Isozyme genotyping of Phytophthora ramorum reveals fixed heterozygosity at three loci suggesting a hybrid origin

Willem A. Man in ’t Veld, Plant Protection Service, P.O. Box 9102, 6700 HC Wageningen, the Netherlands

Five enzyme stains, comprising seven loci were used to characterize thirty seven strains of Phytophthora ramorum originating from Europe and America. All strains were monomorphic at all putative loci, except Ldh-2 which was polymorphic. At the dimeric Gpi and Ldh-2 loci three banded patterns and sometimes five banded patterns (Ldh-2) were present indicating heterozygosity. The presence of four alleles at Ldh-3 indicates polyploidy.

It is concluded that the American and European strains are conspecific. The heterozygosity at Gpi, Ldh-2 and Ldh-3 suggests that P. ramorum has been involved in hybridization events in its evolutionary past, since fixed heterozygosity is generally considered to be the hallmark of hybridization. This hybridization event may have triggered the problematic mating behaviour as observed in laboratory experiments.

When in the presumed common centre of origin outcrossing would have taken place regularly, gene flow between the two mating types should have resulted in homozygosity at the three heterozygous loci forementioned at least in some strains. This, however, was not observed. In addition, sequence analysis of the Cox I gene and AFLP data showed consistent differences between the two mating types. This apparent lack of gene flow suggests that the two mating types have been reproductively isolated in their recent evolutionary past and that they may represent two species in statu nascendi.

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