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An endosymbiont or endobiont is any
organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological ...

organism
that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
: ἔνδον ''endon'' "within", σύν ''syn'' "together" and βίωσις ''biosis'' "living".) Examples are
nitrogen-fixing Nitrogen fixation is a chemical process by which molecular nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific st ...
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
(called
rhizobia Rhizobia are diazotrophic bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit o ...

rhizobia
), which live in the
root nodule Root nodules are found on the root In vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia artēria'' 'windpipe' + φυτά ''p ...
s of
legume A legume () is a plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can ...

legume
s; single-cell
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
inside
reef-building
reef-building
coral Corals are marine invertebrates Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a ver ...

coral
s, and bacterial endosymbionts that provide essential nutrients to about 10–15% of
insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

insect
s. There are two types of symbiont transmissions. In
horizontal transmission Horizontal transmission is the transmission of organisms between biotic and/or abiotic members of an ecosystem that are not in a parent-progeny relationship. This concept has been generalized to include transmissions of infectious agents, symbion ...
, each new generation acquires free living symbionts from the environment. An example is the nitrogen-fixing bacteria in certain plant roots.
Vertical transmission Vertical transmission of symbionts Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction In ecology Ecology (from el, οἶκος, "house ...
takes place when the symbiont is transferred directly from parent to offspring. There is also a combination of these types, where symbionts are transferred vertically for some generation before a switch of host occurs and new symbionts are horizontally acquired from the environment. In vertical transmissions, the symbionts often have a reduced genome and are no longer able to survive on their own. As a result, the symbiont depends on the host, resulting in a highly intimate co-dependent relationship. For instance,
pea aphid The pea is most commonly the small spherical seed A seed is an embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band ...
symbionts have lost genes for essential molecules, now relying on the host to supply them with nutrients. In return, the symbionts synthesize essential amino acids for the aphid host. Other examples include ''
Wigglesworthia ''Wigglesworthia glossinidia'' is a species of gram-negative bacteria which was isolated from the gut of the tsetse fly. ''W. glossinidia'' is a bacterial endosymbiont of the tsetse fly. Because of this relationship, ''Wigglesworthia'' has lost a ...
'' nutritional symbionts of tse-tse flies, or in sponges. When a symbiont reaches this stage, it begins to resemble a cellular
organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
, similar to
mitochondria A mitochondrion (; ) is a double-membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes ...

mitochondria
or
chloroplasts A chloroplast is a type of membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structure ...

chloroplasts
. Many instances of endosymbiosis are obligate; that is, either the endosymbiont or the host cannot survive without the other, such as the
gutless marine worms
gutless marine worms
of the
genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying gr ...
''
Riftia ''Riftia pachyptila'', commonly known as the giant tube worm, is a marine Marine is an adjective meaning of or pertaining to the sea or ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approx ...
'', which get nutrition from their endosymbiotic bacteria. The most common examples of obligate endosymbioses are
mitochondria A mitochondrion (; ) is a double-membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes ...

mitochondria
and
chloroplast A chloroplast is a type of membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structure ...

chloroplast
s. Some human parasites, e.g. ''
Wuchereria bancrofti ''Wuchereria bancrofti'' is a filarial (arthropod-borne) nematode (roundworm) that is the major cause of lymphatic filariasis. It is one of the three parasitic worms, together with ''Brugia malayi'' and ''Brugia timori, B. timori'', that infect ...
'' and ''
Mansonella perstans ''Mansonella perstans'' is a filarial (arthropod-borne) nematode (roundworm), transmitted by tiny blood-sucking fly, flies called midges. ''Mansonella perstans'' is one of two filarial nematodes that causes serous cavity filariasis in humans. T ...
'', thrive in their intermediate insect hosts because of an obligate endosymbiosis with ''
Wolbachia ''Wolbachia'' is a genus of intracellular parasite, intracellular bacteria that infects mainly arthropod species, including a high proportion of insects, and also some nematodes. It is one of the most common parasitic microbes and is possibly the ...

