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The representatives of the family Urosporidae are highly diverse in their morphology at light and electron microscopic level. In addition, they parasitise a wide range of marine and freshwater invertebrates: Annelida, Mollusca, Nemertea, Sipuncula, and Echinodermata. Among species investigated so far on ultrastructural level, there are eugregarines (some Urospora spp.) with typical lecudinid-like morphology: the cell body consisting of a single compartment with an attachment apparatus at the apical end, gliding motility, and the presence of longitudinal epicytic folds on the cell surface. Additionally, there are gregarines (Pterospora spp. and Urospora chiridotae), which are traditionally referred to this family, and possess the completely modified cell-body and cortex organisation. Moreover, some urosporids exhibit the mode of motility that significantly differs from the gliding. Despite these, it is assumed that all urosporid species, sensu lato¸ possess at least one common feature in the oocyst morphology: heteropolar, with a tail or tails at one of the end. It was shown that, in SSU rDNA sequences phylogenetic analyses, gregarines from the family Urosporidae grouped altogether with representatives of the family Lecudinidae into the superclade Lecudinoidea (= Urosporoidea). To this superclade, the unusual gregarine Veloxidium leptosynapta, isolated from a holothurian and initially belonging to the family Archigregarinidae, affiliated as well. We performed the phylogenetic analyses comprising all lecudinid and urosporid species along with environmental sequences available in GenBank. The resulting phylogenetic tree as inferred from SSU rDNA sequences confirms that family Urosporidae comprises several clades to which some lecudinid species affiliated. In contrast, some urosporid species are affiliated to the family Lecudinidae. Interestingly, the coelomic gregarine Urospora chiridotae, described from blood vessels of the holothuroid Chiridota laevis, affiliates to the clade comprising V. leptosynaptae. In conclusion, the taxonomy of the families Urosporidae and Lecudinidae should be reviewed. Financial support provided by GAČR No. P505/12/G112 (ECIP).
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