Endosymbiont

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Endosymbiont

An endosymbiont is any organism

Organism

In biology, an organism is any life thing . In at least some form, all organisms are capable of response to stimulus , reproduction, growth and developmental biology, and maintenance of homeostasis as a stable whole....
that lives within the body or cells of another organism, i.e. forming an endosymbiosis (Greek

Greek language

Greek is an Indo-European languages native to the southern Balkan peninsula, the language of the Greek people. It forms an independent branch within Indo-European....
: ??d?? endon "within", s?? syn "together" and ß??s?? biosis "living"). Examples are nitrogen-fixing bacteria (called rhizobia

Rhizobia

Rhizobia are soil bacterium that Nitrogen fixation nitrogen after becoming established inside root nodules of legumes . Rhizobia require a plant host; they cannot independently fix nitrogen....
) which live in root nodules on legume

Legume

A legume is a plant in the family Fabaceae , or a fruit of these specific plants. A legume fruit is a Fruit#Simple fruit that develops from a simple carpel and usually Dehiscence on two sides....
roots, single-celled algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds....
inside reef-building corals, and bacterial endosymbionts that provide essential nutrients to about 10%–15% of insects.

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

Siboglinidae

Siboglinidae, also known as the beard worms, is a family of polychaete Annelida whose members made up the former phylum Pogonophora and Vestimentifera....
of the genus

Genus

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Encyclopedia

An endosymbiont is any organism

Organism

Greek language

Rhizobia

Legume

Algae

Siboglinidae

Siboglinidae, also known as the beard worms, is a family of polychaete Annelida whose members made up the former phylum Pogonophora and Vestimentifera....
of the genus

Genus

A genus is a low-level taxonomic rank used in the classification of living and fossil organisms. The taxonomic ranks are domain , kingdom , phylum, class , order , family , genus, and species....
Riftia, which get nutrition from their endosymbiotic bacteria. However, not all endosymbioses are obligate. Also, some endosymbioses can be harmful to either of the organisms involved. See symbiosis

Symbiosis

The term symbiosis commonly describes close and often long-term interactions between different biological species. The term was first used in 1879 by the Germany mycology Heinrich Anton de Bary, who defined it as "the living together of unlike organisms"....
for further discussion of this issue.

It is generally agreed that certain organelle

Organelle

In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid membrane....
s of the eukaryotic

Eukaryote

Animals, plants, fungus, and protists are eukaryotes , organisms whose Cell are organized into complex structures enclosed within Cell membrane....
cell, especially mitochondria and plastid

Plastid

Plastids are major organelles found in plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell....
s such as chloroplast

Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryote organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve Thermodynamic free energy in the form of Adenosine triphosphate and reduce NADP to NADPH through a complex set of processes called photosynthesis....
s, originated as bacterial endosymbionts. This theory is called the endosymbiotic theory

Endosymbiotic theory

The endosymbiotic theory concerns the origins of mitochondrion and plastids , which are organelles of eukaryote cells. According to this theory, these organelles originated as separate prokaryote organisms which were taken inside the cell as endosymbionts....
, which was first articulated by the Russia

Russia

Russia , or the Russian Federation , is a list of countries spanning more than one continent country extending over much of northern Eurasia....
n botanist Konstantin Mereschkowski

Konstantin Mereschkowski

Konstantin Sergejewicz Mereschkowsky was a prominent Russian biologist and botanist active mainly around Kazan, whose research on lichens led him to propose the theory of symbiogenesis - that larger, more complexity cell evolution from the symbiotic relationship between less complex ones....
in 1905.

The endosymbiont theory and mitochondria and chloroplasts

The endosymbiont theory attempts to explain the origins of organelles such as mitochondria and chloroplasts in eukaryotic cells. The theory proposes that chloroplasts and mitochondria evolved from certain types of bacteria that prokaryotic cells engulfed through endophagocytosis. These cells and the bacteria trapped inside them entered a symbiotic relationship, a close association between different types of organisms over an extended time. However, more specifically, the relationship was endosymbiotic, meaning that one of the organisms (the bacteria) lived within the other (the prokaryotic cells).

