Metamonad
Encyclopedia
The metamonads are a large group of flagellate
protozoa. Their composition is not entirely settled, but they include the retortamonad
s, diplomonad
s, and possibly the parabasalid
s and oxymonad
s as well. These four groups are all anaerobic
, occurring mostly as symbiote
s of animals.
guts, and play an important role in breaking down the cellulose
found in wood. Some other metamonads are parasites.
These flagellates are unusual in lacking mitochondria
. Originally they were considered among the most primitive eukaryote
s, diverging from the others before mitochondria appeared. However, they are now known to have lost mitochondria secondarily, and retain both organelles and nuclear genes derived from them. Mitochondrial relics include hydrogenosome
s, which produce hydrogen, and small structures called mitosome
s.
All of these groups are united by having flagella
or basal bodies in characteristic groups of four, which are often associated with the nucleus
, forming a structure called a karyomastigont. In addition, the genera Carpediemonas and Trimastix are now known to be close relatives of the retortamonad-diplomonad line and the oxymonads, respectively. Both are free-living and amitochondriate.
s, a eukaryotic supergroup including flagellates with feeding grooves and their close relatives. Their relationships are uncertain, and they do not always appear together on molecular trees. It is possible that the metamonads as defined here do not form a monophyletic subgroup.
Flagellate
Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...
protozoa. Their composition is not entirely settled, but they include the retortamonad
Retortamonad
The retortamonads are a small group of flagellates, mostly found in the intestines of animals, although some are free-living. They are usually around 5-20 μm in length. There are two genera: Retortamonas with two flagella, and Chilomastix with four...
s, diplomonad
Diplomonad
The diplomonads are a group of flagellates, most of which are parasitic. They include most notably Giardia lamblia, which causes giardiasis in humans...
s, and possibly the parabasalid
Parabasalid
The parabasalids are group of flagellate protozoa, most of which are symbiotic in animals. These include a variety of forms found in the guts of termites and cockroaches, many of which have symbiotic bacteria that help them digest wood. Some other species are human pathogens.-Characteristics:The...
s and oxymonad
Oxymonad
The Oxymonads are a group of flagellated protozoa found exclusively in the intestines of termites and other wood-eating insects. Along with the similar parabasalid flagellates, they harbor the symbiotic bacteria that are responsible for breaking down cellulose...
s as well. These four groups are all anaerobic
Anaerobic organism
An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present...
, occurring mostly as symbiote
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
s of animals.
Characteristics
A number of parabasalids and oxymonads are found in termiteTermite
Termites are a group of eusocial insects that, until recently, were classified at the taxonomic rank of order Isoptera , but are now accepted as the epifamily Termitoidae, of the cockroach order Blattodea...
guts, and play an important role in breaking down the cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
found in wood. Some other metamonads are parasites.
These flagellates are unusual in lacking mitochondria
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
. Originally they were considered among the most primitive eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s, diverging from the others before mitochondria appeared. However, they are now known to have lost mitochondria secondarily, and retain both organelles and nuclear genes derived from them. Mitochondrial relics include hydrogenosome
Hydrogenosome
A hydrogenosome is a membrane-enclosed organelle of some anaerobic ciliates, trichomonads and fungi. The hydrogenosomes of trichomonads produce molecular hydrogen, acetate, carbon dioxide and ATP by the combined actions of pyruvate:ferredoxin oxido-reductase, hydrogenase, acetate:succinate CoA...
s, which produce hydrogen, and small structures called mitosome
Mitosome
A mitosome is an organelle found in some unicellular eukaryotic organisms. The mitosome has only recently been found and named, and its function has not yet been well characterized. It was termed a 'crypton' by one group, but that name is no longer in use....
s.
All of these groups are united by having flagella
Flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
or basal bodies in characteristic groups of four, which are often associated with the nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
, forming a structure called a karyomastigont. In addition, the genera Carpediemonas and Trimastix are now known to be close relatives of the retortamonad-diplomonad line and the oxymonads, respectively. Both are free-living and amitochondriate.
Classification
The metamonads make up part of the excavateExcavate
The excavates are a major kingdom of unicellular eukaryotes, often known as Excavata. The phylogenetic category Excavata, proposed by Cavalier-Smith in 2002, contains a variety of free-living and symbiotic forms, and also includes some important parasites of humans.-Characteristics:Many excavates...
s, a eukaryotic supergroup including flagellates with feeding grooves and their close relatives. Their relationships are uncertain, and they do not always appear together on molecular trees. It is possible that the metamonads as defined here do not form a monophyletic subgroup.