PHYLOGENETIC RELATIONSHIP OF SARCOCYSTIS-NEURONA TO OTHER MEMBERS OF THE FAMILY SARCOCYSTIDAE BASED ON SMALL-SUBUNIT RIBOSOMAL-RNA GENE SEQUENCE

Sarcocystis neurona is a coccidial parasite that causes a neurologic d isease of horses in North and South America. The natural host species are not known and classification is based on ultrastructural analysis. The small subunit ribosomal RNA (SSURNA) gene of S. neurona was ampli fied using polymerase chain reaction techniques and sequenced by Sange r sequencing reactions. The sequence was compared with partial sequenc es of S. muris, S. gigantea, S. tenella, S. cruzi, S. arieticanis, S. capracanis, Toxoplasma gondii, Eimeria tenella, and Cryptosporidium pa rvum. Alignments of available sites for al 10 species and alignments o f the entire SSURNA sequence of S. neurona, S. muris, S. cruzi, T. gon dii, and C. parvum were performed. Alignments were analyzed using maxi mum parsimony and maximum likelihood methods to determine relative phy logeny of these organisms. These analyses confirmed placement of S. ne urona in the genus Sarcocystis and suggested a close relationship to S . muris, S. gigantea, and T. gondii. Molecular phylogeny suggests that Sarcocystis spp., which utilize the dog (Canis familaris) as the defi nitive host, evolved from a common ancestor, whereas those species (in cluding T. gondii) that utilize the cat (Felis domesticus) as the defi nitive host evolved from another common ancestor. This suggests a poss ible definitive host for S. neurona.