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A representation of the endosymbiotic theory Endosymbiosis.PNG
A representation of the endosymbiotic theory

An endosymbiont or endobiont [1] is any 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: ἔνδον endon "within", σύν syn "together" and βίωσις biosis "living".) Examples are nitrogen-fixing bacteria (called rhizobia), which live in the root nodules of legumes, single-cell algae inside reef-building corals and bacterial endosymbionts that provide essential nutrients to insects. [2] [3]


There are two types of symbiont transmissions. In horizontal transmission, each new generation acquires free living symbionts from the environment. An example is the nitrogen-fixing bacteria in certain plant roots. Vertical transmission takes place when the symbiont is transferred directly from parent to offspring. [4] [5] It is also possible for both to be involved in a mix-mode transmission, where symbionts are transferred vertically for some generation before a switch of host occurs and new symbionts are horizontally acquired from the environment. [6] [7] [8]

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 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. [9] Other examples include Wigglesworthia nutritional symbionts of tse-tse flies, or in sponges. [8] When a symbiont reaches this stage, it begins to resemble a cellular organelle, similar to mitochondria or 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 of the genus Riftia , which get nutrition from their endosymbiotic bacteria. The most common examples of obligate endosymbioses are mitochondria and chloroplasts. Some human parasites, e.g. Wuchereria bancrofti and Mansonella perstans , thrive in their intermediate insect hosts because of an obligate endosymbiosis with Wolbachia spp.[ citation needed ] They can both be eliminated from hosts by treatments that target this bacterium.[ citation needed ] However, not all endosymbioses are obligate and some endosymbioses can be harmful to either of the organisms involved.

Two major types of organelle in eukaryotic cells, mitochondria and plastids such as chloroplasts, are considered to be bacterial endosymbionts. [10] This process is commonly referred to as symbiogenesis.

Symbiogenesis and organelles

An overview of the endosymbiosis theory of eukaryote origin (symbiogenesis). Endosymbiotic theory.svg
An overview of the endosymbiosis theory of eukaryote origin (symbiogenesis).

Symbiogenesis explains the origins of eukaryotes, 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. 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. [4] [5] [11] [12]

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. [13] A remarkable example of this is the fractionation of the Hodgkinia genome of Magicicada cicadas. Because the cicada life cycle takes years underground, natural selection 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. [14]

Bacterial endosymbionts of invertebrates

The best-studied examples of endosymbiosis are known from invertebrates. These symbioses affect organisms with global impact, including Symbiodinium of corals, or Wolbachia of insects. Many insect agricultural pests and human disease vectors have intimate relationships with primary endosymbionts.[ citation needed ]

Endosymbionts of insects

Diagram of cospeciation, where parasites or endosymbionts speciate or branch alongside their hosts. This process is more common in hosts with primary endosymbionts. Cospeciation (5 processes) - with key.png
Diagram of cospeciation, where parasites or endosymbionts speciate or branch alongside their hosts. This process is more common in hosts with primary endosymbionts.

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 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 with their insect hosts. Secondary endosymbionts exhibit a more recently developed association, are sometimes horizontally transferred between hosts, live in the hemolymph of the insects (not specialized bacteriocytes, see below), and are not obligate. [15]

Primary endosymbionts

Among primary endosymbionts of insects, the best-studied are the pea aphid ( Acyrthosiphon pisum ) and its endosymbiont Buchnera sp. APS, [16] [9] the tsetse fly Glossina morsitans morsitans and its endosymbiont Wigglesworthia glossinidia brevipalpis and the endosymbiotic protists in lower termites. 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.[ citation needed ]

In some insect groups, these endosymbionts live in specialized insect cells called bacteriocytes (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, 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 among colony members.[ citation needed ]

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 acids that the aphid cannot acquire from its natural diet of plant sap. Likewise, the primary role of Wigglesworthia, it is presumed, is to synthesize vitamins that the tsetse fly does not get from the 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 predators 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 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). [17] [18]

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 that causes African sleeping sickness. [19] 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 or plants.

Secondary endosymbionts

Pea aphids are commonly infested by parasitic wasps. Their secondary endosymbionts attack the infesting parasitoid wasp larvae promoting the survival of both the aphid host and its endosymbionts. HEMI Aphididae Aphidius attacking pea aphid.png
Pea aphids are commonly infested by parasitic wasps. Their secondary endosymbionts attack the infesting parasitoid wasp larvae promoting the survival of both the aphid host and its endosymbionts.

