Kinetoplastid

Kinetoplastids are a group of flagellated protists within the phylum Euglenozoa. Their name comes from a distinctive organelle called the kinetoplast, a dense granule packed with a large mass of DNA. This group includes both parasites that cause serious diseases in humans and other animals, as well as free-living species found in soil and water. They are often simply called kinetoplastids or kinetoplasts.

The group was first defined in 1963 by Bronislaw M. Honigberg. Traditionally, kinetoplastids are split into two groups: the biflagellate Bodonidae and the uniflagellate Trypanosomatidae, though the former appears to be paraphyletic relative to the latter. The trypanosomatid family is notable for containing several genera that are exclusively parasitic. A typical free-living genus is *Bodo*, which feeds on bacteria. Other examples include *Cryptobia* and the parasitic *Leishmania*.

Taxonomically, Honigberg created the names Kinetoplastida and Kinetoplastea in 1963, but there is no consensus on which is the definite taxon. Kinetoplastea is more widely used as a class, while Kinetoplastida is mostly used as an order, though it is also sometimes treated as a class. Lynn Margulis initially accepted Kinetoplastida as an order in 1974 but later placed it as a class. Using Kinetoplastida as an order creates confusion because the older name Trypanosomatida (Kent, 1880) already exists and is often used for the same group.

Kinetoplastida is divided into two subclasses: Metakinetoplastina and Prokinetoplastina. It includes several families and orders, such as the families Bordnamonadidae and Trypanophididae, the order Prokinetoplastida (with family Ichthyobodonidae), and the subclass Metakinetoplastina, which contains the orders Neobodonida, Parabodonida, Bodonida, and Trypanosomatida.

Morphologically, kinetoplastids are eukaryotic and have normal organelles like a nucleus, mitochondrion, Golgi apparatus, and flagellum. They also have distinguishing features: the kinetoplast, a sub-pellicular microtubule array, and a paraflagellar rod. The kinetoplast itself is a dense DNA granule inside the single mitochondrion, made of a network of concatenated circular DNA molecules, structural proteins, and DNA and RNA polymerases. It sits at the base of the flagella and is linked to the flagellum's basal body by a cytoskeletal structure.

The cytoskeleton is mainly composed of microtubules arranged in a highly regular sub-pellicular array that runs parallel just under the cell surface along the cell's long axis. There are also specialized rootlet microtubules. Kinetoplastids can form actin microfilaments, but their role in the cytoskeleton is unclear. Other cytoskeletal structures include the attachment between the flagellum and the kinetoplast.

All kinetoplastids have at least one flagellum. Trypanosomatids have one, while bodonids have two. In two-flagellated forms, there is usually a leading and a trailing flagellum, with the latter sometimes attached to the cell's side. Flagella are used for movement and for attaching to surfaces. Their bases sit in a specialized pocket structure that also contains the cytostome.

Life cycles vary. Kinetoplastids can be free-living or parasitic. The order Trypanosomatida is notable for having many exclusively parasitic genera. Trypanosomatids may have simple life cycles in a single host or more complex ones with multiple differentiation stages in two hosts. Dramatic morphological changes can occur between life cycle stages. Diseases caused by trypanosomatids include sleeping sickness and Chagas disease (from *Trypanosoma* species) and leishmaniasis (from *Leishmania* species). *Trypanosoma brucei* can undergo meiosis, likely as part of a sexual cycle, and *Leishmania major* is also capable of a meiotic process that is probably part of a sexual cycle.