Unstable Aneuploid Progenies of Phytophthora ramorum

Phytophthora ramorum has three populations belonging to the same species: the two North American populations (NA1 and NA2) and the European population (EU1). All three populations are genetically distinct lineages, as revealed by different molecular marker systems. P. ramorum is also heterothallic, with two opposite mating types, A1 and A2. All NA1 and NA2 isolates are of the A2 mating type. All EU1 isolates have been identified as A1, except for three A2 isolates collected in Belgium in 2002 and 2003. Since 2004, no isolates of the A2 mating type have been reported in Europe, which limits the risk of sexual recombination in Europe. In contrast, several EU1 isolates have been reported in nurseries in North America, some even at sites where NA1 isolates were also found. Although sexual reproduction in nature has not been observed yet, the presence of isolates of both mating types at a single site might lead to genetic recombination, which could lead to an increase in pathogen fitness and host range. The formation of sexual resting oospores might also increase the long-term survival of P. ramorum. The ability of plant pathogen populations to respond to novel challenges from their environment, such as host resistance and pesticide exposure, depends on their mechanisms for generating genetic variation. Germinating oospores have been obtained in vitro from crosses between EU1-A1 and NA1A2 isolates as well as between EU1-A1 and EU1-A2 isolates (Boutet and others, unpublished), but at a low frequency. To determine the nature of sexual recombination in P. ramorum, germinating oospores of a cross between an NA1-A2 isolate and an EU1-A1 isolate were genotyped with microsatellite markers that were heterozygous in the parental isolates. All crosses contained alleles of both parents, confirming exchange of genetic material, but meiotic irregularities occurred frequently. Non-Mendelian inheritance events were also observed. This included inheritance of more than two alleles at a single locus or loss of alleles from one of the parents at a separate locus. The aneuploid progeny was also mitotically unstable: zoospore and hyphal tip derivatives of the progenies showed genotypic rearrangements as well as phenotypic variation. These data indicate that progenies from crossings between P. ramorum isolates are often aneuploid and genetically unstable.