Parabasalid
Encyclopedia
The parabasalids are group of flagellate
protozoa
, most of which are symbiotic
in animal
s. These include a variety of forms found in the guts of termite
s and cockroach
es, many of which have symbiotic bacteria that help them digest wood. Some other species are human pathogen
s.
, distinctive to the group. Usually they also give rise to a sheet of cross-like microtubule
s that runs down the center of the cell and in some cases projects past the end. This is called the axostyle
, but is different in structure from the axostyles of oxymonad
s.
Parabasalids are anaerobic
, and lack mitochondria
, but this is now known to be a result of secondary loss, and they contain small hydrogenosome
s which apparently developed from reduced mitochondria. Similar relics have been found in other amitochondriate flagellates, and the parabasalids are probably related to them, forming a group called the metamonad
s. They lack the feeding grooves found in most others, but this is probably a secondary loss as well.
.
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
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
, most of which are symbiotic
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...
in animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s. These include a variety of forms found in the guts of termite
Termite
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...
s and cockroach
Cockroach
Cockroaches are insects of the order Blattaria or Blattodea, of which about 30 species out of 4,500 total are associated with human habitations...
es, many of which have symbiotic bacteria that help them digest wood. Some other species are human pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s.
Characteristics
The flagella are arranged in one or more clusters near the anterior of the cell. Their basal bodies are linked to parabasal fibers that attach to prominent Golgi complexesGolgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
, distinctive to the group. Usually they also give rise to a sheet of cross-like microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
s that runs down the center of the cell and in some cases projects past the end. This is called the axostyle
Axostyle
An axostyle is a sheet of microtubules found in certain microbial eukaryotes. It arises from the bases of the flagella, sometimes projecting beyond the end of the cell, and is often flexible or contractile, and so may be involved in movement and provides support for the cell...
, but is different in structure from the axostyles of 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.
Parabasalids are 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...
, and lack 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...
, but this is now known to be a result of secondary loss, and they contain small 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 apparently developed from reduced mitochondria. Similar relics have been found in other amitochondriate flagellates, and the parabasalids are probably related to them, forming a group called the metamonad
Metamonad
The metamonads are a large group of flagellate protozoa. Their composition is not entirely settled, but they include the retortamonads, diplomonads, and possibly the parabasalids and oxymonads as well...
s. They lack the feeding grooves found in most others, but this is probably a secondary loss as well.
Classification
The parabasalids are currently divided into 10 ordersOrder (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
.
- The trichomonadTrichomonadThe trichomonads are an order of anaerobic protists, included with the parabasalids. Most are either parasites or other endosymbionts of animals. They typically have four to six flagella at the cell's apical pole, one of which is recurrent - that is, it runs along a surface wave, giving the...
s have one group of 4-6 flagella, one of which is attached to the side of the cell and often forms an undulating membrane. Many are found in vertebrate hosts, including Trichomonas vaginalisTrichomonas vaginalisTrichomonas vaginalis is an anaerobic, flagellated protozoan, a form of microorganism. The parasitic microorganism is the causative agent of trichomoniasis, and is the most common pathogenic protozoan infection of humans in industrialized countries. Infection rates between men and women are the...
, which causes a sexually transmitted disease in humans. - The other orders, formerly grouped as the hypermastigids, have a large number of flagellar clusters and are found exclusively in the guts of insects. (The term "Hypermastigida" is still occasionally encountered.)