Abstract
There are several distinct groups of anaerobic eukaryotes that lack canonical mitochondria, and live in oxygen-poor habitats. The most diverse of these groups is the taxon Metamonada, which includes diplomonads, parabasalids, oxymonads, and their relatives.
Almost all of these organisms are unicellular flagellates, and most described species are intestinal commensals or parasites, although there are some free-living forms as well. Most diplomonads (Diplomonadida) have a characteristic doubled organization, with two nuclei and two flagellar apparatuses per cell.
Parabasalids (Parabasalia) range from small cells with a few flagella to very large (0.5 mm) multiflagellated cells. Oxymonads are small-to-large cells (~10-200 µm), some of which attach to the gut wall of their hosts.
Almost all larger oxymonads and parabasalids are symbionts of the hindguts of wood-eating insects, especially termites, and many assist in cellulose digestion. Some diplomonads and small parabasalids are parasitic.
The best known of these are Giardia intestinalis, a diplomonad that causes a very common diarrheal disease in humans, and Trichomonas vaginalis, a parabasalid that causes urogenital trichomoniasis, the most prevalent sexually transmitted infection of humans. Other species cause serious diseases in domestic animals.
Most metamonads, including parabasalids and diplomonads, have organelles that share a common ancestry with mitochondria, but differ biochemically. The parabasalid organelle is a hydrogenosome that generates ATP anaerobically with hydrogen gas as one of its waste products.
Giardia has mitosomes, which do not produce ATP but retain the mitochondrial iron-sulfur cluster (ISc) assembly machinery. By contrast, the oxymonad Monocercomonoides is the only living eukaryote shown to have completely dispensed with the mitochondrial organelle.