Excavata

Source: Wikipedia, the free encyclopedia.
(Redirected from
Discoba
)
For an explanation of very similar terms, see
Eukaryota
.
Excavata
Temporal range: Neoproterozoic–present
Giardia lamblia
, a parasitic diplomonad
Scientific classification Edit this classification
(obsolete as paraphyletic)
Domain: Eukaryota
(unranked): Excavata
(Cavalier-Smith), 2002
Phyla and classes

see text

Metamonada, 1-anterior flagella, 2-parabasal body, 3-undulating membrane, 4-posterior flagellum, 5-nucleus, 6-axostyle
.

Excavata is an extensive and diverse but

Protista kingdom.[6] They were distinguished from other lineages based on electron-microscopic information about how the cells are arranged (they have a distinctive ultrastructural identity).[4] They are considered to be a basal flagellate lineage.[7]

On the basis of phylogenomic analyses, the group was shown to contain three widely separated eukaryote groups, the discobids, metamonads, and malawimonads.[8][9][10][11] A current view of the composition of the excavates is given below, indicating that the group is paraphyletic. Except for some Euglenozoa, all are non-photosynthetic.

Characteristics

Most excavates are unicellular, heterotrophic flagellates. Only some

mitochondria, and are called "amitochondriate", although most 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.[4] Many have a conspicuous ventral feeding groove with a characteristic ultrastructure, supported by microtubules—the "excavated" appearance of this groove giving the organisms their name.[3][6] However, various groups that lack these traits are considered to be derived excavates based on genetic evidence (primarily phylogenetic trees of molecular sequences).[6]

The

slime molds
, they live most of their life as single cells, but will sometimes assemble into larger clusters.

Proposed group

See also: Eukaryote § Phylogeny, and wikispecies:Excavata

Excavate relationships were always uncertain, suggesting that they are not a

monophyletic group.[12] Phylogenetic analyses often do not place malawimonads on the same branch as the other Excavata.[13]

Excavates were thought to include multiple groups:

Kingdom/Superphylum Included taxa Representative genera (examples) Description
Discoba or JEH or Eozoa
Tsukubea
Tsukubamonas
Euglenozoa Euglena, Trypanosoma Many important parasites, one large group with plastids (chloroplasts)
Heterolobosea
 (Percolozoa)
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
, Paratrimastix) 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.
Anaeramoeba Anaeramoeba ignava Anaerobic protists with hydrogenosomes instead of mitochondria.
Neolouka
 Malawimonadida Malawimonas

Discoba or JEH clade

Euglenozoa and Heterolobosea (Percolozoa) or Eozoa (as named by Cavalier-Smith

Jakobida,[15] 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.[1]

Metamonads

Metamonads are unusual in not having classical mitochondria—instead they have hydrogenosomes, mitosomes or uncharacterised organelles. The oxymonad Monocercomonoides is reported to have completely lost homologous organelles. There are competing explanations.[16][17]

Malawimonads

The malawimonads have been proposed to be members of Excavata owing to their typical excavate morphology, and phylogenetic affinity to other excavate groups in some molecular phylogenies. However, their position among eukaryotes remains elusive.[2]

Ancyromonads

Ancyromonads are small free-living cells with a narrow longitudinal groove down one side of the cell. The ancyromonad groove is not used for "suspension feeding", unlike in "typical excavates" (e.g. malawimonads, jakobids, Trimastix, Carpediemonas, Kiperferlia, etc). Ancyromonads instead capture prokaryotes attached to surfaces. The phylogenetic placement of ancyromonads is poorly understood (in 2020), however some phylogenetic analyses place them as close relatives of malawimonads.[9]

Evolution

Origin of the Eukaryotes

Further information: Eukaryogenesis

The conventional explanation for the origin of the Eukaryotes is that a

endosymbiont, and that this became the mitochondrion, the organelle providing oxidative respiration to the eukaryotic cell.[19]

Caesar al Jewari and Sandra Baldauf argue instead that the Eukaryotes possibly started with an endosymbiosis event of a

Gammaproteobacterium, accounting for the otherwise unexplained presence of anaerobic bacterial enzymes in Metamonada. The sister of the Preaxostyla within Metamonada represents the rest of the Eukaryotes which acquired an Alphaproteobacterium. In their scenario, the hydrogenosome and mitosome, both conventionally considered "mitochondrion-derived organelles", would predate the mitochondrion, and instead be derived from the earlier symbiotic bacterium.[17]

Phylogeny

In 2023, using molecular phylogenetic analysis of 186 taxa, Al Jewari and Baldauf proposed a phylogenetic tree with the metamonad Parabasalia as basal Eukaryotes. Discoba and the rest of the Eukaryota appear to have emerged as

cladistically considered part of the Excavata yet, the Excavata are in this analysis highly paraphyletic.[17]

Hodarchaeales[19]

Eukaryota

Parabasalia

Fornicata

Preaxostyla

Discoba
Neokaryotes

Amorphea (inc. animals, fungi)

SAR

Archaeplastida (inc. plants)

+ α‑proteobacterium
+ δ/γ‑proteobacterium
"Excavata"

The Anaeramoeba are associated with Parabasalia, but could turn out to be more basal as the root of a tree is often difficult to pinpoint.[20]

See also

Metakaryota

Gallery

References

  1. ^
    PMID 19237557
    .
  2. ^
    PMID 16308337
    .
  3. ^
    doi:10.1016/S0932-4739(99)80044-3
    .
  4. ^
    PMID 14657103
    .
  5. ^
    ISBN 978-0544859937
    .
  6. ^
    PMID 11931142
    .
  7. PMID 23312067
    .
  8. S2CID 204545629
    .
  9. ^
    PMID 29360967
    .
  10. PMID 29765641
    .
  11. PMID 31430481
    .
  12. Wikidata Q21090155
    .
  13. PMID 34358228
    .
  14. PMID 20031978
    .
  15. PMID 17689961
    .
  16. PMID 8790385
    .
  17. ^
    PMID 37115919
    .
  18. PMID 33739376
    .
  19. ^
    PMID 37316666
    .
  20. S2CID 240054026
    .

External links