Excavate

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This article is about the protist group called excavates. For the process of digging, see excavation.

The excavates are a major line of protists, often known as Excavata. The phylogenetic category Excavata includes a variety of free-living and symbiotic forms, as well as microbes varying as to whether they contain mitochondria (that is, amitochondriate versus discicristate). Excavates are usually characterized by having two, four, or more flagella with distinct ultrastructure anterior to a ventral feeding groove supported by microtubules. However, various groups that lack these traits may be considered excavates based on genetic evidence. Most excavates fall into four groups, which may be treated as phyla:

Of these, the Percolozoa and Euglenozoa appear to be particularly close relatives, and are united by the presence of discoid cristae within the mitochondria. The jakobids have tubular cristae, like most other protists, while the metamonads are unusual in having lost their mitochondria. Excavate relationships are still uncertain; it is possible that they are not monophyletic groups.

The excavates are often considered among the most primitive eukaryotes, and may be paraphyletic to the others. However, this placement may be an artifact caused by long branch attraction, as has been seen with some other amitochondriate groups, and in some phylogenies the excavates show up as an advanced group.

[edit] 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.