Invertebrates The best-studied examples of endosymbiosis are in
invertebrates. These symbioses affect organisms with global impact, including
Symbiodinium (corals), or
Wolbachia (insects). Many insect agricultural pests and human disease vectors have intimate relationships with primary endosymbionts.
Insects Scientists classify insect endosymbionts as Primary or Secondary. Primary endosymbionts (P-endosymbionts) have been associated with their
insect hosts for millions of years (from ten to several hundred million years). They form obligate associations and display
cospeciation with their insect hosts. Secondary endosymbionts more recently associated with their hosts, may be horizontally transferred, live in the
hemolymph of the insects (not specialized bacteriocytes, see below), and are not obligate.
Primary Among primary endosymbionts of insects, the best-studied are the pea
aphid (
Acyrthosiphon pisum) and its endosymbiont
Buchnera sp. APS, 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 embryo. In termites, the endosymbionts reside within the hindguts and are transmitted through
trophallaxis among colony members. Primary endosymbionts are thought to help the host either by providing essential nutrients 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 diet of plant sap. The primary role of
Wigglesworthia 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. Primary endosymbionts of insects have among the smallest of known bacterial genomes and have
lost many genes commonly found in closely related bacteria. One theory claimed 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 assumption that primary endosymbionts are transferred only vertically. Attacking obligate bacterial endosymbionts may present a way to control their hosts, many of which are pests or human disease carriers. For example, aphids are crop pests and the tsetse fly carries the organism
Trypanosoma brucei that causes African
sleeping sickness. Studying insect endosymbionts can aid understanding the origins of symbioses in general, as a proxy for understanding endosymbiosis in other species. The best-studied ant endosymbionts are
Blochmannia bacteria, which are the primary endosymbiont of
Camponotus ants. In 2018 a new ant-associated symbiont,
Candidatus Westeberhardia Cardiocondylae, was discovered in
Cardiocondyla. It is reported to be a primary symbiont.
Secondary The pea aphid (
Acyrthosiphon pisum) contains at least three secondary endosymbionts,
Hamiltonella defensa,
Regiella insecticola, and
Serratia symbiotica.
Hamiltonella defensa defends its aphid host from parasitoid wasps. This symbiosis replaces lost elements of the insect's 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 toxins represent one of the first understood 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 do not report a correlation between evolution of
Sodalis and tsetse. Unlike
Wigglesworthia, Sodalis has been cultured
in vitro.
Cardinium is another clade of vertically transmitted bacterial endosymbionts that occurs in a range of arthropod hosts and often manipulates host reproductive biology to favor females that transmit the endosymbiont.
Marine Extracellular endosymbionts are 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. 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's
endosymbiont is the algae Vaucheria litorea.
The jellyfish Mastigias have a similar relationship with an algae. Elysia chlorotica'' forms this relationship intracellularly with the algae's chloroplasts. These chloroplasts retain their photosynthetic capabilities and structures for several months after entering the slug's cells.
Trichoplax have two bacterial endosymbionts. Ruthmannia lives inside the animal's digestive cells. Grellia lives permanently inside the
endoplasmic reticulum (ER), the first known symbiont to do so.
Paracatenula is a
flatworm which have lived in symbiosis with an endosymbiotic bacteria for 500 million years. The bacteria produce numerous small, droplet-like vesicles that provide the host with needed nutrients.
Dinoflagellates Dinoflagellate endosymbionts of the genus
Symbiodinium, commonly known as
zooxanthellae, are found in
corals,
mollusks (esp.
giant clams, the
Tridacna),
sponges, and the unicellular
foraminifera. These endosymbionts capture sunlight and provide their hosts with energy via
carbonate deposition. Previously thought to be a single species, molecular
phylogenetic evidence reported diversity in
Symbiodinium. In some cases, the host requires a specific
Symbiodinium clade. More often, however, the distribution is ecological, with symbionts switching among hosts with ease. When reefs become environmentally stressed, this distribution 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 is an important in coral reef ecology. endosymbiont relationships are especially prevalent in
oligotrophic or nutrient-poor regions of the ocean like that of the North Atlantic. Endosymbiotic bacteria fix nitrogen for their hosts and in turn receive organic carbon from photosynthesis.
Richelia is found within the
diatom frustule of
Hemiaulus spp., and has a reduced genome. A 2011 study measured nitrogen fixation by the
cyanobacterial host
Richelia intracellularis well above intracellular requirements, and found the cyanobacterium was likely fixing nitrogen for its host. 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. The
Chaetoceros-
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.
Paramecium bursaria, a species of
ciliate, has a mutualistic symbiotic relationship with green alga called
Zoochlorella. The algae live in its cytoplasm.
Platyophrya chlorelligera is a freshwater ciliate that harbors
Chlorella that perform photosynthesis.
Strombidium purpureum is a marine ciliate that uses endosymbiotic, purple, non-sulphur bacteria for anoxygenic photosynthesis.
Paulinella chromatophora is a freshwater
amoeboid that has a
cyanobacterium endosymbiont. Many
foraminifera are hosts to several types of algae, such as
red algae,
diatoms,
dinoflagellates and
chlorophyta. 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 algae horizontally following sexual reproduction. Several species of
radiolaria have photosynthetic symbionts. In some species the host digests algae to keep the population at a constant level.
