An eimeriid origin of isosporoid coccidia with stieda bodies as shown by phylogenetic analysis of small subunit ribosomal RNA gene sequences

Morphological and life cycle features of the tissue cyst-forming coccidia h ave been difficult to interpret in devising taxonomic classifications for t he various genera. In this study, we amplified the full small subunit rRNA gene sequence of Isospora robini McQuistion and Holmes, 1988, and the parti al sequence of Isospora gryphoni Olsen, Gissing, Barta, and Middleton, 1998 by PCR. Both of these species vary from Isospora species of mammals in hav ing Stieda bodies on the sporocysts. The sequences were cloned and sequence d and were incorporated into an alignment with other Isospora species lacki ng Stieda bodies as well as with other coccidia. Maximum parsimony analysis of these sequences produced a single most parsimonious tree that placed I. robini and I. gryphoni in a clade containing various other eimeriid specie s. The Isospora species lacking Stieda bodies were in the sarcocystid clade . Similar results were found by maximum likelihood analysis. These findings indicate that the genus Isospora as defined by several authors is polyphyl etic. Taxonomic changes to the genus Isospora would have to incorporate the 2 major clades found by molecular phylogenetic analysis. Isospora species with Stieda bodies should be classified in the family Eimeriidae, whereas t hose without Stieda bodies should remain in the family Sarcocystidae.