The choanoflagellates are a group of free-living unicellular and colonial

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 thei ...

eukaryotes considered to be the closest living relatives of the

animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

s. Choanoflagellates are collared flagellates, having a funnel shaped collar of interconnected

microvilli Microvilli (singular: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume, and are involved in a wide variety of functions, including absorption, secretion, ce ...

at the base of a flagellum. Choanoflagellates are capable of both asexual and sexual reproduction. They have a distinctive cell

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...

characterized by an ovoid or spherical cell body 3–10

µm The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...

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 Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...

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 The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major componen ...

, linking

trophic level The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the number of steps it i ...

s. 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.

Etymology

''Choanoflagellate'' is a

hybrid word A hybrid word or hybridism is a word that etymologically derives from at least two languages. Common hybrids The most common form of hybrid word in English combines Latin and Greek parts. Since many prefixes and suffixes in English are of Latin ...

from Greek ' meaning " funnel" (due to the shape of the collar) and the Latin word '' flagellum''.

Appearance

Each choanoflagellate has a single flagellum, surrounded by a ring of

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ov ...

-filled protrusions called

, forming a cylindrical or conical collar (' in Greek). Movement of the flagellum draws water through the collar, and bacteria and detritus are captured by the microvilli and ingested. Water currents generated by the flagellum also push free-swimming cells along, as in

sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, wh ...

. 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 cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...

in the

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. Th ...

is constant. A flagellar

basal body A basal body (synonymous with basal granule, kinetosome, and in older cytological literature with blepharoplast) is a protein structure found at the base of a eukaryotic undulipodium (cilium or flagellum). The basal body was named by Theodor W ...

sits at the base of the

apical Apical means "pertaining to an apex". It may refer to: *Apical ancestor, refers to the last common ancestor of an entire group, such as a species (biology) or a clan (anthropology) *Apical (anatomy), an anatomical term of location for features loc ...

flagellum, and a second, non-flagellar basal body rests at a right angle to the flagellar base. The

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: * Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...

occupies an apical-to-central position in the cell, and

food vacuole The food vacuole, or digestive vacuole, is an organelle found in simple eukaryotes such as protists. This organelle is essentially a lysosome. During the stage of the symbiont parasites' lifecycle where it resides within a human (or other mammalia ...

s are positioned in the basal region of the cytoplasm. Additionally, the cell body of many choanoflagellates is surrounded by a distinguishing extracellular matrix or

periplast The periplast is one of three types of cell-covering of three classes of algae. The ''Cryptomonads'' have the periplast covering. The '' Dinophyceae'' have a type called the amphiesma, and the ''Euglena'' covering is the pellicle. Structure The pe ...

. 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. 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. Choanoflagellates are either

in the water column or

sessile Sessility, or sessile, may refer to: * Sessility (motility), organisms which are not able to move about * Sessility (botany), flowers or leaves that grow directly from the stem or peduncle of a plant * Sessility (medicine), tumors and polyps that ...

, adhering to the substrate directly or through either the periplast or a thin pedicel. 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 Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...

or pathogenic lifestyle. 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.

Ecology

Over 125 extant species of choanoflagellates are known, distributed globally in marine,

brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuar ...

and freshwater environments from the Arctic to the tropics, occupying both pelagic and

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

zones. Although most sampling of choanoflagellates has occurred between , they have been recovered from as deep as in open water and under Antarctic ice sheets. Many species are hypothesized to be

cosmopolitan Cosmopolitan may refer to: Food and drink * Cosmopolitan (cocktail), also known as a "Cosmo" History * Rootless cosmopolitan, a Soviet derogatory epithet during Joseph Stalin's anti-Semitic campaign of 1949–1953 Hotels and resorts * Cosmopoli ...

on a global scale .g.,_''Diaphanoeca_grandis''_has_been_reported_from_North_America.html" ;"title="Diaphanoeca_grandis.html" ;"title=".g., ''Diaphanoeca grandis">.g., ''Diaphanoeca grandis'' has been reported from North America">Diaphanoeca_grandis.html" ;"title=".g., ''Diaphanoeca grandis">.g., ''Diaphanoeca grandis'' has been reported from North America, Europe and Australia (OBIS)], while other species are reported to have restricted regional distributions. 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. A number of

species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...

, such as those in the genus '' Proterospongia'', form simple

colonies In modern parlance, a colony is a territory subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the '' metropolitan state'' ...

planktonic Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...

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. In October 2019, scientists found a new band behaviour of choanoflagellates: they apparently can coordinate to respond to light. The choanoflagellates feed on bacteria and link otherwise inaccessible forms of carbon to organisms higher in the trophic chain. Even today, they are important in the

and microbial food web. There is some evidence that choanoflagellates feast on viruses as well.

Life cycle

Choanoflagellates grow vegetatively, with multiple species undergoing longitudinal fission; 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. The ploidy level is unknown; however, the discovery of both retrotransposons and key genes involved in meiosis 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. Evidence for sexual reproduction has been reported in the choanoflagellate species '' Salpingoeca rosetta''. Evidence has also been reported for the presence of conserved meiotic genes in the choanoflagellates ''Monosiga brevicollis'' and ''Monosiga ovata''.

