Abstract
A recent survey in Germany revealed the wide presence of ‘Candidatus Phytoplasma ulmi’ in native elm stands. Accessions were studied for their genetic variability and phylogenetic relationship based on the conserved groEL and the variable imp gene. While the groEL sequences revealed a high intraspecific homology of more than 99%, the homology of the imp gene dropped to 71% between distantly related sequences. Twenty-nine groEL and 74 imp genotypes were distinguished based on polymorphic sites. Phylogenetic analysis of the groEL gene clustered all ‘Ca. P. ulmi’ strains and separated them from related phytoplasmas of the 16SrV group. The inferred phylogeny of the imp gene resulted in a different tree topology and separated the ‘Ca. P. ulmi’ genotypes into two clusters, one closely related to the flavescence dorée phytoplasma strain FD-D (16SrV-D), the other affiliated with the flavescence dorée phytoplasma strains FD-C and FD70 and the alder yellows phytoplasma (16SrV-C). In both phylograms, ‘Ca. P. ulmi’ genotypes from Scots elm trees formed a coherent cluster, while genotypes from European white elms and field elms grouped less strictly. The regional distribution pattern was congruent for some of the groEL and imp genotypes, but a strict linkage for all genotypes was not apparent.
Highlights
Phytoplasmas are obligate vector-borne bacterial parasites associated with diseases of more than 1000 plant species[1,2]
The first molecular evidence of a close relationship between American and European strains came from Southern blot hybridisation experiments using random DNA probes[7], supported by restriction fragment length polymorphism (RFLP) and sequence analyses of the 16S rRNA gene thereafter[4]
The conserved groEL gene was selected to provide a better intraspecific resolution, due to a higher sequence variation compared to the 16S rRNA gene[24,25,30], and the variable imp gene was chosen to tap the full range of genetic polymorphism of a variable gene
Summary
Phytoplasmas are obligate vector-borne bacterial parasites associated with diseases of more than 1000 plant species[1,2]. There, ‘Ca. P. ulmi’ infections seem to be associated generally with typical disease symptoms like witches’ brooms, leaf yellowing and premature leaf fall[16,17], albeit the typical destructive phloem necrosis that American elm species display is not present in European elm species[18,19]. This decisive difference and the lack of molecular evidence were the determining factors in classifying this phytoplasma as a quarantine pathogen for the EPPO region[20]. We examine genetic variations, the phylogenetic relationship and the regional distribution of ‘Ca. P. ulmi’ accessions with a groEL gene fragment and the imp gene—two markers of different resolving powers adding specific information to survey results published recently by the authors
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