Electron tomographic and ultrastructural analysis of the Cryptosporidium parvum relict mitochondrion, its associated membranes, and organelles.

Sporozoites of the apicomplexan Cryptosporidium parvum possess a small, membranous organelle sandwiched between the nucleus and crystalloid body. Based upon immunolabelling data, this organelle was identified as a relict mitochondrion. Transmission electron microscopy and tomographic reconstruction reveal the complex arrangement of membranes in the vicinity of this organelle, as well as its internal organization. The mitochondrion is enveloped by multiple segments of rough endoplasmic reticulum that extend from the outer nuclear envelope. In tomographic reconstructions of the mitochondrion, there is either a single, highly-folded inner membrane or multiple internal subcompartments (which might merge outside the reconstructed volume). The infoldings of the inner membrane lack the tubular "crista junctions" found in typical metazoan, fungal, and protist mitochondria. The absence of this highly conserved structural feature is congruent with the loss, through reductive evolution, of the normal oxidative phosphorylation machinery in C. parvum. It is proposed that the retention of a relict mitochondrion in C. parvum is a strategy for compartmentalizing away from the cytosol toxic ferrous iron and sulfide, which are needed for iron sulfur cluster biosynthesis, an essential function of mitochondria in all eukaryotes.