2005 年 38 巻 2 号 p. 171-183
Resent insights into the origin and early evolution of mitochondria come from two approaches: the investigation of mtDNAs from minimally derived (primitive) mitochondriate eukaryotes, in particular jakobid flagellates, and of genomes from intracellular α-proteobacterial symbionts. Of particular interest in this context is Holospora obtusa, an intracellular bacterial endosymbiont that resides and replicates in the somatic nucleus of its eukaryotic host, the ciliate Paramecium caudatum. Currently we have sequenced close to 50% of the ～1.7 Mbp H. obtusa genome, revealing the absence of genes for oxidative phosphorylation, the TCA cycle, and many other metabolic pathways, but the presence of several pathogenesis-related genes and a high number of bacterial IS elements. Phylogenetic analyses with multiple protein sequences place H. obtusa basally to the Rickettsia-Ehrlichia-Wolbachia assemblage of bacterial pathogens. This leads us to postulate that H. obtusa is the closest bacterial relative of mitochondria known to date.
