The parabasalids are a group of flagellated protists within the supergroup Excavata. Most of these eukaryotic organisms form a symbiotic relationship in animals. These include a variety of forms found in the intestines of termites[2] and cockroaches, many of which have symbiotic bacteria that help them digest cellulose in woody plants. Other species within this supergroup are known parasites, and include human pathogens.[3]
| Parabasalid | |
|---|---|
| |
| Two trophozoites of Trichomonas vaginalis stained with Giemsa | |
Scientific classification 
| |
| Domain: | Eukaryota |
| Phylum: | Metamonada |
| Subphylum: | Trichozoa |
| (unranked): | Parabasalia Honigberg 1973 |
| Orders[1] | |
Characteristics
editThe 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.
- Anterior flagellum
- Recurrent flagellum
- Undulating membrane
- Basal bodies
- Golgi apparatus; modifies proteins and sends them out of the cell
- Nucleus
- Costa, a striated fiber associated with the undulating membrane[5]
- Pelta, made of microtubules and creates cell structure[5]
- Parabasal fibre
- Axostyle, may be involved in movement or support for the cell
- Endoplasmic reticulum, the transport network for molecules going to specific parts of the cell
- Vacuole
- Hydrogenosome, produces molecular hydrogen and ATP (energy) in anaerobic conditions
- Lysosome, holds enzymes
Classification
editBefore reclassification, the parabasalids were divided into about seven[6] to 10 orders depending on sources. Present classification divides Parabasalia into four 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.[7])
Evolution
editThe parabasalid Trichomonas vaginalis is not known to undergo meiosis. However, Malik et al.[8] 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.[8]
References
edit- ^ a b Adl, Sina M; et, al (2005). "The new higher level classification of eukaryotes with emphasis on the taxonomy of protists". Journal of Eukaryotic Microbiology. 52 (5): 399–451. doi:10.1111/j.1550-7408.2005.00053.x. PMID 16248873.
- ^ 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. PMID 15878133.
- ^ "Parabasalia - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2023-12-25.
- ^ Bui, ET; Bradley, PJ; Johnson, PJ (Sep 3, 1996). "A common evolutionary origin for mitochondria and hydrogenosomes". Proceedings of the National Academy of Sciences of the United States of America. 93 (18): 9651–6. Bibcode:1996PNAS...93.9651B. doi:10.1073/pnas.93.18.9651. PMC 38483. PMID 8790385.
- ^ a b Simpson, A.G.B.; Čepička, I. (2009). Schaechter, Moselio (ed.). Encyclopedia of Microbiology (Third Edition). Academic Press. ISBN 978-0-12-373944-5.
- ^ 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. PMID 20880033. S2CID 4195962.
- ^ Carpenter KJ, Keeling PJ (2007). "Morphology and phylogenetic position of Eucomonympha imla (Parabasalia: Hypermastigida)". J. Eukaryot. Microbiol. 54 (4): 325–32. doi:10.1111/j.1550-7408.2007.00263.x. PMID 17669157. S2CID 11908419.
- ^ a b Malik SB, Pightling AW, Stefaniak LM, Schurko AM, Logsdon JM (2008). "An expanded inventory of conserved meiotic genes provides evidence for sex in Trichomonas vaginalis". PLOS ONE. 3 (8): e2879. Bibcode:2008PLoSO...3.2879M. doi:10.1371/journal.pone.0002879. PMC 2488364. PMID 18663385.