ULTRASTRUCTURAL FEATURES OF CYSTIC AND MEROGONIC STAGES OF HEPATOZOONSIPEDON (APICOMPLEXA, ADELEORINA) IN NORTHERN LEOPARD FROGS (RANA-PIPIENS) AND NORTHERN WATER SNAKES (NERODIA SIPEDON) FROM ONTARIO, CANADA

The cystic and merogonic stages of the haemogregarine Hepatozoon siped on, infecting Northern water snakes (Nerodia sipedon sipedon) and Nort hern leopard frogs (Rana pipiens), respectively, in Ontario, Canada, w ere investigated by transmission electron microscopy. Cysts, which wer e observed in the liver of Northern leopard frogs (Rana pipiens) after these anurans ingested mosquitoes (Culex pipiens) containing oocysts of the parasite, harboured two cystozoites, each of which contained a large crystalloid inclusion anterior to the nucleus. Two types of mero nts were observed in snakes that were fed the liver of infected frogs. Macromeronts, which matured in endothelial cells of the liver approxi mately 16 d after snakes ingested infected frogs, contained about 50 l arge macromerozoites. Macromerozoites emerged from macromeronts, enter ed the bloodstream of the snake, and reinfected endothelial cells cell s. Mircomeronts, which matured about 34 d post-inoculation. contained about 150 micromerozoites that infected erythrocytes and transformed i nto gamonts. The ultrastructural features of micromeronts and macromer onts differed only slightly: immature macromeronts and macromerozoites contained numerous amylopectin and lipid inclusions, whereas immature micromeronts and micromerozoites did not contain amylopectin inclusio ns and featured fewer, smaller lipid inclusions. A comparison of cysti c stages among Heptatozoon species in different groups of vertebrates is presented with respect to their structure and evolutionary signific ance.