The diplomonads (Greek for "two units") are a group of

flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and thei ...

s, most of which are parasitic. They include ''

Giardia duodenalis ''Giardia duodenalis'', also known as ''Giardia intestinalis'' and ''Giardia lamblia'', is a flagellated parasitic microorganism of the genus ''Giardia'' that colonizes the small intestine, causing a diarrheal condition known as giardiasis. The ...

'', which causes giardiasis in humans. They are placed among the

metamonad The metamonads are microscopic eukaryotic organisms, a large group of flagellate amitochondriate Loukozoa. Their composition is not entirely settled, but they include the retortamonads, diplomonads, and possibly the parabasalids and oxymonads a ...

s, and appear to be particularly close relatives of the

retortamonad The retortamonads are a small group of flagellates, most commonly found in the intestines of animals as commensals, although a free-living species called the ''Chilomastix cuspidata'' exists. They are grouped under the taxon, Archezoa. They are ...

s. Most diplomonads are double cells: they have two nuclei, each with four associated flagella, arranged symmetrically about the body's main axis. Like the retortamonads, they lack both

mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...

and

Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles i ...

es. However, they are now known to possess modified mitochondria, in the case of ''G. duodenalis'', called

mitosome A mitosome is an organelle found in some unicellular eukaryotic organisms, like in members of the supergroup Excavata. The mitosome was found and named in 1999, and its function has not yet been well characterized. It was termed a ''crypton'' by ...

s. These are not used in ATP synthesis the way mitochondria are, but are involved in the maturation of

iron-sulfur protein Iron–sulfur proteins (or iron–sulphur proteins in British spelling) are proteins characterized by the presence of iron–sulfur clusters containing sulfide-linked di-, tri-, and tetrairon centers in variable oxidation states. Iron–sulfur cl ...

Possible sexual reproduction in ''Giardia''

The common intestinal parasite ''

'' (synonyms ''Giardia lamblia'', ''G. intestinalis'') was once considered to be a descendant of a

protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the e ...

lineage that predated the emergence of

meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...

and sex. However, researchers found ''G. duodenalis'' to have a core set of genes that function in meiosis and that are widely present among sexual

eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...

s. These results suggested that ''Giardia duodenalis'' is capable of meiosis and thus sexual reproduction. Furthermore, Cooper et al. found direct evidence in ''Giardia duodenalis'' for infrequent meiotic recombination, indicative of sexual reproduction between individuals. Lasek-Nesselquist et al. also detected molecular signatures consistent with meiotic sex. The possibility of sexual reproduction is still debated. ''Giardia duodenalis'' contains two functionally equivalent nuclei that are inherited independently during mitosis. In the giardial cyst these nuclei fuse ( karyogamy) and undergo

homologous recombination Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may ...

facilitated by meiosis gene homologs. The recombination associated with karyogamy may primarily function to repair DNA damage. ''Giardia duodenalis'' is divided into eight assemblages based on host specificities and

genetic divergence Genetic divergence is the process in which two or more populations of an ancestral species accumulate independent genetic changes ( mutations) through time, often leading to reproductive isolation and continued mutation even after the populations ha ...

of marker genes. Although recombination can occur infrequently within assemblages, Xu et al. found that recombination between individuals from different assemblages is very rare. They suggested that the assemblages are genetically isolated lineages, and thus could be viewed as separated ''

Giardia ''Giardia'' ( or ) is a genus of anaerobic flagellated protozoan parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and ...

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...

References

{{Taxonbar, from=Q136575 Flagellates Metamonads