Alveolate
Encyclopedia
The alveolates are a major line of protist
s.
, which are very divergent in form, but are now known to be close relatives based on various ultrastructural and genetic similarities:
The genus Perkinsus may belong to another clade - Perkinsozoa - based on a number of molecular biological findings.
s packed into a continuous layer supporting the membrane
, typically forming a flexible pellicle. In dinoflagellates they often form armor plates. Alveolates have mitochondria
with tubular cristae, and their flagella or cilia have a distinct structure.
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
, 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. 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.
Relationships between some of these the major groups were suggested during the 1980s, and a specific relationship between all three was confirmed in the early 1990s by genetic studies, most notably by Gajadhar et al. Cavalier-Smith
, introduced the formal name Alveolata in 1991, although at the time he actually considered the grouping to be a paraphyletic assemblage, rather than a monophyletic group.
Some studies suggested the haplosporid
s, mostly parasites of marine invertebrates, might belong here but they lack alveoli and are now placed among the Cercozoa
.
(the chromalveolate
hypothesis). However, as plastids only appear in relatively derived (as opposed to ancestral) groups, others argue the alveolates originally lacked them and possibly the dinoflagellates and Apicomplexa acquired them separately.
s, rhoptries, microneme
s, a polar ring and a coiled open sided conoid
. This ancestor also probably possesed a plastid
but it is presently not clear whether it was photosynthetic. Furthermore it is not clear whether extant perkinsids or colpodellids have retained this organelle
.
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s.
Phyla
There are four phylaPhylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
, which are very divergent in form, but are now known to be close relatives based on various ultrastructural and genetic similarities:
- ApicomplexaApicomplexaThe Apicomplexa are a large group of protists, most of which possess a unique organelle called apicoplast and an apical complex structure involved in penetrating a host's cell. They are unicellular, spore-forming, and exclusively parasites of animals. Motile structures such as flagella or...
- parasitic protozoa that lack axonemalAxonemeNumerous eukaryotic cells carry whip-like appendages whose inner core consists of a cytoskeletal structure called the axoneme....
locomotive structures except in gameteGameteA gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s - Chromerida - a marine phylum of photosynthetic protozoa
- CiliateCiliateThe ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella...
s - very common protozoa with many short ciliaCiliumA cilium is an organelle found in eukaryotic cells. Cilia are slender protuberances that project from the much larger cell body....
arranged in rows - DinoflagellateDinoflagellateThe dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth...
s - mostly marine flagellateFlagellateFlagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...
s many of which have chloroplastChloroplastChloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
s
The genus Perkinsus may belong to another clade - Perkinsozoa - based on a number of molecular biological findings.
Characteristics
The most notable shared characteristic is the presence of cortical alveoli, flattened vesicleVesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
s packed into a continuous layer supporting the membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
, typically forming a flexible pellicle. In dinoflagellates they often form armor plates. Alveolates have mitochondria
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
with tubular cristae, and their flagella or cilia have a distinct structure.
Classification
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 microtubuleMicrotubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
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
Oxyrrhis is a genus of dinoflagellate. It includes the species Oxyrrhis marina....
and Colponema, and parasites in Perkinsus
Perkinsus marinus
Perkinsus marinus is a prevalent pathogen of oysters, causing massive mortality in oyster populations. The disease it causes is known as "Dermo" ,and is characterized by proteolytic degradation of oyster tissues...
, Parvilucifera, Rastrimonas and the ellobiopsids. In 2001, direct amplification of the rRNA gene in marine picoplankton
Picoplankton
Picoplankton is the fraction of plankton composed by cells between 0.2 and 2 μm that can be either :* photosynthetic * heterotrophic Some species can also be mixotrophic....
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
Syndiniales
The Syndiniales are an order of dinoflagellates, found exclusively as endosymbionts of the tintinnid ciliates, crustacea and fish, protozoa, algae, other dinoflagellates. The trophic form is often multinucleate, and ultimately divides to form motile spores, which have two flagella in typical...
dinoflagellate order.
Relationships between some of these the major groups were suggested during the 1980s, and a specific relationship between all three was confirmed in the early 1990s by genetic studies, most notably by Gajadhar et al. Cavalier-Smith
Thomas Cavalier-Smith
Professor Thomas Cavalier-Smith , FRS, FRSC, NERC Professorial Fellow, is a Professor of Evolutionary Biology in the Department of Zoology, at the University of Oxford...
, introduced the formal name Alveolata in 1991, although at the time he actually considered the grouping to be a paraphyletic assemblage, rather than a monophyletic group.
Some studies suggested the haplosporid
Ascetosporea
The Ascetosporea are a group of protists that are parasites of animals, especially marine invertebrates. There are two groups, the haplosporids and paramyxids, which are not particularly similar morphologically but consistently group together on molecular trees, which place them near the base of...
s, mostly parasites of marine invertebrates, might belong here but they lack alveoli and are now placed among the Cercozoa
Cercozoa
The Cercozoa are a group of protists. They are sometimes described as a kingdom.-Characteristics:The group includes most amoeboids and flagellates that feed by means of filose pseudopods. These may be restricted to part of the cell surface, but there is never a true cytostome or mouth as found in...
.
Development
The development of plastids among the alveolates is uncertain. Cavalier-Smith proposed the alveolates developed from a chloroplast-containing ancestor, which also gave rise to the ChromistaChromista
The Chromista are a eukaryotic supergroup, probably polyphyletic, which may be treated as a separate kingdom or included among the Protista. They include all algae whose chloroplasts contain chlorophylls a and c, as well as various colorless forms that are closely related to them...
(the chromalveolate
Chromalveolate
Chromalveolata is a eukaryote supergroup first proposed by Thomas Cavalier-Smith as a refinement of his kingdom Chromista, which was first put forward in 1981. Chromalveolata was proposed to represent the result of a single secondary endosymbiosis between a line descending from a bikont and a red...
hypothesis). However, as plastids only appear in relatively derived (as opposed to ancestral) groups, others argue the alveolates originally lacked them and possibly the dinoflagellates and Apicomplexa acquired them separately.
Evolution
It seems likely that the common ancestor of this group was a myzocytotic predator with two heterodynamic flagella, micropores, trichocystTrichocyst
http://media-2.web.britannica.com/eb-media/68/8968-004-A94DB3DF.jpg A trichocyst is a structure in the cortex of certain ciliate and flagellate protozoans consisting of a cavity and long, thin threads that can be ejected in response to certain stimuli...
s, rhoptries, microneme
Microneme
Micronemes are cellular organs, or organelles, possessed by Apicomplexa protozoans that are restricted to the apical third of the protozoan body. They are surrounded by a typical unit membrane. On electorn microscopy they have an electron-dense matrix due to the high protein content...
s, a polar ring and a coiled open sided conoid
Conoid
In geometry, a conoid is a Catalan surface all of whose rulings intersect a fixed line, called the axis of the conoid. If all its rulings are perpendicular to its axis, then the conoid is called a right conoid....
. This ancestor also probably possesed a plastid
Plastid
Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
but it is presently not clear whether it was photosynthetic. Furthermore it is not clear whether extant perkinsids or colpodellids have retained this organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
.