The pea aphid ( Acyrthosiphon pisum ) is known to contain at least three secondary endosymbionts, Hamiltonella defensa , Regiella insecticola , and Serratia symbiotica . Hamiltonella defensa defends its aphid host from parasitoid wasps. [20] This defensive symbiosis improves the survival of aphids, which have lost some elements of the insect immune response. [21]

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 flies. In Drosophila neotestacea , S. poulsonii has spread across North America owing to its ability to defend its fly host against nematode parasites. [22] This defence is mediated by toxins called "ribosome-inactivating proteins" that attack the molecular machinery of invading parasites. [23] [24] 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.[ citation needed ]

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. [25] Unlike tsetse's primary symbiont Wigglesworthia, though, Sodalis has been cultured in vitro. [26]

Many other insects have secondary endosymbionts not reviewed here. [27] [13]

Endosymbionts of ants

Bacteriocyte-associated symbionts

The most well studied endosymbiont of ants are bacteria of the genus Blochmannia, which are the primary endosymbiont of Camponotus 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. [28]

Endosymbionts of marine invertebrates

Extracellular endosymbionts are also represented in all four extant classes of Echinodermata (Crinoidea, Ophiuroidea, Echinoidea, and Holothuroidea). Little is known of the nature of the association (mode of infection, transmission, metabolic requirements, etc.) but phylogenetic analysis indicates that these symbionts belong to the class Alphaproteobacteria, 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. [29]

Some marine 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 chemoautotrophic bacteria lacking any digestive or excretory system (no gut, mouth, or nephridia). [30]

The sea slug Elysia chlorotica lives in endosymbiotic relationship with the algae Vaucheria litorea , and the jellyfish Mastigias have a similar relationship with an algae.[ citation needed ]

Dinoflagellate endosymbionts

Dinoflagellate endosymbionts of the genus Symbiodinium, commonly known as zooxanthellae, are found in corals, mollusks (esp. giant clams, the Tridacna), sponges, and foraminifera. These endosymbionts drive the formation of coral reefs by capturing sunlight and providing their hosts with energy for carbonate deposition. [31]

Previously thought to be a single species, molecular 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. 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 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. [31]

Endosymbionts of phytoplankton

In marine environments, bacterial endosymbionts have more recently been discovered. [32] [33] [34] [35] These endosymbiotic relationships are especially prevalent in oligotrophic or nutrient-poor regions of the ocean like that of the North Atlantic. [32] [36] [33] [34] In these oligotrophic waters, cell growth of larger phytoplankton like that of diatoms is limited by low nitrate concentrations. [37]   Endosymbiotic bacteria fix nitrogen for their diatom hosts and in turn receive organic carbon from photosynthesis. [36] These symbioses play an important role in global carbon cycling in oligotrophic regions. [38] [33] [34]

One known symbiosis between the diatom Hemialus spp. and the cyanobacterium Richelia intracellularis has been found in the North Atlantic, Mediterranean, and Pacific Ocean. [32] [33] [39] 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. [40]   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. [37]  Additionally, both host and symbiont cell growth were much greater than free-living Richelia intracellularis or symbiont-free Hemiaulus spp. [37]  The Hemaiulus-Richelia symbiosis is not obligatory especially in areas with excess nitrogen (nitrogen replete). [32]

Richelia intracellularis is also found in Rhizosolenia spp., a diatom found in oligotrophic oceans. [36] [37] [34] Compared to the Hemaiulus host, the endosymbiosis with Rhizosolenia is much more consistent, and Richelia intracellularis is generally found in Rhizosolenia. [32] There are some asymbiotic (occurs without an endosymbiont) Rhizosolenia, however there appears to be mechanisms limiting growth of these organisms in low nutrient conditions. [41] 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. [41]

Other endosymbiosis with nitrogen fixers in open oceans include Calothrix in Chaetocerous spp. and UNCY-A in prymnesiophyte microalga. [42]   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. [40]  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. [40]  This endosymbiont reduction in genome size, might be a step that occurred in the evolution of organelles (above). [42]

Endosymbionts of protists

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.[ citation needed ]

Paramecium bursaria , a species of ciliate, has a mutualistic symbiotic relationship with green alga called Zoochlorella . The algae live inside the cell, in the cytoplasm.[ citation needed ]

Paulinella chromatophora is a freshwater amoeboid which has recently (evolutionarily speaking) taken on a cyanobacterium as an endosymbiont.

