Submitted to: Infection, Genetics and Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/2/2006
Publication Date: 9/1/2006
Citation: Asmundsson, I.M., Dubey, J.P., Rosenthal, B.M. 2006. A genetically diverse but distinct north american population of sarcocystis neurona includes an overrepresented clone described by twelve microsatellite alleles; infection, genetics and evolution. Infection, Genetics and Evolution. 6(5):352-360. Interpretive Summary: Modern population genetic methods have revealed much about the formation of genetically stable strains in Toxoplasma gondii, a prominent food safety parasitic threat. However, these same tools have yet to be applied to any other related coccidian parasite. Here, we have developed a new suite of genetic tools and applied them to a broad sample of parasites representing Sarcocystis neurona, the primary cause of Equine Protozoal Myeloencephalitis, and to other closely related species. We reach several original and important conclusions: 1) most isolates are characterized by unique genotypes, 2) isolates from a broad geographic North American range, although genotypically variable, are clearly identifiable as a distinct genetic population, 3) one particular genotype is signficantly over-represented in our sample, suggesting the possibility that strictly asexual propegation may be possible in some circumstances, and 4) that isolates from ecologically diverse sources (opossums, horses, and a sea otter) belong to the same genetic population. Ours is also the first attempt at investigating how genotypic variation in S. neurona genotype may contribute to variation in disease manifestation.
Technical Abstract: The population genetics and systematics of most coccidians, protozoan parasites that cause diseases in humans and animals, remain poorly defined. Non-recombinant parasite clones characterized by distinct transmission and pathogenesis traits persist in the coccidian Toxoplasma gondii despite opportunities for sexual recombination. In order to determine whether this may be generally true for tissue-cyst forming coccidia, and to address evolutionary and taxonomic problems within the genus Sarcocystis, we characterized polymorphic microsatellite markers in Sarcocystis neurona, the major causative agent of equine protozoal myeloencephalitis (EPM). Bayesian statistical modeling, phylogenetic reconstruction based on genotypic chord distances, and analyses of linkage disequilibrium were employed to examine the population structure within S. neurona and closely related S. falcatula isolates from North and South America. North American S. neurona were clearly differentiated from those of South America and also from isolates of S. falcatula. Although S. neurona is characterized by substantial allelic and genotypic diversity typical of interbreeding populations, one genotype occurs with significantly excessive frequency; thus, some degree of asexual propagation of S. neurona clones may naturally occur. Finally, S. neurona isolated from disparate North American localities and diverse hosts (opossums, a Southern sea otter, and horses) comprise a single genetic population. Isolates associated with clinical neurological disease bear no obvious distinction as measured by these neutral genetic markers.