Wolbachia
spp.'' They can both be eliminated from hosts by treatments that target this bacterium. However, not all endosymbioses are obligate and some endosymbioses can be
harmful Harmful is a rock band from Frankfurt, Germany, founded in 1992 and frequently compared to early Helmet (band), Helmet and more occasionally to Blackmail (band), Blackmail.sefor an overview of reviewer opinions The band has released eight albums t ...

harmful
to either of the organisms involved. Two major types of
organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
in
eukaryotic cells Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryotic cells
, mitochondria and
plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ' ...
s such as chloroplasts, are considered to be bacterial endosymbionts. This process is commonly referred to as
symbiogenesis Symbiogenesis, endosymbiotic theory, or serial endosymbiotic theory is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibly oth ...

symbiogenesis
.


Symbiogenesis and organelles

Symbiogenesis explains the origins of
eukaryote Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryote
s, whose cells contain two major kinds of organelle: mitochondria and chloroplasts. The theory proposes that these organelles evolved from certain types of bacteria that eukaryotic cells engulfed 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 Endocytosis is a cellular process i ...

phagocytosis
. These cells and the bacteria trapped inside them entered an endosymbiotic relationship, meaning that the bacteria took up residence and began living exclusively within the eukaryotic cells. Numerous insect species have endosymbionts at different stages of symbiogenesis. A common theme of symbiogenesis involves the reduction of the genome to only essential genes for the host and symbiont collective genome. A remarkable example of this is the fractionation of the '' Hodgkinia'' genome of ''
Magicicada ''Magicicada'' is the genus of the 13-year and 17-year periodical cicadas of eastern North America, consisting of seven species. Although they are sometimes called "locusts", this is a misnomer, as cicadas belong to the taxonomic order Hemiptera ...
''
cicadas The cicadas () are a superfamily, the Cicadoidea, of insects in the order Hemiptera (true bugs). They are in the suborder Auchenorrhyncha, along with smaller jumping bugs such as leafhoppers and froghoppers. The superfamily is divided into two ...
. Because the cicada life cycle takes years underground,
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
on endosymbiont populations is relaxed for many bacterial generations. This allows the symbiont genomes to diversify within the host for years with only punctuated periods of selection when the cicadas reproduce. As a result, the ancestral '' Hodgkinia'' genome has split into three groups of primary endosymbiont, each encoding only a fraction of the essential genes for the symbiosis. The host now requires all three sub-groups of symbiont, each with degraded genomes lacking most essential genes for bacterial viability.


Bacterial endosymbionts of invertebrates

The best-studied examples of endosymbiosis are known from
invertebrates Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebrat ...
. These symbioses affect organisms with global impact, including ''
Symbiodinium : ''This is about the genus sometimes called Zoox. For the company, see Zoox (company)'' ''Symbiodinium'' is a genus of dinoflagellates that encompasses the largest and most prevalent group of endosymbiotic dinoflagellates known. These unicellul ...

Symbiodinium
'' of corals, or ''
Wolbachia ''Wolbachia'' is a genus of intracellular parasite, intracellular bacteria that infects mainly arthropod species, including a high proportion of insects, and also some nematodes. It is one of the most common parasitic microbes and is possibly the ...

Wolbachia
'' of insects. Many insect agricultural pests and human disease vectors have intimate relationships with primary endosymbionts.


Endosymbionts of insects

Scientists classify insect endosymbionts in two broad categories, 'Primary' and 'Secondary'. Primary endosymbionts (sometimes referred to as P-endosymbionts) have been associated with their
insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

insect
hosts for many millions of years (from 10 to several hundred million years in some cases). They form obligate associations (see below), and display
cospeciation Cospeciation is a form of coevolution In biology, coevolution occurs when two or more species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of bio ...
with their insect hosts. Secondary endosymbionts exhibit a more recently developed association, are sometimes horizontally transferred between hosts, live in the
hemolymph Hemolymph, or haemolymph, is a fluid, analogous to the blood Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics ...
of the insects (not specialized bacteriocytes, see below), and are not obligate.


Primary endosymbionts

Among primary endosymbionts of insects, the best-studied are the pea
aphid Aphids are small sap-sucking insects and members of the Taxonomic rank, superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white Erio ...

aphid
(''
Acyrthosiphon pisum ''Acyrthosiphon pisum'', commonly known as the pea aphid (and colloquially known as the green dolphin, pea louse, and clover louse ), is a sap-sucking insect in the family Aphididae The Aphididae are a very large insect family (biology), fami ...
'') and its endosymbiont '' Buchnera sp.'' APS, the
tsetse fly Tsetse ( , or ), sometimes spelled tzetze and also known as tik-tik flies, are large biting flies Many species of arthropods (insects, arachnids and others) regularly or occasionally bite or sting human beings. Insect saliva contains antico ...

