Parabasalid
| Parabasalid | |
|---|---|
| |
| Two Giemsa
| |
Scientific classification 
| |
| Domain: | Eukaryota |
| Phylum: | Metamonada |
| Subphylum: | Trichozoa |
| (unranked): | Parabasalia Honigberg 1973 |
| Orders[1] | |
| |
The parabasalids are a group of flagellated
Characteristics
The flagella are arranged in one or more clusters near the anterior of the cell. Their
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]