The alveolates (meaning "pitted like a honeycomb") are a group of

protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the e ...

s, considered a major

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

and superphylum within Eukarya. They are currently grouped with the

stramenopile Stramenopile is a clade of organisms distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, and in some they have be ...

s and

Rhizaria The Rhizaria are an ill-defined but species-rich supergroup of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus Paulinella in the phylum Cercozoa, they are all non-photosynthethic, but many forami ...

among the protists with tubulocristate mitochondria, the group being referred to as SAR.

Characteristics

The most notable shared characteristic is the presence of cortical (near the surface) alveoli (sacs). These are flattened

vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry) In cell biology, a vesicle is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer. Vesicles form nat ...

s (sacs) arranged as a layer just under the

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. ...

and supporting it, typically contributing to a flexible pellicle (thin skin). In armored

dinoflagellates The dinoflagellates ( Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates ...

they may contain stiff plates. Alveolates have

mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...

with tubular cristae (invaginations), and cells often have pore-like intrusions through the cell surface. The group contains free-living and parasitic organisms, predatory flagellates, and photosynthetic organisms. EM alveoli ciliate paramecium putrinum

Almost all sequenced mitochondrial genomes of ciliates and apicomplexa are linear. The mitochondria almost all carry

mtDNA Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial DNA ...

of their own but with greatly reduced genome sizes. Exceptions are ''

Cryptosporidium ''Cryptosporidium'', sometimes informally called crypto, is a genus of apicomplexan parasitic alveolates that can cause a respiratory and gastrointestinal illness (cryptosporidiosis) that primarily involves watery diarrhea (intestinal crypt ...

'' which are left with only a

mitosome A mitosome is an organelle found in some unicellular eukaryotic organisms, like in members of the supergroup Excavata. The mitosome was found and named in 1999, and its function has not yet been well characterized. It was termed a ''crypton'' by ...

; ciliates; and Janouškovec et al 2013 demonstrated that '' Acavomonas'' diverged early and thus have retained some gene-encoding mtDNA. The mitochondrial genome of '' Babesia microti'' is circular. This species is also now known not to belong to either of the genera ''

Babesia ''Babesia'', also called ''Nuttallia'', is an apicomplexan parasite that infects red blood cells and is transmitted by ticks. Originally discovered by the Romanian bacteriologist Victor Babeș in 1888, over 100 species of ''Babesia'' have since ...

'' or ''

Theileria ''Theileria'' is a genus of parasites that belongs to the phylum Apicomplexa, and is closely related to '' Plasmodium''. Two ''Theileria'' species, ''T. annulata'' and ''T. parva'', are important cattle parasites. ''T. annulata'' causes tropical ...

'' 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''.

Cavalier-Smith Thomas (Tom) Cavalier-Smith, FRS, FRSC, NERC Professorial Fellow (21 October 1942 – 19 March 2021), was a professor of evolutionary biology in the Department of Zoology, at the University of Oxford. His research has led to disc ...

introduced the formal name Alveolata in 1991, although at the time he was unconvinced that the group was monophyletic, and considered the grouping to be a

paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...

assemblage. Many biologists prefer the use of the colloquial name 'alveolate'.

Classification

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: *

Ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a differen ...

s – very common protozoa with many short

cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike proje ...

arranged in rows, and two nuclei * Acavomonidia * Colponemidia *

Dinoflagellate The dinoflagellates ( Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates are ...

s s.l. – mostly marine

flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and thei ...

s many of which have

chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it i ...

s *

Perkinsozoa Perkinsozoa is a proposed phylum of intracellular parasites in the superphylum Alveolata, which was suggested to account for the genus '' Perkinsus'' and other protist A protist () is any eukaryotic organism (that is, an organism whose cell ...

Chromerida Chromerida is a phylum of unicellular alveolates, which includes photosynthetic species '' Chromera velia'' and '' Vitrella brassicaformis''. General features of the phylum include spherical cells each with a thick cell wall, chloroplast present w ...

– a marine phylum of photosynthetic protozoa * Colpodellida * Voromonadida *

Apicomplexa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. Th ...

– parasitic and secondary non-photosynthetic protozoa that lack axonemal locomotive structures except in

gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...

s The Acavomonidia and Colponemidia were previously grouped together as colponemids, a taxon now split because each has a distinctive organization or ultrastructural identity. The Acavomonidia are closer to the dinoflagellate/perkinsid group than the Colponemidia are. As such, the informal term "colponemids", as it stands currently, covers two non-sister groups within Alveolata: the Acavomonidia and the Colponemidia. The Apicomplexa and dinoflagellates may be more closely related to each other than to the ciliates. Both have

plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracellular endosymbiotic cyan ...

s, and most share a bundle or cone of

microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 1 ...

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 ''Oxyrrhis'' is a genus of dinoflagellates. It includes the species ''Oxyrrhis marina''. It is sometimes considered to be a monotypic genus.Lowe, C. D., et al. (2011)Who is ''Oxyrrhis marina''? Morphological and phylogenetic studies on an unusua ...