tsetse fly
''Glossina morsitans morsitans'' and its endosymbiont '' Wigglesworthia glossinidia brevipalpis'' and the endosymbiotic
protists A protist () is any eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...
in lower
termite Termites are eusocial Eusociality (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its populati ...

termite
s. As with endosymbiosis in other insects, the symbiosis is obligate in that neither the bacteria nor the insect is viable without the other. Scientists have been unable to cultivate the bacteria in lab conditions outside of the insect. With special nutritionally-enhanced diets, the insects can survive, but are unhealthy, and at best survive only a few generations. In some insect groups, these endosymbionts live in specialized insect cells called
bacteriocyte A bacteriocyte (Greek language, Greek for ''bacteria cell''), also known as a mycetocyte, is a specialized adipocyte found primarily in certain insect groups such as aphids, tsetse flies, German cockroaches, weevils. These cells contain Endosymbios ...
s (also called ''mycetocytes''), and are maternally-transmitted, i.e. the mother transmits her endosymbionts to her offspring. In some cases, the bacteria are transmitted in the
egg An egg is the organic vessel containing the in which an develops until it can survive on its own, at which point the animal hatches. An egg results from of an . Most s, (excluding s), and lay eggs, although some, such as s, do not. eg ...
, as in ''Buchnera''; in others like ''Wigglesworthia'', they are transmitted via milk to the developing insect embryo. In termites, the endosymbionts reside within the hindguts and are transmitted through
trophallaxis Trophallaxis () is the transfer of food or other fluids among members of a community A community is a social unit The term "level of analysis" is used in the social sciences to point to the location, size, or scale of a research target. "Level ...

trophallaxis
among colony members. The primary endosymbionts are thought to help the host either by providing nutrients that the host cannot obtain itself or by metabolizing insect waste products into safer forms. For example, the putative primary role of ''Buchnera'' is to synthesize
essential amino acid An essential amino acid, or indispensable amino acid, is an amino acid that cannot be synthesized from scratch by the organism fast enough to supply its demand, and must therefore come from the diet. Of the 21 amino acids common to all life forms ...
s that the aphid cannot acquire from its natural diet of plant sap. Likewise, the primary role of ''Wigglesworthia'', it is presumed, is to synthesize
vitamin A vitamin is an organic molecule , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's ability to Catenation, ...
s that the tsetse fly does not get from the
blood Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers t ...

blood
that it eats. In lower termites, the endosymbiotic protists play a major role in the digestion of lignocellulosic materials that constitute a bulk of the termites' diet. Bacteria benefit from the reduced exposure to
predator Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical en ...

predator
s and competition from other bacterial species, the ample supply of nutrients and relative environmental stability inside the host. Genome sequencing reveals that obligate bacterial endosymbionts of insects have among the smallest of known bacterial genomes and have lost many genes that are commonly found in closely related bacteria. Several theories have been put forth to explain the loss of genes. It is presumed that some of these genes are not needed in the environment of the host insect cell. A complementary theory suggests that the relatively small numbers of bacteria inside each insect decrease the efficiency of natural selection in 'purging' deleterious mutations and small mutations from the population, resulting in a loss of genes over many millions of years. Research in which a parallel
phylogeny A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram A diagram is a symbolic representation Representation may refer to: Law a ...

phylogeny
of bacteria and insects was inferred supports the belief that the primary endosymbionts are transferred only vertically (i.e., from the mother), and not horizontally (i.e., by escaping the host and entering a new host). Attacking obligate bacterial endosymbionts may present a way to control their insect hosts, many of which are pests or carriers of human disease. For example, aphids are crop pests and the tsetse fly carries the organism ''
Trypanosoma brucei ''Trypanosoma brucei'' is a species of parasitic kinetoplastid Kinetoplastida (or Kinetoplastea, as a class) is a group of flagellated A flagellum (; plural: flagella) is a lash-like appendage that protrudes from the soma (biology), cell ...

Trypanosoma brucei
'' that causes African sleeping sickness. Other motivations for their study involve understanding the origins of symbioses in general, as a proxy for understanding e.g. how chloroplasts or mitochondria came to be obligate symbionts of
eukaryotes Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for "Outline ...
or plants.


