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 Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...

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 ...

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, ...

at the base of a

flagellum A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have f ...

. Choanoflagellates are capable of both asexual and sexual reproduction. They have a distinctive cell morphology 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 Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

and

detritus In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts comm ...

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 compon ...

, 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 ...

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 A funnel is a tube or pipe that is wide at the top and narrow at the bottom, used for guiding liquid or powder into a small opening. Funnels are usually made of stainless steel, aluminium, glass, or plastic. The material used in its construct ...

" (due to the shape of the collar) and the Latin word ''

''.

Appearance

Each choanoflagellate has a single

, 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, ...

. 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. ...

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 Wi ...

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 mammali ...

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

extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide s ...

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, 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 heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

, 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 ha ...

pathogenic In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a ger ...

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 Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military ...

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 estua ...

and

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does incl ...

environments from the Arctic to the tropics, occupying both

pelagic The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or w ...

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 " ...

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 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 the appropriat ...

, such as those in the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

Proterospongia ''Proterospongia'' is a genus of single-celled aquatic organisms which form colonies. It belongs to the choanoflagellate class. As a colony-forming choanoflagellate, ''Proterospongia'' is of interest to scientists studying the mechanisms of interce ...

'', 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

and link otherwise inaccessible forms of

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

to organisms higher in the trophic chain. Even today, they are important in the

and

microbial A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Another name for food web is consumer-resource system. Ecologists can broadly lump all life forms into one o ...

. 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 Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectiv ...

level is unknown; however, the discovery of both retrotransposons and key genes involved in meiosis previously suggested that they used

sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...

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 ''Salpingoeca rosetta'' is a species of Choanoflagellates in the family Salpingoecidae. It is a rare marine eukaryote consisting of a number of cells embedded in a jelly-like matrix. This organism demonstrates a very primitive level of cell diffe ...

''. 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 mei ...

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 one ...

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 A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...

and other silica-forming

stramenopiles Stramenopile is a clade of organisms distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, and in some they have be ...

. 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 Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). H ...

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 Dujardin is a French surname, meaning "from the garden", and may refer to: * Charlotte Dujardin, British dressage rider * Édouard Dujardin, French writer * Félix Dujardin (1801–1860), French biologist * Jean Dujardin, French actor and comedian ...

, a French biologist interested in protozoan evolution, recorded the morphological similarities of choanoflagellates and sponge

choanocytes 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 ...

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 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 ...

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 Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...

(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 Nemertea is a phylum of animals also known as ribbon worms or proboscis worms, consisting of 1300 known species. Most ribbon worms are very slim, usually only a few millimeters wide, although a few have relatively short but wide bodies. Many ...

, 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 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 The Mesomycetozoea (or DRIP clade, or Ichthyosporea) are a small group of Opisthokonta in Eukaryota (formerly protists), mostly parasites of fish and other animals. Significance They are not particularly distinctive morphologically, appearin ...

. 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'', whic ...

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 ...

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 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 riboso ...

, 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 A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...

) demonstrate that choanoflagellates are strongly supported as

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gr ...

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 comm ...

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 wa ...

or other

polysaccharides Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with w ...

. These divisions are now known to be

paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...

, 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 In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to hav ...

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 te ...

. 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 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 flagellate ...

Cavalier-Smith 1997 em. Nitsche et al. 2011 ** Family

Salpingoecidae Salpingoecidae is a family 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 flagell ...

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 ''Salpingoeca'' is a genus 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 flagell ...

'' 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 *** ''

'' Kent 1882 *** '' Sphaeroeca'' Lauterborn 1894 non Meyrick 1895 *** '' Stylochromonas'' Lackey 1940 * Order Acanthoecida Norris 1965 em. Nitsche et al. 2011 (Loricate choanoflagellates) ** '' Conioeca'' Thomsen & Ostergaard 2019 ** Family Acanthoecidae 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 ''Acanthocorbis'' is a genus of choanoflagellates within the family Acanthoecidae Acanthoecidae is a family of choanoflagellates. Genera '' Acanthocorbis'' - '' Acanthoeca'' - '' Amoenoscopa'' - '' Apheloecion'' - '' Bicosta'' - '' Calliacant ...

'' Hara & Takahashi 1984 *** '' Amoenoscopa'' Hara & Takahashi 1987 *** '' Apheloecion'' Thomsen 1983 *** '' Bicosta'' Leadbeater 1978 *** '' Calliacantha'' Leadbeater 1978 *** ''

Calotheca ''Calotheca'' is the name used for a genus of choanoflagellates in the family Acanthoecidae, though this name is a junior homonym of the name ''Calotheca'' Heyden, 1887, and it must be replaced under the rules of the ICZN The International C ...

'' 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 ''Diaphanoeca'' is a genus of choanoflagellates belonging to the family Acanthoecidae Acanthoecidae is a family of choanoflagellates. Genera '' Acanthocorbis'' - '' Acanthoeca'' - '' Amoenoscopa'' - '' Apheloecion'' - '' Bicosta'' - '' Callia ...

'' 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 Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...

. Carr et al. (2010) screened the ''M. brevicollis'' genome for known

eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

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 ''

'' 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 anim ...

''. 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 pathwa ...

. ''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

and have evolved through

. 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, ''

'' under PCM Image:Salpingoeca sp..jpg, ''Salpingoeca'' sp. section under TEM File:Desmarella moniliformis.jpg, '' Desmarella moniliformis'' colony under PCM File:0803col.jpg, '' Codosiga'' colony under

light microscopy Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of micr ...

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 Nicole King (born 1970) is an American biologist and faculty member at the University of California, Berkeley in molecular and cell biology and integrative biology. She was awarded a MacArthur Fellowship in 2005. She has been an investigator wit ...

laboratory at the

University of California, Berkeley The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California) is a public land-grant research university in Berkeley, California. Established in 1868 as the University of California, it is the state's first land-grant un ...

{{Taxonbar, from1=Q129012, from2=Q21446923 Proterozoic first appearances