Many foraminifera are hosts to several types of algae, such as red algae, diatoms, dinoflagellates and chlorophyta. [43] 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 gametes, they need to acquire new algae again after sexual reproduction. [44]

Several species of radiolaria have photosynthetic symbionts. In some species the host will sometimes digest algae to keep their population at a constant level. [45]

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 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 had been infected by bacteria that lived inside the cytoplasmic vacuoles. [46] 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 and protists had occurred. [47] [48] [49]

Endosymbionts of vertebrates

The spotted salamander ( Ambystoma maculatum ) lives in a relationship with the algae Oophila amblystomatis , which grows in the egg cases. [50]

Endosymbionts of plants

Chloroplasts are primary endosymbionts of plants that provide energy to the plant by generating sugars.

Of all the plants, Azolla has the most intimate relationship with a symbiont, as its cyanobacterium symbiont Anabaena is passed on directly from one generation to the next. [51] [52]

Endosymbionts of bacteria

It has been observed that some Betaproteobacteria have Gammaproteobacteria endosymbionts. [53]

Virus-host associations

The human genome project found several thousand endogenous retroviruses, endogenous viral elements in the genome that closely resemble and can be derived from retroviruses, organized into 24 families. [54] [ citation needed ] [55]

See also

Related Research Articles

Symbiosis Close, long-term biological interaction between distinct organisms (usually species)

Symbiosis is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasitic. The organisms, each termed a symbiont, must be of different species. In 1879, Heinrich Anton de Bary defined it as "the living together of unlike organisms". The term was subject to a century-long debate about whether it should specifically denote mutualism, as in lichens. Biologists have now abandoned that restriction.

<span class="mw-page-title-main">Symbiogenesis</span> Evolutionary theory holding that eukaryotic organelles evolved through symbiosis with prokaryotes

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 other organelles of eukaryotic cells are descended from formerly free-living prokaryotes taken one inside the other in endosymbiosis. Mitochondria appear to be phylogenetically related to Rickettsiales bacteria, while chloroplasts seem to be related to nitrogen-fixing filamentous cyanobacteria.

<i>Buchnera aphidicola</i> Species of bacterium

Buchnera aphidicola, a member of the Pseudomonadota and the only species in the genus Buchnera, is the primary endosymbiont of aphids, and has been studied in the pea aphid, Acyrthosiphon pisum. Buchnera is believed to have had a free-living, Gram-negative ancestor similar to a modern Enterobacterales, such as Escherichia coli. Buchnera is 3 µm in diameter and has some of the key characteristics of their Enterobacterales relatives, such as a Gram-negative cell wall. However, unlike most other Gram-negative bacteria, Buchnera lacks the genes to produce lipopolysaccharides for its outer membrane. The long association with aphids and the limitation of crossover events due to strictly vertical transmission has seen the deletion of genes required for anaerobic respiration, the synthesis of amino sugars, fatty acids, phospholipids, and complex carbohydrates. This has resulted not only in one of the smallest known genomes of any living organism, but also one of the most genetically stable.


Aposymbiosis occurs when symbiotic organisms live apart from one another. Studies have shown that the lifecycles of both the host and the symbiont are affected in some way, usually negative, and that for obligate symbiosis the effects can be drastic. Aposymbiosis is distinct from exsymbiosis, which occurs when organisms are recently separated from a symbiotic association. Because symbionts can be vertically transmitted from parent to offspring or horizontally transmitted from the environment, the presence of an aposymbiotic state suggests that transmission of the symbiont is horizontal. A classical example of a symbiotic relationship with an aposymbiotic state is the Hawaiian bobtail squid Euprymna scolopes and the bioluminescent bacteria Vibrio fischeri. While the nocturnal squid hunts, the bacteria emit light of similar intensity of the moon which camouflages the squid from predators. Juveniles are colonized within hours of hatching and Vibrio must outcompete other bacteria in the seawater through a system of recognition and infection.