Secondary endosymbionts

The pea aphid (''
Acyrthosiphon pisum ''Acyrthosiphon pisum'', commonly known as the pea aphid (and colloquially known as the green dolphin, pea louse, and clover louse ), is a sap-sucking insect in the family Aphididae The Aphididae are a very large insect family (biology), fami ...
'') is known to contain at least three secondary endosymbionts, ''
Hamiltonella defensa ''Hamiltonella defensa'' (''H. defensa'') is a species of bacteria. It is maternally or sexually transmitted and lives as an endosymbiont of whiteflies and aphids, meaning that it lives within a host, protecting its host from attack. It does this ...
'', ''
Regiella insecticola ''Regiella insecticola'' is a species of bacteria, that lives as a symbiont Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction ...
'', and ''
Serratia symbiotica ''Serratia symbiotica'' is a species of bacteria that lives as a symbiont Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction b ...
''. ''Hamiltonella defensa'' defends its aphid host from parasitoid wasps. This defensive symbiosis improves the survival of aphids, which have lost some elements of the insect immune response. One of the best-understood defensive symbionts is the spiral bacteria '' Spiroplasma poulsonii''. ''Spiroplasma sp.'' can be reproductive manipulators, but also defensive symbionts of ''
Drosophila ''Drosophila'' () is a genus of fly, flies, belonging to the family (biology), family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the character ...

Drosophila
'' flies. In '' Drosophila neotestacea'', ''S. poulsonii'' has spread across North America owing to its ability to defend its fly host against nematode parasites. This defence is mediated by toxins called "ribosome-inactivating proteins" that attack the molecular machinery of invading parasites. These ''Spiroplasma'' toxins represent one of the first examples of a defensive symbiosis with a mechanistic understanding for defensive symbiosis between an insect endosymbiont and its host. '' Sodalis glossinidius'' is a secondary endosymbiont of tsetse flies that lives inter- and intracellularly in various host tissues, including the midgut and hemolymph. Phylogenetic studies have not indicated a correlation between evolution of '' Sodalis'' and tsetse. Unlike tsetse's primary symbiont ''Wigglesworthia'', though, ''Sodalis'' has been cultured ''in vitro''. Many other insects have secondary endosymbionts not reviewed here.


Endosymbionts of ants


Bacteriocyte-associated symbionts

The most well studied endosymbiont of ants are bacteria of the genus
Blochmannia ''Blochmannia'' is a genus of symbiotic bacteria found in carpenter ant. There are over 1000 species of these ants and, as of 2014, of the over 30 species of carpenter ant that have been investigated, all contain some form of ''Blochmannia''. T ...
, which are the primary endosymbiont of ants. In 2018 a new ant-associated symbiont was discovered in Cardiocondyla ants. This symbiont was named Candidatus Westeberhardia Cardiocondylae and it is also believed to be a primary symbiont.


Endosymbionts of marine invertebrates

Extracellular endosymbionts are also represented in all four extant classes of
Echinodermata An echinoderm () is any member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of ...

Echinodermata
(
Crinoidea Crinoids are marine animals that make up the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class ...

Crinoidea
,
Ophiuroidea Brittle stars, serpent stars, or ophiuroids are echinoderm An echinoderm () is any member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of ...

Ophiuroidea
,
Echinoidea Sea urchins () are spiny, globular echinoderm An echinoderm () is any member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values ...

Echinoidea
, and
Holothuroidea Sea cucumbers are echinoderm An echinoderm () is any member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatica ...

Holothuroidea
). Little is known of the nature of the association (mode of infection, transmission, metabolic requirements, etc.) but
phylogenetic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...

phylogenetic
analysis indicates that these symbionts belong to the alpha group of the class
Proteobacteria Proteobacteria is a major phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The pl ...
, relating them to ''Rhizobium'' and ''Thiobacillus''. Other studies indicate that these subcuticular bacteria may be both abundant within their hosts and widely distributed among the Echinoderms in general. Some marine
oligochaeta Oligochaeta () is a subclass of animals in the phylum Annelida, which is made up of many types of aquatic and terrestrial worms, including all of the various earthworms. Specifically, oligochaetes comprise the terrestrial megadrile earthworms ( ...

