Excavates

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  • Excavata
    • Metamonads
      • Fornicata
        • Diplomonads (Giardia & relatives)
        • Retortomonads
        • Carpediemonas
      • Parabasalians
        • Trichomonads
        • Hypermastigotes
      • Oxymonads (termite gut symbiont)
      • Trimastix
    • Discobans
      • Loukozoans (Jacobids, e.g. Reclinomonas)
      • Heterolobosea (Naegleria & relatives)
      • Euglenozoa
        • Euglenids
        • Kinetoplastids (Trypanosomes)
        • Diplonemids
      • Malawimonas

About this Superkingdom

The excavates are flagellates with a well-developed oral groove, the “excavate”. They usually lack typical mitochondria. In most cases, they don’t entirely lack mitochondria, but have organelles (e.g. kinetoplasts, mitosomes, or hydrogenosomes) that probably are very highly specialized forms of mitochondria. Those with more-or-less recognizable mitochondria (which have DNA) are sometimes grouped into the Discobians, and those without into the Metamonads. The metamonads were thought to lack mitochondria entirely, but mitochondria-like genes were found in the nuclear genome, and small organelles called “mitosomes” probably represent the DNA-less relic of mitochondria. On the other hand, the Jacobids have primitive mitochondria, retaining many more bacterial features than do other mitochondria.

The excavates may not be a valid phylogentic group (clad); if the root of the eukaryotic tree is amongst the excavates, as shown above, then they would be a “grade”, like fish or reptiles … or “protist”. The metamonads are sometimes referred to as the Archaeozoans, because they were (or are, depending on the tree) deep branches in the eukaryotic tree, but even if this is the case this is a misnomer, because they are not primitive (short) branches.

Discobans

Loukozoans (Jakobids) are a small group of free-living aquatic biflagellates that feed on bacteria. They have primitive mitochondria (usually only one), which have larger and more complete bacterial-like genomes than other eukaryotes. Some are free-swimming, others have an organic (non-mineralized) lorica that covers most of the cell except for the oral groove, and a stalk for attachment to a surface.

Heterolobosea (Percolozoans) have both amoeboid and flagellated (and cysts) stages in their life cycle, and so are also sometimes referred to as schizopyrenids or amoeboflagellates. Common in soil, feces and freshwater, one species, Naegleria fowleri is an opportunistic pathogen that invades the nervous system and causes necrosis after entering via the nasal mucosa, usually while swimming. Once the central nervous system tissue is infected by these “brain-eating amoeba”, mortality is greater than 99%.

Euglenozoans fall mostly into two main phenotypically distinct groups; the euglenids and the kinetoplastids. The free-living euglenids are phagocytic or photosynthetic; chloroplasts, when present, are secondary endosymbionts. They are biflagellated, but often one is very short or trailing. Often striped, some can move in an “inchworm” fashion in addition to flagellar swimming. Kinetoplastids are generally parasitic, and include Trypanosoma (the cause of sleeping sickness) and Leishmania (the cause of Chaga’s disease). Their mitochondrion is a large single organelle is contains a compact solid mass of DNA, the kineoplast. The kinetoplast is located at the anterior end of the cell, and is associated with the basal body of the flagella.

Example species

Reclinomonas americana

Reclinomonas americana
Franz Lang

Reclinomonas americana is a freshwater Jacobid that “reclines” in a wine glass-shaped loroica (the stem of the lorica points down in the image above). One flagellum is fre and beats to generate a current, bringing bacteris into the oral groove where the other flagellum is part of an oral vane to trap them. The R. americana mitochondrial genome is relatively large (97Kbp), with 97 genes, including a a number not usually seen in mitochondrial genomes, e,g, a RNA polymerase (most mitochondria have an RNA polymerase more closely related to phage T3/T3/SP6 RNA polymerases), succinate dehydrogenase, sec genes, ribosomal proteins, and a bacterial-type RNase P.

Trypanosoma brucei

Trypanosoma brucei
T. brucei in blood, Unattributed, Parasite Museum
http://www.parasitemuseum.com/trypanosome/

Trypanosoma brucei is a euglenoidozoan, and the cause of African sleeping sickness. It is an obligate parasite with a complex life cycle, including a variety of developmental morphotypes, an insect vector (the tsetse fly, Glossina sp.) and mammalian host. In the insect, T. brucei is an gut parasite that moves to the salivary gland so that it is injected into the mammalian host when the fly feeds. The parasite lives in the bloodstream of the mammalian host, where it can, in turn, be transmitted to the fly when the fly feeds on an infected host.

Trypanosoma life cycle

Metamonads

Metamonads have a central bundle of microtubules, the axostyle, that run the length of the cell. Undulation of the axostyle seems to be involved in motility, but metamonads are also flagellated.

Fornicata are mostly anaerobic gut symbionts, often parasitic. They lack both typical mitochondria and Golgi, along at least some of them contain small DNA-less organelles (mitosomes) that may represent the evolutionary relic of mitochondria. The function of mitosomes is unclear. The major groups of Fornicates are the Diplomonads and the Retortomonads; the diplomonads live as sort-of “Siamese twins”, with two nuclei, each with its associated flagella.

Parabasalians are mostly insect gut symbionts or animal pathogens. Like other metamonads they lack typical mitochondria, and so are anaerobic. However, they contain hydrogenosomes, small DNA-less organelles that probably represent highly modified mitochondria. They usually have several anterior flagella, the basal bodies of which are connected by fibers (parabasal fibers, thus the name of the group), with obvious Golgi, and the axostyle often protrudes from the posterior end.

Oxymonads are prominent gut symbionts of wood-eating insects, that live attached by an anterior stalk to the gut mucosa. They harbor bacterial endo- and ecto-symbionts that are involved in lignin and/or cellulse digestion. They lack both mitochondria and Golgi. Their surfaces contain specialized pit structures that serve as receptors for spirochaete and other bacterial attachment. The relationship between oxymonads and their symbionts and host and poorly understood; none have been grown in culture.

Example species

Giardia lamblia

Giardia lamblia
Dick Despommier

These organisms are binucleated, flagellated anaerobic gut parasites, the causative agent of “beaver fever” or “runner’s diarrhea”. Division occurs by binary fission, creating 2 uninucleate daughter cells, which quickly undergo mitosis to regenerate the normal binucleate structure - mitosis is after cytokinesis, not before. Chromatin is condensed throughout the life cycle. A large ventral disk is used for attachment to the gut mucosa. The function of the paired “medial bodies” (the smile of their “face”) is unknown. They contain 4 pairs of flagella, but not mitochondria (although they do have mitosomes), Golgi, endoplasmic reticulum, or lysosomes.

Streblomastix strix


S. strix SEM and cross section, showing symbiotic surface bacteria.
from Leander & Keeling, J. Euk Microbiol 2004 51:291-300

Streblomastix strix is an oxymonad gut symbiont of the damp-wood termite. The anterior end of the cell has 4 flagella, and sometimes an attachment stalk. The remainder of the cell is covered in 100-200 very long rod-shaped bacteria, probably of at least 3 different species. The cell is star-shaped in cross-section, providing 6 or 7 long lateral “vanes”, along which the bacteria are attached. Small symbiotic bacteria are found in a central vacuole and in the cytoplasm of the lateral vanes. Mitosomes are apparently absent.