User:Madunn99/Endosymbiont

Outline of Article

-Lead Section

-The Origin: Symbiogenesis and Symbiont transmission

-Endosymbiotic Theory: Symbiogenesis and organelles

-Symbiont transmission

-Endosymbionts of invertebrates

-Endosymbionts of terrestrial invertebrates

-Endosymbionts of insects

(Endosymbionts of ants would be moved here--Bacteriocyte-associated symbionts)

-Primary endosymbionts

-Secondary endosymbionts

-Endosymbionts of marine invertebrates

-Dinoflagellate endosymbionts

-Endosymbionts of phytoplankton

-Endosymbionts of protists

-Endosymbionts of vertebrates- This one is listed as a subheading under endosymbionts of invertebrates, but needs to be its own heading because its talking about vertebrates

           - Endosymbionts of Humans

-Endosymbionts of plants (Endophytes) - this section redefines endosymbiosis as it does in the beginning of article, it should be cut down to remove repeated information

-Fungi as plant endosymbionts

(Endophytic Fungi would be moved here)

-Arbuscular Mycorrhizal Fungi (AMF)

- Bacteria as plant endosymbionts - needs some more information.

- Archaea as plant endosymbionts

Endosymbionts of bacteria

Endosymbionts of fungi

Virus-host associations. Debatable section to have, thoughts on motion to remove?

See also

References

Lead Section
An endosymbiont or endobiont 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.

The history behind the concept of endosymbiosis stems from the postulates of the endosymbiotic theory. The endosymbiotic theory (symbiogenesis) pushes the notion of bacteria exclusively living in eukaryotic organisms after being engulfed by them. This is popular with the concept of organelle development observed with eukaryotes. Two major types of organelle in eukaryotic cells, mitochondria and plastids such as chloroplasts, are considered to be obtained from bacterial endosymbionts. There are two main 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. An example is pea aphid symbionts. Also, it is possible for both to be involved in a mixed-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.

The Origin: Symbiogenesis and Symbiont transmission
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.

Proposed Change- Endosymbiotic Theory: Symbiogenesis
Symbiogenesis explains the origins of eukaryotes, whose cells contain two major kinds of organelle: mitochondria and chloroplasts. The theory proposes the process through which these organelles evolved within the ancestors of eukaryotic organisms through the formation of an endosymbiotic relationship between a bacteria and an archaea This idea is that the ancient Archaean organism engulfed a bacteria that possessed aerobic capabilities. This initial endosymbiotic relationship is believed to have arisen between an Asgard superphylum archaea and an ancient bacteria related to the Rickettsiales lineage. The bacterial endosymbiont would have been used for its metabolic capabilities, and is believed to have eventually developed into the mitochondria that is found in eukaryotic cells. Evidence pointing to this theory involves the evolutionary history of the two groups and their DNA structure. Eukaryotes are believed to have arisen from a common ancestor they possess with archaeans, which also points to archaeans being more closely related to eukaryotes than bacteria. When looking at the DNA found in the nucleus of a cells and the mitochondria. the nuclear DNA of a eukaryote more closely resembles the DNA of an organism belonging to the domain Archaea, while the mitochondrial DNA is more closely related in structure to the DNA of a bacteria organism. The chloroplast is theorized to have arisen from a eukaryotic cell already possessing a mitochondria enveloping and taking in a cyanobacteria that possessed photosynthetic capabilities.

Symbiont transmission
Symbiont transmission is the process where the host in a symbiotic relationship between two organisms acquires an organism (internally or externally) that serves as its symbiont. Most symbionts are either obligatory (require their host to survive) or facultative (do not necessarily need their host to survive). 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 endosymbiosis 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. They can both be eliminated from hosts by treatments that target this bacterium.

Horizontal (lateral), vertical, and mix-mode (hybrid of horizonal and vertical) transmission are the three paths for symbiont transfer. Horizontal symbiont transfer (horizontal transmission) is a process where a host acquires a facultative symbiont from the environment or from another host. The Rhizobia-Legume symbiosis (bacteria-plant endosymbiosis) is a prime example of horizontal symbiont transmission. The Rhizobia-legume symbiotic relationship is important for processes like the formation of root nodules. It starts with flavonoids released by the plant host (Legume), which causes the rhizobia species (endosymbiont) to activate its nod genes. These Nod genes generate lipooligosaccharide signals which the legume(host) detects, thus leading to root nodule formation. This process bleeds on to other unique processes like nitrogen fixation in plants. The evolutionary advantage of such an interaction allows genetic exchange between both organisms involved increases the propensity for novel functions as seen in the plant-bacterium interaction (holobiont formation).

In vertical transmission, 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. Other examples include Wigglesworthia nutritional symbionts of tsetse flies, or in sponges. When a symbiont reaches this stage, it begins to resemble a cellular organelle, similar to mitochondria or chloroplasts. The evolutionary consequences causes the host and the symbiont to be dependent and form a holobiont, and in the event of a bottleneck a decrease in symbiont diversity could affect the host-symbiont interactions adversely, when deleterious mutations build up over time.

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.

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 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. In insects such as ants, the most well studied endosymbionts 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.

Primary endosymbionts
Among primary endosymbionts of insects, the best-studied are the pea aphid (Acyrthosiphon pisum) and its endosymbiont Buchnera sp. APS, 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.

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.

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).

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. 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
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. 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 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 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.

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).

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. Elysia chlorotica forms this relationship intracellularly with the chloroplasts from the algae. These chloroplast retain their photosynthetic capabilities and structures for several months after being taken into the cells of the slug.

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.

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.

Endosymbiosis in Vertebrates- Information updated and changed
Endosymbiotic relationships can be difficult to develop in vertebrates, largely due to the immune capabilities found within the class of organisms.

The bacteria that constitute the microbiome of the human are not considered endosymbiotic but rather an example of ectosymbiosis. The digestive tract is considered to be an external surface and thus the organisms that inhabit it are not endosymbionts.

Endosymbionts of marine invertebrates
Elysia chlorotica forms this relationship intracellularly with the chloroplasts from the algae. These chloroplast retain their photosynthetic capabilities and structures for several months after being taken into the cells of the slug.

Endophytic Bacteria
The most common genera of endophytic bacterial community isolated from forest trees include Pseudomonas, Bacillus, Acinetobacter, Actinobacteria, Sphingomonas and some genera belong to the Enterobacteriacae family (Pirttila and Frank, 2011). Endophytic bacteria mostly colonize the leaf tissues from plant roots. Endophytic bacteria can also enter the plant through the leaves from the phyllosphere via leaf stomata (Senthilkumar et al., 2011).

References

10. Frank, A. C., & Pirttilä, A. M. (Eds.). (2018). Endophytes of forest trees: biology and applications. Springer International Publishing.