Breviatea

Breviatea, commonly known as breviate amoebae, are a group of free-living, amitochondriate protists with uncertain phylogenetic position. They are biflagellate, and can live in anaerobic (oxygen-free) environments. They are currently placed in the Obazoa clade. They likely do not possess vinculin proteins. Their metabolism relies on fermentative production of ATP as an adaptation to their low-oxygen environment.

The lineage emerged roughly one billion years ago, at a time when the oxygen content of the Earth's oceans was low, and they thus developed anaerobic lifestyles. Together with Apusomonads, they are the closest relatives of the Opisthokonts, a group that includes animals and fungi.

Mitochondrion-related organelles
Mitochondrion-related organelles (MROs) are organelles that evolved from a degradation of ancestral, fully functional mitochondria. Among Breviatea, MROs are present in Pygsuia, Breviata and Subulatomonas. In the cells of Pygsuia, for which the complete transcriptome is known, there is a single smooth MRO that lacks a mitochondrial genome and most components of the electron transport chain. Of the citric acid cycle enzymes, which are present in the mitochondria in other organisms, only two are present in Pygsuia: fumarase and succinate dehydrogenase. In contrast, Lenisia cells contain multiple MROs with cristae.

Evolution
Breviatea is a clade of basal eukaryotes. They are closely related to the apusomonads and the Opisthokonta supergroup, and together they compose the larger clade Obazoa, which is the sister group to Amoebozoa. Within Breviatea, the four known species are distributed into smaller clades of two species each: one uniting Breviata with Subulatomonas, and one uniting Lenisia with Pygsuia.

History
The class Breviatea was created in 2004 by British protozoologist Thomas Cavalier-Smith to group a problematic taxon previously known as ‘Mastigamoeba invertens’. This organism, initially classified in the Archamoebae within phylum Amoebozoa, appeared to strongly diverge in phylogenetic trees based on ribosomal RNA and had a structure very different from other Archamoebae. Because of these results, ''‘M. invertens’'' was separated into the order Breviatida, contained in the monotypic class Breviatea. The organism was eventually renamed Breviata anathema. A family-level rank for these amoebae, Breviatidae, was formally described by the same author in 2013.

Classification
There are currently four accepted genera, each containing only one species.
 * Breviata
 * Breviata anathema
 * Lenisia
 * Lenisia limosa
 * Pygsuia
 * Pygsuia biforma
 * Subulatomonas
 * Subulatomonas tetraspora