Sequences of nuclear-encoded small-subunit rRNA genes have been determined for representatives of the enigmatic genera Dermocystidium, Ichthyophonus, and Psorospermium, protistan parasites of fish and crustaceans. The small-subunit rRNA genes from these parasites and from the "rosette agent" (also a parasite of fish) together form a novel, statistically supported clade. Phylogenetic analyses demonstrate this clade to diverge near the animal-fungal dichotomy, although more precise resolution is problematic. In the most parsimonious and maximally likely phylogenetic frameworks inferred from the most stably aligned sequence regions, the clade constitutes the most basal branch of the metazoa; but within a limited range of model parameters, and in some analyses that incorporate less well-aligned sequence regions, an alternative topology in which it diverges immediately before the animal-fungal dichotomy was recovered. Mitochondrial cristae of Dermocystidium spp. are flat, whereas those of Ichthyophonus hoferi appear tubulovesiculate. These results extend our understanding of the types of organisms from which metazoa and fungi may have evolved.

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ABSTRACT-Leslons found in bay scallops Argopecten ~r r a d imore than 1 organism associated with this name which ans imported into Atlantic Canada for culture were attributed may later be described outside the genus Perkinsus, to a parasite Perklnsus karlssonj It appears, however, that Bay scallops Argopecten irradians imported into stages froin several organisms were combined to create the life cvcle of P karlsson~. (McGladdery et al. 1991, McGladdery et al. 1993). There are significant differences, however, between P. karlssoni and the other 3 described species of Perkinsus. P. karlssoni is used hereafter as a matter of convenience with the recognition that there may be 'Present address CSIRO Divls~on of Fishenes, GPO Box 1538, Hobart, Tasmania 7001, Australia E-mall louise goggin@ml csiro au Perkinsus. P. karlssoni which were healthy when preserved have not been seen under the electron microscope. The organisms identified as P. karlssoni in electron micrographs published by Whyte et al. (1994) (Fig. 18a, b) are dead organisms encapsulated by ceroid and do not resemble Perkinsus.In histological sections, the meront of species of Perkinsus is distinguished by a large eccentric vacuole which sometimes contains a vacuoplast. A vacuole was rarely observed in P. karlssoni, but a dense basophilic body was found next to the nucleus which could b e equivalent to the vacuoplast of P. marinus (McGladdery et al. 1991). The basophilic body of P. karlssoni, however, was not contained in a large eccentric vacuole, which makes this association unlikely. Furthermore, the presence of a basophilic body does not diagnose a Perkinsus spp. because the vacuoplast can be either basophilic or eosinophilic in P. marinus (Mackin et al. 1950) or slightly eosinophilic (when present) in P. olseni (Lester & Davis 1981).Perkinsus karlssoni differs from described species of Perkinsus in other ways. P. karlssoni was not recognised by an antibody raised against the meront of P. O Inter-Research 1996 Resale of full a r t~c l e not p e r m~t t e d

Improved rates of in vitro excystation of sporozoites from sporocysts of Sarcocystis capracanis, Sarcocystis cruzi, and Sarcocystis tenella were obtained by pretreating sporocysts with an aqueous sodium hypochlorite (NaOCl) solution followed by incubation in excysting fluid (EF). After pretreatment with NaOCl, sporocysts were washed 4 times in Hanks' balanced salt solution and then incubated in various EF (pH 7.4) at 38.5 C in 5% CO2-95% air. Maximum rates of excystation (free sporozoites/(sporozoites in sporocysts + free sporozoites) X 100) for all 3 species of Sarcocystis occurred at 4 hr after incubation in EF. These rates were 17% for S. capracanis after incubation in EF containing 2% trypsin + 10% caprine bile; 90% for S. cruzi in 2% trypsin + 10% bovine bile; and 20% for S. tenella in 2% trypsin + 10% caprine bile. Only a 40% excystation rate occurred in sporocysts of S. cruzi that had been stored previously for 14 days in aqueous potassium dichromate. Excysted sporozoites of S. capracanis, S. cruzi, and S. tenella penetrated and developed to mature meronts in bovine pulmonary artery endothelial cells or bovine monocytes.

The development of the sarcocyst of Sarcocystis rauschorum in its intermediate host was studied. Lemmings were orally administered sporocysts of S. rauschorum obtained from snowy owls (Nyctea scandiaca). Beginning at 9 days postinoculation (DPI) and at various intervals to 84 DPI, skeletal muscle tissue taken from the infected lemmings was examined by electron microscopy. At 9 DPI the sarcocysts contained few metrocytes and the cyst wall was flat. The metrocytes underwent endodyogeny, and within a few days the cyst wall of the rapidly growing sarcocyst developed numerous tubulovesicular invaginations into the electron-dense layer, and the wall had a few irregular infoldings. By 21 DPI, banana-shaped bradyzoites appeared, and by 84 DPI the mature cysts were filled with bradyzoites in groups subdivided by septa and by deep infoldings of the cyst wall. The fine structure of the wall remained simple throughout maturation, with no conspicuous invagination or protrusion. The sarcocyst produced in response to S. rauschorum is unlike those from many species of Sarcocystis, which have complex walls that change markedly as the sarcocysts mature; however, its simple appearance is similar to other species that have rodents as intermediate hosts and raptorial birds as definitive hosts.