Excavate

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Excavata
File:Giardia lamblia.jpg
Giardia lamblia, a parasitic diplomonad
Scientific classification e
Domain: Eukaryota
(unranked): Excavata
(Cavalier-Smith) Simpson, 2003
Phyla
Three types of excavate cells. Top: Jakobida, 1-nucleus, 2-anterior flagellum, 3-ventral/posterior flagellum, 4-ventral feeding groove. Middle: Euglenozoa, 1-nucleus, 2-flagellar pocket/reservoir, 3-dorsal/anterior flagellum, 4-ventral/posterior flagellum, 5-cytostome/feeding apparatus. Bottom: Metamonada, 1-anterior flagella, 2-parabasal body, 3-undulating membrane, 4-posterior flagellum, 5-nucleus, 6-axostyle.

The excavates are a major subgroup of unicellular[1] eukaryotes,[2] often known as Excavata. The phylogenetic category Excavata, proposed by Cavalier-Smith in 2002, contains a variety of free-living and symbiotic forms, and also includes some important parasites of humans. Excavates were formerly considered to be included in the now obsolete Protista kingdom.

Characteristics

Many excavates lack 'classical' mitochondria - these organisms are often referred to as 'amitochondriate', although most, perhaps all, retain a mitochondrial organelle in greatly modified form (e.g. a hydrogenosome or mitosome). Among those with mitochondria, the mitochondrial cristae may be tubular, discoidal, or in some cases, laminar. Most excavates have two, four, or more flagella[3] and many have a conspicuous ventral feeding groove with a characteristic ultrastructure, supported by microtubules.[4] However, various groups that lack these traits may be considered excavates based on genetic evidence (primarily phylogenetic trees of molecular sequences).

The closest that the excavates come to multicellularity are the Acrasidae slime molds. Like other cellular slime molds, they live most of their life as single cells, but will sometimes assemble into a larger cluster.

Subgroups

Excavates are classified into six major subdivisions at the phylum/class level. These are shown in the table below. An additional organism, Malawimonas, may also be included amongst excavates, though phylogenetic evidence is equivocal.

Superphylum Phylum/Class Representative genera (examples) Description
Discoba or JEH Euglenozoa EuglenaTrypanosoma Many important parasites, one large group with plastids (chloroplasts)
Heterolobosea (Percolozoa) Naegleria, Acrasis Most alternate between flagellate and amoeboid forms
Jakobea Jakoba, Reclinomonas Free-living, sometimes loricate flagellates, with very gene-rich mitochondrial genomes
Metamonada or POD Preaxostyla Oxymonads, Trimastix Amitochondriate flagellates, either free-living (Trimastix) or living in the hindguts of insects
Fornicata Giardia, Carpediemonas Amitochondriate, mostly symbiotes and parasites of animals.
Parabasalia Trichomonas Amitochondriate flagellates, generally intestinal commensals of insects. Some human pathogens.

Discoba or JEH clade

Euglenozoa and Heterolobosea (Percolozoa) appear to be particularly close relatives, and are united by the presence of discoid cristae within the mitochondria (Superphylum Discicristata). More recently a close relationship has been shown between Discicristata and Jakobida,[5] the latter having tubular cristae like most other protists, and hence were united under the taxon name Discoba which was proposed for this apparently monophyletic group.[2]

Metamonads

Metamonads are unusual in having lost classical mitochondria—instead they have hydrogenosomes, mitosomes or uncharacterised organelles.

Monophyly

Excavate relationships are still uncertain; it is possible that they are not a monophyletic group. The monophyly of the excavates is far from clear, although it seems like there are several clades within the excavates which are monophyletic.[6]

Certain excavates are often considered among the most primitive eukaryotes, based partly on their placement in many evolutionary trees. This could encourage proposals that excavates are a paraphyletic grade that includes the ancestors of other living eukaryotes. However, the placement of certain excavates as 'early branches' may be an analysis artifact caused by long branch attraction, as has been seen with some other groups, for example, microsporidia.

Malawimonas

In addition to the groups mentioned in the table above, the genus Malawimonas is generally considered to be a member of Excavata owing to its typical excavate morphology, and phylogenetic affinity to excavate groups in some molecular phylogenies. However, its position among excavates remains elusive.[citation needed]

Malawimonas appears not to belong to Excavata as it is close to Collodictyon which either appears close to Amoebozoa or they form a basal Unikont clade together.[7]

References

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External links