Diversity and reductive evolution of mitochondria among microbial eukaryotes

Abstract

All extant eukaryotes are now considered to possess mitochondria in one form or another. Many parasites or anaerobic protists have highly reduced versions of mitochondria, which have generally lost their genome and the capacity to generate ATP through oxidative phosphorylation. These organelles have been called hydrogenosomes, when they make hydrogen, or remnant mitochondria or mitosomes when their functions were cryptic. More recently, organelles with features blurring the distinction between mitochondria, hydrogenosomes and mitosomes have been identified. These organelles have retained a mitochondrial genome and include the mitochondrial-like organelle of Blastocystis and the hydrogenosome of the anaerobic ciliate Nyctotherus. Studying eukaryotic diversity from the perspective of their mitochondrial variants has yielded important insights into eukaryote molecular cell biology and evolution. These investigations are contributing to understanding the essential functions of mitochondria, defined in the broadest sense, and the limits to which reductive evolution can proceed while maintaining a viable organelle.

Biochemistry and Evolution of Anaerobic Energy Metabolism in Eukaryotes

“…Numerous aspects of the mitochondria of anaerobes and parasites have recently been reviewed elsewhere, including protein import (121), FeS cluster assembly (454,520), bioenergetic aspects (266,267), and reductive evolutionary trends (193). Monographs on the topic of hydrogenosomes and mitosomes have appeared (84,310,480), as have books on the evolutionary significance of mitochondria, including anaerobic forms (264,265).…”

Symbiosis as a General Principle in Eukaryotic Evolution

“…(We cannot, however, exclude the alternative scenario that the ancestor of mitochondria was acquired by bona fide eukaryotic cells, with the subsequent extinction of all amitochondriate eukaryotic lineages.) Furthermore, the mitochondria have undergone substantial evolutionary diversification, including the transformation into hydrogenosomes, functionally distinct organelles that generate ATP with the production of hydrogen and lack oxidative phosphorylation (Hjort et al 2010). Hydrogenosomes have evolved multiple times in secondarily anaerobic eukaryotes, including various ciliate protists and the lorificeran animals in anoxic marine sediments (van der Giezen et al 2005;Danovaro et al 2010).…”

Mitochondrial and plastid genome architecture: Reoccurring themes, but significant differences at the extremes

“…With few exceptions (1,4), mitochondria and plastids contain genomes-chromosomal relics of the bacterial endosymbionts from which they evolved (2,5). Mitochondrial and plastid DNAs (mtDNAs and ptDNAs) have many traits in common, which is not surprising given their similar evolutionary histories.…”

“…Each of these two types of energy-producing eukaryotic organelle independently arose from the endosymbiosis, retention, and integration of a free-living bacterium into a host cell more than 1.4 billion years ago. Mitochondria came first, evolving from an alphaproteobacterial endosymbiont in an ancestor of all known living eukaryotes, and still exist, in one form or another (1), in all its descendants (2). Plastids came later, via the "primary" endosymbiosis of a cyanobacterium by the eukaryotic ancestor of the Archaeplastida, and then spread laterally to disparate groups through eukaryote-eukaryote endosymbioses (3).…”