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The '''alveolates''' (meaning "with alveoli")<ref>{{cite web |url=http://www.memidex.com/alveolate |title=alveolate |work=Memidex (WordNet) Dictionary/Thesaurus |access-date=2011-01-26 |archive-url=https://web.archive.org/web/20160411233722/http://www.memidex.com/alveolate |archive-date=2016-04-11 |url-status=dead }}</ref> are a group of [[protist]]s, considered a major [[clade]]<ref name="Adl2012">{{cite journal | last1 = Adl | first1 = S.M. | display-authors = etal | year = 2012 | title = The revised classification of eukaryotes | journal = Journal of Eukaryotic Microbiology | volume = 59 | issue = 5| pages = 429–514 | doi=10.1111/j.1550-7408.2012.00644.x | pmid=23020233 | pmc=3483872}}</ref> and [[Biological classification|superphylum]]<ref name=Ruggiero15>{{cite journal |vauthors=Ruggiero MA, Gordon DP, Orrell TM, Bailly N, Bourgoin T, Brusca RC, Cavalier-Smith T, Guiry MD, Kirk PM |title=A higher level classification of all living organisms |journal=PLOS ONE |volume=10 |issue=4 |pages=e0119248 |year=2015 |pmid=25923521 |pmc=4418965 |doi=10.1371/journal.pone.0119248 |bibcode=2015PLoSO..1019248R |doi-access=free }}</ref> within [[Eukarya]], and are also called '''Alveolata'''.<ref name="Cavalier-Smith, T. 1991 pp. 113-131">{{cite book |last=Cavalier-Smith |first=T. |chapter=Cell diversification in heterotrophic flagellates |pages=113–131 |editor-first=David J. |editor-last=Patterson |editor2-first=Jacob |editor2-last=Larsen |editor3=Systematics Association |title=The Biology of free-living heterotrophic flagellates |chapter-url=https://books.google.com/books?id=-JUWAQAAIAAJ |year=1991 |publisher=Oxford University Press |isbn=978-0-19-857747-8}}</ref>
The '''alveolates''' (meaning "with alveoli")<ref>{{cite web |url=http://www.memidex.com/alveolate |title=alveolate |work=Memidex (WordNet) Dictionary/Thesaurus |access-date=2011-01-26 |archive-url=https://web.archive.org/web/20160411233722/http://www.memidex.com/alveolate |archive-date=2016-04-11 |url-status=dead }}</ref> are a group of [[protist]]s, considered a major [[clade]]<ref name="Adl2012">{{cite journal | last1 = Adl | first1 = S.M. | display-authors = etal | year = 2012 | title = The revised classification of eukaryotes | journal = Journal of Eukaryotic Microbiology | volume = 59 | issue = 5| pages = 429–514 | doi=10.1111/j.1550-7408.2012.00644.x | pmid=23020233 | pmc=3483872}}</ref> and [[Biological classification|superphylum]]<ref name=Ruggiero15>{{cite journal |vauthors=Ruggiero MA, Gordon DP, Orrell TM, Bailly N, Bourgoin T, Brusca RC, Cavalier-Smith T, Guiry MD, Kirk PM |title=A higher level classification of all living organisms |journal=PLOS ONE |volume=10 |issue=4 |pages=e0119248 |year=2015 |pmid=25923521 |pmc=4418965 |doi=10.1371/journal.pone.0119248 |bibcode=2015PLoSO..1019248R |doi-access=free }}</ref> within [[Eukarya]]. They are currently grouped with the [[stramenopile]]s and Rhizaria among the protists with tubulocristate mitochondria, the group being referred to as SAR.


==Characteristics==
==Characteristics==
Line 46: Line 46:
| pmid = 1904987
| pmid = 1904987
|display-authors=etal}}.
|display-authors=etal}}.
</ref> [[Thomas Cavalier-Smith|Cavalier-Smith]] introduced the formal name Alveolata in 1991,<ref name="Cavalier-Smith, T. 1991 pp. 113-131"/> although at the time he was unconved that the group was monophyletic - considered the grouping to be a [[paraphyletic]] assemblage.
</ref> [[Thomas Cavalier-Smith|Cavalier-Smith]] introduced the formal name Alveolata in 1991, <ref name="Cavalier-Smith, T. 1991 pp. 113-131">{{cite book |last=Cavalier-Smith |first=T. |chapter=Cell diversification in heterotrophic flagellates |pages=113–131 |editor-first=David J. |editor-last=Patterson |editor2-first=Jacob |editor2-last=Larsen |editor3=Systematics Association |title=The Biology of free-living heterotrophic flagellates |chapter-url=https://books.google.com/books?id=-JUWAQAAIAAJ |year=1991 |publisher=Oxford University Press |isbn=978-0-19-857747-8}}</ref> although at the time he was unconvinced that the group was monophyletic - considered the grouping to be a [[paraphyletic]] assemblage. Many biologists preferthe use of the colloquial name 'alveolate'<ref>Kumar, S. & Rzhetsky, A. 1996. Evolutionary relationships of eukaryotic kingdoms. Journal of Molecular Evolution, 42: 183–193</ref>.