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 Cytokinesis () is the part of the cell division process during which the cytoplasm of a single eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of nuclear division in mitosis and meios ...

and assembles its own lorica using only these previously produced strips. Choanoflagellate biosilicification requires the concentration of

silicic acid Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

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.

Classification

Relationship 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. 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; 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 Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

and negate the possibility that choanoflagellates evolved from metazoans (Lavrov, et al., 2005). Finally, a 2001 study of genes expressed in choanoflagellates have revealed that choanoflagellates synthesize homologues of metazoan cell signaling and adhesion genes. Genome sequencing shows that, among living organisms, the choanoflagellates are most closely related to animals. 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 Choanocytes (also known as "collar cells") are cells that line the interior of asconoid, syconoid and leuconoid body types of sponges that contain a central flagellum, or ''cilium,'' surrounded by a collar of microvilli which are connected by a ...

s (also known as "collared cells") of sponges (considered among the most basal metazoa) have the same basic structure as choanoflagellates. Collared cells are found in other animal groups, such as ribbon worms, suggesting this was the

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 eumetazoan animals was a multicellular organism, with differentiated tissues, a definite "body plan", and embryonic development (including gastrulation). The timing of the splitting of these lineages is difficult to constrain, but was probably in the late Precambrian, >. Some recent papers do not support the sister relationship of Choanoflagellates with Animals. Choanoflagellates may be related to Ichthyosporea. External relationships of Choanoflagellatea.

Phylogenetic relationships

The choanoflagellates were included in

Chrysophyceae The Chrysophyceae, usually called chrysophytes, chrysomonads, golden-brown algae or golden algae are a large group of algae, found mostly in freshwater. Golden algae is also commonly used to refer to a single species, ''Prymnesium parvum'', which ...

until Hibberd, 1975. Recent molecular

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

reconstruction of the internal relationships of choanoflagellates allows the polarization of character evolution within the clade. Large fragments of the nuclear SSU and

LSU Louisiana State University (officially Louisiana State University and Agricultural and Mechanical College, commonly referred to as LSU) is a Public university, public Land-grant university, land-grant research university in Baton Rouge, Louisian ...

ribosomal RNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosomal ...

, alpha tubulin, and heat-shock protein 90 coding genes were used to resolve the internal relationships and character polarity within choanoflagellates. Each of the four genes showed similar results independently and analysis of the combined data set (

concatenated In formal language theory and computer programming, string concatenation is the operation of joining character strings end-to-end. For example, the concatenation of "snow" and "ball" is "snowball". In certain formalisations of concatenat ...

) along with sequences from other closely related species (

s and fungi) demonstrate that choanoflagellates are strongly supported as 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

electron microscopy An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a hi ...

. 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 In biology, a theca (plural 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 commo ...

is a secreted covering predominately composed of

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...

or other polysaccharides. 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 Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

costal strips arranged into a species-specific lorica pattern." The choanoflagellate tree based on molecular phylogenetics divides into three well supported

clades A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, t ...

. 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. The mapping of character traits on to this phylogeny indicates that the

last common ancestor In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...

of choanoflagellates was a marine organism with a differentiated

life cycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring *Life-cycle hypothesis, ...

with

sedentary Sedentary lifestyle is a lifestyle type, in which one is physically inactive and does little or no physical movement and or exercise. A person living a sedentary lifestyle is often sitting or lying down while engaged in an activity like soci ...

and

motile Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...

stages.

Taxonomy

Choanoflagellates; * Order

Craspedida Craspedida is an order of choanoflagellate 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 flagellat ...

Cavalier-Smith 1997 em. Nitsche et al. 2011 ** Family Salpingoecidae Kent 1880-1882 *** ?'' Dicraspedella'' 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 *** '' 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 *** '' Codosiga'' James-Clark 1866 *** '' Desmarella'' Kent 1880-1882 *** '' Kentrosiga'' Schiller 1953 *** '' Monosiga'' Kent 1880-1882 *** '' Proterospongia'' Kent 1882 *** '' Sphaeroeca'' Lauterborn 1894 non Meyrick 1895 *** '' Stylochromonas'' Lackey 1940 * Order

Acanthoecida Acanthoecida is an order of Choanoflagellates belonging to the 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 ...

Norris 1965 em. Nitsche et al. 2011 (Loricate choanoflagellates) ** '' Conioeca'' Thomsen & Ostergaard 2019 ** Family

Acanthoecidae Acanthoecidae is a family of choanoflagellates. Genera '' Acanthocorbis'' - '' Acanthoeca'' - '' Amoenoscopa'' - '' Apheloecion'' - '' Bicosta'' - '' Calliacantha'' - '' Calotheca'' - '' Campyloacantha'' - '' Conion'' - '' Cosmoeca'' - '' Crinol ...

