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Alveolate

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Alveolate
Temporal range:Ediacaran–Recent[1]
Ceratium furca
Scientific classificationEdit this classification
Domain: Eukaryota
Clade: Diaphoretickes
Clade: TSAR
Clade: SAR
Clade: Alveolata
Cavalier-Smith,1991
Phyla
Synonyms
  • Alveolatobiontes

Thealveolates(meaning "pitted like a honeycomb" )[2]are a group ofprotists,considered a majorclade[3]andsuperphylum[4]withinEukarya.They are currently grouped with thestramenopilesandRhizariaamong the protists with tubulocristate mitochondria into theSAR supergroup.

Characteristics[edit]

The most notable shared characteristic is the presence ofcortical (near the surface) alveoli (sacs).These are flattenedvesicles(sacs) arranged as a layer just under themembraneand supporting it, typically contributing to a flexible pellicle (thin skin). In armoreddinoflagellatesthey may contain stiff plates. Alveolates havemitochondriawith tubular cristae (invaginations), and cells often have pore-like intrusions through the cell surface. The group contains free-living andparasiticorganisms, predatoryflagellates,andphotosyntheticorganisms.

Transmission electron micrograph of a thin section of the surface of the ciliateParameciumputrinum,showing the alveoli (red arrows) under the cell surface

Almost all sequenced mitochondrial genomes of ciliates and apicomplexa are linear.[5]The mitochondria almost all carrymtDNAof their own but with greatly reduced genome sizes. Exceptions areCryptosporidiumwhich are left with only amitosome,the circular mitochondrial genomes ofAcavomonasandBabesia microti,[6][7]andToxoplasma's highly fragmented mitochondrial genome, consisting of 21 sequence blocks which recombine to produce longer segments.[8][9]

History[edit]

The relationship of apicomplexa, dinoflagellates and ciliates had been suggested during the 1980s, and this was confirmed in the early 1990s by comparisons of ribosomal RNA sequences, most notably by Gajadharet al.[10]Cavalier-Smithintroduced the formal name Alveolata in 1991,[11]although at the time he considered the grouping to be aparaphyleticassemblage. Many biologists prefer the use of the colloquial name 'alveolate'.[12]

Classification[edit]

Alveolata include around nine major and minor groups. They are diverse in form, and are known to be related by various ultrastructural and genetic similarities:[13]

The Acavomonidia and Colponemidia were previously grouped together as colponemids, a taxon now split because each has a distinctive organization orultrastructural identity.The Acavomonidia are closer to the dinoflagellate/perkinsid group than the Colponemidia are.[13]As such, the informal term "colponemids", as it stands currently, covers two non-sister groups within Alveolata: the Acavomonidia and the Colponemidia.[13]

The Apicomplexa and dinoflagellates may be more closely related to each other than to the ciliates. Both haveplastids,and most share a bundle or cone ofmicrotubulesat 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 inOxyrrhisandColponema,and parasites inPerkinsus,[14]Parvilucifera,Rastrimonasand theellobiopsids.In 2001, direct amplification of therRNAgene in marinepicoplanktonsamples revealed the presence of two novel alveolate lineages, called group I and II.[15][16]Group I has no cultivated relatives, while group II is related to the dinoflagellate parasiteAmoebophrya,which was classified until now in theSyndinialesdinoflagellate order.

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

The ellobiopsids are of uncertain relation within the alveolates. Silberman et al 2004 establish that theThalassomycesgenus of ellobiopsids are alveolates usingphylogeneticanalysis, however as of 2016no more certainty exists on their place.[17][18]

Phylogeny[edit]

In 2017,Thomas Cavalier-Smithdescribed the phylogeny of the Alveolata as follows:[19]

Alveolata

Taxonomy[edit]

AlveolataCavalier-Smith 1991[Alveolatobiontes]

Development[edit]

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 theChromista(thechromalveolatehypothesis). 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 theheterokont algaeacquired their plastids from a red alga with evidence of a common origin of this organelle in all these four clades.[20]

Evolution[edit]

A Bayesian estimate places the evolution of the alveolate group at ~850million years ago.[21]The Alveolata consist ofMyzozoa,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.[22]The ancestral alveolate probably possessed aplastid.Chromerids, apicomplexans, and peridinin dinoflagellates have retained thisorganelle.[23]Going one step even further back, the chromerids, the peridinin dinoflagellates and theheterokont algaehave been argued to possess a monophyletic plastid lineage in common, i.e. acquired their plastids from ared alga,[20]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 thedinoflagellates,apicomplexans,Colpodella,Chromerida,andVoromonaswas a myzocytotic predator with two heterodynamicflagella,micropores,trichocysts,rhoptries,micronemes,a polar ring and a coiled open sidedconoid.[24]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.[13]

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 four membranes, and that peridinin dinoflagellates possess a plastid surrounded by three membranes, Petersen et al.[25]have been unable to rule out 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 andendosymbiosis.

Ciliates are a model alveolate, having been genetically studied in great depth over the longest period of any alveolate lineage. They are unusual among eukaryotes in that reproduction involves amicronucleusand amacronucleus.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[22]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.

Epigenetics[edit]

Fewalgaehave been studied forepigenetics.[26]Those for which epigenetic data are available include some algal alveolates.[26]

References[edit]

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  2. ^"alveolate".Memidex (WordNet) Dictionary/Thesaurus.Archived fromthe originalon 2016-04-11.Retrieved2011-01-26.
  3. ^Adl S, et al. (2012)."The revised classification of eukaryotes".Journal of Eukaryotic Microbiology.59(5): 429–514.doi:10.1111/j.1550-7408.2012.00644.x.PMC3483872.PMID23020233.
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  13. ^abcdefTikhonenkov DV, Janouškovec J, Mylnikov AP, Mikhailov KV, Simdyanov TG, Aleoshin VV, Keeling PJ (2014)."Description ofColponema vietnamicasp.n. andAcavomonas peruvianan. gen. n. sp., two new alveolate phyla (Colponemidia nom. nov. and Acavomonidia nom. nov.) and their contributions to reconstructing the ancestral state of alveolates and eukaryotes ".PLOS ONE.9(4): e95467.Bibcode:2014PLoSO...995467T.doi:10.1371/journal.pone.0095467.PMC3989336.PMID24740116.
  14. ^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 genusPerkinsusas a unique group of Alveolates ".PLOS ONE.6(5): e19933.Bibcode:2011PLoSO...619933Z.doi:10.1371/journal.pone.0019933.PMC3101222.PMID21629701.
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External links[edit]

Wikispecieshas information related toAlveolata.
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