Diplomonadida

The diplomonads ("two units") are characterized by their possession of two nuclei that are similar in appearance, replication, and function. Together with the Carpediemonas-like organisms and retortamonads, the diplomonads are classified within Fornicata. Each "unit" of the diplomonad cell includes a karyomastigont that has one nucleus and (usually) four flagella, which are used for locomotion. Thus, most diplomonads have two karyomastigonts. However, the "enteromonads" present an exception in that they have a single karyomastigont per cell. The diplomonads have anaerobic metabolism and lack conventional mitochondria, so they were thought to be pre-mitochondriate organ- isms. However, they have subsequently been shown to have highly reduced mitochondria called mitochondrion-related organelles (MRO) that perform some of the functions of conventional mitochondria. The most studied diplomonads are the Giardia species, which are intestinal pathogens or commen- sals for a variety of vertebrates from amphibians to mammals and include pathogens of humans. Like Giardia spp., the Spironucleus species also replicate in the host intestine, in this case in vertebrates or invertebrates and include notable fish pathogens. In contrast, Hexamita and Trepomonas species can be either free- living or parasitic.