Parabasalid
Twotrophozoites ofTrichomonas vaginalis stained withGiemsa
Scientific classification
Domain:	Eukaryota
Clade:	Metamonada
Phylum:	Parabasalia Honigberg 1973
Orders[1]
Trichomonadida Cristamonadida Spirotrichonymphida Trichonymphida

Theparabasalids are a group of flagellatedprotists within the supergroupExcavata. Most of theseeukaryoticorganisms form asymbiotic relationship inanimals. These include a variety of forms found in the intestines oftermites^[2] andcockroaches, many of which have symbioticbacteria that help them digestcellulose inwoody plants. Other species within this supergroup are knownparasites, and include humanpathogens.^[3][4]

The flagella are arranged in one or more clusters near the anterior of the cell. Theirbasal bodies are linked toparabasal fibers that are associated with a prominentGolgi complex, together forming a parabasal apparatus distinctive to the group.^[5] Attachment of a parabasal fiber to the first Golgi cisterna by thin filaments has been reported inTritrichomonas foetus.^[6] Usually they also give rise to a sheet of cross-likemicrotubules that runs down the center of the cell and in some cases projects past the end. This is called theaxostyle, but is different in structure from the axostyles ofoxymonads.^{[citation needed]}

Parabasalids areanaerobic, and lackmitochondria, but this is now known to be a result of secondary loss, and they contain smallhydrogenosomes which apparently developed from reduced mitochondria.^[7] Similar relics have been found in other amitochondriate flagellates, and the parabasalids are probably related to them, forming a group called themetamonads. They lack the feeding grooves found in most others, but this is probably a secondary loss as well.^{[citation needed]}

Before reclassification, the parabasalids were divided into about seven^[9] to 10orders depending on sources. Present classification divides Parabasalia into four orders, that is, Trichonymphida, Spirotrichonymphida, Cristamonadida, and Trichomonadida.^[1]

• Thetrichomonads 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, includingTrichomonas vaginalis, which causes a sexually transmitted disease in humans.
• The other orders, formerly grouped as thehypermastigids, have a large number of flagellar clusters and are found exclusively in the guts of insects. (The term "Hypermastigida" is still occasionally encountered.^[10])

The parabasalidTrichomonas vaginalis is not known to undergomeiosis. However, Maliket al.^[11] examinedT. vaginalis for the presence of 29 genes that function in meiosis and found 27 such genes, including eight genes specific to meiosis inmodel organisms. These findings suggested that the capability for meiosis, and hence sexual reproduction, was likely present in a recent parabasalid ancestor ofT. vaginalis.^[11]

• Flagellates
• Metamonads

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