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 the ...

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

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

'', which causes giardiasis in humans. They are placed among the metamonads, and appear to be particularly close relatives of the retortamonads.

Morphology

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 us ...

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 Cell (biology), cells. Part of the endomembrane system in the cytoplasm, it protein targeting, packages proteins ...

es. However, they are now known to possess modified mitochondria, in the case of ''G. duodenalis'', called mitosomes. These are not used in ATP synthesis the way mitochondria are, but are involved in the maturation of iron-sulfur proteins. 2023 Diplomonad

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 ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancest ...

lineage that predated the emergence of

meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

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 The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

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 Cell (biology), cellular organi ...

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 h ...

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 parasites of the phylum Metamonada that colonise and reproduce in the small intestines of several vertebrates, causing the disease giardiasis. Their life cycle alternates be ...

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

References

{{Authority control Flagellates Metamonads