Trypanosomatida is a group of
kinetoplastidunicellular organisms distinguished by having only a single
flagellum. The name is derived from the
Greektrypano (borer) and soma (body) because of the corkscrew-like motion of some trypanosomatid species. All members are exclusively
parasitic, found primarily in
insects.[1] 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.[2] Some trypanosomatida are
intracellular parasites, with the important exception of Trypanosoma brucei.
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 heteroxenous life 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,[7] and other organisms such as
plants. Different species go through a range of different morphologies at different stages of the life cycle, with most having 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. [citation needed]
Among commonly studied examples, T. brucei, T. congolense, and T. vivax are extracellular, while T. cruzi and Leishmania spp. are intracellular.[8] Trypanosomatids with intracellular stages express δ-amastin proteins on their surfaces.[8] de Paiva et al., 2015 illuminates δ-amastins' roles in intracellular success.[8]
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 (referring to the flagellum), and a prefix which indicates the location of the flagellum on the cell. For example, the amastigote (prefix "a-", meaning no flagellum) form is also known as the leishmanial form as all Leishmania have an amastigote life cycle stage.[citation needed]
Amastigote (leishmanial).[10] Amastigotes are a common morphology during an intracellular lifecycle stage in a mammalian host. All Leishmania have an amastigote stage of the lifecycle. Leishmania amastigotes are particularly small and are among the smallest eukaryotic cells. The flagellum is very short, projecting only slightly beyond the flagellar pocket.
Promastigote (leptomonad).[10] 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 (crithidial).[10] 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 (trypanosomal).[10] 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 (herpetomonad).[10] A rarer morphology where the flagellum posterior of nucleus, passing through a long groove in the cell.
Endomastigote.[11] A morphotype where the flagellum does not extend beyond the deep flagellar pocket.
Amastigote: False colour
SEM micrograph of
amastigote form Leishmania mexicana. The cell body is shown in orange and the flagellum is in red. 219 pixels/μm.
Promastigote: False colour
SEM micrograph of
promastigote form Leishmania mexicana. The cell body is shown in orange and the flagellum is in red. 119 pixels/μm.
Trypomastigote: False colour
SEM micrograph of procyclic form Trypanosoma brucei. The cell body is shown in orange and the flagellum is in red. 84 pixels/μm.
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.[12]
Six species of trypanosomatids are known to carry an additional proteobacterial
endosymbiont, termed TPE (trypanosomatid proteobacterial endosymbionts). These trypansomatids (Strigomonas oncopelti, S. culicis, S. galati, Angomonas desouzai, and A. deanei) are in turn known as SHTs, for symbiont-harboring trypanosomatids. All such symbionts have a shared evolutionary origin and are classified in the
Candidatus genus "Kinetoplastibacterium".[13]
As with many symbionts, the bacteria have a much reduced genome compared to their free-living relatives of genera Taylorella and Achromobacter. (
GTDB finds the genus sister to Proftella, a symbiont of Diaphorina citri.)[14] Reflecting their inability to live alone, they have lost genes dedicated to essential biological functions, relying on the host instead. They have modified their division to become synchronized with the host. In S. culicis at least, the TPE helps the host by synthesizing
heme[13] and producing essential enzymes, staying tethered to the
kinetoplast.[15]
^Poinar, G. (2005). "Triatoma dominicana sp. n. (Hemiptera: Reduviidae: Triatominae), and Trypanosoma antiquus sp. n. (Stercoraria: Trypanosomatidae), the First Fossil Evidence of a Triatomine-Trypanosomatid Vector Association". Vector-Borne and Zoonotic Diseases. 5 (1): 72–81.
doi:
10.1089/vbz.2005.5.72.
PMID15815152.
^Merzlyak, Ekaterina; Yurchenko, Vyacheslav; Kolesnikov, Alexander A.; Alexandrov, Kirill; Podlipaev, Sergei A.; Maslov, Dmitri A. (2001-03-01). "Diversity and Phylogeny of Insect Trypanosomatids Based on Small Subunit rRNA Genes: Polyphyly of Leptomonas and Blastocrithidia". The Journal of Eukaryotic Microbiology. 48 (2): 161–169.
doi:
10.1111/j.1550-7408.2001.tb00298.x.
PMID12095103.
S2CID13880469.