Ultrastructural Features of Cystic and Merogonic Stages of Hepatozoon sipedon (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 sipedon , infecting Northern water snakes ( Nerodia sipedon sipedon ) and Northern leopard frogs ( Rana pipiens ), respectively, in Ontario, Canada, were investigated by transmission electron microscopy. Cysts, which were 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 meronts were observed in snakes that were fed the liver of infected frogs. Macromeronts, which matured in endothelial cells of the liver approximately 16 d after snakes ingested infected frogs, contained about 50 large macromerozoites. Macromerozoites emerged from macromeronts, entered the bloodstream of the snake, and reinfected endothelial cells. Micromeronts, which matured about 34 d post-inoculation, contained about 150 micromerozoites that infected erythrocytes and transformed into gamonts. The ultrastructural features of micromeronts and macromeronts differed only slightly: immature macromeronts and macromerozoites contained numerous amylopectin and lipid inclusions, whereas immature micromeronts and micromerozoites did not contain amylopectin inclusions and featured fewer, smaller lipid inclusions. A comparison of cystic stages among Hepatozoon species in different groups of vertebrates is presented with respect to their structure and evolutionary significance.