PHYLOGENETIC ANALYSIS OF COCCIDIA BASED ON 18S RDNA SEQUENCE COMPARISON INDICATES THAT ISOSPORA IS MOST CLOSELY-RELATED TO TOXOPLASMA AND NEOSPORA

The phylogenetic relationships and taxonomic affinities of coccidia wi th isosporan-type oocysts have been unclear as overlapping characters, recently discovered lift: cycle features, and even recently discovere d taxa, continue to be incorporated into biological classifications of the group. We determined the full or partial 18S ribosomal RNA gene s equences of three mammalian Isospora spp., Isospora Sells. Isospora oh ioensis and Isospora suis, and a Sarcocystis sp. of a rattlesnake, and used these sequences for a phylogenetic analysis of the genus Isospor a and the cyst-forming coccidia. Various alveolate 18S rDNA sequences were aligned and analyzed using maximum parsimony to obtain a phylogen etic hypothesis for the group. The three Isospora spp. were found to b e most closely related to Toxoplasma gondii and Neospora caninum. This clade in turn formed the sister group to the Sarcocystis spp. include d in the analysis. The results confirm that the genus Isospora does no t belong to the family Eimeriidae, but should be classified together w ith the cyst-forming coccidia in the family Sarcocystidae. Furthermore , there appear to be two lineages within the Sarcocystidae. One lineag e comprises Isospora and the Toxoplasma/Neospora clade which share the characters of having a proliferative phase of development preceding g amogony in the definitive host and an exogenous phase of sporogony. Th e other lineage comprises the Sarcocystis spp. which have no prolifera tive phase in thr definitive host and an endogenous phase of sporogony .