Alveolate
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The alveolates (meaning "pitted like a honeycomb") are a group of
protist A protist ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancest ...
s, considered a major unranked
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
or superphylum within
Eukaryota The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
. They are currently grouped with the
Stramenopile The stramenopiles, also called heterokonts, are protists 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, an ...
s and
Rhizaria The Rhizaria are a diverse and species-rich clade of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus '' Paulinella'' in the phylum Cercozoa, they are all non-photosynthetic, but many Foraminifera ...
among the protists with tubulocristate mitochondria into the SAR supergroup.


Characteristics

The most notable shared characteristic is the presence of cortical (near the surface) alveoli (sacs). These are flattened vesicles (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. Bi ...
and supporting it, typically contributing to a flexible pellicle (thin skin). In armored 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 us ...
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 Photosynthesis ( ) is a Biological system, system of biological processes by which Photoautotrophism, photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical ener ...
organisms. Almost all sequenced mitochondrial genomes of ciliates and apicomplexa are linear. The mitochondria almost all carry
mtDNA Mitochondrial DNA (mtDNA and mDNA) is the DNA located in the mitochondria organelles in a eukaryotic cell that converts chemical energy from food into adenosine triphosphate (ATP). Mitochondrial DNA is a small portion of the DNA contained in ...
of their own but with greatly reduced genome sizes. Exceptions are ''
Cryptosporidium ''Cryptosporidium'', sometimes called crypto, is an apicomplexan genus of alveolates which are parasitism, parasites that can cause a respiratory and gastrointestinal illness (cryptosporidiosis) that primarily involves watery diarrhea (inte ...
'' which are left with only a mitosome, the circular mitochondrial genomes of '' Acavomonas'' and '' Babesia microti'', and '' Toxoplasmas highly fragmented mitochondrial genome, consisting of 21 sequence blocks which recombine to produce longer segments.


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, Royal Society, FRS, Royal Society of Canada, FRSC, Natural Environment Research Council, NERC Professorial Fellow (21 October 1942 – 19 March 2021), was a professor of evolutionary biology in the Departme ...
introduced the formal name Alveolata in 1991, although at the time he considered the grouping to be a
paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
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 flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a ...
s – very common protozoa with many short
cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...
arranged in rows, and two nuclei * Acavomonidia * Colponemidia *
Dinoflagellate The Dinoflagellates (), also called Dinophytes, are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered protists. Dinoflagellates are mostly marine plankton, but they are also commo ...
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 the ...
s many of which have
chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
s * Perkinsozoa * Chromerida – a marine phylum of photosynthetic protozoa * Colpodellida * Voromonadida *
Apicomplexa The Apicomplexa (also called Apicomplexia; single: apicomplexan) are organisms of a large phylum of mainly parasitic alveolates. Most possess a unique form of organelle structure that comprises a type of non-photosynthetic plastid called an ap ...
– parasitic and secondary non-photosynthetic protozoa that lack axonemal locomotive structures except in
gamete A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...
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 A plastid is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. Plastids are considered to be intracellular endosymbiotic cyanobacteria. Examples of plastids include chloroplasts ...
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 nanometer, nm and have an inner diameter bet ...
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 ''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 ...
'', '' 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 ribosomal ...
gene in marine
picoplankton Picoplankton is the fraction of plankton composed by cell (biology), cells between 0.2 and 2 μm that can be either prokaryotic and eukaryotic phototrophs and heterotrophs: * photosynthetic * heterotrophic They are prevalent amongst microbial p ...
samples revealed the presence of two novel alveolate lineages, 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 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 (now synonymised with Filosa) is a phylum of diverse single-celled eukaryotes. They lack shared morphological characteristics at the microscopic level, and are instead united by phylogeny, molecular phylogenies of rRNA and actin or Ubiqu ...
. 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 () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
analysis, however no more certainty exists on their place.


Phylogeny

In 2017,
Thomas 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 discov ...
described the phylogeny of the Alveolata as follows:


