Excavata
Excavata is a major supergroup of unicellular organisms belonging to the domain Eukaryota.[1][2][3] It was first suggested by Simpson and Patterson in 1999[4][5] and introduced by Thomas Cavalier-Smith in 2002 as a formal taxon. It contains a variety of free-living and symbiotic forms, and also includes some important parasites of humans, including Giardia and Trichomonas.[6] Excavates were formerly considered to be included in the now obsolete Protista kingdom.[7] They are classified based on their flagellar structures,[5] and they are considered to be the most basal flagellate lineage.[8] Phylogenomic analyses split the members of Excavata into three different and not all closely related groups: Discobids, Metamonads and Malawimonads.[9][10][11] Except for Euglenozoa, they are all non-photosynthetic.
Most excavates are unicellular, heterotrophic flagellates. Only the Euglenozoa are photosynthetic. In some (particularly anaerobic intestinal parasites), the mitochondria have been greatly reduced.[6] Some excavates lack "classical" 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.[5] 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.[4][7] However, various groups that lack these traits may be considered excavates based on genetic evidence (primarily phylogenetic trees of molecular sequences).[7]
The Acrasidae slime molds are the only excavates to exhibit limited multicellularity. Like other cellular slime molds, they live most of their life as single cells, but will sometimes assemble into larger clusters.
Excavates are classified into six major subdivisions at the phylum/class level. These are shown in the table below. An additional group, Malawimonadida (e.g. Malawimonas), may also be included amongst excavates, though phylogenetic evidence is equivocal.
Euglenozoa and Heterolobosea (Percolozoa) or Eozoa (Cavalier-Smith) appear to be particularly close relatives, and are united by the presence of discoid cristae within the mitochondria (Superphylum Discicristata). A close relationship has been shown between Discicristata and Jakobida,[12] 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.[2]
Metamonads are unusual in having lost classical mitochondria—instead they have hydrogenosomes, mitosomes or uncharacterised organelles. The oxymonad Monocercomonoides is reported to have completely lost homologous organelles.
