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| classis = '''Choanoflagellatea'''
| classis = '''Choanoflagellatea'''
}}
}}
The '''choanoflagellates''' are a group of free-living unicellular and colonial [[flagellate]] [[eukaryotes]] considered to be the closest living relatives of the [[animal]]s. As the name suggests, choanoflagellates (collared flagellates) have a distinctive cell [[morphology (biology) | morphology]] characterized by an ovoid or spherical cell body 3-10 µm in diameter with a single apical flagellum surrounded by a collar of 30-40 [[microvilli]] (see figure). Movement of the flagellum creates water currents that can propel free-swimming choanoflagellates through the water column and trap [[bacteria]] and [[detritus]] against the collar of [[microvilli]] where these foodstuffs are engulfed. This feeding provides a critical link the global [[carbon cycle]], linking [[trophic levels]]. In addition to their critical ecological roles, choanoflagellates are of particular interest to evolutionary biologists studying the origins of multicellularity in [[animals]]. As the closest living relatives of [[animals]], choanoflagellates serve as a useful model for reconstructions of the last unicellular ancestor of [[animals]].
The '''choanoflagellates''' are a group of free-living unicellular and colonial [[flagellate]] [[eukaryotes]] considered to be the closest living relatives of the [[animal]]s. As the name suggests, choanoflagellates (collared flagellates) have a distinctive cell [[morphology (biology)|morphology]] characterized by an ovoid or spherical cell body 3-10 µm in diameter with a single apical flagellum surrounded by a collar of 30-40 [[microvilli]] (see figure). Movement of the flagellum creates water currents that can propel free-swimming choanoflagellates through the water column and trap [[bacteria]] and [[detritus]] against the collar of [[microvilli]] where these foodstuffs are engulfed. This feeding provides a critical link the global [[carbon cycle]], linking [[trophic levels]]. In addition to their critical ecological roles, choanoflagellates are of particular interest to evolutionary biologists studying the origins of multicellularity in [[animals]]. As the closest living relatives of [[animals]], choanoflagellates serve as a useful model for reconstructions of the last unicellular ancestor of [[animals]].


==Appearance and Growth==
==Appearance and Growth==
Each choanoflagellate has a single [[flagellum]], surrounded by a ring of [[actin]]-filled protrusions called [[microvilli]], forming a cylindrical or conical collar (''choanos'' in Greek). Movement of the [[flagellum]] draws water through the collar, and [[bacteria]] and detritus are captured by the microvilli and ingested.<ref name=King2008>{{cite journal
Each choanoflagellate has a single [[flagellum]], surrounded by a ring of [[actin]]-filled protrusions called [[microvilli]], forming a cylindrical or conical collar (''choanos'' in Greek). Movement of the [[flagellum]] draws water through the collar, and [[bacteria]] and detritus are captured by the microvilli and ingested.<ref name=King2008>{{cite journal
| author = [[Nicole King|King, N.]]
| author = [[Nicole King|King, N.]]
| coauthors = Westbrook, M.J.; Young, S.L.; Kuo, A.; Abedin, M.; Chapman, J.; Fairclough, S.; Hellsten, U.; Isogai, Y.; Letunic, I.; Others,
| coauthors = Westbrook, M.J.; Young, S.L.; Kuo, A.; Abedin, M.; Chapman, J.; Fairclough, S.; Hellsten, U.; Isogai, Y.; Letunic, I.; Others,
| year = 2008
| year = 2008
| title = The genome of the choanoflagellate ''Monosiga brevicollis'' and the origin of metazoans
| title = The genome of the choanoflagellate ''Monosiga brevicollis'' and the origin of metazoans
Line 24: Line 24:
| doi = 10.1038/nature06617
| doi = 10.1038/nature06617
}}</ref>
}}</ref>
Water currents generated by the [[flagellum]] also push free-swimming cells along, as in [[animal]] [[spermatozoon|sperm]]. In contrast, most other flagellates are ''pulled'' by their flagella.
Water currents generated by the [[flagellum]] also push free-swimming cells along, as in [[animal]] [[spermatozoon|sperm]]. In contrast, most other flagellates are ''pulled'' by their flagella.