Norris 1965 em. Nitsche et al. 2011 (Nudiform choanoflagellates) *** '' Acanthoeca'' Ellis 1930 *** '' Enibas'' Schiwitza, Arndt & Nitsche 2019 *** '' Helgoeca'' Leadbeater 2008 *** '' Polyoeca'' Kent 1880 *** '' Savillea'' Loeblich III 1967 ** Family Stephanoecidae Leadbeater 2011 (Tectiform choanoflagellates) *** ?'' Conion'' Thomsen 1982 *** ?'' Spiraloecion'' Marchant & Perrin 1986 *** '' Acanthocorbis'' Hara & Takahashi 1984 *** '' Amoenoscopa'' Hara & Takahashi 1987 *** '' Apheloecion'' Thomsen 1983 *** '' Bicosta'' Leadbeater 1978 *** '' Calliacantha'' Leadbeater 1978 *** '' Calotheca'' Thomsen & Moestrup 1983 non Desv. 1810 non Spreng. 1817 non Heyden 1887 *** '' Cosmoeca'' Thomsen 1984 *** '' Crinolina'' Thomsen 1976 non Smetana 1982 *** '' Crucispina'' Espeland & Throndsen 1986 *** '' Diaphanoeca'' Ellis 1930 *** '' Didymoeca'' Doweld 2003 *** '' Kakoeca'' Buck & Marchant 1991 *** '' Monocosta'' Thomsen 1979 non Monocostus Schumann 1904 *** '' Nannoeca'' Thomsen 1988 *** '' Parvicorbicula'' Deflandre 1960 *** '' Pleurasiga'' Schiller 1925 *** '' Polyfibula'' Manton 1981 *** '' Saepicula'' Leadbeater 1980 *** '' Saroeca'' Thomsen 1979 *** '' Spinoeca'' Thomsen, Ostergaard & Hansen 1995 non Poulsen 1973 *** '' Stephanacantha'' Thomsen 1983 *** '' Stephanoeca'' Ellis 1930 *** '' Syndetophyllum'' Thomsen & Moestrup 1983 *** '' Thomsenella'' Özdikmen 2009

Genomes and transcriptomes

''Monosiga brevicollis'' genome

The genome of ''

Monosiga brevicollis 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 interconne ...

'', with 41.6 million base pairs, is similar in size to filamentous fungi and other free-living unicellular eukaryotes, but far smaller than that of typical animals. 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. Carr et al. (2010) 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 Sex is the trait that determines whether a sexually reproducing animal or plant produces male or female gametes. Male plants and animals produce smaller mobile gametes (spermatozoa, sperm, pollen), while females produce larger ones (ova, ...

is present within the choanoflagellates.

''Salpingoeca rosetta'' genome

The genome of '' Salpingoeca rosetta'' is 55 megabases in size. 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. 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 ''Aliivibrio fischeri'' (also called ''Vibrio fischeri'') is a Gram-negative, rod-shaped bacterium found globally in marine environments. This species has bioluminescent properties, and is found predominantly in symbiosis with various marine an ...

''. 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.

Transcriptomes

An EST dataset from ''Monosiga ovata'' was published in 2006. 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 The Hedgehog signaling pathway is a signaling pathway that transmits information to embryonic cells required for proper cell differentiation. Different parts of the embryo have different concentrations of hedgehog signaling proteins. The pathway ...

. ''M. ovata'' has at least four eukaryotic meiotic genes. The transcriptiome of ''Stephanoeca diplocostata'' was published in 2013. This first transcriptome of a loricate choanoflagellate 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. An additional 19 transcriptomes were published in 2018. A large number of

gene families A gene family is a set of several similar genes, formed by duplication of a single original gene, and generally with similar biochemical functions. One such family are the genes for human hemoglobin subunits; the ten genes are in two clusters on ...

previously thought to be animal-only were found.

Gallery

File:Monosiga Brevicollis Phase.jpg, ''

'' under

PCM Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the am ...

File:Protero-7.png, '' Salpingoeca'' under PCM Image:Salpingoeca sp..jpg, ''Salpingoeca'' sp. section under

TEM Tem or TEM may refer to: Acronyms * Threat and error management, an aviation safety management model. * Telecom Expense Management * Telecom Equipment Manufacturer * TEM (currency), local to Volos, Greece * TEM (nuclear propulsion), a Russian ...

File:Desmarella moniliformis.jpg, '' Desmarella moniliformis'' colony under PCM File:0803col.jpg, '' Codosiga'' colony under light microscopy File:Sphaeroeca-colony.jpg, '' Sphaeroeca'' colony (approx. 230 individuals) under light microscopy.

References

External links

ChoanoWiki
a collaborative resource maintained by the Choanoflagellate research community
Tree of Life Webpage for Choanoflagellates

* ttps://web.archive.org/web/20040804013410/http://mcb.berkeley.edu/labs/king/choano/ Choanobase the Choanoflagellate genetic library, developed and maintained by the Nicole King laboratory at the University of California, Berkeley {{Taxonbar, from1=Q129012, from2=Q21446923 Proterozoic first appearances