Direct evidence for secondary loss of mitochondria in Entamoeba histolytica.

Archezoan protists are though to represent lineages that diverged from other eukaryotes before acquisition of the mitochondrion and other organelles. The parasite Entamoeba histolytica was originally included in this group. Ribosomal RNA based phylogenies, however, place E. histolytica on a comparatively recent branch of the eukaryotic tree, implying that its ancestors had these structures. In this study, direct evidence for secondary loss of mitochondrial function was obtained by isolating two E. histolytica genes encoding proteins that in other eukaryotes are localized in the mitochondrion: the enzyme pyridine nucleotide transhydrogenase and the chaperonin cpn60. Phylogenetic analysis of the E. histolytica homolog of cpn60 confirmed that it is specifically related to the mitochondrial lineage. The data suggest that a mitochondrial relic may persist in this organism. Similar studies are needed in archezoan protists to ascertain which, if any, eukaryotic lineages primitively lack mitochondria.

Elimination of paternal mitochondrial DNA in intraspecific crosses during early mouse embryogenesis.

To examine whether mtDNA is uni- or biparentally transmitted in mice, we developed an assay that can detect sperm mtDNA in a single mouse embryo. In intraspecific hybrids of Mus musculus, paternal mtDNA was detected only through the early pronucleus stage, and its disappearance co-incided with loss of membrane potential in sperm-derived mitochondria. By contrast, in interspecific hybrids between M. musculus and Mus spretus, paternal mtDNA was detected throughout development from pronucleus stage to neonates. We propose that oocyte cytoplasm has a species-specific mechanism that recognizes and eliminates sperm mitochondria and mtDNA. This mechanism must recognize nuclearly encoded proteins in the sperm midpiece, and not the mtDNA or the proteins it encodes, because sperm mitochondria from the congenic strain B6.mtspr, which carries M. spretus mtDNA on background of M. musculus (B6) nuclear genes, were eliminated early by B6 oocytes as in intraspecific crosses. We conclude that cytoplasmic genomes are transmitted uniparentally in intraspecific crosses in mammals as in Chlamydomonas and that leakage of parental mtDNA is limited to interspecific crosses, which rarely occur in nature.