In addition to the single apical [[flagellum]] surrounded by [[actin]]-filled [[microvilli]] that characterizes choanoflagellates, the internal organization of [[organelles]] in the [[cytoplasm]] is constant<ref name=Leadbeater2000(1)>{{cite journal
In addition to the single apical [[flagellum]] surrounded by [[actin]]-filled [[microvilli]] that characterizes choanoflagellates, the internal organization of [[organelles]] in the [[cytoplasm]] is constant<ref name=Leadbeater2000(1)>{{cite journal
| author = Leadbeater, B.S.C.
| author = Leadbeater, B.S.C.
| coauthors = Thomsen, H.
| coauthors = Thomsen, H.
| year = 2000
| year = 2000
| title = Order choanoflagellida.
| title = Order choanoflagellida.
Line 54: Line 54:
Choanoflagellates are either free-swimming in the water column or [[Sessility (zoology)|sessile]], adhering to the substrate directly or through either the periplast or a thin pedicel. <ref name=Leadbeater1983>{{cite journal
Choanoflagellates are either free-swimming in the water column or [[Sessility (zoology)|sessile]], adhering to the substrate directly or through either the periplast or a thin pedicel. <ref name=Leadbeater1983>{{cite journal
| author = Leadbeater, B.S.C.
| author = Leadbeater, B.S.C.
| coauthors =
| coauthors =
| title = Life-History and Ultrastrucutre of a New Marine Species of Proterospongia (Choanoflagellida).
| title = Life-History and Ultrastrucutre of a New Marine Species of Proterospongia (Choanoflagellida).
| journal = J. Mar. Biol. Ass. U.K.
| journal = J. Mar. Biol. Ass. U.K.
| volume =
| volume =
| issue = 63
| issue = 63
| pages = 135–160
| pages = 135–160
Line 65: Line 65:
| title = The class mesomycetozoea: a heterogeneous group of microorganisms at the animal-fungal boundary.
| title = The class mesomycetozoea: a heterogeneous group of microorganisms at the animal-fungal boundary.
| journal = Ann. Rev. Microbiol.
| journal = Ann. Rev. Microbiol.
| volume =
| volume =
| issue = 56
| issue = 56
| pages = 315–44
| pages = 315–44
Line 79: Line 79:
| pages = 433–439
| pages = 433–439
| doi = 10.1111/j.1550-7408.2000.tb00071.x
| doi = 10.1111/j.1550-7408.2000.tb00071.x
}}</ref>. Further examination of the choanoflagellate [[Biological life cycle | life cycle]] will be informative about mechanisms of colony formation and attributes present before the the [[evolution]] of [[animal]] multicellularity.
}}</ref>. Further examination of the choanoflagellate [[Biological life cycle|life cycle]] will be informative about mechanisms of colony formation and attributes present before the the [[evolution]] of [[animal]] multicellularity.


==Colonial behaviour==
==Colonial behaviour==
A number of [[species]] such as those in the [[genus]] ''[[Proterospongia]]'' form simple [[Colony (biology)|colonies]],<ref name=King2008/> [[planktonic]] clumps that resemble a miniature cluster of grapes in which each cell in the colony is flagellated or clusters of cells on a single stalk.<ref name=Carr2008>{{cite journal
A number of [[species]] such as those in the [[genus]] ''[[Proterospongia]]'' form simple [[Colony (biology)|colonies]],<ref name=King2008/> [[planktonic]] clumps that resemble a miniature cluster of grapes in which each cell in the colony is flagellated or clusters of cells on a single stalk.<ref name=Carr2008>{{cite journal
| author = Carr, M.
| author = Carr, M.
| coauthors =
| coauthors =
| year = 2008
| year = 2008
| title = Molecular phylogeny of choanoflagellates, the sister group to Metazoa
| title = Molecular phylogeny of choanoflagellates, the sister group to Metazoa
Line 92: Line 92:
| pages = 16641–16646
| pages = 16641–16646
| doi = 10.1073/pnas.0801667105
| doi = 10.1073/pnas.0801667105
}}</ref><ref name=Leadbeater2000(1)/>
}}</ref><ref name=Leadbeater2000(1)/>
[[Image:Sphaeroeca wiki.jpg|thumb|400px|center|Sphaeroeca, a colony of choanoflagellates (aprox. 230 individuals)]]
[[Image:Sphaeroeca-colony.jpg|thumb|400px|center|Sphaeroeca, a colony of choanoflagellates (aprox. 230 individuals)]]


