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 complexes, distinctive to the group. Usually they also give rise to a sheet of cross-like
microtubules 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
oxymonads.
Parabasalids are
anaerobic, and lack
mitochondria, but this is now known to be a result of secondary loss, and they contain small
hydrogenosomes which apparently developed from reduced mitochondria.[4] 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.
Classification
Before reclassification, the parabasalids were divided into about 7[5] to 10
orders depending on sources. Present classification divides Parabasalia into 4 orders, that is, Trichonymphida, Spirotrichonymphida, Cristamonadida, and Trichomonadida.
[1]
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, 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.[6])
Evolution
The parabsalid Trichomonas vaginalis is not known to undergo
meiosis. However, Malik et al.[7] 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 organisms. These findings suggested that the capability for meiosis, and hence sexual reproduction, was likely present in a recent parabasalid ancestor of T. vaginalis.[7]
^Ohkuma M, Iida T, Ohtoko K, et al. (June 2005). "Molecular phylogeny of parabasalids inferred from small subunit rRNA sequences, with emphasis on the Hypermastigea". Mol. Phylogenet. Evol. 35 (3): 646–55.
doi:
10.1016/j.ympev.2005.02.013.
PMID15878133.
^Yubuki, Naoji; Céza, VÍT; Cepicka, Ivan; Yabuki, Akinori; Inagaki, Yuji; Nakayama, Takeshi; Inouye, Isao; Leander, Brian S (2010). "Cryptic Diversity of Free-Living Parabasalids, Pseudotrichomonas keilini and Lacusteria cypriaca n. G., n. Sp., as Inferred from Small Subunit rDNA Sequences". Journal of Eukaryotic Microbiology. 57 (6): 554–61.
doi:
10.1111/j.1550-7408.2010.00509.x.
PMID20880033.
S2CID4195962.