According to this endosymbiont theory, an anaerobic cell probably ingested an aerobic

Aerobic organism

An aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment....
bacterium but failed to digest it. The aerobic bacterium flourished within the cell because the cell's cytoplasm

Cytoplasm

The cytoplasm is the part of a Cell that is enclosed within the plasma membrane. In eukaryote cells the cytoplasm contains organelles, such as mitochondrion, that are filled with liquid kept separate from the rest of the cytoplasm by biological membranes....
was abundant in half-digested food molecule

Molecule

In chemistry, a molecule is defined as a sufficiently stable, electric charge neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds....
s. The bacterium digested these molecules with oxygen

Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
and gained great amounts of energy. Because the bacterium had so much energy, it probably leaked some of it as Adenosine triphosphate

Adenosine triphosphate

This article is about the chemical used by cells as an energy carrier. For other uses, see ATP .Adenosine-5'-triphosphate is a multifunctional nucleotide, and plays an important role in cell biology as a coenzyme that is the "molecule unit of currency" of intracellular energy transfer....
into the cell's cytoplasm. This benefited the anaerobic cell because it enabled it to digest food aerobically. Eventually, the aerobic bacterium could no longer live independently from the cell, and it therefore became a mitochondrion. The origin of the chloroplast is very similar to that of the mitochondrion. A cell must have captured a photosynthetic

Photosynthesis

File:Seawifs global biosphere.jpgPhotosynthesis is a metabolic pathway that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight....
cyanobacterium and failed to digest it. The cyanobacterium thrived in the cell and eventually evolved into the first chloroplast. Other eukaryotic organelles may have also evolved through endosymbiosis. Scientists believe that cilia, flagella, centriole

Centriole

A centriole is a barrel-shaped organelle found in most animal eukaryotic Cell s, though absent in higher plants and most fungi. The walls of each centriole are usually composed of nine triplets of microtubules ....
s, and microtubule

Microtubule

Microtubules are one of the components of the cytoskeleton. They have a diameter of 25 Nanometre and length varying from 200 nanometers to 25 micrometers....
s may have come from a symbiosis between a spirilla-like bacterium and an early eukaryotic cell.

There are several examples of evidence that support the endosymbiont theory. Mitochondria and chloroplasts contain their own small supply of DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetics instructions used in the development and functioning of all known living organisms and some viruses....
, which may be remnants of the genome

Genome

In classical genetics, the genome of a diploid organism including eukarya refers to a full set of chromosomes or genes in a gamete; thereby, a regular somatic cell contains two full sets of genomes....
the organelles had when they were independent aerobic bacteria. The single most convincing evidence of the descent of organelles from bacteria is the position of mitochondria and plastid DNA sequences in phylogenetic tree

TREE

TREE was a Boston hardcore punk band formed in the summer of 1990. They were active in the Boston music scene until disbanding in 2002....
s of bacteria. Mitochondria have sequences that clearly indicate origin from a group of bacteria called the alpha-Proteobacteria. Plastids have DNA sequences that indicate origin from the cyanobacteria (blue-green algae). In addition, there are organisms alive today, called living intermediates, that are in a similar endosymbiotic condition to the prokaryotic cells and the aerobic bacteria. Living intermediates show that the evolution proposed by the endosymbiont theory is possible. For example, the giant amoeba Pelomyxa
Pelomyxa

Pelomyxa are giant Amoebozoae, usually 500-800 micrometre but occasionally up to 5 mm in length. The most notable species is P. palustris; other described species may be synonyms, or have been moved to the unrelated genus Chaos ....
lacks mitochondria but has aerobic bacteria that carry out a similar role. A variety of coral

Coral

Corals are marine organisms from the class Anthozoa and exist as small sea anemone?like polyps, typically in colonies of many identical individuals....
s, clam

Clam

Clam is a word which can be used for all, some, or only a few species of bivalve mollusks; the word is a common name which has no real Taxonomy significance in biology....
s, snail

Snail

The word snail is a common name for almost all members of the molluscan class Gastropoda that have coiled animal shells in the adult stage. When the word snail is used in a general sense, it includes sea snails, land snails and freshwater snails....
s, and one species of Paramecium
Paramecium