==Ecology==
==Ecology==

There are over 125 extant species of choanoflagellates.<ref name=King2008/> distributed globally in [[marine]], [[brackish]] and [[freshwater]] environments from the Arctic to the tropics, occupying both [[pelagic]] and [[benthic]] zones. Although most sampling of choanoflagellates has occurred between 0 m and 25 m, they have been recovered from as deep as 300 m in open water<ref name=Thomsen1982>{{cite journal
There are over 125 extant species of choanoflagellates.<ref name=King2008/> distributed globally in [[marine]], [[brackish]] and [[freshwater]] environments from the Arctic to the tropics, occupying both [[pelagic]] and [[benthic]] zones. Although most sampling of choanoflagellates has occurred between 0 m and 25 m, they have been recovered from as deep as 300 m in open water<ref name=Thomsen1982>{{cite journal
| author = Thomsen, H.
| author = Thomsen, H.
Line 115: Line 114:
| pages = 263–269
| pages = 263–269
| doi = http://www.springerlink.com/index/N141343025V6736T.pdf
| doi = http://www.springerlink.com/index/N141343025V6736T.pdf
}}</ref> Many species are hypothesized to be [[cosmopolitan (biology) | cosmopolitan]] on a global scale [e.g., Diaphanoeca grandis has been reported from North America, Europe and Australia (OBIS)], while other species are reported to have restricted regional distributions.<ref name=Thomsen1991>{{cite journal
}}</ref> Many species are hypothesized to be [[cosmopolitan (biology)|cosmopolitan]] on a global scale [e.g., Diaphanoeca grandis has been reported from North America, Europe and Australia (OBIS)], while other species are reported to have restricted regional distributions.<ref name=Thomsen1991>{{cite journal
| author = Thomsen, H.
| author = Thomsen, H.
| coauthors = Buck, K. and Chavez, F.
| coauthors = Buck, K. and Chavez, F.
Line 123: Line 122:
| volume = 33
| volume = 33
| pages = 131–164
| pages = 131–164
| doi =
| doi =
}}</ref> Co-distributed choanoflagellate species can occupy quite different microenvironments, but in general, the factors that influence the distribution and dispersion of choanoflagellates remain to be elucidated.
}}</ref> Co-distributed choanoflagellate species can occupy quite different microenvironments, but in general, the factors that influence the distribution and dispersion of choanoflagellates remain to be elucidated.


The choanoflagellates feed on [[bacteria]] and link otherwise inaccessible forms of [[carbon]] to organisms higher in the [[trophic level | trophic]] chain.<ref name=Butterfield1997>{{cite journal
The choanoflagellates feed on [[bacteria]] and link otherwise inaccessible forms of [[carbon]] to organisms higher in the [[trophic level|trophic]] chain.<ref name=Butterfield1997>{{cite journal
| author = Butterfield, N.J.
| author = Butterfield, N.J.
| date = [[1997-04-01]]
| date = [[1997-04-01]]
Line 134: Line 133:
| issue = 2
| issue = 2
| pages = 247–262
| pages = 247–262
| url = http://paleobiol.geoscienceworld.org/cgi/content/abstract/23/2/247
| url = http://paleobiol.geoscienceworld.org/cgi/content/abstract/23/2/247
| accessdate = 2007-08-19
| accessdate = 2007-08-19
}}</ref> Even today they are important in the [[carbon cycle]] and [[microbial]] [[food web]].<ref name=King2008/>
}}</ref> Even today they are important in the [[carbon cycle]] and [[microbial]] [[food web]].<ref name=King2008/>


==Classification==
==Classification==

'''Phylogenetic Relationships'''
'''Phylogenetic Relationships'''


