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Choanoflagellate

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Choanoflagellate

Choanoflagellates
Temporal range: 900–0Ma No fossils known, molecular clock evidence for origin 1050-800Ma[1]
Scientific classification
Domain: Eukarya
(unranked): Opisthokonta
(unranked): Choanozoa or Holozoa
(unranked): Filozoa
Class: Choanoflagellatea
Cavalier-Smith, 1998[2]
Families

Codonosigidae
Salpingoecidae
Acanthoecidae

Synonyms
  • Craspedmonadina Stein, 1878
  • Choanoflagellata Kent, 1880[3]
  • Craspedomonadaceae Senn, 1900
  • Craspedophyceae Chadefaud, 1960
  • Craspédomonadophycidées Bourrelly, 1968
  • Craspedomonadophyceae Hibberd, 1976
  • Choanomonadea Krylov et al., 1980
  • Choanoflagellida Levine et al., 1980, Lee et al., 1985
  • Choanoflagellea Cavalier-Smith, 1997
  • Choanomonada Adl et al., 2005[4]

The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. Choanoflagellates are collared flagellates having a funnel shaped collar of interconnected microvilli at the base of a flagellum. They 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 within 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.

Contents

  • Etymology 1
  • Appearance and growth 2
  • Classification 3
    • Phylogenetic relationships 3.1
    • Relationship of choanoflagellates to metazoans 3.2
  • Colonial behaviour 4
  • Silicon biomineralization 5
  • Ecology 6
  • Genomes and transcriptomes 7
    • Monosiga brevicollis genome 7.1
    • Salpingoeca rosetta genome 7.2
    • Monosiga ovata transcriptome 7.3
    • Stephanoeca diplocostata transcriptome 7.4
  • References 8
  • External links 9

Etymology

From Greek Khoanē meaning "funnel" (due to the shape of the collar) and the English word "flagellum".

Appearance and growth

Salpingoeca sp., in transmission electron microscopy (TEM).

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.[5] 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

  • ChoanoWiki a collaborative resource maintained by the Choanoflagellate research community.
  • Tree of Life Webpage for Choanoflagellates
  • genome browserMonosiga brevicollis
  • Your brain chemistry existed before animals did
  • Choanobase, the Choanoflagellate genetic library, developed and maintained by the Nicole King laboratory at the University of California, Berkeley
  • [2] A 2014 book bringing together the evolution, biology and ecology of the choanoflagellates.

External links

  1. ^
  2. ^ Cavalier-Smith, T. 1998. Neomonada and the origin of animals and fungi. In: Coombs GH, Vickerman K, Sleigh MA, Warren A (ed.) Evolutionary relationships among protozoa. Kluwer, London, pp. 375-407.
  3. ^ Saville-Kent, W. (1880). A manual of Infusoria. London, vol. 1, p. 324, [1].
  4. ^ Nitsche, F., Carr, M., Arndt, H., & Leadbeater, B. S. (2011). Higher level taxonomy and molecular phylogenetics of the Choanoflagellatea. Journal of Eukaryotic Microbiology, 58(5), 452-462.
  5. ^ a b c d e f g h i
  6. ^ a b c d e
  7. ^ a b
  8. ^ a b c
  9. ^ a b
  10. ^ a b
  11. ^ Claus Nielsen. Animal Evolution: Interrelationships of the Living Phyla. 3rd ed. Claus Nielsen. Oxford, UK: Oxford University Press, 2012, p. 14.
  12. ^
  13. ^
  14. ^ Reviers, B. de. (2006). Biologia e Filogenia das Algas. Editora Artmed, Porto Alegre, p. 156.
  15. ^ a b c d e
  16. ^
  17. ^
  18. ^
  19. ^ a b
  20. ^
  21. ^
  22. ^
  23. ^
  24. ^
  25. ^
  26. ^

References

The first transcriptome of a loricate choanoflagellate [19] led to the discovery of choanoflagellate silicon transporters. Subsequently, similar genes were identified in a second loricate species, Diaphanoeca grandis. Analysis of these genes found that the choanoflagellate SITs show homology to the SIT-type silicon transporters of diatoms and have evolved through horizontal gene transfer.

Stephanoeca diplocostata transcriptome

An EST dataset from Monosiga ovata was published in 2006.[26] The major finding of this transcriptome was the choanoflagellate Hoglet domain and shed light on the role of domain shuffling in the evolution of the Hedgehog signaling pathway.

Monosiga ovata transcriptome

The genome is 55 megabases in size.[25] Homologs of cell adhesion, neuropeptide and glycosphingolipid metabolism genes are present in the genome.