'' and '' Colponema'', and parasites in ''

Perkinsus ''Perkinsus'' is a genus of alveolates in the phylum Perkinsozoa. The genus was erected in 1978 to better treat its type species, ''Perkinsus marinus'', known formerly as ''Dermocystidium marinum''. These are parasitic protozoans that infect mol ...

'', '' Parvilucifera'', '' Rastrimonas'' and the ellobiopsids. In 2001, direct amplification of the

rRNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribos ...

gene in marine picoplankton samples revealed the presence of two novel alveolate linages, called group I and II. Group I has no cultivated relatives, while group II is related to the dinoflagellate parasite '' Amoebophrya'', which was classified until now in the

Syndiniales The Syndiniales are an order of early branching dinoflagellates (also known as Marine Alveolates, "MALVs"), found as parasites of crustaceans, fish, algae, cnidarians, and protists (ciliates, radiolarians, other dinoflagellates). The trophic ...

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 Cercozoa is a phylum of diverse single-celled eukaryotes. They lack shared morphological characteristics at the microscopic level, and are instead defined by molecular phylogenies of rRNA and actin or polyubiquitin. They were the first major euk ...

. The ellobiopsids are of uncertain relation within the alveolates. Silberman et al 2004 establish that the '' Thalassomyces'' genus of ellobiopsids are alveolates using

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...

analysis, however no more certainty exists on their place.

Phylogeny

Based on a compilation of the following works. Recent research indicates that the Dinoflagelllata actually emerged in the Perkinsea. Furthermore, the Perkinsea emerged in the Apicomplexa.

Taxonomy

Alveolata Cavalier-Smith 1991 lveolatobiontes* Phylum

Ciliophora The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different ...

Doflein 1901 stat. n. Copeland 1956 iliata Perty 1852; Infusoria Bütschli 1887; Ciliae, Ciliozoa, Cytoidea, Eozoa, Heterocaryota, Heterokaryota** Subphylum

Postciliodesmatophora Postciliodesmatophora is a subphylum of ciliates. Members of this subphylum share stacks of postciliary microtubular ribbons associated with somatic kinetosome A basal body (synonymous with basal granule, kinetosome, and in older cytologic ...

Gerassimova & Seravin 1976 *** Class Heterotrichea Stein 1859 *** Class Karyorelictea Corliss 1974 ** Subphylum Intramacronucleata Lynn 1996 *** Class ? Mesodiniea Chen et al. 2015 *** Infraphylum Lamellicorticata **** Class Litostomatea Small & Lynn 1981 **** Class Armophorea Lynn 2004 **** Class Cariacotrichea Orsi et al. 2011 **** Class Spirotrichea Bütschli 1889 **** Infraphylum Ventrata Cavalier-Smith 2004 onthreep Lynn 2012**** Order ? Discotrichida Chen et al. 2015 **** Class Protocruziea Chen et al. 2015 rotocruziidia de Puytorac, Grain & Mignot 1987**** Class Colpodea Small & Lynn 1981 **** Class Nassophorea Small & Lynn 1981 **** Class Phyllopharyngea de Puytorac et al. 1974 **** Class Prostomatea Schewiakoff 1896 **** Class Plagiopylea Small & Lynn 1985 sensu Lynn 2008 **** Class

Oligohymenophorea The Oligohymenophorea are a large class of ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter ...

de Puytorac et al. 1974 * Phylum

Miozoa Myzozoa is a grouping of specific phyla within Alveolata, that either feed through myzocytosis, or were ancestrally capable of feeding through myzocytosis. Many protozoan orders are included within Myzozoa. It is sometimes described as a ph ...

Cavalier-Smith 1987 ** Subphylum Colponemidia Tikhonenkov, Mylnikov & Keeling 2013 *** Class Colponemea Cavalier-Smith 1993 ** Subphylum Acavomonadia Tikhonenkov et al. 2014 *** Class Acavomonadea Tikhonenkov et al. 2014 ** Subphylum

Myzozoa Myzozoa is a grouping of specific phyla within Alveolata, that either feed through myzocytosis, or were ancestrally capable of feeding through myzocytosis. Many protozoan orders are included within Myzozoa. It is sometimes described as a ph ...

Cavalier-Smith 2004 *** Infraphylum

Levine 1970 emend. Adl et al. 2005 **** Order ? Vitrellida Cavalier-Smith 2017 **** Class ? Myzomonadea Cavalier-Smith & Chao 2004 sensu Ruggiero et al. 2015 **** Class Chromerea **** Order Colpodellida Patterson & Zölffel 1991 piromonadida Krylov & Mylnikov 1986**** Superclass

Sporozoa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. The ...