Paramecia, also known as Lady Slippers, due to their appearance, are a group of unicellular ciliate protozoa, which are commonly studied as a representative of the ciliate group, and range from about 50 to 350 micrometre in length, Simple cilia cover the body, which allow the cell to move with a synchronous motion ....
permanently host algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds....
in their cells. Many of the insect endosymbionts have been shown to have ancient associations with their hosts, involving strictly vertical inheritance. In addition, these insect symbionts have similar patterns of genome evolution

Evolution

In biology, evolution is change in the heritability trait of a population of organisms from one generation to the next. These changes are caused by a combination of three main processes: variation, reproduction, and selection....
to those found in true organelles: genome reduction, rapid rates of gene evolution, and bias in nucleotide

Nucleotide

Nucleotides are molecules that comprise the structural units of RNA and DNA. Additionally, nucleotides play central roles in metabolism. In that capacity, they serve as sources of chemical energy , participate in cell signaling , and are incorporated into important cofactors of enzymatic reactions ....
base composition favoring adenine

Adenine

Adenine is a nucleobase with a variety of roles in biochemistry including cellular respiration, in the form of both the energy-rich adenosine triphosphate and the cofactor s nicotinamide adenine dinucleotide and flavin adenine dinucleotide , and Protein biosynthesis, as a chemical component of DNA and RNA....
and thymine

Thymine

Thymine is one of the four bases in the nucleic acid of DNA that make up the letters GCAT. The others are adenine, guanine, and cytosine. Thymine always pairs with adenine....
, at the expense of guanine

Guanine

Guanine is one of the five main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, thymine, and uracil. In DNA, guanine is paired with cytosine....
and cytosine

Cytosine

Cytosine is one of the five main bases found in DNA and RNA. It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached ....
.

Further evidence of endosymbiosis are the prokaryotic ribosomes found within chloroplasts and mitochondria as well as the double-membrane enclosing them. It used to be widely assumed that the inner membrane is the original membrane of the once independent prokaryote, while the outer one is the food vacuole (phagosomal membrane) it was enclosed in initially. However, this view neglects the fact that i) both modern cyanobacteria and alpha-proteobacteria are Gram negative bacteria, which are surrounded by double membranes; ii) the outer membranes of the endosymbiotic organelles (chloroplasts and mitochondria) are very similar to those of these bacteria in their lipid and protein compositions. Accumulating biochemical data strongly suggest that the double membrane enclosing chloroplasts and mitochondria derived from those of the ancestral bacteria, and the phagosomal membrane disappeared during organelle evolution. Triple or quadruple membranes are found among certain algae, probably resulting from repeated endosymbiosis (although little else was retained of the engulfed cell).

These modern organisms with endosymbiotic relationships with aerobic bacteria have verified the endosymbiotic theory, which explains the origin of mitochondria and chloroplasts from bacteria. Researchers in molecular and evolutionary biology no longer question this theory, although some of the details, such as the mechanisms for loss of gene

Gene

A gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cell and pass genetic trait to offspring....
s from organelles to host nuclear genomes, are still being worked out.

Bacterial endosymbionts in marine oligochaetes

Some marine oligochaeta

Oligochaeta

Oligochaeta is a scientific classification in the biological phylum Annelida and includes various earthworms. Specifically, it contains the terrestrial megadrile earthworms , and freshwater or semi-terrestrial microdrile forms including the Tubificidaes, pot worms and ice worms , Lumbriculus variegatus and several interstitial marine worms...
(e.g Olavius or Inanidrillus) 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).

Bacterial endosymbionts in other marine invertebrates

Extracellular endosymbionts are also represented in all 5 extant classes of Echinodermata (Crinoidea, Ophiuroidea, Asteroidea, 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 alpha group of the class Proteobacteria

Proteobacteria

The Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera....
, 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.

