A study of the fine structure and the mitosis of a lamellicristate amoeba, Micronuclearia podoventralis gen. et sp nov (Nucleariidae, Rotosphaerida)

A study of the fine structure and the mitosis of a new member of the naked filose amoebae (Nucleariiadae, Rotosphaerida), Micronuclearia podoventralis is performed. The amoeba has a single large dictyosome adjoining to the nucleus, 2-4 spherical mitochondria with lamellate cristae, its cell membrane is supported by a thin proteinous pseudopellicle. M. podoventralis has normally a single branching filopodium or 2-4 smaller ones with bases that lie close to each other. The bases of the filopodia arise from the middle of a shallow depression, the curved edge of which is supported by a mucopolysaccharide layer under the plasma membrane and by a thin cuticle over it. The mitosis of M. podoventralis occurs as an open orthomitosis, but the microtubules appear inside the nucleus at prophase before dissolution of the nuclear envelope and before disaggregation of the nucleolus. The nuclear envelope has broken down by metaphase by which time sister dictyosomes are found at the poles of the spindle. The phylogenetic source of the lamellicristate sarcodines is discussed.