The cortical alveolum () is a cellular

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 ...

consisting of a vesicle located under the

cytoplasmic membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the cytoplasm, interior of a Cell (biology), cell from the extrac ...

, to which they give support. The term " corticate" comes from an evolutionary hypothesis about the common origin of

kingdoms Kingdom commonly refers to: * A monarchic state or realm ruled by a king or queen. ** A monarchic chiefdom, represented or governed by a king or queen. * Kingdom (biology), a category in biological taxonomy Kingdom may also refer to: Arts and me ...

Plantae Plants are the eukaryotes that form the kingdom Plantae; they are predominantly photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with cyanobacteria to produce sugars fro ...

and

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 ...

, because both kingdoms have cortical alveoli in at least one phylum. At least three

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 ...

lineages exhibit these structures:

Telonemia Telonemia is a phylum of microscopic eukaryotes commonly known as telonemids. They are unicellular free-living flagellates with a unique combination of Cell (biology), cell structures, including a highly complex cytoskeleton unseen in other eu ...

, Alveolata and

Glaucophyta The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of unicellular algae found in freshwater and moist terrestrial environments, less common today than they were during the Proterozoic. The stated number of spec ...

Definition

Cortical alveoli have been defined as flattened

membranous 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. B ...

sacs or vesicles that strengthen the cellular cortex through the firm fixation to the underlying membrane and

microtubules 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 11 an ...

. They typically form a continuous layer that acts as a flexible film, although they can also constitute a semi-rigid structure or the scales of a

theca In biology, a theca (: thecae) is a sheath or a covering. Botany In botany, the theca is related to plant's flower anatomy. The theca of an angiosperm consists of a pair of microsporangia that are adjacent to each other and share a common ar ...

Occurrence

Cortical alveoli are the defining cellular characteristic of the

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 ...

Alveolata, otherwise united solely by

molecular phylogeny Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

. The name 'Alveolata' is a reference to this trait, present in all representatives of this group: the parasitic

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 ...

ns and perkinsozoans, the predatory

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 and colpodellids, the chromerid algae, and the ecologically diverse

dinoflagellates 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 ...

, as well as small basal

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 collectively known as

colponemid Colponemids are free-living alveolates, unicellular flagellates related to dinoflagellates, apicomplexans and ciliates. They are predators of other small eukaryotes, found in freshwater, marine and soil environments. They do not form a solid clad ...

s. A small

phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...

of eukaryotes,

, contains flagellated species that exhibit flattened alveoli, much like in Alveolata. Both groups share evolutionary affinity, comprising the

TSAR Tsar (; also spelled ''czar'', ''tzar'', or ''csar''; ; ; sr-Cyrl-Latn, цар, car) is a title historically used by Slavic monarchs. The term is derived from the Latin word '' caesar'', which was intended to mean ''emperor'' in the Euro ...

supergroup along with

Stramenopiles The stramenopiles, also called heterokonts, are Protist, 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 sur ...

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 ...

. In addition to the TSAR supergroup, the

Archaeplastida The Archaeplastida (or kingdom Plantae '' sensu lato'' "in a broad sense"; pronounced ) are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor group glaucophytes. It als ...

supergroup contains a lineage of algae with cortical alveoli, known as

. All species of glaucophytes exhibit flattened membranous alveoli lying immediately below the

plasma membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

, in every region of the cell except for the flagellar furrow. In addition, these alveoli may contain rigid plates that, similarly to thecate dinoflagellates, define the shape of the otherwise amorphous cell. The common morphological trait with alveolates has been used to propose a common evolutionary origin between the kingdoms Plantae (defined as Archaeplastida) and

(containing alveolates). This is known as the " corticate" hypothesis, postulated by protozoologist

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 ...

, but later rejected due to the non-

monophyly In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent comm ...

of kingdom Chromista. Instead, both Archaeplastida and the TSAR supergroup are united in a clade known as

Diaphoretickes Diaphoretickes is a major group of eukaryote, eukaryotic organisms spanning over 400,000 species. The majority of the earth's biomass that carries out photosynthesis belongs to Diaphoretickes. In older classification systems, members of the Diaph ...

