Degenerate mitochondria

Abstract

Mitochondria are the main sites of biological energy generation in eukaryotes. These organelles are remnants of a bacterial endosymbiont that took up residence inside a host cell over 1,500 million years ago. Comparative genomics studies suggest that the mitochondrion is monophyletic in origin. Thus, the original mitochondrial endosymbiont has evolved independently in anaerobic and aerobic environments that are inhabited by diverse eukaryotic lineages. This process has resulted in a collection of morphologically, genetically and functionally heterogeneous organelle variants that include anaerobic and aerobic mitochondria, hydrogenosomes and mitosomes. Current studies aim to determine whether a central common function drives the retention of mitochondrial organelles in different eukaryotic organisms.

Figure 1

Electron micrographs of different mitochondrial manifestations. (A) Mitochondrion from chicken cerebellum. (B,C) Hydrogenosomes from (B) the anaerobic fungus Neocallimastix patriciarum and (C) the cattle parasite Tritrichomonas foetus. (D–F) Mitosomes from (D) the intestinal parasite Entamoeba histolytica, (E) the microsporidian Trachipleistophora hominis and (F) the diplomonad Giardia intestinalis. Scale bars: (A–D) 100 nm and (E,F) 50 nm. (A) Kindly provided by T.G. Frey, San Diego State University, CA, USA, and G.A. Perkins, University of California, San Diego, CA, USA; (B,C) kindly provided by M. Benchimol, Universidade Santa Ursula, Brazil; (D) reproduced with permission from the American Society for Microbiology; (E,F) reproduced with permission from Nature Publishing Group, Macmillan Limited.

Figure 2

Schematic representation of the distribution of anaerobic microbial eukaryotes on a universal tree of life. The presence of mitochondrial-derived organelles is indicated: H, hydrogenosome; M, mitosome; ?, unconfirmed or suspected. Anaerobic microbial eukaryotes are shown in brown.

Figure 3

Evolution of mitochondria and mitochondrion-related organelles. (A) Vertical descent from the original mitochondrial endosymbiont. (B) Chimeric origins of mitosomes and hydrogenosomes from sequential endosymbionts. See text for details. Mitochondria/α-proteobacteria are shown in blue, hydrogenosomes in red, mitosomes in lime and anaerobic eubacterium in turquoise. N, nucleus.