Kinetoplastid

Kinetoplastids are a group of flagellated protists within the phylum Euglenozoa. Their name comes from a unique organelle called the kinetoplast, a dense granule packed with a large mass of DNA. This group includes 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. The Bodonidae appear to be paraphyletic, meaning they gave rise to the Trypanosomatidae. The trypanosomatid family is notable for containing several genera that are exclusively parasitic. A typical genus is *Bodo*, which includes many free-living species that feed on bacteria. Other genera include *Cryptobia* and the parasitic *Leishmania*.

**Taxonomy**

There is no consensus on whether the group should be called Kinetoplastida or Kinetoplastea. Honigberg introduced both names in 1963. Kinetoplastea is more commonly used as a class, while Kinetoplastida is often used as an order, though it is also 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) is already in use for the same group.

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

**Morphology**

Kinetoplastids are eukaryotic cells with normal organelles like a nucleus, mitochondrion, Golgi apparatus, and flagellum. They also have distinctive features, including the kinetoplast, a sub-pellicular array of microtubules, and a paraflagellar rod.

The kinetoplast is a dense granule of DNA inside the cell's single mitochondrion. It contains many copies of the mitochondrial genome, made up of a network of concatenated circular DNA molecules along with structural proteins and polymerases. The kinetoplast sits at the base of the flagellum and is linked to the flagellum's basal body by a cytoskeletal structure.

The cytoskeleton is mostly made of microtubules. These form a regular array called the sub-pellicular array, running parallel just under the cell surface along the cell's long axis. Other specialized microtubules, like rootlet microtubules, are also present. Kinetoplastids can form actin microfilaments, but their role in the cytoskeleton is unclear. Other cytoskeletal structures include the specialized attachment between the flagellum and the kinetoplast.

All kinetoplastids have at least one flagellum. Trypanosomatids have one, while bodonids have two. In species with two flagella, one is usually leading and the other trailing, with the trailing one sometimes attached to the cell's side. Flagella are used for movement and for attaching to surfaces. The flagella bases are located in a specialized pocket, which also contains the cytostome (mouth).

**Life Cycle**

Kinetoplastids can be free-living or parasitic. The order Trypanosomatida is notable for containing many exclusively parasitic genera. Trypanosomatids may have simple life cycles in a single host or more complex ones involving 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.