Willem A. Man in 't Veld1, Hans de Gruyter2, Peter J.M. Bonants3, and Robert P. Baayen2
Three enzyme stains, malate dehydrogenase (MDH), malic enzyme (MDHP) and lactate dehydrogenase (LDH), altogether comprising five putative loci (Mdh-2, Mdhp, Ldh-1, Ldh-2 and Ldh-3) were used to characterize twenty strains of Phytophthora ramorum. All enzymes used are known to be powerful diagnostic enzymes in the genus Phytophthora. The tested strains were isolated from Rhododendron spp. and Viburnum spp. from six European countries, and from various hosts in the USA (Arbutus menziesii, Lithocarpus densiflorus, Quercus agrifolia, Rhododendron spp., Umbellularia californica, Vaccinium spp. and Viburnum spp.). Strikingly, so far all European strains were of the A1 mating type whereas all American strains belonged to the A2 mating type.
All strains contained identical isozyme alleles at all putative loci. At the Ldh-2 locus, which is known to encode for dimeric enzymes, two different alleles were present in all strains, indicating the heterozygous state. Similarly, at Ldh-3, which is known to encode for monomeric enzymes, two different alleles were present also indicating heterozygosity.
Interestingly, P. lateralis, which is phylogenetically the closest neighbour of P. ramorum, was homozygous at all isozyme loci except Ldh-3, where it had one of the two alleles in common with P. ramorum; P. lateralis had also one allele in common with P. ramorum at Ldh-2.
Although a limited number of P. ramorum strains have been subjected to isozyme analysis so far, and a limited number of enzymes have been used, the first results indicate that P.ramorum is a very homogeneous species, in spite of different mating types, geographic origins and hosts. The apparent genetic homogeneity, as demonstrated by isozyme analysis, provides no indication of differences in pathogenicity between strains originating from America and Europe.
The heterozygosity at Ldh-2 and Ldh-3 loci of P. ramorum is intriguing because first attempts to cross the European A1 and the American A2 mating types have not been successful. It is tempting to speculate that the heterozygosity at two isozyme loci of P. ramorum, having one allele at both loci in common with P. lateralis, may be the result of reticulation events in its evolutionary past, possibly involving P. lateralis. Alternatively, the lineages of P. ramorum that have been introduced into Europe and California originate from a sexually outcrossing ancestral population elsewhere in the world. In that case, fine scale differences between geographic populations as recently found with AFLP need not have any meaning in terms of natural species boundaries or pathogenic potential.