oligochaeta
(e.g., '' Olavius algarvensis'' and '' Inanidrillus spp.'') have obligate extracellular endosymbionts that fill the entire body of their host. These marine worms are nutritionally dependent on their symbiotic
chemoautotroph Chemotrophs are organisms that obtain energy by the oxidation of electron donors in their environments. These molecules can be organic molecule, organic (chemoorganotrophs) or inorganic compound, inorganic (chemolithotrophs). The chemotroph design ...
ic bacteria lacking any digestive or excretory system (no gut, mouth, or
nephridia The nephridium (plural ''nephridia'') is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart ...
). The sea slug ''
Elysia chlorotica ''Elysia chlorotica'' (common name Common may refer to: Places * Common, a townland in County Tyrone, Northern Ireland * Boston Common Boston Common (also known as the Common) is a central public park in downtown Boston, Massachusetts. It i ...
'' lives in endosymbiotic relationship with the algae ''
Vaucheria litorea ''Vaucheria litorea'' is a species of yellow-green algae (Xanthophyceae). It grows in a Algae#Morphology, filamentous fashion (forming long tubular cells connected end to end). ''V. litorea'' is a common intertidal species of coastal brackish wa ...
'', and the jellyfish ''
Mastigias ''Mastigias'' is a genus of true jellyfish in the family Mastigiidae. It contains seven described species. Members of this genus are found widely in coastal regions of the Indo-Pacific, including saline lakes of Palau (e.g., Jellyfish Lake), but t ...
'' have a similar relationship with an algae.


Dinoflagellate endosymbionts

Dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 m ...
endosymbionts of the genus ''Symbiodinium'', commonly known as
zooxanthella Zooxanthellae is a colloquial term for single-celled dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a coun ...

zooxanthella
e, are found in
corals Corals are marine invertebrates within the class (biology), class Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important C ...

corals
,
mollusk Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number ...
s (esp.
giant clam The giant clams are the members of the clam genus ''Tridacna'' that are the largest extant taxon, living bivalve mollusks. There are actually several species of "giant clams" in the genus ''Tridacna'', which are often misidentified for ''Tridac ...

giant clam
s, the ''Tridacna''),
sponges Sponges, the members of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. T ...

sponges
, and
foraminifera Foraminifera (; Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...
. These endosymbionts drive the formation of
coral reefs ''Coral Reefs'' is a quarterly peer-review Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work ( peers). It functions as a form of self-regulation by qualified members of a prof ...
by capturing sunlight and providing their hosts with energy for
carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its natural form as a crystallinity, crystalline mi ...

carbonate
deposition. Previously thought to be a single species, molecular
phylogenetic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...

phylogenetic
evidence over the past couple decades has shown there to be great diversity in ''Symbiodinium''. In some cases, there is specificity between host and ''Symbiodinium''
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), lineal descendants - on a phylogenetic tree. R ...

clade
. More often, however, there is an ecological distribution of ''Symbiodinium'', the symbionts switching between hosts with apparent ease. When reefs become environmentally stressed, this distribution of symbionts is related to the observed pattern of
coral bleaching Coral bleaching is the process when corals become white due to various stressors, such as changes in temperature, light, or nutrients. Bleaching occurs when coral polyp (zoology), polyps expel the algae (zooxanthellae) that live inside their tissue ...

coral bleaching
and recovery. Thus, the distribution of ''Symbiodinium'' on coral reefs and its role in coral bleaching presents one of the most complex and interesting current problems in reef
ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms In biol ...
.


Endosymbionts of phytoplankton

In marine environments, bacterial endosymbionts have more recently been discovered. These endosymbiotic relationships are especially prevalent in
oligotrophic The Trophic State Index (TSI) is a classification system designed to rate water bodies based on the amount of biological productivity they sustain. Although the term "trophic index" is commonly applied to lakes, any surface water body may be inde ...
or nutrient-poor regions of the ocean like that of the North Atlantic. In these oligotrophic waters, cell growth of larger phytoplankton like that of
diatom Diatoms (''diá-tom-os'' 'cut in half', from ''diá'', 'through' or 'apart', and the root of ''tém-n-ō'', 'I cut') are a major group of algae Algae (; singular alga ) is an informal term for a large and diverse group of s. It is a grou ...

diatom
s is limited by low nitrate concentrations.  Endosymbiotic bacteria fix nitrogen for their diatom hosts and in turn receive organic carbon from photosynthesis. These symbioses play an important role in global
carbon cycling The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere of the Earth. Carbon is the main component of biological compounds as well as ...

