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 flagellate protozoa, including a variety of free-living and symbiotic forms. They 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:

Metamonads e.g. Giardia, Trichomonas Mostly amitochondriate, 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 with chloroplasts, some important parasites

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, and the metamonads are unusual in having lost their mitochondria. Their relationships are still uncertain, and 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 may be an artifact caused by rapid changes in particular genes, as has been seen in some other amitochondriate groups, and in some phylogenies the excavates show up as an advanced group.

References

  • Alastair G. B. Simpson (2003). Cytoskeletal organization, phylogenetic affinities and systematics in the contentious taxon Excavata. International Journal of Systematic and Evoluionary Microbiology 53: 1759-1777.
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