Symbiodinium dinoflagellate endosymbionts in marine metazoa and protists

Dinoflagellate

The dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on sea surface temperature, salinity, or depth....
endosymbionts of the genus Symbiodinium, commonly known as zooxanthella

Zooxanthella

Zooxanthellae are golden-brown intracellular endosymbionts of various marine animals and protozoa, especially anthozoans such as the Scleractinia corals and the tropical sea anemone, Aiptasia....
e, are found in corals, mollusks (esp. giant clam

Giant clam

The giant clam, Tridacna gigas, or traditionally, pa?ua, is the largest living bivalve mollusk. One of a number of large clam species native to the shallow coral reefs of the South Pacific ocean and Indian oceans, they can weigh more than 200 kilograms , measure as much as 1.2 metres across, and have an average lifespan in the wild...
s, the Tridacna), sponges

Sea sponge

The sponges or poriferans are animals of the phylum Porifera . Their bodies consist of an outer thin layer of cells, the pinacoderm and an inner mass of cells and skeletal elements, the choanoderm....
, and foraminifera

Foraminifera

The Foraminifera, or forams for short, are a large group of amoeboid protists with reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net....
. These endosymbionts drive the amazing formation of coral reefs by capturing sunlight and providing their hosts with energy for carbonate

Carbonate

In chemistry, a carbonate is a salt or ester of carbonic acid....
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

Coral bleaching

Coral bleaching is the loss of color of corals, due to stress-induced expulsion of symbiotic unicellular algae or due to the loss of pigmentation within the algae....
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

Ecology is the science study of the distribution and Abundance of life and the interactions between organisms and their nature environment ....
.

Endosymbionts in protists

Mixotricha paradoxa
Mixotricha paradoxa

Mixotricha paradoxa is a species of protozoan that lives inside the termite species Mastotermes darwiniensis and has multiple bacterial symbionts....
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

Paramecium bursaria is a species of ciliate protozoan that has a mutualistic symbiosis relationship with green alga called Zoochlorella....
, a species of ciliate

Ciliate

The ciliates are a group of protists characterized by the presence of hair-like organelles called cilium, which are identical in structure to flagellum but typically shorter and present in much larger numbers with a different undulating pattern than flagella....
, has a mutualistic symbiotic relationship with green alga called Zoochlorella. The algae live inside the cell, in the cytoplasm.

Bacterial obligate endosymbionts in 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

Insect

Insects are the biggest class of arthropods and the only ones with wings. They are the most diverse group of animals on the planet. They are most diverse at the equator and their diversity declines toward the poles....
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

Hemolymph

Hemolymph or haemolymph is the blood analogue used by all arthropods and most mollusks that have an open circulatory system.In these animals there is no distinction between blood and interstitial fluid....
of the insects (not specialized bacteriocytes, see below), and are not obligate.

Among primary endosymbionts of insects, the best studied are the pea aphid

Aphid

Aphids, also known as plant lice , are small plant-eating insects, and members of the Taxonomic rank Aphidoidea. Aphids are among the most destructive insect pests on cultivated plants in temperate regions....
(Acyrthosiphon pisum) and its endosymbiont Buchnera
Buchnera (proteobacteria)

Buchnera aphidicola a member of the Proteobacteria, is the primary Endosymbiosis of aphids . It is believed that Buchnera was once a free living gram negative ancestor similar to a modern Enterobacteriaceae such as Escherichia coli....
sp. APS, the tsetse fly

Tsetse fly

Tsetse are large biting flies from Africa which live by feeding on the blood of vertebrate animals. Tsetse include all the species in the genus Glossina, which are generally placed in their own family, Glossinidae....
Glossina morsitans morsitans and its endosymbiont Wigglesworthia glossinidia brevipalpis
Wigglesworthia glossinidia brevipalpis

Wigglesworthia glossinidia brevipalpis is a Gram-negative bacterium in the family Enterobacteriaceae, related to E. coli, which lives in the gut of the tsetse fly....
and the endosymbiotic protists in lower termite

Termite

The termites are a group of social insects usually classified at the Taxonomy of Order Isoptera . As truly social animals, they are termed eusocial along with the ants and some bees and wasps which are all placed in the separate Order Hymenoptera....
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

Bacteriocyte

A bacteriocyte , also called a mycetocyte, is a specialized adipocyte found in some insect groups such as aphids, tsetse flies, german cockroaches, and many others....
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

Egg (biology)

In most birds and reptiles, an egg is the zygote, resulting from fertilization of the ovum. To enable incubation the egg is usually kept within a favourable temperature range as it nourishes and protects the growing embryo....
, as in Buchnera; in others like Wigglesworthia, they are transmitted via milk