==Classification==
==Classification==
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*[[Sporozoa|Apicomplexa]] – parasitic and secondary non-photosynthetic protozoa that lack [[Axoneme|axonemal]] locomotive structures except in [[gamete]]s
*[[Sporozoa|Apicomplexa]] – parasitic and secondary non-photosynthetic protozoa that lack [[Axoneme|axonemal]] locomotive structures except in [[gamete]]s


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.<ref name=Tikhonenkov14/> As such, the informal term "colponemids", as it stands currently, covers two non-sister groups within Alveolata: the Acavomonidia and the Colponemidia.<ref name=Tikhonenkov14/>
The Acavomonidia and Colponemidia were previously grouped together as colponemids, a taxon now split because of each has a distinctive organization or [[ultrastructural identity]]. The Acavomonidia are closer to the dinoflagellate/perkinsid group than the Colponemidia are.<ref name=Tikhonenkov14/> As such, the informal term "colponemids", as it stands currently, covers two non-sister groups within Alveolata: the Acavomonidia and the Colponemidia.<ref name=Tikhonenkov14/>


The Apicomplexa and dinoflagellates may be more closely related to each other than to the ciliates. Both have [[plastid]]s, and most share a bundle or cone of [[microtubule]]s 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 marinus|Perkinsus]]'',<ref name=Zhang2011>{{cite journal | last1 = Zhang | first1 = H | last2 = Campbell | first2 = DA | last3 = Sturm | first3 = NR | last4 = Dungan | first4 = CF | last5 = Lin | first5 = S | year = 2011 | title = Spliced leader RNAs, mitochondrial gene frameshifts and multi-protein phylogeny expand support for the genus ''Perkinsus'' as a unique group of Alveolates | journal = PLOS ONE | volume = 6 | issue = 5| page = e19933 | doi=10.1371/journal.pone.0019933 | pmid=21629701 | pmc=3101222| bibcode = 2011PLoSO...619933Z | doi-access = free }}</ref> ''[[Parvilucifera]]'', ''[[Rastrimonas]]'' and the [[ellobiopsid]]s. 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.<ref>{{cite journal |vauthors=López-García P, Rodríguez-Valera F, Pedrós-Alió C, Moreira D |title=Unexpected diversity of small eukaryotes in deep-sea Antarctic plankton |journal=Nature |volume=409 |issue=6820 |pages=603–7 |year=2001 |pmid=11214316 |doi=10.1038/35054537 |bibcode=2001Natur.409..603L |s2cid=11550698 }}</ref><ref>{{cite journal |vauthors=Moon-van der Staay SY, De Wachter R, Vaulot D |title=Oceanic 18S rDNA sequences from picoplankton reveal unsuspected eukaryotic diversity |journal=Nature |volume=409 |issue=6820 |pages=607–10 |year=2001 |pmid=11214317 |doi=10.1038/35054541 |bibcode=2001Natur.409..607M |s2cid=4362835 }}</ref> 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.
The Apicomplexa and dinoflagellates may be more closely related to each other than to the ciliates. Both have [[plastid]]s, and most share a bundle or cone of [[microtubule]]s 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 marinus|Perkinsus]]'',<ref name=Zhang2011>{{cite journal | last1 = Zhang | first1 = H | last2 = Campbell | first2 = DA | last3 = Sturm | first3 = NR | last4 = Dungan | first4 = CF | last5 = Lin | first5 = S | year = 2011 | title = Spliced leader RNAs, mitochondrial gene frameshifts and multi-protein phylogeny expand support for the genus ''Perkinsus'' as a unique group of Alveolates | journal = PLOS ONE | volume = 6 | issue = 5| page = e19933 | doi=10.1371/journal.pone.0019933 | pmid=21629701 | pmc=3101222| bibcode = 2011PLoSO...619933Z | doi-access = free }}</ref> ''[[Parvilucifera]]'', ''[[Rastrimonas]]'' and the [[ellobiopsid]]s. 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.<ref>{{cite journal |vauthors=López-García P, Rodríguez-Valera F, Pedrós-Alió C, Moreira D |title=Unexpected diversity of small eukaryotes in deep-sea Antarctic plankton |journal=Nature |volume=409 |issue=6820 |pages=603–7 |year=2001 |pmid=11214316 |doi=10.1038/35054537 |bibcode=2001Natur.409..603L |s2cid=11550698 }}</ref><ref>{{cite journal |vauthors=Moon-van der Staay SY, De Wachter R, Vaulot D |title=Oceanic 18S rDNA sequences from picoplankton reveal unsuspected eukaryotic diversity |journal=Nature |volume=409 |issue=6820 |pages=607–10 |year=2001 |pmid=11214317 |doi=10.1038/35054541 |bibcode=2001Natur.409..607M |s2cid=4362835 }}</ref> 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.