A bacteriocyte, 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 endosymbiotic organisms such as bacteria and fungi, which provide essential amino acids and other chemicals to their host. Bacteriocytes may aggregate into a specialized organ called the bacteriome.

Symbiotic bacteria are bacteria living in symbiosis with another organism or each other. For example, rhizobia living in root nodules of legumes provide nitrogen fixing activity for these plants. Symbiosis was first defined by Marko de Bary in 1869 in a work entitled "Die Erscheinung der Symbiose" in which he defined the term as "namely, the living together of parasite and host". The definition of symbiosis has evolved to encompass a sustained relationship between two or more different organisms "over a considerable fraction of the life of the host." In addition, this relationship is often beneficial for at least one of the organisms involved. There are three main types of symbiotic relationships: commensalism, mutualism, and parasitism. Commensalism is when one organism benefits and the other is neither harmed nor benefits. Mutualism is when both organisms benefit. Lastly, parasitism is when one organism benefits while the other organism is harmed. Organisms can also be involved in multiple of these symbiotic relationships simultaneously.

<i>Paulinella</i> Genus of single-celled organisms

Paulinella is a genus of at least eleven species including both freshwater and marine amoeboids.

Cyanobionts are cyanobacteria that live in symbiosis with a wide range of organisms such as terrestrial or aquatic plants; as well as, algal and fungal species. They can reside within extracellular or intracellular structures of the host. In order for a cyanobacterium to successfully form a symbiotic relationship, it must be able to exchange signals with the host, overcome defense mounted by the host, be capable of hormogonia formation, chemotaxis, heterocyst formation, as well as possess adequate resilience to reside in host tissue which may present extreme conditions, such as low oxygen levels, and/or acidic mucilage. The most well-known plant-associated cyanobionts belong to the genus Nostoc. With the ability to differentiate into several cell types that have various functions, members of the genus Nostoc have the morphological plasticity, flexibility and adaptability to adjust to a wide range of environmental conditions, contributing to its high capacity to form symbiotic relationships with other organisms. Several cyanobionts involved with fungi and marine organisms also belong to the genera Richelia, Calothrix, Synechocystis, Aphanocapsa and Anabaena, as well as the species Oscillatoria spongeliae. Although there are many documented symbioses between cyanobacteria and marine organisms, little is known about the nature of many of these symbioses. The possibility of discovering more novel symbiotic relationships is apparent from preliminary microscopic observations.

Nancy A. Moran is an American evolutionary biologist and entomologist, University of Texas Leslie Surginer Endowed Professor, and co-founder of the Yale Microbial Diversity Institute. Since 2005, she has been a member of the United States National Academy of Sciences. Her seminal research has focused on the pea aphid, Acyrthosiphon pisum and its bacterial symbionts including Buchnera (bacterium). In 2013, she returned to the University of Texas at Austin, where she continues to conduct research on bacterial symbionts in aphids, bees, and other insect species. She has also expanded the scale of her research to bacterial evolution as a whole. She believes that a good understanding of genetic drift and random chance could prevent misunderstandings surrounding evolution. Her current research goal focuses on complexity in life-histories and symbiosis between hosts and microbes, including the microbiota of insects.

<i>Acyrthosiphon pisum</i> Species of true bug

Acyrthosiphon pisum, commonly known as the pea aphid, is a sap-sucking insect in the family Aphididae. It feeds on several species of legumes worldwide, including forage crops, such as pea, clover, alfalfa, and broad bean, and ranks among the aphid species of major agronomical importance. The pea aphid is a model organism for biological study whose genome has been sequenced and annotated.

The hologenome theory of evolution recasts the individual animal or plant as a community or a "holobiont" – the host plus all of its symbiotic microbes. Consequently, the collective genomes of the holobiont form a "hologenome". Holobionts and hologenomes are structural entities that replace misnomers in the context of host-microbiota symbioses such as superorganism, organ, and metagenome. Variation in the hologenome may encode phenotypic plasticity of the holobiont and can be subject to evolutionary changes caused by selection and drift, if portions of the hologenome are transmitted between generations with reasonable fidelity. One of the important outcomes of recasting the individual as a holobiont subject to evolutionary forces is that genetic variation in the hologenome can be brought about by changes in the host genome and also by changes in the microbiome, including new acquisitions of microbes, horizontal gene transfers, and changes in microbial abundance within hosts. Although there is a rich literature on binary host–microbe symbioses, the hologenome concept distinguishes itself by including the vast symbiotic complexity inherent in many multicellular hosts. For recent literature on holobionts and hologenomes published in an open access platform, see the following reference.

