Genetic diversity and origins of the homoploid-type hybrid Phytophthora ×alni

Abstract

Assessing the process that gives rise to hybrid pathogens is central to understanding the evolution of emerging plant diseases. Phytophthora ×alni, a pathogen of alder, results from the homoploid hybridization of two related species, Phytophthora uniformis and Phytophthora×multiformis. Describing the genetic characteristics of P.×alni should help us understand how reproductive mechanisms and historical processes shaped the population structure of this emerging hybrid pathogen. The population genetic structure of P.×alni and the relationship with its parental species were investigated using 12 microsatellites and one mitochondrial DNA (mtDNA) marker on a European collection of 379 isolates. Populations of P.×alni were dominated by one multilocus genotype (MLG). The frequency of this dominant MLG increased after the disease emergence together with a decline in diversity, suggesting that it was favored by a genetic mechanism such as drift or selection. Combined microsatellite and mtDNA results confirmed that P.×alni originated from multiple hybridization events that involved different genotypes of the progenitors. Our detailed analyses point to a geographic structure that mirrors that observed for P. uniformis in Europe. The study provides more insights on the contribution of P. uniformis, an invasive species in Europe, to the emergence of Phytophthora- induced alder decline.