Alveolate

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Alveolata
Temporal range: Ediacaran [1] - Recent
Ceratium furca.jpg
Ceratium furca
Scientific classification
Domain:
Eukarya
(unranked):
SAR
Superphylum:
Alveolata

Cavalier-Smith, 1991
Phyla

Apicomplexa
Chromerida
Ciliophora
Dinoflagellata

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The alveolates ("with cavities")[2] is one of the major groups of protists, considered a clade[3] and a superphylum[4] of Eukarya called Alveolata.[5]

Phyla

Within Alveolata, there are around 9 major and minor phyla, which are very diverse in form, but are now known to be related on various ultrastructural and genetic similarities:[6]

Acavomonidia and Colponemidia were previously grouped together as colponemids, a taxon now split based on ultrastructural analysis. Acavomonidia is closer to the dinoflagellate/perkinsid group than Colponemidia is.[7]

Characteristics

Two tachyzoites of Toxoplasma gondii, transmission electron microscopy

The most notable shared characteristic is the presence of cortical alveoli, flattened vesicles packed into a continuous layer supporting the membrane, typically forming a flexible pellicle. In dinoflagellates they often form armor plates. Alveolates have mitochondria with tubular cristae and their flagella or cilia have a distinct structure.

The ancestors of this group may have been photosynthetic.[8]

Almost all sequenced mitochondrial genomes of ciliates and apicomplexia are linear.[9] The mitochondrial genome of Babesia microti is circular.[10] This species is also now known not to belong to either of the genera Babesia or Theileria and a new genus will have to be created for it.

Classification

The Apicomplexa and dinoflagellates may be more closely related to each other than to the ciliates. Both have plastids, and most share a bundle or cone of microtubules at the top of the cell. In apicomplexans this forms part of a complex used to enter host cells, while in some colorless dinoflagellates it forms a peduncle used to ingest prey. Various other genera are closely related to these two groups, mostly flagellates with a similar apical structure. These include free-living members in Oxyrrhis and Colponema, and parasites in Perkinsus,[11] Parvilucifera, Rastrimonas and the ellobiopsids. In 2001, direct amplification of the rRNA gene in marine picoplankton samples revealed the presence of two novel alveolate linages, called group I and II.[12][13] Group I has no cultivated relatives, while group II is related to the dinoflagellate parasite Amoebophrya, which was classified until now in the Syndiniales dinoflagellate order.

Relationships between some of these the major groups were suggested during the 1980s, and a specific relationship between all three was confirmed in the early 1990s by genetic studies, most notably by Gajadhar et al.[14] Cavalier-Smith, introduced the formal name Alveolata in 1991,[5] although at the time he actually considered the grouping to be a paraphyletic assemblage, rather than a monophyletic group.

Some studies suggested the haplosporids, mostly parasites of marine invertebrates, might belong here but they lack alveoli and are now placed among the Cercozoa.

Development

The development of plastids among the alveolates is intriguing. Cavalier-Smith proposed the alveolates developed from a chloroplast-containing ancestor, which also gave rise to the Chromista (the chromalveolate hypothesis). Other researchers have speculated that the alveolates originally lacked plastids and possibly the dinoflagellates and Apicomplexa acquired them separately. However, it now appears that the alveolates, the dinoflagellates, the Chromerida and the heterokont algae acquired their plastids from a red algae with evidence of a common origin of this organelle in all these four clades.[15]

Evolution

The Alveolata consist of Myzozoa, Ciliates, and Colponemids. It seems likely that the common ancestor of the Myzozoa was a myzocytotic predator with two heterodynamic flagella, micropores, trichocysts, rhoptries, micronemes, a polar ring and a coiled open sided conoid.[16] The common ancestor of Myzozoa, Ciliates, and Colponemids may also have possessed some of these characteristics, but it has been argued that Myzocytosis was not one of these characteristics.[6] The informal term colponemids, as its stands currently, covers two non-sister groups within Alveolata, the Acavomonidia and the Colponemidia[6]

The ancestral alveolate probably possessed a plastid. Chromerids, apicomplexans, and peridinin dinoflagellates have retained this organelle.[17] Going one step even further back, the chromerids, the peridinin dinoflagellates and the heterokont algae possess a monophyletic plastid lineage in common, i.e. acquired their plastids from a red alga,[15] and so it seems likely that the common ancestor of aveolates and heterokonts was also photosynthetic.

References

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External links

  1. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ Adl, S.M. et al. 2012. The revised classification of eukaryotes. Journal of Eukaryotic Microbiology, 59(5), 429-514
  4. ↑ Ruggiero, M. A., Gordon, D. P., Orrell, T. M., Bailly, N., Bourgoin, T., Brusca, R. C., Cavalier-Smith, T., Guiry, M.D. y Kirk, P. M. (2015). A Higher Level Classification of All Living Organisms.
  5. ↑ 5.0 5.1 Cavalier-Smith, T. (1991). Cell diversification in heterotrophic flagellates. In The Biology of Free-living Heterotrophic Flagellates, ed. D.J. Patterson & J. Larsen. pp. 113-131. Oxford University Press.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0095467
  7. ↑ http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3989336/
  8. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  9. ↑ Barth D, Berendonk TU (2011) The mitochondrial genome sequence of the ciliate Paramecium caudatum reveals a shift in nucleotide composition and codon usage within the genus Paramecium. BMC Genomics. 12:272
  10. ↑ Cornillot E, Hadj-Kaddour K, Dassouli A, Noel B, Ranwez V, Vacherie B, Augagneur Y, Brès V, Duclos A, Randazzo S, Carcy B, Debierre-Grockiego F, Delbecq S, Moubri-Ménage K, Shams-Eldin H, Usmani-Brown S, Bringaud F, Wincker P, Vivarès CP, Schwarz RT, Schetters TP, Krause PJ, Gorenflot A, Berry V, Barbe V, Ben Mamoun C (2012) Sequencing of the smallest Apicomplexan genome from the human pathogen Babesia microti{dagger} Nucleic Acids Res
  11. ↑ Zhang H, Campbell DA, Sturm NR, Dungan CF, Lin S (2011) Spliced leader RNAs, mitochondrial gene frameshifts and multi-protein phylogeny expand support for the genus Perkinsus as a unique group of Alveolates. PLoS One. 2011;6(5):e19933
  12. ↑ López-García, P. et al. (2001). Unexpected diversity of small eukaryotes in deep-sea Antarctic plankton. Nature 409: 603-7.
  13. ↑ Moon-van der Staay, S. Y. et al. (2001). Oceanic 18S rDNA sequences from picoplankton reveal unsuspected eukaryotic diversity. Nature 409: 607-10.
  14. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  15. ↑ 15.0 15.1 Janouskovec J, Horák A, Oborník M, Lukes J, Keeling PJ (2010) A common red algal origin of the apicomplexan, dinoflagellate, and heterokont plastids. Proc Natl Acad Sci USA 107(24):10949-10954
  16. ↑ Kuvardina ON, Leander BS, Aleshin VV, Myl'nikov AP, Keeling PJ, Simdyanov TG (2002) The phylogeny of colpodellids (Alveolata) using small subunit rRNA gene sequences suggests they are the free living sister group to apicomplexans. J Eukaryot Microbiol 49(6):498-504
  17. ↑ http://www.nature.com/nature/journal/v451/n7181/full/nature06635.html
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