Milk

Milk is an opaque white liquid produced by the mammary glands of female mammals . It provides the primary source of nutrition for newborn mammals before they are able to digestion other types of food....
to the developing insect embryo. In termites, the endosymbionts reside within the hindguts and are transmitted through trophallaxis

Trophallaxis

Trophallaxis is the transfer of food or other fluids among members of a community through mouth-to-mouth or anus-to-mouth feeding. It is most highly developed in social insects such as ants, termites, wasps and bees....
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

Essential amino acid

File:BakedFish.jpgAn essential amino acid or indispensable amino acid is an amino acid that cannot be synthesized de novo synthesis by the organism , and therefore must be supplied in the diet....
s that the aphid cannot acquire from its natural diet of plant sap. Similarly, the primary role of Wigglesworthia is probably to synthesize vitamin

Vitamin

A vitamin is an organic compound required as a nutrient in tiny amounts by an organism. A compound is called a vitamin when it cannot be biosynthesis in sufficient quantities by an organism, and must be obtained from the diet....
s that the tsetse fly does not get from the blood

Blood

Blood is a specialized bodily fluid that delivers necessary substances to the body's Cell s ? such as nutrients and oxygen ? and transports waste products away from those same cells....
that it eats. In lower termites, the endosymbiotic protists play a major role in the digestion of lignocellulosic materials which constitutes a bulk of the termites' diet.

Bacteria benefit from the reduced exposure to predators, 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. Presumably 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
Trypanosoma brucei

'Trypanosoma BRYAN LI' is a parasitic protist species that causes African trypanosomiasis in humans and nagana in animals in Africa. There are 3 sub-species of T....
that causes African sleeping sickness

Sleeping sickness

Sleeping sickness or human African trypanosomiasis is a parasitic disease of people and animals, caused by protozoa of species Trypanosoma brucei and transmitted by the tsetse fly....
. Other motivations for their study is to understand symbiosis, and to understand how bacteria with severely depleted genomes are able to survive, thus improving our knowledge of genetics

Genetics

Genetics , a discipline of biology, is the science of heredity and Genetic variation in living organisms. The fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding....
and molecular biology

Molecular biology

Molecular biology is the study of biology at a molecule level. The field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry....
.

Less is known about secondary endosymbionts. The pea aphid (Acyrthosiphon pisum) is known to contain at least three secondary endosymbionts, Hamiltonella defensa, Regiella insecticola, and Serratia symbiotica. H. defensa aids in defending the insect from parasitoids. Sodalis glossinidius is a secondary endosymbiont 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 P-symbiont Wigglesworthia, though, Sodalis has been cultured in vitro.

Viral endosymbionts, endogenous retrovirus (ERV)

During pregnancy in viviparous mammals, ERVs are activated and produced in high quantities during the implantation of the embryo. On one hand they act as immunodepressors, and protect the embryo from the immune system of the mother and on the other hand viral fusion proteins cause the formation of the placental syncytium

Syncytium

In biology, a syncytium is a large cell-like structure filled with cytoplasm containing many cell nucleus....
in order to limit the exchange of migratory cells between the developing embryo and the body of the mother, an epithelium

Epithelium

In biology and medicine, epithelium is a Biological tissue composed of cell s that line the cavities and surfaces of structures throughout the body....
won't do because certain blood cells are specialized to be able to insert themselves between adjacent epithelial cells. The ERV is a virus similar to HIV

HIV

Human immunodeficiency virus is a lentivirus that can lead to AIDS , a condition in humans in which the immune system begins to fail, leading to life-threatening opportunistic infections....
(the virus causing AIDS

AIDS

Acquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the HIV ....
in humans). The immunodepressive action was the initial normal behavior of the virus, similar to HIV. The fusion proteins was a way to spread the infection to other cells by simply merging them with the infected one (similar to HIV). It is believed that the ancestors of modern vivipary

Vivipary

A viviparous animal is an animal employing vivipary: the embryo develops inside the body of the mother, as opposed to outside in an Egg ....
mammals evolved after an accidental infection of an ancestor with this virus, that permitted to the fetus to survive the immune system of the mother.

The human genome project found several thousand ERVs, which are organized into 24 families.

