Genetic Diversity and Origins of the Homoploid-Type HybridPhytophthora ×alni
ABSTRACTAssessing 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 uniformisandPhytophthora×multiformis. Describing the genetic characteristics ofP. ×alnishould help us understand how reproductive mechanisms and historical processes shaped the population structure of this emerging hybrid pathogen. The population genetic structure ofP. ×alniand 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 ofP. ×alniwere 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 thatP. ×alnioriginated from multiple hybridization events that involved different genotypes of the progenitors. Our detailed analyses point to a geographic structure that mirrors that observed forP. uniformisin Europe. The study provides more insights on the contribution ofP. uniformis, an invasive species in Europe, to the emergence ofPhytophthora-induced alder decline.IMPORTANCEOur study describes an original approach to assess the population genetics of polyploid organisms using microsatellite markers. By studying the parental subgenomes present in the interspecific hybridP. ×alni, we were able to assess the geographical and temporal structure of European populations of the hybrid, shedding new light on the evolution of an emerging plant pathogen. In turn, the study of the parental subgenomes permitted us to assess some genetic characteristics of the parental species ofP. ×alni,P. uniformis, andP. ×multiformis, which are seldom sampled in nature. The subgenomes found inP. ×alnirepresent a picture of the “fossilized” diversity of the parental species.