Genetic Variation and Phylogeny of Spongospora subterranea f.sp. subterranea Based on Ribosomal DNA Sequence Analysis

American Journal of Potato Research, Nov/Dec 2004 by Qu, Xinshun, Christ, Barbara J

In recent taxonomic treatments, Cavalier-Smith (1998) created a new phylum Cercozoa under the Kingdom Protozoa by renaming a radically modified phylum Rhizopoda. The new phylum Cercozoa comprises several diverse groups previously of uncertain affinity, including chlorarachneans, euglyphids, sarcomonads, and plasmodiophorids (Cavalier-Smith 1998). In the present SSU rDNA phylogenetic analysis, S. subterranea and other species of plasmodiophorids formed a deep branch within the Cercozoa clade, and this result supplies further evidence to support Cavalier-Smith's proposal (Cavalier-Smith 1998) to place the Plasmodiophorida within the phylum Cercozoa. Species of plasmodiophorids represent an important group of plant pathogens that cause serious losses to crops. The classification of this group within the phylum Cercozoa represents a significant step in our understanding of plasmodiophorid pathogens.

There are two forma speciales in S. subterranea: S. subterranea f.sp. subteiranea, the cause of powdery scab of potato, and S. subterranea f.sp. nasturtii, the cause of crook root of watercress (Tomlinson 1958). Considering the host specificity and differences in morphological characters, Dick (2001) has recently raised this taxon to specific rank. In this study, comparison of ITS rDNA sequences and phylogenetic analyses of SSU rDNA sequences have shown that S. subterranea f.sp. subterranea is distinct from S. subterranea f.sp. nasturtii. These molecular data support the view of Dick (2001) that S. subterranea f.sp. subteiranea and S. subterranea, f.sp. nasturtii should be separated into distinct species.

In this study genetic variation within 52 isolates of S. subterranea obtained from the Republic of Ireland, Northern Ireland, Scotland, Canada, and USA was investigated and two distinct genetic groups (I and II) were identified based on the ITS sequence diversity among the isolates. The two types of ITS sequences identified in this investigation were previously reported by Bulman and Marshall (1998) in 20 isolates of S. subterranea obtained from Australia, New Zealand, Scotland, Peru, the Netherlands, and Switzerland. In their study, Bulman and Marshall (1998) found that no sequence variation was detected among any of the Australasian or European isolates with the exception of one from Inverness (Scotland), which was identical to the two Peruvian samples. In this investigation, however, isolates from both Ireland and Scotland have the two types of ITS sequences indicating that the genetic variations of S. subterranea exist in these countries. However, no variation was found in North American samples and all isolates from different locations in Canada and USA have the same type (II) of ITS sequences.

From the results of this investigation and the Bulman and Marshall (1998) study, it is interesting to note that all Australasian and North American isolates consisted of only one genetic group (II) and South American isolates consisted of only one genetic group (I), whereas European isolates consisted of both genetic groups (I and II). Karling (1968) suggested that powdery scab originated from the highlands of South America (Peru), was introduced with potato to Europe in post-Colombian times, and then back to North America and other parts of the world by the shipment and importations of infected tubers for food and seed. It appears that genetic group I might originate from South America and was then introduced to Europe, but it is not clear where and when the genetic group II originated. If European populations of S. subterranea are the source of Australasian and North American populations as suggested by Karling (1968), the genetic group I should also exist in Australia and North America.