Genetic variation and phylogeny of Spongospora subterranea f.sp. subterranea based on ribosomal DNA sequence analysis

The nuclear rDNA regions of the two internal transcribed spacers (ITS1 and ITS2) and 5.8S rRNA gene from 52 field isolates of Spongospora subterranea f.sp. subterranea obtained from the British Isles and North America were polymerase chain reaction-amplified, sequenced, and assessed for genetic variation. Two genetically distinct groups (I and II) were identified based on the ITS sequence diversity among the isolates, representing 34.6% and 65.4% of the isolates, respectively. British Isles isolates occurred in groups I and II, whereas North American isolates belonged only to group II. The British Isles groups of S. subterranea were associated with particular potato cultivars. The full-length small-subunit rRNA gene of S. subterranea was sequenced and analyzed by both neighbor-joining and parsimony methods to clarify the taxonomic position of this pathogen. The results of phylogenetic analysis showed that S. subterranea grouped together with other species of plasmodiphorids, and this group clustered with the phylum Cercozoa, an assemblage of filose and reticulose amoebae and phylogenetically related zooflagellates. The recognition of the existence of different genetic groups within S. subterranea will be important for the design of plant-breeding programs and in testing for plant resistance.