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

The taxonomic positions of Isospora belli and other members of the genus Isospora are controversial. We determined the small-subunit ribosomal RNA of I. belli and used this sequence in combination with other coccidian RNA sequences for analysis of the taxonomic position of I. belli. The phylogenetic trees we obtained provide molecular evidence for three clades within a monophyletic group that represents the suborder Eimeriina. The clade 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 species, contains members of the genus Sarcocystis. The third one consists of members of the family Eimeriidae, including Eimeria and Cyclospora species. This shows that although I. belli as well as other members of the genus Isospora belong to the suborder Eimeriina, the family to which they belong is not Eimeriidae but rather Sarcocystidae. We suggest that the genus Isospora should be removed from the family Eimeriidae and placed into the family Sarcocystidae within the suborder Eimeriina.