The parabasalids are a group of flagellated

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 ...

s within the supergroup Excavata. Most of these

eukaryotic 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 ...

organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...

s form a

symbiotic Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biolo ...

relationship in

animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...

s. These include a variety of forms found in the intestines of

termite Termites are a group of detritivore, detritophagous Eusociality, eusocial cockroaches which consume a variety of Detritus, decaying plant material, generally in the form of wood, Plant litter, leaf litter, and Humus, soil humus. They are dist ...

s and

cockroach Cockroaches (or roaches) are insects belonging to the Order (biology), order Blattodea (Blattaria). About 30 cockroach species out of 4,600 are associated with human habitats. Some species are well-known Pest (organism), pests. Modern cockro ...

es, many of which have symbiotic

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

that help them digest

cellulose Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...

woody plant A woody plant is a plant that produces wood as its structural tissue and thus has a hard stem. In cold climates, woody plants further survive winter or dry season above ground, as opposed to Herbaceous plant, herbaceous plants that die back to t ...

s. Other species within this supergroup are known

parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...

s, and include human

pathogen In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...

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 are associated with a prominent

Golgi complex 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 ...

, together forming a parabasal apparatus distinctive to the group. Attachment of a parabasal fiber to the first Golgi cisterna by thin filaments has been reported in '' Tritrichomonas foetus''. Usually they also give rise to a sheet of cross-like

microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nanometer, nm and have an inner diameter bet ...

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 The Oxymonads (or Oxymonadida) are a group of flagellated protists found exclusively in the intestines of animals, mostly termites and other Xylophagy, wood-eating insects. Along with the similar parabasalid flagellates, they harbor the Symbiosi ...

s. Parabasalids are anaerobic, and lack

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 ...

, but this is now known to be a result of secondary loss, and they contain small hydrogenosomes 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 metamonads. They lack the feeding grooves found in most others, but this is probably a secondary loss as well. 2023 Parabasalian

Classification

Before reclassification, the parabasalids were divided into about seven to 10 orders depending on sources. Present classification divides Parabasalia into four orders, that is, Trichonymphida, Spirotrichonymphida, Cristamonadida, and Trichomonadida. * The trichomonads 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 vaginalis ''Trichomonas vaginalis'' is an Anaerobic organism, anaerobic, flagellated protozoan parasite and the causative agent of a Sexually transmitted infection, sexually transmitted disease called trichomoniasis. It is the most common pathogenic protoz ...

'', 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.)

Evolution

The parabasalid ''Trichomonas vaginalis'' is not known to undergo

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 ...

. However, Malik ''et al.'' examined ''T. vaginalis'' for the presence of 29 genes that function in meiosis and found 27 such genes, including eight genes specific to meiosis in

model organism A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms. Mo ...

s. These findings suggested that the capability for meiosis, and hence sexual reproduction, was likely present in a recent parabasalid ancestor of ''T. vaginalis''.

References

{{Taxonbar, from=Q134585 Flagellates Metamonads