, which contains the remaining chromist groups

Haptista Haptista is a proposed group of protists made up of centrohelids and haptophytes. Phylogenomic studies indicate that Haptista, together with ''Ancoracysta twista'', forms a sister clade to the SAR supergroup#Internal phylogeny, TSAR Supergroup (b ...

and

Cryptista Cryptista is a clade of alga-like eukaryotes. It is most likely related to Archaeplastida which includes plants and many algae, within the larger group Diaphoretickes. Other characteristic features of cryptophyte mtDNAs include large syntenic ...

Function

Although cortical alveoli are very diverse in shape and function among the different groups of protists, they always share the function of supporting the cytoplasmic membrane. In the case of apicomplexan parasites, they're related to the mobility and facilitate the invasion of host cells, and thus they have a considerable importance in

medicine Medicine is the science and Praxis (process), practice of caring for patients, managing the Medical diagnosis, diagnosis, prognosis, Preventive medicine, prevention, therapy, treatment, Palliative care, palliation of their injury or disease, ...

. In

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 the alveoli contain

cellulose Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...

and compose the scales of their armor. Lastly, in

s they are part of the cortical complex that supports the extrusomes, the basal bodies of the

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 ...

and the intricate cortical shell.

References

{{Reflist, refs= {{cite journal, title=Kingdom Chromista and its eight phyla: a new synthesis emphasising periplastid protein targeting, cytoskeletal and periplastid evolution, and ancient divergences, first1=Thomas, last1=Cavalier-Smith, date=2017, journal=Protoplasma, pages=297–357, volume=255, issue=1 , doi=10.1007/s00709-017-1147-3, pmid=28875267 , pmc=5756292 {{cite journal, vauthors=Stelly N, Mauger JP, Claret M, Adoutte A, date=1991, url=http://jcb.rupress.org/content/113/1/103.full.pdf, title=Cortical alveoli of Paramecium: a vast submembranous calcium storage compartment, journal=The Journal of Cell Biology, volume=113, issue=1, pages=103–112, doi=10.1083/jcb.113.1.103 , pmid=1848863 , pmc=2288916 , s2cid=2395284 {{cite journal, vauthors=Adl, etal., date=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 {{cite journal, vauthors=Gould SB, Tham WH, Cowman AF, McFadden GI, Waller RF, date=2008, url=http://mbe.oxfordjournals.org/content/25/6/1219.full, title=Alveolins, a new family of cortical proteins that define the protist infrakingdom Alveolata, journal=Molecular Biology and Evolution, volume=25, issue=6, pages=1219–1230, doi=10.1093/molbev/msn070 , pmid=18359944 , url-access=subscription {{cite journal, last1 = Cavalier-Smith, first1=Thomas, last2=Chao, first2=Ema E., last3=Lewis, first3=Rhodri, date = December 2015, title = Multiple origins of Heliozoa from flagellate ancestors: New cryptist subphylum Corbihelia, superclass Corbistoma, and monophyly of Haptista, Cryptista, Hacrobia and Chromista , journal = Molecular Phylogenetics and Evolution , volume = 93, issue = 1, pages = 331–362, doi = 10.1016/j.ympev.2015.07.004, pmid = 26234272, doi-access = free, bibcode=2015MolPE..93..331C {{cite Q, Q28212529 {{cite Q, Q24601474 {{cite Q, Q29616064 {{cite Q, Q82839650 {{cite Q, Q35062245 {{cite Q, Q62109160 {{cite journal , title=A photosynthetic alveolate closely related to apicomplexan parasites , journal=Nature , volume=451 , issue=7181 , pages=959–963 , date=February 2008 , pmid=18288187 , doi=10.1038/nature06635 , author1=Moore RB , author2=Oborník M , author3=Janouskovec J , author4=Chrudimský T , author5=Vancová M , author6=Green DH , author7=Wright SW , author8=Davies NW , author9=Bolch CJ, display-authors=8 , last10=Heimann , first10=Kirsten , last11=Šlapeta , first11=Jan , last12=Hoegh-Guldberg , first12=Ove , last13=Logsdon , first13=John M. , last14=Carter , first14=Dee A. , bibcode=2008Natur.451..959M , s2cid=28005870 {{cite Q, Q34270501 {{cite Q, Q112831588 {{cite Q, Q38368966 {{cite Q, Q33346647 Organelles