Alveolate

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Alveolata
Temporal range: Ediacaran [1] - Recent
Ceratium furca.jpg
Ceratium furca
Scientific classification
Domain: Eukarya
Kingdom: Chromalveolata
Superphylum: Alveolata
Cavalier-Smith, 1991
Phyla

Apicomplexa
Chromerida
Ciliophora
Dinoflagellata

The alveolates ("with cavities")[2] are a major superphylum of protists.

Phyla[edit]

There are five phyla, which are very diverse in form, but are now known to be close relatives based on various ultrastructural and genetic similarities:

The genus Perkinsus may belong to another clade, Perkinsozoa, based on a number of molecular biological findings.[3]

Characteristics[edit]

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.[4]

Almost all sequenced mitochondrial genomes of ciliates and apicomplexia are linear.[5] The mitochondrial genome of Babesia microti is circular.[6] 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[edit]

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, 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.[7][8] 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.[9] Cavalier-Smith, introduced the formal name Alveolata in 1991,[10] 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[edit]

The development of plastids among the alveolates is uncertain. Cavalier-Smith proposed the alveolates developed from a chloroplast-containing ancestor, which also gave rise to the Chromista (the chromalveolate hypothesis). However, as plastids only appear in relatively derived (as opposed to ancestral) groups, others argue the alveolates originally lacked them and possibly the dinoflagellates and Apicomplexa acquired them separately.

It appears that the alveolates, the dinoflagellates and the heterokont algae acquired their plastids from a red algae suggesting a common origin of this organelle in all these clades.[11]

Evolution[edit]

It seems likely that the common ancestor of this group was a myzocytotic predator with two heterodynamic flagella, micropores, trichocysts, rhoptries, micronemes, a polar ring and a coiled open sided conoid.[12] This ancestor also probably possessed a plastid but it is not clear whether it was photosynthetic. Furthermore it is not clear whether extant perkinsids or colpodellids have retained this organelle.

Given that the alveolates, the dinoflagellates and the heterokont algae acquired their plastids from a red alga[11] it seems likely that their ancestor was photosynthetic.

In most of the species in this clade the primary extrusosome is the rhoptry suggesting its presence in their common ancestor.

The most likely path of evolution of the parasitic Apicomplexa seems to be from colpodellid like predators, to archigregarine like parasites and then to completely intracellular forms.

References[edit]

  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. ^ 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
  4. ^ 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. 
  5. ^ 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
  6. ^ 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
  7. ^ López-García, P. et al. (2001). Unexpected diversity of small eukaryotes in deep-sea Antarctic plankton. Nature 409: 603-7.
  8. ^ Moon-van der Staay, S. Y. et al. (2001). Oceanic 18S rDNA sequences from picoplankton reveal unsuspected eukaryotic diversity. Nature 409: 607-10.
  9. ^ 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. 
  10. ^ 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.
  11. ^ a b 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
  12. ^ 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

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