Excavate

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:

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, 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.

References

From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.