Choanoflagellate

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"Choanoflagellates" redirects here. For the clade that groups these organisms with animals, see Choanozoa.

Choanoflagellates
Temporal range: Neoproterozoic-present, fossils from 100.5–0 Ma Only possible fossils are known from Cretaceous (Cenomanian/Turonian)[1][2]
Codosiga sp.
Scientific classification Edit this classification
Domain: Eukaryota
Clade: Amorphea
Clade: Obazoa
Clade: Opisthokonta
Clade: Holozoa
Clade: Filozoa
Clade: Choanozoa
Class: Choanoflagellata
Kent, 1880–1882[3][4]
Type species
Monosiga brevicollis[5]
Orders & families
Synonyms
  • Craspedmonadina Stein, 1878
  • 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[6]
  • Choanoflagellatea Cavalier-Smith, 1998[7][8]

Choanoflagellates are a group of free-living unicellular and colonial

multicellularity
in animals.

The flagellum of choanoflagellates is surrounded by microvilli at its base. Movement of the flagellum creates water currents that can propel

free-swimming choanoflagellates through the water column and trap bacteria and detritus against the microvilli, where these foodstuffs are engulfed. This feeding plays an ecological role in the carbon cycle by linking different trophic levels
.

Choanoflagellates bear morphological similarities to the choanocyte, a type of cell in sponges. As the proposed sister group to Animalia, choanoflagellates serve as a useful model for reconstructions of the last unicellular ancestor of animals. According to a 2021 study, crown group craspedids (and perhaps crown group choanoflagellates if Acanthoecida arose within Craspedida[9]) appeared 422.78 million years ago,[10] although a previous study from 2017 recovered the divergence of the crown group choanoflagellates (craspedids) at 786.62 million years.[11]

Etymology

Choanoflagellate is a hybrid word from Greek χοάνη khoánē meaning "funnel" (due to the shape of the collar) and the Latin word flagellum (whence English flagellum).[citation needed]

Appearance

Detailed morphology

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.[12] 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.[citation needed]

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.[13] 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.[13][14] Additionally, the cell body of many choanoflagellates is surrounded by a distinguishing extracellular 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.[13] The functional significance of the periplast is unknown, but in sessile organisms, it is thought to aid 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.[15]

Choanoflagellates are either

pathogenic lifestyle.[17] 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 still retain a solitary stage.[16]

Ecology

Metchnikoff
, 1886

Over 125 extant species of choanoflagellates

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.[20] 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.[citation needed
]

A number of

cluster of grapes in which each cell in the colony is flagellated or clusters of cells on a single stalk.[13][21] A colonial species from Mono Lake, Barroeca monosierra, forms spheres filled with a branched network of an extracellular matrix where a microbiome of different species of symbiotic bacteria live.[22][23] In October 2019, scientists found a new band behaviour of choanoflagellates: they apparently can coordinate to respond to light.[24]

The choanoflagellates feed on

microbial food web.[12] There is some evidence that choanoflagellates feast on viruses as well.[26]

Life cycle

sperm cell (B) looks very similar to that of an ancient choanoflagellate (A). Farnesol is very ancient in evolution, and its use goes back at least as far as the choanoflagellates which preceded the animals.[27][clarification needed
]

Choanoflagellates grow vegetatively, with multiple species undergoing longitudinal fission;[14] however, the reproductive life cycle of choanoflagellates remains to be elucidated. A paper released in August 2017 showed that environmental changes, including the presence of certain bacteria, trigger the swarming and subsequent sexual reproduction of choanoflagellates.[28] The ploidy level is unknown;[29] however, the discovery of both retrotransposons and key genes involved in meiosis[30] previously suggested that they used sexual reproduction as part of their life cycle. 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.[31]

Evidence for sexual reproduction has been reported in the choanoflagellate species Salpingoeca rosetta.[32][33] Evidence has also been reported for the presence of conserved meiotic genes in the choanoflagellates Monosiga brevicollis and Monosiga ovata.[34]

Silicon biomineralization

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.[35]

Choanoflagellate biosilicification requires the concentration of

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.[36]

Classification

Relationship to metazoans

Metazoa (animals, also known as Animalia) and negate the possibility that choanoflagellates evolved from metazoans (Lavrov, et al., 2005). Finally, a 2001 study of genes expressed in choanoflagellates has revealed that choanoflagellates synthesize homologues of metazoan cell signaling and adhesion genes.[37] Genome sequencing shows that, among living organisms, the choanoflagellates are most closely related to animals.[12]