carbon cycling
in oligotrophic regions. One known symbiosis between the diatom '''' spp. and the cyanobacterium ''Richelia intracellularis'' has been found in the North Atlantic, Mediterranean, and Pacific Ocean. The ''Richelia'' endosymbiont is found within the diatom frustule of ''Hemiaulus'' spp., and has a reduced genome likely losing genes related to pathways the host now provides.  Research by Foster et al. (2011) measured nitrogen fixation by the cyanobacterial host ''Richelia intracellularis'' well above intracellular requirements, and found the cyanobacterium was likely fixing excess nitrogen for Hemiaulus host cells. Additionally, both host and symbiont cell growth were much greater than free-living ''Richelia intracellularis'' or symbiont-free ''Hemiaulus'' spp. The ''Hemaiulus''-''Richelia'' symbiosis is not obligatory especially in areas with excess nitrogen (nitrogen replete). ''Richelia intracellularis'' is also found in ''Rhizosolenia'' spp., a diatom found in oligotrophic oceans. Compared to the ''Hemaiulus'' host, the endosymbiosis with ''Rhizosolenia'' is much more consistent, and ''Richelia intracellularis'' is generally found in ''Rhizosolenia''. There are some asymbiotic (occurs without an endosymbiont) Rhizosolenia, however there appears to be mechanisms limiting growth of these organisms in low nutrient conditions. Cell division for both the diatom host and cyanobacterial symbiont can be uncoupled and mechanisms for passing bacterial symbionts to daughter cells during cell division are still relatively unknown. Other endosymbiosis with nitrogen fixers in open oceans include Calothrix in Chaetocerous spp. and UNCY-A in prymnesiophyte microalga.  The Chaetocerous-Calothrix endosymbiosis is hypothesized to be more recent, as the Calothrix genome is generally intact. While other species like that of the UNCY-A symbiont and Richelia have reduced genomes. This reduction in genome size occurs within nitrogen metabolism pathways indicating endosymbiont species are generating nitrogen for their hosts and losing the ability to use this nitrogen independently. This endosymbiont reduction in genome size, might be a step that occurred in the evolution of organelles (above).


Endosymbionts of protists

''
Mixotricha paradoxa ''Mixotricha paradoxa'' is a species of protozoan Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of single-celled eukaryote Eukaryotes () are organism In biology, a ...

Mixotricha paradoxa
'' is a protozoan that lacks mitochondria. However, spherical bacteria live inside the cell and serve the function of the mitochondria. ''Mixotricha'' also has three other species of symbionts that live on the surface of the cell. ''
Paramecium bursaria ''Paramecium bursaria'' is a species of ciliate found in marine and brackish waters. It has a mutualistic endosymbiotic relationship with green algae called ''Zoochlorella''. The algae live inside the ''Paramecium'' in its cytoplasm and provide i ...

Paramecium bursaria
'', a species of
ciliate The ciliates are a group of protozoan Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of single-celled eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a c ...

ciliate
, has a mutualistic symbiotic relationship with green alga called ''
Zoochlorella ''Zoochlorella'' is a ''nomen rejiciendum Nomen may refer to: *Nomen (Roman name), the middle part of Ancient Roman names *Nomen (Ancient Egypt), the personal name of Ancient Egyptian pharaohs *Jaume Nomen (born 1960), Catalan astronomer *Nomen, ...
''. The algae live inside the cell, in the cytoplasm. '' Paulinella chromatophora'' is a freshwater
amoeboid An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism A unicellular organism, also known as a single-celled organism, is an organism ...
which has recently (evolutionarily speaking) taken on a
cyanobacterium Cyanobacteria (), also known as Cyanophyta, are a phylum (biology), phylum of Gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), giving them their other name, "blue-green algae", ...

cyanobacterium
as an endosymbiont. Many
foraminifera Foraminifera (; Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...
are hosts to several types of algae, such as
red algae Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organism ...

red algae
,
diatom Diatoms (''diá-tom-os'' 'cut in half', from ''diá'', 'through' or 'apart', and the root of ''tém-n-ō'', 'I cut') are a major group of algae Algae (; singular alga ) is an informal term for a large and diverse group of s. It is a grou ...

diatom
s,
dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 m ...
s and
chlorophyta Chlorophyta or Prasinophyte, Prasinophyta is a taxon of green algae informally called chlorophytes. The name is used in two very different senses, so care is needed to determine the use by a particular author. In older classification systems, i ...
. These endosymbionts can be transmitted vertically to the next generation via asexual reproduction of the host, but because the endosymbionts are larger than the foraminiferal
gamete A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply ...
s, they need to acquire new algae again after sexual reproduction. Several species of
radiolaria The Radiolaria, also called Radiozoa, are protozoa Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasitism, ...