Leuckart 1879 stat. nov. Cavalier-Smith 2013 amontozoa***** Class

Blastogregarinida The gregarines are a group of Apicomplexan alveolates, classified as the Gregarinasina or Gregarinia. The large (roughly half a millimeter) parasites inhabit the intestines of many invertebrates. They are not found in any vertebrates. However, gr ...

Chatton & Villeneuve 1936 lastogregarinina; Blastogregarinorina Chatton & Villeneuve 1936***** Class Paragregarea Cavalier-Smith 2014 ***** Class Gregarinomorphea Grassé 1953 ***** Class Coccidiomorphea Doflein 1901 *** 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

Norén et al. 1999 s.s. ***** Class

Perkinsea Perkinsea is a class of alveolates. Taxonomy ''Perkinsus'' is a genus in the class Perkinsea that is a parasite of bivalve molluscs; it displays a number of features characteristic of the dinoflagellates including laterally inserted heterodyna ...

Levine 1978 erkinsasida Levine 1978**** Superclass Dinoflagellata Butschli 1885 stat. nov. Cavalier-Smith 1999 sensu Cavalier-Smith 2013 inozoa Cavalier-Smith 1981***** Class Pronoctilucea ***** Class Ellobiopsea Cavalier-Smith 1993 llobiophyceae Loeblich III 1970; Ellobiopsida Whisler 1990***** Class Myzodinea Cavalier-Smith 2017 ***** Class Oxyrrhea Cavalier-Smith 1987 ***** Class Syndinea Chatton 1920 s.l. yndiniophyceae Loeblich III 1970 s.s.; Syndina Cavalier-Smith***** Class Endodinea Cavalier-Smith 2017 ***** Class Noctiluciphyceae Fensome et al. 1993 octilucae Haeckel 1866; Noctilucea Haeckel 1866 stat. nov.; Cystoflagellata Haeckel 1873 stat. nov. Butschli 1887***** Class Dinophyceae Pascher 1914 eridinea 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 Chromista is a biological kingdom consisting of single-celled and multicellular eukaryotic species that share similar features in their photosynthetic organelles ( plastids). It includes all protists whose plastids contain chlorophyll ''c'', ...

(the

chromalveolate Chromalveolata was a eukaryote supergroup present in a major classification of 2005, then regarded as one of the six major groups within the eukaryotes. It was a refinement of the kingdom Chromista, first proposed by Thomas Cavalier-Smith in ...

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.

Evolution

A Bayesian estimate places the evolution of the alveolate group at ~. The Alveolata consist of

, 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. The ancestral alveolate probably possessed a

. Chromerids, apicomplexans, and peridinin dinoflagellates have retained this

organelle In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' th ...

. 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 Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority ...

, 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

, apicomplexans, ''

Colpodella ''Colpodella'' is a genus of alveolates comprising 5 species, and two further possible species: They share all the synapomorphies of apicomplexans, but are free-living, rather than parasitic. Many members of this genus were previously assigned to ...

'',

, and ''

Voromonas ''Voromonas'' is a genus of predatory alveolates. The genus and species were described by Mylnikov in 2000. It was originally described as ''Colpodella pontica'' but was later renamed by Cavalier-Smith and Chao in 2004. Taxonomy There is one ...

'' was a myzocytotic predator with two heterodynamic

flagella A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have fro ...

, micropores,

trichocyst A trichocyst is an organelle found in certain ciliates and dinoflagellate The dinoflagellates ( Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the ...

s, rhoptries, micronemes, a polar ring and a coiled open sided conoid. 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. 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. 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 An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...

. 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 Micronucleus is the name given to the small nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. It usually is a sign of genotoxic events and chromosomal i ...

and a

macronucleus A macronucleus (formerly also meganucleus) is the larger type of nucleus in ciliates. Macronuclei are polyploid and undergo direct division without mitosis. It controls the non-reproductive cell functions, such as metabolism. During conjugati ...

. 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 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. File:Paramecium caudatum Ehrenberg, 1833.jpg, ''

Paramecium caudatum ''Paramecium caudatum'' is a species of unicellular protist in the phylum Ciliophora. They can reach 0.33 mm in length and are covered with minute hair-like organelles called cilia. The cilia are used in locomotion and feeding. The species ...

'' (

) File:Mikrofoto.de-Glockentierchen-1.jpg, '' Vorticella'' (

) (left) File:Plasmodium.jpg, ''

Plasmodium falciparum ''Plasmodium falciparum'' is a unicellular protozoan parasite of humans, and the deadliest species of ''Plasmodium'' that causes malaria in humans. The parasite is transmitted through the bite of a female '' Anopheles'' mosquito and causes the ...

'' (

) in blood File:Emaxima oocysts usda.jpg, '' Eimeria maxima'' (

) File:Dinophysis acuminata.jpg, '' Dinophysis acuminata'' ( Dinoflagellata)

References

External links

Tree of Life: Alveolates
{{Taxonbar, from=Q448746 Infrakingdoms Taxa named by Thomas Cavalier-Smith