Recent molecular [[phylogenetic]] reconstruction of the internal relationships of choanoflagellates and allows the polarization character evolution within the clade. Large fragments of the nuclear [[18S ribosomal RNA | SSU]] and [[28S ribosomal RNA | LSU]] [[ribosomal RNA]], [[tubulin | alpha tubulin]], and [[heat-shock protein]] 90 coding genes were used to resolve of the internal relationships and character polarity within choanoflagellates.<ref name=Carr2008/> Each of the four genes showed similar results independently and analysis of the combined data set ([[concatenated]]) along with sequences from other closely related species ([[animals]] and [[fungi]]) demonstrate that choanoflagellates and are strongly supported as [[monophyletic]] and the closest known living relative of animals. The choanoflagellate tree divides into three well supported [[clades]].<ref name=Carr2008/> Clade 1 and Clade 2 each consist of a combination of species traditionally attributed to the Codonosigidae and Salpingoecidae while Clade 3 comprises species from the group taxonomically classified as Acanthoecidae.<ref name=Carr2008/> Previously, Choanoflagellida was divided into these three families based upon the composition and structure of their periplast: Codonosigidae, Salpingoecidae and Acanthoecidae. Members of the family Codonosigidae appear to lack a periplast when examined by light microscopy, but may have a fine outer coat visible only by [[electron microscopy]]. The family Salpingoecidae consists of species whose cells are encased in a firm theca that is visible by both light and electron microscopy. The [[theca]] is a secreted covering predominately composed of [[cellulose]] or other [[polysaccharides]] (Adl, et al., 2005). The third family of choanoflagellates, the Acanthoecidae, contains species whose cells rest in a basket-like lorica composed of [[siliceous]] ribs or “costae.” <ref name=Leadbeater2001/><ref name=Leadbeater2000(1)/> The mapping of character traits on to this phylogeny indicates that the [[last common ancestor]] of choanoflagellates was a marine organisms with a differentiated [[Biological life cycle | life cycle]] with [[sedentary]] and [[motile]] stages.<ref name=Carr2008/>
Recent molecular [[phylogenetic]] reconstruction of the internal relationships of choanoflagellates and allows the polarization character evolution within the clade. Large fragments of the nuclear [[18S ribosomal RNA|SSU]] and [[28S ribosomal RNA|LSU]] [[ribosomal RNA]], [[tubulin|alpha tubulin]], and [[heat-shock protein]] 90 coding genes were used to resolve of the internal relationships and character polarity within choanoflagellates.<ref name=Carr2008/> Each of the four genes showed similar results independently and analysis of the combined data set ([[concatenated]]) along with sequences from other closely related species ([[animals]] and [[fungi]]) demonstrate that choanoflagellates and are strongly supported as [[monophyletic]] and the closest known living relative of animals. The choanoflagellate tree divides into three well supported [[clades]].<ref name=Carr2008/> Clade 1 and Clade 2 each consist of a combination of species traditionally attributed to the Codonosigidae and Salpingoecidae while Clade 3 comprises species from the group taxonomically classified as Acanthoecidae.<ref name=Carr2008/> Previously, Choanoflagellida was divided into these three families based upon the composition and structure of their periplast: Codonosigidae, Salpingoecidae and Acanthoecidae. Members of the family Codonosigidae appear to lack a periplast when examined by light microscopy, but may have a fine outer coat visible only by [[electron microscopy]]. The family Salpingoecidae consists of species whose cells are encased in a firm theca that is visible by both light and electron microscopy. The [[theca]] is a secreted covering predominately composed of [[cellulose]] or other [[polysaccharides]] (Adl, et al., 2005). The third family of choanoflagellates, the Acanthoecidae, contains species whose cells rest in a basket-like lorica composed of [[siliceous]] ribs or “costae.” <ref name=Leadbeater2001/><ref name=Leadbeater2000(1)/> The mapping of character traits on to this phylogeny indicates that the [[last common ancestor]] of choanoflagellates was a marine organisms with a differentiated [[Biological life cycle|life cycle]] with [[sedentary]] and [[motile]] stages.<ref name=Carr2008/>