Salpingoeca rosetta genome

The genome of Monosiga brevicollis, with 41.6 million base pairs,[5] is similar in size to filamentous fungi and other free-living unicellular eukaryotes, but far smaller than that of typical animals.[5] Recently, a phylogenomic study revealed that several algal genes are present in the genome of Monosiga brevicollis. This could be due to the fact that, in early evolutionary history, choanoflagellates consumed algae as food through phagocytosis.[24]

Monosiga brevicollis genome

Two choanoflagellate species have had their genomes fully sequenced, with another two species having had transcriptome data published.

Genomes and transcriptomes

The choanoflagellates feed on trophic chain.[23] Even today they are important in the carbon cycle and microbial food web.[5]

There are over 125 extant species of choanoflagellates[5] 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[20] and 100 m under Antarctic ice sheets.[21] 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.[22] 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.

Ecology

Choanoflagellate biosilicification requires the concentration of silicic acid within the cell. This is carried out by Silicon Transporter (SIT) proteins. Analysis of choanoflagellate SITs shows that they are similar to the SIT-type silicon transporters of diatoms and other silica-forming stramenopiles. The SIT gene family shows little or no homology to any other genes, even to genes in non-siliceous choanoflagellates or stramenopiles. This suggests that the SIT gene family evolved via a lateral gene transfer event between Acanthoecids and Stramenopiles. This is a remarkable case of horizontal gene transfer between two distantly related eukaryotic groups, and has provided clues to the biochemistry and silicon-protein interactions of the unique SIT gene family.[19]

The Acanthoecid choanoflagellates produce an extracellular basket structure known as a lorica. The lorica is composed of individual costal strips, made of a silica-protein biocomposite. Each costal strip is formed within the choanoflagellate cell and is then secreted to the cell surface. In nudiform choanoflagellates, lorica assembly takes place using a number of tentacles once sufficient costal strips have been produced to comprise a full lorica. In tectiform choanoflagellates, costal strips are accumulated in a set arrangement below the collar. During cell division, the new cell takes these costal strips as part of cytokinesis and assembles its own lorica using only these previously produced strips.[18]

Silicon biomineralization

Sphaeroeca, a colony of choanoflagellates (approx. 230 individuals), in light microscopy.

A number of species, such as those in the genus Proterospongia, form simple colonies,[5] 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.[6][15]

Colonial behaviour

[5] The timing of the splitting of these lineages is difficult to constrain, but was probably in the late Precambrian, >.[5]

Relationship of choanoflagellates to metazoans

The choanoflagellate tree based on molecular phylogenetics divides into three well supported life cycle with sedentary and motile stages.[15]

Previously, Choanoflagellida was divided into these three families based on 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). These divisions are now known to be paraphyletic, with convergent evolution of these forms widespread. The third family of choanoflagellates, the Acanthoecidae, has been supported as a monophyletic group. This clade possess a synapomorphy of the cells being found within a basket-like lorica, providing the alternative name of "Loricate Choanoflagellates". The Acanthoecid lorica is composed of a series of siliceous costal strips arranged into a species-specific lorica pattern."[6][8]

The choanoflagellates were included in Chrysophyceae until Hibberd, 1975.[14] Recent molecular phylogenetic reconstruction of the internal relationships of choanoflagellates allows the polarization of 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 the internal relationships and character polarity within choanoflagellates.[15] 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 are strongly supported as monophyletic and confirm their position as the closest known unicellular living relative of animals.

Phylogenetic relationships

Classification

Choanoflagellates grow vegetatively, with many species undergoing longitudinal fission;[7] 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, and the ploidy level is unknown;[11] however, the discovery of both retrotransposons and key genes involved in meiosis [12] suggests that they are cryptically sexual. Interestingly, some choanoflagellates can undergo encystment, which involves the retraction of the flagellum and collar and encasement in an electron dense fibrillar wall. On transfer to fresh media, excystment occurs; though it remains to be directly observed.[13] Further examination of the choanoflagellate life cycle will be informative about mechanisms of colony formation and attributes present before the evolution of animal multicellularity.

[9] 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.[10] lifestyle.pathogenic or parasitic, follow a Ichthyosporea or Mesomycetozoa, a number of choanoflagellate relatives, such as members of heterotrophic Although choanoflagellates are thought to be strictly free-living and [9], adhering to the substrate directly or through either the periplast or a thin pedicel.sessile Choanoflagellates are either free-swimming in the water column or [8]

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