Excavate
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 Giardia lamblia, a parasitic diplomonad
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Metamonada |
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Overview
The excavates are a major assemblage of protists, often known as Excavata. The phylogenetic category Excavata contains a variety of free-living and symbiotic forms, and includes some important parasites of humans. 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. Others have mitochondria with tubular, discoidal, or in some cases, flat cristae. Most excavates have two, four, or more flagella and many have a conspicuous ventral feeding groove with a characteristic ultrastructure, supported by microtubules. However, various groups that lack these traits may be considered excavates based on genetic evidence (primarily phylogenetic trees of molecular sequences). Most excavates fall into one of four groups, which may be treated as phyla:
| Metamonads | e.g. Giardia, Trichomonas | Amitochondriate, mostly symbiotes of animals |
| Loukozoa or jakobids | e.g. Jakoba | |
| Percolozoa or Heterolobosea | e.g. Naegleria, Acrasis | Most alternate between flagellate and amoeboid forms |
| Euglenozoa | e.g. Euglena, Trypanosoma | Many important parasites, one large group with plastids (chloroplasts) |
Of these, the Percolozoa (Heterolobosea) and Euglenozoa appear to be particularly close relatives, and are united by the presence of discoid cristae within the mitochondria. Most jakobids have tubular cristae, like most other protists, while the metamonads are unusual in having lost classical mitochondria - instead they have 'hydrogenosomes', 'mitosomes' or uncharacterised organelles. Excavate relationships are still uncertain; it is possible that they are not a monophyletic group.
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.
References
- Cavalier-Smith, T. (2002). "The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa". International Journal of Systematic and Evolutionary Microbiology. 52: 297–354.
- Alastair G. B. Simpson (2003). "Cytoskeletal organization, phylogenetic affinities and systematics in the contentious taxon Excavata". International Journal of Systematic and Evolutionary Microbiology. 53: 1759–1777.