Taxonomic position of the human intestinal protozoan parasite Isospora belli based on ribosomal RNA sequences

The taxonomic positions of Isospora belli and other members of the genus Is ospora are controversial. We determined the small-subunit ribosomal RNA of I. belli and used this sequence in combination with other coccidian RNA seq uences for analysis of the taxonomic position of I. belli. The phylogenetic trees we obtained provide molecular evidence for three clades within a mon ophyletic group that represents the suborder Eimeriina. The dade containing I. belli consists of tissue-cyst-forming coccidia (Toxoplasma and Neospora ) and members of the genus Isospora (I. ohioensis, I. suis, I, belli). The second clade, representing a sister clade of that containing the Isospora s pecies, contains members of the genus Sarcocystis. The third one consists o f members of the family Eimeriidae, including Eimeria and Cyclospora specie s. This shows that although I belli as well as other members of the genus I sospora belong to the suborder Eimeriina, the family to which they belong i s not Eimeriidae but rather Sarcocystidae. We suggest that the genus Isospo ra should be removed from the family Eimeriidae and placed into the family Sarcocystidae within the suborder Eimeriina.