Hamiltonella 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 through bypassing the host's immune responses by protecting its host against parasitoid wasps. However, H. defensa is only defensive if infected by a virus. H. defensa shows a relationship with Photorhabdus species, together with Regiella insecticola. Together with other endosymbionts, it provides aphids protection against parasitoids. It is known to habitate Bemisia tabaci.

Sodalis is a genus of bacteria within the family Pectobacteriaceae. This genus contains several insect endosymbionts and also a free-living group. It is studied due to its potential use in the biological control of the tsetse fly. Sodalis is an important model for evolutionary biologists because of its nascent endosymbiosis with insects.

"Candidatus Karelsulcia muelleri" is an aerobic, gram-negative, bacillus bacterium that is a part of the phylum Bacteroidota. "Ca. K. muelleri" is an obligate and mutualistic symbiotic microbe commonly found occupying specialized cell compartments of sap-feeding insects called bacteriocytes. A majority of the research done on "Ca. K. muelleri" has detailed its relationship with the host Homalodisca vitripennis. Other studies have documented the nature of its residency in other insects like the maize leafhopper (Cicadulina) or the spittlebug (Cercopoidea). "Ca. K. muelleri" is noted for its exceptionally minimal genome and it is currently identified as having the smallest known sequenced Bacteroidota genome at only 245 kilobases.

Marine microbial symbiosis

Microbial symbiosis in marine animals was not discovered until 1981. In the time following, symbiotic relationships between marine invertebrates and chemoautotrophic bacteria have been found in a variety of ecosystems, ranging from shallow coastal waters to deep-sea hydrothermal vents. Symbiosis is a way for marine organisms to find creative ways to survive in a very dynamic environment. They are different in relation to how dependent the organisms are on each other or how they are associated. It is also considered a selective force behind evolution in some scientific aspects. The symbiotic relationships of organisms has the ability to change behavior, morphology and metabolic pathways. With increased recognition and research, new terminology also arises, such as holobiont, which the relationship between a host and its symbionts as one grouping. Many scientists will look at the hologenome, which is the combined genetic information of the host and its symbionts. These terms are more commonly used to describe microbial symbionts.

Fungal-bacterial endosymbiosis encompasses the mutualistic relationship between a fungus and intracellular bacteria species residing within the fungus. Many examples of endosymbiotic relationships between bacteria and plants, algae and insects exist and have been well characterized, however fungal-bacteria endosymbiosis has been less well described.


A symbiosome is a specialised compartment in a host cell that houses an endosymbiont in a symbiotic relationship.

Reductive evolution is the process by which microorganisms remove genes from their genome. It can occur when bacteria found in a free-living state enter a restrictive state or are completely absorbed by another organism becoming intracellular (symbiogenesis). The bacteria will adapt to survive and thrive in the restrictive state by altering and reducing its genome to get rid of the newly redundant pathways that are provided by the host. In an endosymbiont or symbiogenesis relationship where both the guest and host benefit, the host can also undergo reductive evolution to eliminate pathways that are more efficiently provided for by the guest.

Novymonas esmeraldas is a protist and member of flagellated trypanosomatids. It is an obligate parasite in the gastrointestinal tract of a bug, and is in turn a host to symbiotic bacteria. It maintains strict mutualistic relationship with the bacteria as a sort of cell organelle (endosymbiont) so that it cannot lead an independent life without the bacteria. Its discovery in 2016 suggests that it is a good model in the evolution of prokaryotes into eukaryotes by symbiogenesis. The endosymbiotic bacterium was identified as member of the genus Pandoraea.

Richelia is a genus of nitrogen-fixing filamentous heterocystous cyanobacteria. It contains the single species Richelia intracellularis. They exist as both free-living organisms as well as symbionts within potentially up to 13 diatoms distributed throughout the global ocean. As a symbiont, Richelia can associate epiphytically and as endosymbionts within the periplasmic space between the cell membrane and cell wall of diatoms.


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