radiolaria
have photosynthetic symbionts. In some species the host will sometimes digest algae to keep their population at a constant level. '' Hatena arenicola'' is a flagellate protist with a complicated feeding apparaturs that feed on other microbes. But when it engulfs a green alga from the genus ''Nephroselmis'', the feeding apparatus disappears and it becomes photosynthetic. During
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

mitosis
the algae is transferred to only one of the two cells, and the cell without the algae needs to start the cycle all over again. In 1976, biologist Kwang W. Jeon found that a lab strain of ''
Amoeba proteus ''Amoeba proteus (type species of Amoeba)'', of which ''Chaos diffluens'' is one of many synonyms, is a large amoeba An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type ...

Amoeba proteus
'' had been infected by bacteria that lived inside the cytoplasmic
vacuoles A vacuole () is a membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts ...
. This infection killed all the protists except for a few individuals. After the equivalent of 40 host generations, the two organisms gradually became mutually interdependent. Over many years of study, it has been confirmed that a genetic exchange between the
prokaryotes A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contig ...

prokaryotes
and protists had occurred.


Endosymbionts of vertebrates

The spotted salamander (''
Ambystoma maculatum The spotted salamander or yellow-spotted salamander (''Ambystoma maculatum'') is a mole salamander The mole salamanders (genus ''Ambystoma'') are a group of Salamandroidea, advanced salamanders endemic to North America. The group has become fa ...

Ambystoma maculatum
'') lives in a relationship with the algae ''
Oophila amblystomatis ''Chlorococcum amblystomatis'', Synonym (taxonomy), synonym ''Oophila amblystomatis'', commonly known as chlamydomonad algae or salamander algae, is a species of single-celled green algae. When placed in the genus ''Oophila'', it was the only s ...
'', which grows in the egg cases.


Endosymbionts of plants

Chloroplasts are primary endosymbionts of plants that provide energy to the plant by generating sugars. Of all the plants, ''
Azolla ''Azolla'' (mosquito fern, duckweed fern, fairy moss, water fern) is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of ...

Azolla
'' has the most intimate relationship with a symbiont, as its cyanobacterium symbiont ''
Anabaena ''Anabaena'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumsc ...
'' is passed on directly from one generation to the next.


Endosymbionts of bacteria

It has been observed that some
Betaproteobacteria Betaproteobacteria are a class of Gram-negative bacteria, and one of the eight classes of the phylum Proteobacteria. The ''Betaproteobacteria'' are a class comprising over 75 genus, genera and 400 species of bacteria. Together, the ''Betaproteob ...
have
Gammaproteobacteria The class Gammaproteobacteria belongs to the Proteobacteria phylum and contains about 250 genera, which makes it the most genera-rich taxon of the Prokaryotes. Several medically, ecologically, and scientifically important groups of bacteria belon ...
endosymbionts.


Virus-host associations

The human genome project found several thousand
endogenous retrovirus Endogeny (biology), Endogenous Retrovirus, retroviruses (ERVs) are endogenous viral elements in the genome that closely resemble and can be derived from retroviruses. They are abundant in the genomes of Gnathostomata, jawed vertebrates, and they ...
es,
endogenous viral element An endogenous viral element (EVE) is a DNA sequence derived from a virus, and present within the germline of a Organism#Semantics, non-viral organism. EVEs may be entire viral genomes (proviruses), or fragments of viral genomes. They arise when a vi ...
s in the
genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...

genome
that closely resemble and can be derived from
retrovirus A retrovirus is a type of virus A virus is a submicroscopic infectious agent In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical ...

retrovirus
es, organized into 24 families.


See also

*
Epibiont An epibiont (from the Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek ...
, organism living ''on the surface'' of another organism *
Anagenesis Anagenesis is the gradual evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexua ...
*
Endophyte An endophyte is an endosymbiont An endosymbiont or endobiont is any organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). O ...
*
Ectosymbiosis Ectosymbiosis is a form of symbiotic Symbiosis (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in So ...
* List of symbiotic organisms *
List of symbiotic relationships Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological organisms, be it Mutualism (biolog ...
* Multigenomic organism * Protocell


References

{{Self-replicating organic structures Symbiosis Microbial population biology Environmental microbiology Endosymbiotic events