'''Relationship of Choanoflagellates to Metazoans'''
'''Relationship of Choanoflagellates to Metazoans'''
Line 148: Line 146:
Dujardin, a French biologist interested in protozoan evolution, recorded the morphological similarities of choanoflagellates and sponge choanocytes and proposed the possibility of a close relationship as early as 1841.<ref name=Leadbeater2001/> Over the past decade, this hypothesized relationship between choanoflagellates and animals has been upheld by independent analyses of multiple unlinked sequences: 18S rDNA, nuclear protein-coding genes, and mitochondrial genomes (Steenkamp, et al., 2006; Burger, et al., 2003;<ref name=Mendoza2002/>; Wainright, et al., 1993). Importantly, comparisons of mitochondrial genome sequences from a choanoflagellate and three sponges confirm the placement of choanoflagellates as an outgroup to Metazoa and negate the possibility that choanoflagellates evolved from metazoans (Lavrov, et al., 2005). Finally, recent studies of genes expressed in choanoflagellates have revealed that choanoflagellates synthesize homologues of metazoan cell signaling and adhesion genes.<ref name=King2001>{{cite journal
Dujardin, a French biologist interested in protozoan evolution, recorded the morphological similarities of choanoflagellates and sponge choanocytes and proposed the possibility of a close relationship as early as 1841.<ref name=Leadbeater2001/> Over the past decade, this hypothesized relationship between choanoflagellates and animals has been upheld by independent analyses of multiple unlinked sequences: 18S rDNA, nuclear protein-coding genes, and mitochondrial genomes (Steenkamp, et al., 2006; Burger, et al., 2003;<ref name=Mendoza2002/>; Wainright, et al., 1993). Importantly, comparisons of mitochondrial genome sequences from a choanoflagellate and three sponges confirm the placement of choanoflagellates as an outgroup to Metazoa and negate the possibility that choanoflagellates evolved from metazoans (Lavrov, et al., 2005). Finally, recent studies of genes expressed in choanoflagellates have revealed that choanoflagellates synthesize homologues of metazoan cell signaling and adhesion genes.<ref name=King2001>{{cite journal
| author = [[Nicole King|King, N.]]
| author = [[Nicole King|King, N.]]
| coauthors = [[Sean B. Carroll|Carroll, S.B.]]
| coauthors = [[Sean B. Carroll|Carroll, S.B.]]
| year = 2001
| year = 2001
| title = A receptor tyrosine kinase from choanoflagellates: molecular insights into early animal evolution.
| title = A receptor tyrosine kinase from choanoflagellates: molecular insights into early animal evolution.
| journal = ''Pnas''
| journal = ''Pnas''
| volume =
| volume =
| issue = 98
| issue = 98
| pages = 15032–15037
| pages = 15032–15037
| doi =
| doi =
}}</ref> (King, 2003) Genome sequencing shows that among living organisms, the choanoflagellates are most closely related to animals.<ref name=King2008/>
}}</ref> (King, 2003) Genome sequencing shows that among living organisms, the choanoflagellates are most closely related to animals.<ref name=King2008/>
Because choanoflagellates and metazoans are closely related, comparisons between the two groups promise to provide insights into the biology of their [[last common ancestor]] and the earliest events in [[metazoan]] evolution. The [[choanocyte]]s (also known as "collared cells") of [[sea sponge|sponge]]s (considered the most basal metazoa) have the same basic structure as choanoflagellates. Collared cells are found in other [[animal]] groups, such as [[flatworm]]s.{{Fact|date=March 2008}} suggesting this was the [[morphology (biology)|morphology]] of their [[last common ancestor]]. The [[last common ancestor]] of [[animals]] and choanoflagellates was unicellular, perhaps forming simple colonies; in contrast, the [[last common ancestor]] of all [[animals]] was a multicellular organism, with differentiated tissues, a definite "body plan", and embryonic development (including gastrulation).<ref name=King2008/> The timing of the splitting of these lineages is difficult to constrain, but was probably in the late Precambrian, >{{ma|600}}.<ref name=King2008/>
Because choanoflagellates and metazoans are closely related, comparisons between the two groups promise to provide insights into the biology of their [[last common ancestor]] and the earliest events in [[metazoan]] evolution. The [[choanocyte]]s (also known as "collared cells") of [[sea sponge|sponges]] (considered the most basal metazoa) have the same basic structure as choanoflagellates. Collared cells are found in other [[animal]] groups, such as [[flatworm]]s.{{Fact|date=March 2008}} suggesting this was the [[morphology (biology)|morphology]] of their [[last common ancestor]]. The [[last common ancestor]] of [[animals]] and choanoflagellates was unicellular, perhaps forming simple colonies; in contrast, the [[last common ancestor]] of all [[animals]] was a multicellular organism, with differentiated tissues, a definite "body plan", and embryonic development (including gastrulation).<ref name=King2008/> The timing of the splitting of these lineages is difficult to constrain, but was probably in the late Precambrian, >{{ma|600}}.<ref name=King2008/>

