Hyperamoeba isolated from human feces: Description and phylogenetic affinity

The morphology and phylogenetic affinity of Hyperamoeba isolated from human feces is described. During its life cycle, it switches reversibly from fla gellate to aflagellated amoebae and is capable of forming cysts. It grows a erobically. Under anaerobic conditions it persists but does not replicate. The amoeboflagellate has a single nucleus with a distinct nucleolus. Its mi tochondria possess tubular cristae and a central electron dense body, simil ar to that of plasmodial slime molds. A single contractile vacuole is evide nt. The flagellate has one detectable anterior flagellum but two basal bodi es are visible at the ultrastructure level. The flagellar apparatus is very similar to that found in some Eumycetozoa, especially the myxogastrids. Th e uninucleate cyst has a bi-layered endocyst and a membranous, irregular sh aped, faintly laminated ectocyst that harbors bacterial inclusions. Phyloge netic reconstructions based on nuclear small subunit ribosomal gene sequenc e comparisons show that Hyperamoeba is closely related to the plasmodial sl ime mold Physarum polycephalum. These protists share a most recent common a ncestry that excludes all other taxa in the database. This phylogenetic rel ationship is supported by detailed similarities in both mitochondrial and f lagellar apparatus ultrastructure.