Some studies suggested the [[Ascetosporea|haplosporid]]s, mostly parasites of marine invertebrates, might belong here, but they lack alveoli and are now placed among the [[Cercozoa]].
Some studies suggested the [[Ascetosporea|haplosporid]]s, mostly parasites of marine invertebrates, might belong here, but they lack alveoli and are now placed among the [[Cercozoa]].
Line 245: Line 243:
A Bayesian estimate places the evolution of the alveolate group at ~{{ma|850}}.<ref name=Berney2006>{{cite journal | last1 = Berney | first1 = C | last2 = Pawlowski | first2 = J | year = 2006 | title = A molecular time-scale for eukaryote evolution recalibrated with the continuous microfossil record | journal = Proc Biol Sci | volume = 273 | issue = 1596| pages = 1867–72 | doi=10.1098/rspb.2006.3537| pmc = 1634798 | pmid=16822745}}</ref> 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.
A Bayesian estimate places the evolution of the alveolate group at ~{{ma|850}}.<ref name=Berney2006>{{cite journal | last1 = Berney | first1 = C | last2 = Pawlowski | first2 = J | year = 2006 | title = A molecular time-scale for eukaryote evolution recalibrated with the continuous microfossil record | journal = Proc Biol Sci | volume = 273 | issue = 1596| pages = 1867–72 | doi=10.1098/rspb.2006.3537| pmc = 1634798 | pmid=16822745}}</ref> 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.<ref name=Reyes-Prieto>{{cite journal|last1=Reyes-Prieto|first1=A|last2=Moustafa|first2=A|last3=Bhattacharya|first3=D|title=Multiple genes of apparent algal origin suggest ciliates may once have been photosynthetic.|journal=Curr. Biol.|date=2008|volume=18|issue=13|pages=956–62|doi=10.1016/j.cub.2008.05.042|pmid=18595706|pmc=2577054}}</ref> The ancestral alveolate probably possessed a [[plastid]]. Chromerids, apicomplexans, and peridinin dinoflagellates have retained this [[organelle]].<ref>{{cite journal |vauthors=Moore RB, Oborník M, Janouskovec J, Chrudimský T, Vancová M, Green DH, Wright SW, Davies NW, Bolch CJ, Heimann K, Slapeta J, Hoegh-Guldberg O, Logsdon JM, Carter DA |title=A photosynthetic alveolate closely related to apicomplexan parasites |journal=Nature |volume=451 |issue=7181 |pages=959–963 |year=2008 |pmid=18288187 |doi=10.1038/nature06635 |bibcode=2008Natur.451..959M |s2cid=28005870 }}</ref> Going one step even further back, the chromerids, the peridinin dinoflagellates and the [[Heterokont|heterokont algae]] possess a monophyletic plastid lineage in common, i.e. acquired their plastids from a [[red alga]],<ref name="Janouskovec2010"/> and so it seems likely that the common ancestor of alveolates and heterokonts was also photosynthetic.
