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.