Trypanosomatida is a group of kinetoplastid excavates distinguished by having only a single flagellum. The name is derived from the Greektrypano and soma because of the corkscrew-like motion of some trypanosomatid species. All members are exclusively parasitic, found primarily in insects. A few genera have life-cycles involving a secondary host, which may be a vertebrate, invertebrate or plant. These include several species that cause major diseases in humans.
Three genera are dixenous – Leishmania, Phytomonas and Trypanosoma and the remainder are monoxenous. Paratrypanosoma appears to be the first evolving branch in this order. Fifteen genera are recognised in the Trypanosomatidae and there are three subfamilies – Blechomonadinae, Leishmaniinae and Strigomonadinae. The genera in the subfamily Strigomonadinae are characterised by the presence of obligatory intracellular bacteria of the Kinetoplastibacterium genus.
Some trypanosomatids only occupy a single host, while many others are heteroxenous: they live in more than one host species over their life cycle. This heteroxenouslife cycle typically includes the intestine of a bloodsucking insect and the blood and/or tissues of a vertebrate. Rarer hosts include other bloodsucking invertebrates, such as leeches, and other organisms such as plants. Different species go through a range of different morphologies at different stages of the life cycle, most have at least two different morphologies. Typically the promastigote and epimastigote forms are found in insect hosts, trypomastigote forms in the mammalian bloodstream and amastigotes in intracellular environments.
Morphologies
A variety of different morphological forms appear in the life cycles of trypanosomatids, distinguished mainly by the position, length and the cell body attachment of the flagellum. The kinetoplast is found closely associated with the basal body at the base of the flagellum and all species of trypanosomatid have a single nucleus. Most of these morphologies can be found as a life cycle stage in all trypanosomatid genera however certain morphologies are particularly common in a particular genus. The various morphologies were originally named from the genus where the morphology was commonly found, although this terminology is now rarely used because of potential confusion between morphologies and genus. Modern terminology is derived from the Greek; "mastig", meaning whip, and a prefix which indicates the location of the flagellum on the cell. For example, the amastigote form is also known as the leishmanial form as all Leishmania have an amastigote life cycle stage.
Amastigote. Amastigotes are a common morphology during an intracellular lifecycle stage in a mammalian host. All Leishmania have an amastigote stage of the lifecycle. Leishmania amastigoes are particularly small and are among the smallest eukaryotic cells. The flagellum is very short, projecting only slightly beyond the flagellar pocket.
Promastigote. The promastigote form is a common morphology in the insect host. The flagellum is found anterior of nucleus and flagellum not attached to the cell body. The kinetoplast is located in front of the nucleus, near the anterior end of the body.
Epimastigote. Epimastigotes are a common form in the insect host and Crithidia and Blastocrithidia, both parasites of insects, exhibit this form during their life cycles. The flagellum exits the cell anterior of nucleus and is connected to the cell body for part of its length by an undulating membrane. The kinetoplast is located between the nucleus and the anterior end.
Trypomastigote. This stage is characteristic of the genus Trypanosoma in the mammalian host bloodstream as well as infective metacyclic stages in the fly vector. In trypomastigotes the kinetoplast is near the posterior end of the body, and the flagellum lies attached to the cell body for most of its length by an undulating membrane.
Opisthomastigote. A rarer morphology where the flagellum posterior of nucleus, passing through a long groove in the cell.
Endomastigote. A morphotype where the flagellum does not extend beyond the deep flagellar pocket.
Other features
Notable characteristics of trypanosomatids are the ability to perform trans-splicing of RNA and possession of glycosomes, where much of their glycolysis is confined to. The acidocalcisome, another organelle, was first identified in trypanosomes.
