Monocercomonoides

Monocercomonoides is a genus of flagellate Excavata belonging to the order Oxymonadida. It was established by Bernard V. Travis and was first described as those with "polymastiginid flagellates having three anterior flagella and a trailing one originating at a single basal granule located in front of the anteriorly positioned nucleus, and a more or less well-defined axostyle". It is the first eukaryotic genus to be found to completely lack mitochondria, and all hallmark proteins responsible for mitochondrial function. The genus also lacks any other mitochondrion-related organelles such as hydrogenosomes or mitosomes. Data suggests that the absence of mitochondria is not an ancestral feature, but rather due to secondary loss. Monocercomonoides sp. was found to obtain energy through an enzymatic action of nutrients absorbed from the environment. The genus has replaced the Iron–sulfur cluster assembly pathway with a cytosolic sulfur mobilization system, likely acquired by horizontal gene transfer from a eubacterium to a common ancestor of oxymonads. These organisms are significant because they undermine assumptions that eukaryotes must have mitochondria to function properly. The genome of Monocercomonoides exilis has approximately 82 million base pairs (82 Mbp), with 18 152 predicted protein-coding genes.

== Habitat and ecology == Most Monocercomonoides species are obligate animal symbionts that live in the digestive tracts of insects, amphibians, reptiles, and mammals. Monocercomonoides are common in insect orders Orthoptera and Coleoptera. The species Monocercomonoides qadrii are found in the rectum of the larva of the dung-beetle (Oryctes rhinoceros). M. caviae, M. wenrichi, M. quadrifunilis, and M. exilis are found in the caecum of guinea pigs, and M. caprae has been found in the rumen of goats. Interestingly, some Monocercomonoides species were isolated from cesspits, suggesting that they might be able to survive outside of the host in certain environmental conditions. The organism uses enzymes found in its cytoplasm to break down food and furnish energy since there is no mitochondria or oxygen presence. It has been noted that Monocercomonoides ingest bacteria or wood and feed by pinocytosis, however, limited studies have been done on feeding style.