Taxonomy

Alveolata Cavalier-Smith 1991 lveolatobiontes* Phylum Ciliophora Doflein 1901 stat. n. Copeland 1956 iliata Perty 1852; Infusoria Bütschli 1887; Ciliae, Ciliozoa, Cytoidea, Eozoa, Heterocaryota, Heterokaryota** Subphylum Postciliodesmatophora Gerassimova & Seravin 1976 *** Class Heterotrichea Stein 1859 *** Class
Karyorelictea Karyorelictea is a class (biology), class of ciliates in the subphylum Postciliodesmatophora. Most species are members of the microbenthos community, that is, microscopic organisms found in the marine interstitial habitat, though one genus, ''Lox ...
Corliss 1974 ** Subphylum
Intramacronucleata Intramacronucleata is a subphylum of ciliates. The group is characterized by the manner in which division of the macronucleus is accomplished during Cell division, binary fission of the cell. In ciliates of this subphylum, division of the macronu ...
Lynn 1996 *** Class ? Mesodiniea Chen et al. 2015 *** Infraphylum Lamellicorticata **** Class Litostomatea Small & Lynn 1981 **** Class
Armophorea Armophorea is a class of ciliates in the subphylum Intramacronucleata. . It was first resolved in 2004 and comprises three orders: Metopida, Clevelandellida, and Armophorida. Previously members of this class were thought to be heterotrichs beca ...
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 Protocruziea is a class of ciliates in the subphylum Intramacronucleata Intramacronucleata is a subphylum of ciliates. The group is characterized by the manner in which division of the macronucleus is accomplished during Cell division, binary ...
Chen et al. 2015 rotocruziidia de Puytorac, Grain & Mignot 1987**** Class
Colpodea The Colpodea are a class of ciliates, of about 200 species common in freshwater and soil habitats. The body cilia are typically uniform, and are supported by dikinetids of characteristic structure, with cilia on both kinetosomes. The mouth may ...
Small & Lynn 1981 **** Class Nassophorea Small & Lynn 1981 **** Class Phyllopharyngea de Puytorac et al. 1974 **** Class
Prostomatea Prostomatea is a 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 flagellum, eukaryotic flagella, but are in general shorter and pr ...
Schewiakoff 1896 **** Class Plagiopylea Small & Lynn 1985 sensu Lynn 2008 **** Class Oligohymenophorea de Puytorac et al. 1974 * Phylum Miozoa 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 Cavalier-Smith 2004 *** Infraphylum
Apicomplexa The Apicomplexa (also called Apicomplexia; single: apicomplexan) are organisms of a large phylum of mainly parasitic alveolates. Most possess a unique form of organelle structure that comprises a type of non-photosynthetic plastid called an ap ...
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 Leuckart 1879 stat. nov. Cavalier-Smith 2013 amontozoa***** Class Blastogregarinida 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 Perkinsozoa Norén et al. 1999 s.s. ***** Class
Perkinsea Perkinsids are single-celled protists that live as intracellular parasites of a variety of other organisms. They are classified as the class Perkinsea within the monotypic phylum Perkinsozoa. It is part of the eukaryotic supergroup Alveolata, ...
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 Dinophyceae is a class of dinoflagellates. Taxonomy * Class Dinophyceae Pascher 1914 eridinea Ehrenberg 1830 stat. nov. Wettstein; Blastodiniphyceae Fensome et al. 1993 orthog. emend.** Order Haplozoonales aplozooidea Poche 1913*** Famil ...
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 proposed but polyphyletic obsolete Biology, biological Kingdom (biology), kingdom, refined from the Chromalveolata, consisting of single-celled and multicellular eukaryotic species that share similar features in their Photosynthesi ...
(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.


Evolution

A Bayesian estimate places the evolution of the alveolate group at ~. The Alveolata consist of Myzozoa,
Ciliates 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 ...
, 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
plastid A plastid is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. Plastids are considered to be intracellular endosymbiotic cyanobacteria. Examples of plastids include chloroplasts ...
. Chromerids, apicomplexans, and peridinin dinoflagellates have retained this
organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to 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 (, ; ), make up one of the oldest groups of eukaryotic algae. The Rhodophyta comprises one of the largest Phylum, phyla of algae, containing over 7,000 recognized species within over 900 Genus, genera amidst ongoing taxon ...
, 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 A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...
, micropores,
trichocyst A trichocyst is an organelle found in certain ciliates and dinoflagellate The Dinoflagellates (), also called Dinophytes, are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered pr ...
s, rhoptries,
microneme Micronemes are secretory organelles, possessed by parasitic apicomplexans. Micronemes are located on the apical third of the protozoan body. They are surrounded by a typical unit membrane. On electron microscopy they have an electron-dense ma ...
s, 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 an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...
. 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 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 Metabolism (, ...
. 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'' ( Ciliophora) File:Mikrofoto.de-Glockentierchen-1.jpg, '' Vorticella'' ( Ciliophora) (left) File:Plasmodium.jpg, ''
Plasmodium falciparum ''Plasmodium falciparum'' is a Unicellular organism, unicellular protozoan parasite of humans and is the deadliest species of ''Plasmodium'' that causes malaria in humans. The parasite is transmitted through the bite of a female ''Anopheles'' mos ...
'' (
Apicomplexa The Apicomplexa (also called Apicomplexia; single: apicomplexan) are organisms of a large phylum of mainly parasitic alveolates. Most possess a unique form of organelle structure that comprises a type of non-photosynthetic plastid called an ap ...
) in blood File:Emaxima oocysts usda.jpg, '' Eimeria maxima'' (
Apicomplexa The Apicomplexa (also called Apicomplexia; single: apicomplexan) are organisms of a large phylum of mainly parasitic alveolates. Most possess a unique form of organelle structure that comprises a type of non-photosynthetic plastid called an ap ...
) File:Dinophysis acuminata.jpg, '' Dinophysis acuminata'' ( Dinoflagellata)


Epigenetics

Few algae have been studied for
epigenetics In biology, epigenetics is the study of changes in gene expression that happen without changes to the DNA sequence. The Greek prefix ''epi-'' (ἐπι- "over, outside of, around") in ''epigenetics'' implies features that are "on top of" or "in ...
. Those for which epigenetic data are available include some algal alveolates.


References


External links


Tree of Life: Alveolates
{{Taxonbar, from=Q448746 Infrakingdoms