The ancestors of the alveolate group may have been photosynthetic.<ref name=Reyes-Prieto>{{cite journal|last1=Reyes-Prieto|first1=A|last2=Moustafa|first2=A|last3=Bhattacharya|first3=D|title=Multiple genes of apparent algal origin suggest ciliates may once have been photosynthetic.|journal=Curr. Biol.|date=2008|volume=18|issue=13|pages=956–62|doi=10.1016/j.cub.2008.05.042|pmid=18595706|pmc=2577054}}</ref> The ancestral alveolate probably possessed a [[plastid]]. Chromerids, apicomplexans, and peridinin dinoflagellates have retained this [[organelle]].<ref>{{cite journal |vauthors=Moore RB, Oborník M, Janouskovec J, Chrudimský T, Vancová M, Green DH, Wright SW, Davies NW, Bolch CJ, Heimann K, Slapeta J, Hoegh-Guldberg O, Logsdon JM, Carter DA |title=A photosynthetic alveolate closely related to apicomplexan parasites |journal=Nature |volume=451 |issue=7181 |pages=959–963 |year=2008 |pmid=18288187 |doi=10.1038/nature06635 |bibcode=2008Natur.451..959M |s2cid=28005870 }}</ref> Going one step even further back, the chromerids, the peridinin dinoflagellates and the [[Heterokont|heterokont algae]] have been argued to possess a monophyletic plastid lineage in common, i.e. acquired their plastids from a [[red alga]],<ref name="Janouskovec2010"/> 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]], [[micropore]]s, [[trichocyst]]s, [[rhoptries]], [[microneme]]s, a polar ring and a coiled open sided [[conoid]].<ref name=Kuvardina2002>{{cite journal | last1 = Kuvardina | first1 = ON | last2 = Leander | first2 = BS | last3 = Aleshin | first3 = VV | last4 = Myl'nikov | first4 = AP | last5 = Keeling | first5 = PJ | last6 = Simdyanov | first6 = TG | year = 2002 | title = The phylogeny of colpodellids (Alveolata) using small subunit rRNA gene sequences suggests they are the free living sister group to apicomplexans | journal = J Eukaryot Microbiol | volume = 49 | issue = 6| pages = 498–504 | doi=10.1111/j.1550-7408.2002.tb00235.x | pmid=12503687| title-link = colpodellid | s2cid = 4283969 }}</ref> 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.<ref name=Tikhonenkov14/>
In one school of thought the common ancestor of the [[dinoflagellates]], [[apicomplexans]], [[Colpodella]], [[Chromerida]], and [[Voromonas]] was a myzocytotic predator with two heterodynamic [[flagella]], [[micropore]]s, [[trichocyst]]s, [[rhoptries]], [[microneme]]s, a polar ring and a coiled open sided [[conoid]].<ref name=Kuvardina2002>{{cite journal | last1 = Kuvardina | first1 = ON | last2 = Leander | first2 = BS | last3 = Aleshin | first3 = VV | last4 = Myl'nikov | first4 = AP | last5 = Keeling | first5 = PJ | last6 = Simdyanov | first6 = TG | year = 2002 | title = The phylogeny of colpodellids (Alveolata) using small subunit rRNA gene sequences suggests they are the free living sister group to apicomplexans | journal = J Eukaryot Microbiol | volume = 49 | issue = 6| pages = 498–504 | doi=10.1111/j.1550-7408.2002.tb00235.x | pmid=12503687| title-link = colpodellid | s2cid = 4283969 }}</ref> 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.<ref name=Tikhonenkov14/>