⚫
=='''''Monosiga brevicollis'' Genome'''==


⚫
==''Monosiga brevicollis'' Genome==
The genome of ''Monosiga brevicollis'', with 41.6 million base pairs,<ref name=King2008/> is similar in size to filamentous fungi and other free-living unicellular eukaryotes, but far smaller than that of typical animals.<ref name=King2008/>
The genome of ''Monosiga brevicollis'', with 41.6 million base pairs,<ref name=King2008/> is similar in size to filamentous fungi and other free-living unicellular eukaryotes, but far smaller than that of typical animals.<ref name=King2008/>


Revision as of 18:36, 27 May 2009

Choanoflagellates
Scientific classification
Domain:
Eukaryota
(unranked):
Opisthokonta
Phylum:
Choanozoa
Class:
Choanoflagellatea

The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. As the name suggests, choanoflagellates (collared flagellates) have a distinctive cell morphology characterized by an ovoid or spherical cell body 3-10 µm in diameter with a single apical flagellum surrounded by a collar of 30-40 microvilli (see figure). Movement of the flagellum creates water currents that can propel free-swimming choanoflagellates through the water column and trap bacteria and detritus against the collar of microvilli where these foodstuffs are engulfed. This feeding provides a critical link the global carbon cycle, linking trophic levels. In addition to their critical ecological roles, choanoflagellates are of particular interest to evolutionary biologists studying the origins of multicellularity in animals. As the closest living relatives of animals, choanoflagellates serve as a useful model for reconstructions of the last unicellular ancestor of animals.

Appearance and Growth

Each choanoflagellate has a single flagellum, surrounded by a ring of actin-filled protrusions called microvilli, forming a cylindrical or conical collar (choanos in Greek). Movement of the flagellum draws water through the collar, and bacteria and detritus are captured by the microvilli and ingested.[1] Water currents generated by the flagellum also push free-swimming cells along, as in animal sperm. In contrast, most other flagellates are pulled by their flagella.

In addition to the single apical flagellum surrounded by actin-filled microvilli that characterizes choanoflagellates, the internal organization of organelles in the cytoplasm is constant[2]. A flagellar basal body sits at the base of the apical flagellum, and a second, non-flagellar basal body rests at a right angle to the flagellar base. The nucleus occupies an apical-to-central position in the cell, and food vacuoles are positioned in the basal region of the cytoplasm. [2] [3] Additionally, the cell body of many choanoflagellates is surrounded by a distinguishing extracelluar matrix or periplast. These cell coverings vary greatly in structure and composition and are used by taxonomists for classification purposes. Many choanoflagellates build complex basket-shaped "houses" called lorica, from several silica strips cemented together.[2] The functional significance of the periplast is unknown, but in sessile organisms, it is thought to aid in attachment to the substrate. In planktonic organisms, there is speculation that the periplast increases drag, thereby counteracting the force generated by the flagellum and increasing feeding efficiency. [4] Choanoflagellates are either free-swimming in the water column or sessile, adhering to the substrate directly or through either the periplast or a thin pedicel. [5] Although choanoflagellates are thought to be strictly free-living and heterotrophic, a number of choanoflagellate relatives such as members of Ichthyosporea or Mesomycetozoa follow a parasitic or pathogenic lifestyle. [6] The life histories of choanoflagellates are poorly understood. Many species are thought to be solitary; however coloniality seems to have arisen independently several times within the group and colonial species retain a solitary stage.[5]

Choanoflagellates grow vegetatively, with many species undergoing longitudinal fission[3]; however, the reproductive life cycle of choanoflagellates remains to be elucidated. Currently, it is unclear whether there is a sexual phase to the choanoflagellate life cycle. Interestingly, some choanoflagellates can undergo encystment, which involves the retraction of the flagellum and collar and encasement in an electron dense fibrillar wall. Upon transfer to fresh media excystment occurs, though it remains to be directly observed [7]. Further examination of the choanoflagellate life cycle will be informative about mechanisms of colony formation and attributes present before the the evolution of animal multicellularity.