Obligate bacterial endosymbiosis in marine oligochaetes:

Endosymbiotic sulphate-reducing and sulphide-oxidizing bacteria in an oligochaete worm. Dubilier N., Mülders C.,Ferdelman T., De Beer D.,Pernthaler A.,Klein M., Wagner M., Erseus C., Thiermann F., Krieger J., Giere O & Amann R. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11357130

Bacterial endosymbionts in echinoderms:

Subcuticular bacteria from the brittle star Ophiactis balli (Echinodermata: Ophiuroidea) represent a new lineage of extracellular marine symbionts in the alpha subdivision of the class Proteobacteria. Burnett, W J and J D McKenzie http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=168468&rendertype=abstract

Symbiodinium dinoflagellate endosymbionts in marine metazoa and protists

Excellent review paper covering the role of Symbiodinium in reef ecology and the current state of research:

Obligate bacterial endosymbionts in insects:

PLOS Biology Primer- Endosymbiosis: lessons in conflict resolution http://www.plosbiology.org/plosonline/?request=get-document&doi=10.1371/journal.pbio.0020068
A general review of bacterial endosymbionts in insects. P. Baumann, N. A. Moran and L. Baumann, Bacteriocyte-associated endosymbionts of insects in M. Dworkin, ed., The prokaryotes, Springer, New York, 2000. http://link.springer.de/link/service/books/10125/
An excellent review of insect endosymbionts that focuses on genetic issues. Jennifer J. Wernegreen (2002), Genome evolution in bacterial endosymbionts of insects, Nature Reviews Genetics, 3, pp. 850-861. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12415315&dopt=Abstract
A review article on aphids and their bacterial endosymbionts. A. E. Douglas (1998), Nutritional interactions in insect-microbial symbioses: Aphids and Their Symbiotic Bacteria Buchnera, Annual Reviews of Entomology, 43, pp. 17-37.
Describes possible methods to control the human pathogen causing African sleeping sickness, which is transmitted by tsetse flies. Focuses on methods using the primary and secondary endosymbionts of the tsetse fly. Serap Aksoy, Ian Maudlin, Colin Dale, Alan S. Robinsonand and Scott L. O'Neill (2001), Prospects for control of African trypanosomiasis by tsetse vector, TRENDS in Parasitology, 17 (1), pp. 29-35. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11137738&dopt=Abstract
Announces and analyzes the full genome sequence of Buchnera sp. APS, the endosymbiont of the pea aphid, and the first endosymbiont to have its genome sequenced. S. Shigenobu, H. Watanabe, M. Hattori, Y. Sakaki and H. Ishikawa (2000), Genome sequence of the endocellular bacterial symbiont of aphids Buchnera sp. APS, Nature
Nature (journal)

Nature is a prominent scientific journal, first published on 4 November 1869. Although most scientific journals are now highly specialized, Nature is one of the few journals, along with other weekly journals such as Science and Proceedings of the National Academy of Sciences, that still publishes original research articles ac...
, 407, pp. 81-86. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10993077&dopt=Abstract
An article that presents for the first time a theory on how obligate endosymbionts may have their genomes degraded, in a freely-available journal. Nancy A. Moran (1996), Accelerated evolution and Muller's ratchet in endosymbiotic bacteria, Proceedings of the National Academy of Sciences of the USA
Proceedings of the National Academy of Sciences

The Proceedings of the National Academy of Sciences of the United States of America, usually referred to as PNAS, is the official journal of the United States National Academy of Sciences....
, 93, pp. 2873-2878. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8610134&dopt=Abstract

Endosymbiont

The endosymbiont theory and mitochondria and chloroplasts

Bacterial endosymbionts in marine oligochaetes

Bacterial endosymbionts in other marine invertebrates

Symbiodinium dinoflagellate endosymbionts in marine metazoa and protists

Endosymbionts in protists

Bacterial obligate endosymbionts in insects

Viral endosymbionts, endogenous retrovirus (ERV)

Obligate bacterial endosymbiosis in marine oligochaetes:

Bacterial endosymbionts in echinoderms:

Symbiodinium dinoflagellate endosymbionts in marine metazoa and protists

Obligate bacterial endosymbionts in insects:

See also