SUMMARY: The taxonomic status of the parasitic protozoal species depends on the morphological similarity of all isolated from humans and the presumption that are host-specific. On the basis of electrophoretic data derived from examination of 26 enzyme loci in Australian isolates, it has been proposed that is a species complex comprising three or four genetically distinct (but morphologically cryptic) species. These received the tentative designations of genetic groups I-IV (R. H. Andrews, M. Adams, P. F. L. Boreham, G. Mayrhofer & B. P. Meloni. 19, 183–190, 1989). In the present study, two unrelated DNA probes (one specific for a gene encoding a trophozoite surface protein, the other detecting a non-coding repetitive sequence within the genome) were used in Southern hybridization analyses to examine 10 axenic isolates of , established from diverse geographical regions in Australia, together with the Portland-1 isolate from the USA. Both probes identified every isolate unambiguously as belonging to one or other of two genetic clusters. Electrophoretic analysis of the same samples indicated that these clusters correspond to the previously defined genetic groups I and II. No heterogeneity was apparent within the seven group I isolates using either probe. However, when probed with the repetitive sequence, the four isolates belonging to group II exhibited small differences in banding patterns, suggesting that this group may be less homogeneous than group I. The consistency of the Southern blot data with the allozyme groupings confirms group I and group II as distinct genetic subgroups of and establishes that selected restriction sites can be used as taxonomic markers for the purposes of a genetically based systematics for .


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