Colonial behaviour

A number of species such as those in the genus Proterospongia form simple colonies,[1] planktonic clumps that resemble a miniature cluster of grapes in which each cell in the colony is flagellated or clusters of cells on a single stalk.[8][2]

Sphaeroeca, a colony of choanoflagellates (aprox. 230 individuals)

Ecology

There are over 125 extant species of choanoflagellates.[1] distributed globally in marine, brackish and freshwater environments from the Arctic to the tropics, occupying both pelagic and benthic zones. Although most sampling of choanoflagellates has occurred between 0 m and 25 m, they have been recovered from as deep as 300 m in open water[9] and 100 m under Antarctic ice sheets.[10] Many species are hypothesized to be cosmopolitan on a global scale [e.g., Diaphanoeca grandis has been reported from North America, Europe and Australia (OBIS)], while other species are reported to have restricted regional distributions.[11] Co-distributed choanoflagellate species can occupy quite different microenvironments, but in general, the factors that influence the distribution and dispersion of choanoflagellates remain to be elucidated.

The choanoflagellates feed on bacteria and link otherwise inaccessible forms of carbon to organisms higher in the trophic chain.[12] Even today they are important in the carbon cycle and microbial food web.[1]

Classification

Phylogenetic Relationships

Recent molecular phylogenetic reconstruction of the internal relationships of choanoflagellates and allows the polarization character evolution within the clade. Large fragments of the nuclear SSU and LSU ribosomal RNA, alpha tubulin, and heat-shock protein 90 coding genes were used to resolve of the internal relationships and character polarity within choanoflagellates.[8] Each of the four genes showed similar results independently and analysis of the combined data set (concatenated) along with sequences from other closely related species (animals and fungi) demonstrate that choanoflagellates and are strongly supported as monophyletic and the closest known living relative of animals. The choanoflagellate tree divides into three well supported clades.[8] Clade 1 and Clade 2 each consist of a combination of species traditionally attributed to the Codonosigidae and Salpingoecidae while Clade 3 comprises species from the group taxonomically classified as Acanthoecidae.[8] Previously, Choanoflagellida was divided into these three families based upon the composition and structure of their periplast: Codonosigidae, Salpingoecidae and Acanthoecidae. Members of the family Codonosigidae appear to lack a periplast when examined by light microscopy, but may have a fine outer coat visible only by electron microscopy. The family Salpingoecidae consists of species whose cells are encased in a firm theca that is visible by both light and electron microscopy. The theca is a secreted covering predominately composed of cellulose or other polysaccharides (Adl, et al., 2005). The third family of choanoflagellates, the Acanthoecidae, contains species whose cells rest in a basket-like lorica composed of siliceous ribs or “costae.” [4][2] The mapping of character traits on to this phylogeny indicates that the last common ancestor of choanoflagellates was a marine organisms with a differentiated life cycle with sedentary and motile stages.[8]

Relationship of Choanoflagellates to Metazoans

Dujardin, a French biologist interested in protozoan evolution, recorded the morphological similarities of choanoflagellates and sponge choanocytes and proposed the possibility of a close relationship as early as 1841.[4] Over the past decade, this hypothesized relationship between choanoflagellates and animals has been upheld by independent analyses of multiple unlinked sequences: 18S rDNA, nuclear protein-coding genes, and mitochondrial genomes (Steenkamp, et al., 2006; Burger, et al., 2003;[6]; Wainright, et al., 1993). Importantly, comparisons of mitochondrial genome sequences from a choanoflagellate and three sponges confirm the placement of choanoflagellates as an outgroup to Metazoa and negate the possibility that choanoflagellates evolved from metazoans (Lavrov, et al., 2005). Finally, recent studies of genes expressed in choanoflagellates have revealed that choanoflagellates synthesize homologues of metazoan cell signaling and adhesion genes.[13] (King, 2003) Genome sequencing shows that among living organisms, the choanoflagellates are most closely related to animals.[1] Because choanoflagellates and metazoans are closely related, comparisons between the two groups promise to provide insights into the biology of their last common ancestor and the earliest events in metazoan evolution. The choanocytes (also known as "collared cells") of sponges (considered the most basal metazoa) have the same basic structure as choanoflagellates. Collared cells are found in other animal groups, such as flatworms.[citation needed] suggesting this was the morphology of their last common ancestor. The last common ancestor of animals and choanoflagellates was unicellular, perhaps forming simple colonies; in contrast, the last common ancestor of all animals was a multicellular organism, with differentiated tissues, a definite "body plan", and embryonic development (including gastrulation).[1] The timing of the splitting of these lineages is difficult to constrain, but was probably in the late Precambrian, >600 million years ago.[1]

