Monocercomonoides

Monocercomonoides is a genus of flagellate Excavata belonging to the order Oxymonadida. Monocercomonoides species have been discovered living in the guts of small mammals, snakes, and insects. The genome of Monocercomonoides has approximately 75 million base pairs (75 Mbp), with 16629 predicted protein-coding genes.

Many excavates lack "classical" mitochondria. Oxymonads lack true mitochondria and Golgi apparatus. Monocercomonoides has been characterized as the first example of a eukaryotic organism devoid of mitochondria. Its genome contains no mitochondrial DNA (mtDNA), and no genes for cardiolipin, a lipid for energy-transducing membranes.

In 2016, Monocercomonoides sp. PA 203, a Monocercomonoides species which resides in the intestines of the chinchilla, was found to possess no mitochondria at all. Monocercomonoides sp. PA 203 obtains its energy by enzymatic action on nutrients absorbed from the environment. It has also acquired, by horizontal gene transfer, a cytosolic sulfur mobilisation system to provide essential iron–sulfur clusters required for protein synthesis. The normal mitochondrial iron–sulfur cluster pathway is considered to have been lost secondarily.