THE PHYLOGENY OF NEOSPORA-CANINUM AND TOXOPLASMA-GONDII BASED ON RIBOSOMAL-RNA SEQUENCES

Neospora caninum is a newly described cyst-forming coccidium which is the cause of severe neurological disease in dogs. The parasite is morp hologically similar to Toxoplasma gondii, but the two species can be d ifferentiated serologically. In order to define the phylogenetic posit ion of N. caninum, we have determined 16S-like rRNA sequences from thr ee members of the family of Sarcocystidae: N. caninum, T. gondii, and Sarcocystis fusiformis. The 16S-like rRNA genes from the three parasit es were amplified by polymerase chain reaction and the sequences were determined by direct solid-phase sequencing. The sequences derived wer e computer aligned with several other 16S-like rRNA sequences from pro tozoan parasites to construct phylogenetic trees. The study confirmed that N. caninum should be classified as a member of the family Sarcocy stidae. However, because of the close relationship to T. gondii it see ms questionable that N. caninum should be placed in a new genus.