Ultrastructure of Sarcocystis Sporocysts from Passerine Birds and Opossums: Comments on Classification of the Genus Isospora

Abstract

Sporocysts were obtained from the feces of opossums (Didelphis virginiana) which had been fed muscles of passerine birds (Molothrus ater and Cassidix mexicanus) infected with Sarcocystis. Sporocysts were examined by phase microscopy and scanning and transmission electron microscopy. Ridges on the surface of the sporocysts outlined 4 plates. Thin sections of the sporocyst wall showed thickened regions and gaps interpreted as cross sections of the ridges. The sporocyst wall has 4 major layers with a thick, granular inner layer of sporocysts of related species. Excystation structures are discussed as a basis for classifying disporocystic coccidia to correlate with Frenkel''s (1977) life cycle classification of this group.