Monosiga brevicollis Genome

The genome of Monosiga brevicollis, with 41.6 million base pairs,[1] is similar in size to filamentous fungi and other free-living unicellular eukaryotes, but far smaller than that of typical animals.[1]

External links

References

  1. ^ a b c d e f g h i King, N. (2008). "The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans". Nature. 451 (7180): 783–8. doi:10.1038/nature06617. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: extra punctuation (link)
  2. ^ a b c d e Leadbeater, B.S.C. (2000). "Order choanoflagellida". An Illustrated Guide to the Protozoa, Second Edition. Lawrence : Society of Protozoologists. 451: 14–38. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  3. ^ a b Karpov S. (1998). "Cytoskeleton structure and composition in choanoflagellates". J. Eukaryot. Microbiol. 45: 361–367. doi:0.1111/j.1550-7408.1998.tb04550.x. {{cite journal}}: Check |doi= value (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  4. ^ a b c Leadbeater, B.S.C. "Evolution of animals choanoflagellates and sponges". Water and Atmosphere Online. 9 (2): 9–11. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  5. ^ a b Leadbeater, B.S.C. "Life-History and Ultrastrucutre of a New Marine Species of Proterospongia (Choanoflagellida)". J. Mar. Biol. Ass. U.K. (63): 135–160. {{cite journal}}: Cite has empty unknown parameter: |coauthors= (help)
  6. ^ a b Mendoza L. "The class mesomycetozoea: a heterogeneous group of microorganisms at the animal-fungal boundary". Ann. Rev. Microbiol. (56): 315–44. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  7. ^ Leadbeater, B.S.C. (2000). "Cyst Formation in a Freshwater Strain of the Choanoflagellate Desmarella moniliformis Kent". J. Eukaryot. Microbiol. 47: 433–439. doi:10.1111/j.1550-7408.2000.tb00071.x. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  8. ^ a b c d e Carr, M. (2008). "Molecular phylogeny of choanoflagellates, the sister group to Metazoa". Pnas. 105 (43): 16641–16646. doi:10.1073/pnas.0801667105. {{cite journal}}: Cite has empty unknown parameter: |coauthors= (help); Italic or bold markup not allowed in: |journal= (help)
  9. ^ Thomsen, H. (1982). "Planktonic choanoflagellates from Disko Bugt, West Greenland, with a survey of the marine nanoplankton of the area". Meddelelser om Gronland, Bioscience. 8: 3–63. doi:http://www.springerlink.com/index/N141343025V6736T.pdf. {{cite journal}}: Check |doi= value (help); Cite has empty unknown parameter: |coauthors= (help); External link in |doi= (help)
  10. ^ Buck, K. (1988). "Distribution and abundance of choanoflagellates (Acanthoecidae) across the ice-edge zone in the Weddell Sea, Antarctica". Mar. Biol. , 98 pp. 263-269. 98: 263–269. doi:http://www.springerlink.com/index/N141343025V6736T.pdf. {{cite journal}}: Check |doi= value (help); External link in |doi= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  11. ^ Thomsen, H. (1991). "Choanoflagellates of the central California waters: Taxonomy, morphology and species assemblages". Ophelia. 33: 131–164. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  12. ^ Butterfield, N.J. (1997-04-01). "Plankton ecology and the Proterozoic-Phanerozoic transition". Paleobiology. 23 (2): 247–262. Retrieved 2007-08-19. {{cite journal}}: Check date values in: |date= (help)
  13. ^ King, N. (2001). "A receptor tyrosine kinase from choanoflagellates: molecular insights into early animal evolution". Pnas (98): 15032–15037. {{cite journal}}: Italic or bold markup not allowed in: |journal= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
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