Alveolate

Alveolata
Temporal range: Ediacaran [1] - Recent
Ceratium furca
Scientific classification
Domain: Eukarya
Kingdom: Protista
(unranked): SAR
Superphylum: Alveolata
Cavalier-Smith, 1991
Phyla

Apicomplexa
Chromerida
Ciliophora
Dinoflagellata

The alveolates (meaning "with cavities")[2] are a group of protists, considered a major clade[3] and superphylum[4] within Eukarya, and are also called Alveolata.[5]

Phyla

The Alveolata have 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]

The Acavomonidia and Colponemidia were previously grouped together as colponemids, a taxon now split based on ultrastructural analysis. The Acavomonidia are closer to the dinoflagellate/perkinsid group than the Colponemidia are.[7] As such, the informal term "colponemids", as its stands currently, covers two non-sister groups within Alveolata: the Acavomonidia and the Colponemidia.[6]

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.

Almost all sequenced mitochondrial genomes of ciliates and apicomplexia are linear.[8] The mitochondrial genome of Babesia microti is circular.[9] 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,[10] 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.[11][12] 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.[13] 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.[14]

Evolution

A Bayesian estimate places the evolution of the alveolate group at ~850 million years ago.[15] The Alveolata consist of Myzozoa, Ciliates, and Colponemids. In other words, The term Myzozoa meaning "to siphon the contents from prey", may be applied informally to the common ancestor of the subset of alveolates that are neither ciliates nor colponemids. Predation upon algae is an important driver in alveolate evolution, as it can provide sources for endosymbiosis of novel plastids. The term Myzozoa is therefore a handy concept for tracking the history of the alveolate phylum.

The ancestors of the alveolate group may have been photosynthetic.[16] 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,[14] and so it seems likely that the common ancestor of alveolates and heterokonts was also photosynthetic.

In one school of thought the common ancestor of the dinoflagellates, apicomplexans, Colpodella, Chromerida, and Voromonas was a myzocytotic predator with two heterodynamic flagella, micropores, trichocysts, rhoptries, micronemes, a polar ring and a coiled open sided conoid.[18] While the common ancestor of alveolates may also have possessed some of these characteristics, it has been argued that Myzocytosis was not one of these characteristics, as ciliates ingest prey by a different mechanism.[6]

An ongoing debate concerns the number of membranes surrounding the plastid across apicomplexans and certain dinoflagellates, and the origin of these membranes. This ultrastructural character can be used to group organisms and if the character is in common, it can imply that phyla had a common photosynthetic ancestor. On the basis that apicomplexans possess a plastid surrounded by 4 membranes, and that peridinin dinoflagellates possess a plastid surrounded by 3 membranes, Petersen et al.[19] have been unable to rule our that the shared stramenopile-alveolate plastid could have been recycled multiple times in the alveolate phylum, the source being stramenopile-alveolate donors, through the mechanism of ingestion and endosymbiosis.

Ciliates are a model apicomplexan, having been genetically studied in great depth over the longest period of any apicomplexan lineage. They are unusual among eukaryotes in that reproduction involves a micronucleus and a macronucleus. Their reproduction is easily studied in the lab, and made them a model eukaryote historically. Being entirely predatory and lacking any remnant plastid, their development as a phylum illustrates how predation and autotrophy[16] are in dynamic balance and that the balance can swing one way or other at the point of origin of a new phylum from mixotrophic ancestors, causing one ability to be lost.

References

  1. ↑ Li, C.-W.; et al. (2007). "Ciliated protozoans from the Precambrian Doushantuo Formation, Wengan, South China". Geological Society, London, Special Publications 286: 151–156. doi:10.1144/SP286.11.
  2. ↑ "alveolate". Memidex (WordNet) Dictionary/Thesaurus. Retrieved 2011-01-26.
  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. 1 2 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. 1 2 3 4 5 http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0095467
  7. ↑ http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3989336/
  8. ↑ 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
  9. ↑ 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
  10. ↑ 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
  11. ↑ López-García, P. et al. (2001). Unexpected diversity of small eukaryotes in deep-sea Antarctic plankton. Nature 409: 603-7.
  12. ↑ Moon-van der Staay, S. Y. et al. (2001). Oceanic 18S rDNA sequences from picoplankton reveal unsuspected eukaryotic diversity. Nature 409: 607-10.
  13. ↑ Gajadhar, A. A.; et al. (1991). "Ribosomal RNA sequences of Sarcocystis muris, Theilera annulata, and Crypthecodinium cohnii reveal evolutionary relationships among apicomplexans, dinoflagellates, and ciliates". Molecular and Biochemical Parasitology 45: 147–153. doi:10.1016/0166-6851(91)90036-6.
  14. 1 2 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
  15. ↑ Berney C, Pawlowski J (2006) A molecular time-scale for eukaryote evolution recalibrated with the continuous microfossil record. Proc Biol Sci 273(1596):1867-1872
  16. 1 2 Reyes-Prieto, A; Moustafa, A; Bhattacharya, D (2008). "Multiple genes of apparent algal origin suggest ciliates may once have been photosynthetic.". Curr Biol. 18 (13): 956–62. doi:10.1016/j.cub.2008.05.042.
  17. ↑ http://www.nature.com/nature/journal/v451/n7181/full/nature06635.html
  18. ↑ 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
  19. ↑ http://gbe.oxfordjournals.org/content/6/3/666.full?sid=c3b27728-b9f7-4018-9c25-8c8563b8da60

External links

Wikispecies has information related to: Alveolata
This article is issued from Wikipedia - version of the Thursday, May 05, 2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.