefore discarded for further tests.…”

“…Polymorphism was detected in P. ϫalni, P. uniformis, and P. ϫmultiformis with markers obtained from the P. uniformis microsatellite-enriched library. This set of markers comprised five microsatellite markers developed by Aguayo et al (19) and another five additional loci that exhibited polymorphism in P. ϫalni and P. ϫmultiformis. Additionally, two polymorphic microsatellite loci derived from P. ϫalni, PA17 and PA23, described by Ioos et al (25), were added to the marker set.…”

“…In all, we used 12 microsatellite markers to characterize our isolate collection. The PCR amplification conditions were those described by Aguayo et al (19). The mitochondrial type of the P. ϫalni isolates, either U or M mitotype (15), was studied using the cox1 gene primers (COXF4N/COXR4N) designed by Kroon et al (26) with PCR mix and run conditions as described by Ioos et al (15).…”

“…uniformis, one of the parental species of P. ϫalni, is a diploid species and has been isolated both in Europe and in North America (18)(19)(20). Genetic characterization of European P. uniformis populations revealed low diversity and a clonal structure, while North American populations were more diverse and exhibited signs of outcrossing (19).…”