Excavata

From Palaeos

Parent taxa: Eukaryota, Bikonta.

Description: Mostly flagellate heterotrophic unicellular eukaryotes, ancestrally with an excavate feeding groove (Simpson, 2003), a single anterior/dorsal and two ventral ciliary microtubular roots, additional cortical microtubules and without cortical alveoli (Cavalier-Smith, 2002). These characters are not universal, and have been lost or altered in various excavates. Other characters found in at least some members of all four daughter taxa listed below that may be ancestral for Excavata include the I fibre (a band of material adhering to the ventral/inner face of flagellar root 1 at its most anterior end) and a split of root 1 shortly after its origin into distinct inner and outer portions (Simpson, 2003).

Daughter taxa: Malawimonas, Metamonada, Jakobida, Discicristata.

Comments: The Excavata are one of the six 'superclades' of eukaryotes that have been widely accepted in recent years (Simpson & Roger, 2002), along with Opisthokonta, Amoebozoa, Rhizaria, Plantae and Chromalveolata. The excavates are probably the most contentious of these clades - no molecular studies to date have recovered a monophyletic Excavata (though nor do they strongly reject one - Simpson et al., 2002), and while the clade is based on morphological characters, no unique characters are shared by all Excavata. The ventral feeding groove shared by many excavates (but lost in Giardia, oxymonads, parabasalids, euglenozoans and many heteroloboseans) is probably the most characteristic feature of the clade, but superficially similar structures are found in probably unrelated taxa such as Diphylleiidae and the alveolate Colponema.

Relationships of Excavata within Eukaryota are even more uncertain than excavate monophyly. Amitochondriate excavates such as diplomonads and parabasalids were originally placed in rRNA trees as basal to almost all other eukaryotes, suggesting that they may have diverged before the origin of mitochondria, but these trees are now generally regarded as affected by long-branch attraction, as also indicated by the recent recognition of mitochondrial ancestry in these taxa (Cavalier-Smith, 2003). Excavates share a biciliate ancestry and fused dihydrofolate reductase-thymidylate synthase (DHFR-TS) genes with other bikonts (Cavalier-Smith, 2002; Simpson & Roger, 2002). Cavalier-Smith (2003) has suggested that the green algal-derived chloroplasts of Euglenozoa may be homologous with those of chlorarachneans (Cercozoa) and hence indicate a sister-relationship between the Excavata and Rhizaria (both together forming the Cabozoa), but as this would require losses of plastids in all other excavate and rhizarian taxa this theory has so far gained little support.

Phylogeny

<==Excavata [Axostylaria, Eozoa, Loukozoa]
   |  i. s.: Triflagellum
   |           |--T. diaphanum Ruinen 1938
   |           `--T. hardyi Ruinen 1938
   |--+--Metamonada
   |  `--Malawimonas [Malawimonadea, Malawimonadida, Malawimonadidae]
   |       |--M. californiana
   |       `--M. jakobiformis
   `--+--Discicristata
      `--Jakobida [Jakobea]
           |  i. s.: Stenocodon
           |         Stomatochone
           |--Jakoba Patterson 1990 [Jakobidae]
           |    |--J. bahamensis
           |    |--J. incarcerata Bernard, Simpson & Patterson 2000
           |    `--J. libera
           `--+--Seculamonas ecuadoriensis
              `--Histionidae
                   |--Histiona Voigt 1902
                   |    `--H. aroides
                   `--Reclinomonas Flavin & Nerad 1993
                        |--R. americana
                        `--R. campanula [=Histiona campanula]

* Type species of generic name indicated

References

Cavalier-Smith, T. 1993. The protozoan phylum Opalozoa. Journal of Eukaryotic Microbiology 40 (5): 609-615.

Cavalier-Smith, T. 2002. The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa. International Journal of Systematic and Evolutionary Microbiology 52: 297-354.

Cavalier-Smith, T. 2003. The excavate protozoan phyla Metamonada Grassé emend. (Anaeromonadea, Parabasalia, Carpediemonas, Eopharyngia) and Loukozoa emend. (Jakobea, Malawimonas): their evolutionary affinites and new higher taxa. International Journal of Systematic and Evolutionary Microbiology 53: 1741-1758.

Gray, M. W., B. F. Lang & G. Burger. 2004. Mitochondria of protists. Annual Review of Genetics 38: 477-524.

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

Simpson, A. G. B., & D. J. Patterson. 2001. On core jakobids and excavate taxa: The ultrastructure of Jakoba incarcerata. Journal of Eukaryotic Microbiology 48 (4): 480-492.

Simpson, A. G. B., & A. J. Roger. 2002. Eukaryotic evolution: getting to the root of the problem. Current Biology 12: R691-R693.

Simpson, A. G. B., A. J. Roger, J. D. Silberman, D. D. Leipe, V. P. Edgcomb, L. S. Jermiin, D. J. Patterson & M. L. Sogin. 2002. Evolutionary history of "early-diverging" eukaryotes: the excavate taxon Carpediemonas is a close relative of Giardia. Molecular Biology and Evolution 19 (10): 1782-1791.

Credits

Descriptive section CKT070821; phylogeny Christopher 03:14, 1 September 2008 (PDT).