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

eumetazoan animals was a multicellular organism, with differentiated tissues, a definite "body plan", and embryonic development (including gastrulation).[12] The timing of the splitting of these lineages is difficult to constrain, but was probably in the late Precambrian, >600 million years ago.[12]

External relationships of Choanoflagellatea.[39]

Opisthokonta
Holomycota
Holozoa

Ichthyosporea

Corallochytrea

Filozoa

Filasterea

Choanozoa

Animalia (Metazoa)

Choanoflagellatea

Phylogenetic relationships

The choanoflagellates were included in

monophyletic
and confirm their position as the closest known unicellular living relative of animals.

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

siliceous costal strips arranged into a species-specific lorica pattern."[13][15]

The choanoflagellate tree based on molecular phylogenetics divides into three well supported

sedentary and motile stages.[21]

Salpingoeca sp. section under TEM

Taxonomy

Choanoflagellates;[8]

Ellis 1930
  • ?Diploeca Ellis 1930
  • ?Diplosigopsis Francé 1897
  • ?Pachysoeca Ellis 1930
  • ?Piropsis Meunier 1910
  • ?Salpingorhiza Klug 1936
  • ?
    Sphaerodendron
     Zhukov, Mylnikov & Moiseev 1976 non Seemann 1865
  • ?Stelexomonas Lackey 1942
  • Astrosiga Kent 1880-1882
  • Aulomonas Lackey 1942
  • light microscopy.
    Choanoeca
    • Ellis 1930
  • Cladospongia
     Iyengar & Ramathan 1940
  • Codonosigopsis Senn 1900
  • Diplosiga Frenzel 1891
  • Hartaetosiga Carr, Richter & Nitsche 2017
  • Mylnosiga Carr, Richter & Nitsche 2017
  • Lagenoeca Kent 1881
  • Microstomoeca Carr, Richter & Nitsche 2017
  • Paramonosiga Jeuck, Arndt & Nitsche 2014
  • Salpingoeca James-Clark 1868 non Ellis 1933
  • Stagondoeca Carr, Richter & Nitsche 2017
  • Family Codonosigaceae Kent 1880-1882
  • Order Acanthoecida Norris 1965 em. Nitsche et al. 2011 (Loricate choanoflagellates)

    • Genomes and transcriptomes

    Monosiga brevicollis genome

    The genome of Monosiga brevicollis, with 41.6 million base pairs,[12] is similar in size to filamentous fungi and other free-living unicellular eukaryotes, but far smaller than that of typical animals.[12] In 2010, 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.[42] Carr et al. (2010)[34] screened the M. brevicollis genome for known eukaryotic meiosis genes. Of 19 known eukaryotic meiotic genes tested (including 8 that function in no other process than meiosis), 18 were identified in M. brevicollis. The presence of meiotic genes, including meiosis specific genes, indicates that meiosis, and by implication, sex is present within the choanoflagellates.

    Salpingoeca rosetta genome

    The genome of Salpingoeca rosetta is 55 megabases in size.[43] Homologs of cell adhesion, neuropeptide and glycosphingolipid metabolism genes are present in the genome. S. rosetta has a sexual life cycle and transitions between haploid and diploid stages.[33] In response to nutrient limitation, haploid cultures of S. rosetta become diploid. This ploidy shift coincides with mating during which small, flagellated cells fuse with larger flagellated cells. There is also evidence of historical mating and recombination in S. rosetta.

    S. rosetta is induced to undergo sexual reproduction by the marine bacterium Vibrio fischeri.[32] A single V. fischeri protein, EroS fully recapitulates the aphrodisiac-like activity of live V. fisheri.

    Other genomes

    The single-cell amplified genomes of four uncultured marine choanoflagellates, tentatively called UC1–UC4, were sequenced in 2019. The genomes of UC1 and UC4 are relatively complete.[44]

    Transcriptomes

    An EST dataset from Monosiga ovata was published in 2006.[45] 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. M. ovata has at least four eukaryotic meiotic genes.[34]

    The transcriptome of Stephanoeca diplocostata was published in 2013. This first transcriptome of a loricate choanoflagellate

    diatoms and have evolved through horizontal gene transfer
    .

    An additional 19 transcriptomes were published in 2018. A large number of

    gene families previously thought to be animal-only were found.[46]

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