Revision as of 17:38, 27 April 2022

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

The alveolates (meaning "with alveoli")[2] are a group of protists, considered a major clade[3] and superphylum[4] within Eukarya. They are currently grouped with the stramenopiles and Rhizaria among the protists with tubulocristate mitochondria, the group being referred to as SAR.

Characteristics

The most notable shared characteristic is the presence of cortical (outer) alveoli (sacs). These are flattened vesicles (sacs) packed into a continuous layer just under the membrane and supporting it, typically forming a flexible pellicle (thin skin). In dinoflagellates they often form armor plates. Alveolates have mitochondria with tubular cristae (ridges), and their flagellae or cilia have a distinct structure.

Transmission electron micrograph of a thin section of the surface of the ciliate Paramecium putrinum, showing the alveoli (red arrows) under the cell surface.


Almost all sequenced mitochondrial genomes of ciliates and apicomplexia are linear.[5] The mitochondria almost all carry mtDNA of their own but with greatly reduced genome sizes. Exceptions are Cryptosporidium which are left with only a mitosome; ciliates; and Janouškovec et al 2013 demonstrated that Acavomonas diverged early and thus have retained some gene-encoding mtDNA.[6] The mitochondrial genome of Babesia microti is circular.[7] 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.

History

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 Gajadhar et al.[8] Cavalier-Smith introduced the formal name Alveolata in 1991, [9] although at the time he was unconvinced that the group was monophyletic - considered the grouping to be a paraphyletic assemblage. Many biologists preferthe use of the colloquial name 'alveolate'[10].

Classification

Alveolata comprises around 9 major and minor groups, which are very diverse in form, and are known to be related by various ultrastructural and genetic similarities:[11]

The Acavomonidia and Colponemidia were previously grouped together as colponemids, a taxon now split because of each has a distinctive organization or ultrastructural identity. The Acavomonidia are closer to the dinoflagellate/perkinsid group than the Colponemidia are.[11] As such, the informal term "colponemids", as it stands currently, covers two non-sister groups within Alveolata: the Acavomonidia and the Colponemidia.[11]

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,[12] 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.[13][14] 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.

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

The ellobiopsids are of uncertain relation within the alveolates. Silberman et al 2004 establish that the Thalassomyces genus of ellobiopsids are alveolates using phylogenetic analysis however as of 2016 no more certainty exists on their place.[15][16]

Phylogeny

Based on a compilation of the following works.[4][17][18][19]

Alveolata

Recent research indicates that the Dinoflagelllata actually emerged in the Perkinsea. Furthermore, the Perkinsea emerged in the Apicomplexa.[20]

Taxonomy

Alveolata Cavalier-Smith 1991 [Alveolatobiontes]

  • Phylum Ciliophora Doflein 1901 stat. n. Copeland 1956 [Ciliata Perty 1852; Infusoria Bütschli 1887; Ciliae, Ciliozoa, Cytoidea, Eozoa, Heterocaryota, Heterokaryota]
  • Phylum Miozoa Cavalier-Smith 1987
    • Subphylum Colponemidia Tikhonenkov, Mylnikov & Keeling 2013
    • Subphylum Acavomonadia Tikhonenkov et al. 2014
    • Subphylum Myzozoa Cavalier-Smith 2004
      • Infraphylum Apicomplexa Levine 1970 emend. Adl et al. 2005
      • Infraphylum Dinozoa Cavalier-Smith 1981 emend. 2003
        • Order ?Acrocoelida Cavalier-Smith & Chao 2004
        • Order ?Rastromonadida Cavalier-Smith & Chao 2004
        • Class Squirmidea Norén 1999 stat. nov. Cavalier-Smith 2014
        • Superclass Perkinsozoa Norén et al. 1999 s.s.
          • Class Perkinsea Levine 1978 [Perkinsasida Levine 1978]
        • Superclass Dinoflagellata Butschli 1885 stat. nov. Cavalier-Smith 1999 sensu Cavalier-Smith 2013 [Dinozoa Cavalier-Smith 1981]
          • Class Pronoctilucea
          • Class Ellobiopsea Cavalier-Smith 1993 [Ellobiophyceae Loeblich III 1970; Ellobiopsida Whisler 1990]
          • Class Myzodinea Cavalier-Smith 2017
          • Class Oxyrrhea Cavalier-Smith 1987
          • Class Syndinea Chatton 1920 s.l. [Syndiniophyceae Loeblich III 1970 s.s.; Syndina Cavalier-Smith]
          • Class Endodinea Cavalier-Smith 2017
          • Class Noctiluciphyceae Fensome et al. 1993 [Noctilucae Haeckel 1866; Noctilucea Haeckel 1866 stat. nov.; Cystoflagellata Haeckel 1873 stat. nov. Butschli 1887]
          • Class Dinophyceae Pascher 1914 [Peridinea Ehrenberg 1830 stat. nov. Wettstein]

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 alga with evidence of a common origin of this organelle in all these four clades.[21]

Evolution

A Bayesian estimate places the evolution of the alveolate group at ~850 million years ago.[22] 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.[23] The ancestral alveolate probably possessed a plastid. Chromerids, apicomplexans, and peridinin dinoflagellates have retained this organelle.[24] Going one step even further back, the chromerids, the peridinin dinoflagellates and the heterokont algae have been argued to possess a monophyletic plastid lineage in common, i.e. acquired their plastids from a red alga,[21] 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.[25] 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.[11]

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.[26] 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 and endosymbiosis.

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 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[23] 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

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