Hatena arenicola is a flagellate
protist with a complicated feeding apparatus that feeds on other microbes. When it engulfs a green
Nephroselmis alga, the feeding apparatus disappears and it becomes photosynthetic. During
mitosis the algae is transferred to only one of the daughter cells, while the other cell restarts the cycle. In 1966, biologist Kwang W. Jeon found that a lab strain of
Amoeba proteus had been infected by bacteria that lived inside the cytoplasmic
vacuoles. This infection killed almost all of the infected protists. After the equivalent of 40 host generations, the two organisms become mutually interdependent. A genetic exchange between the
prokaryotes and protists occurred.
Vertebrates The spotted salamander (
Ambystoma maculatum) lives in a relationship with the algae
Oophila amblystomatis, which grows in its egg cases.
Plants All vascular plants harbor endosymbionts or endophytes in this context. They include
bacteria,
fungi,
viruses,
protozoa and even
microalgae. Endophytes aid in processes such as growth and development, nutrient uptake, and defense against biotic and abiotic stresses like
drought,
salinity, heat, and herbivores. Plant symbionts can be categorized into
epiphytic,
endophytic, and
mycorrhizal. These relations can also be categorized as beneficial,
mutualistic, neutral, and
pathogenic.
Microorganisms living as endosymbionts in plants can enhance their host's primary productivity either by producing or capturing important resources. These endosymbionts can also enhance plant productivity by producing toxic metabolites that aid plant defenses against
herbivores. Plants are dependent on
plastid or
chloroplast organelles. The chloroplast is derived from a cyanobacterial primary endosymbiosis that began over one billion years ago. An oxygenic, photosynthetic free-living
cyanobacterium was engulfed and kept by a heterotrophic
protist and eventually evolved into the present intracellular organelle. Mycorrhizal endosymbionts appear only in
fungi. Typically, plant endosymbiosis studies focus on a single category or species to better understand their individual biological processes and functions.
Fungal endophytes Fungal endophytes can be found in all plant tissues. Fungi living below the ground amidst plant roots are known as
mycorrhiza, but are further categorized based on their location inside the root, with prefixes such as ecto, endo, arbuscular, ericoid, etc. Fungal endosymbionts that live in the roots and extend their extraradical
hyphae into the outer
rhizosphere are known as ectendosymbionts.
Arbuscular Mycorrhizal Fungi (AMF) Arbuscular mycorrhizal fungi or AMF are the most diverse plant microbial endosymbionts. With exceptions such as the
Ericaceae family, almost all vascular plants harbor
AMF endosymbionts as endo and ecto as well. AMF plant endosymbionts systematically colonize
plant roots and help the plant host acquire soil
nutrients such as nitrogen. In return it absorbs plant organic carbon products. AMF
Gigaspora margarita lives as a plant endosymbiont and also harbors further endosymbiont intracytoplasmic bacterium-like organisms. AMF generally promote plant health and growth and alleviate
abiotic stresses such as salinity, drought, heat, poor nutrition, and
metal toxicity. Individual AMF species have different effects in different hosts – introducing the AMF of one plant to another plant can reduce the latter's growth.
Endophytic fungi Endophytic fungi in
mutualistic relations directly benefit and benefit from their host plants. They also can help their hosts succeed in polluted environments such as those contaminated with toxic metals. Fungal
endophytes are taxonomically diverse and are divided into categories based on mode of transmission,
biodiversity, in planta colonization and host plant type. Clavicipitaceous fungi systematically colonize temperate season grasses. Non-clavicipitaceous fungi colonize higher plants and even roots and divide into subcategories.
Aureobasidium and
preussia species of endophytic fungi isolated from
Boswellia sacra produce
indole acetic acid hormone to promote plant health and development.
Aphids can be found in most plants. Carnivorous
ladybirds are aphid predators and are used in
pest control. Plant endophytic fungus
Neotyphodium lolii produces
alkaloid mycotoxins in response to
aphid invasions. In response, ladybird predators exhibited reduced
fertility and abnormal reproduction, suggesting that the mycotoxins are transmitted along the
food chain and affect the
predators. Some endophytic bacteria genera additionally belong to the
Enterobacteriaceae family. Endophytic bacteria typically colonize the leaf tissues from plant roots, but can also enter the plant through the leaves through leaf
stomata. Generally, the endophytic bacteria are isolated from the plant tissues by surface
sterilization of the plant tissue in a sterile environment. Passenger endophytic bacteria eventually colonize inner tissue of plant by
stochastic events while True endophytes possess adaptive traits because of which they live strictly in association with plants. The
in vitro-cultivated endophytic
bacteria association with plants is considered a more intimate relationship that helps plants acclimatize to conditions and promotes health and growth. Endophytic bacteria are considered to be plant's essential endosymbionts because virtually all plants harbor them, and these endosymbionts play essential roles in host survival. This endosymbiotic relation is important in terms of
ecology,
evolution and diversity. Endophytic bacteria such as
Sphingomonas sp. and
Serratia sp. that are isolated from arid land plants regulate endogenous
hormone content and promote growth.
Archaea endosymbionts Archaea are members of most
microbiomes. While archaea are abundant in extreme environments, they are less abundant and diverse in association with eukaryotic hosts. Nevertheless, archaea are a substantial constituent of plant-associated ecosystems in the aboveground and belowground phytobiome, and play a role in host plant's health, growth and survival amid biotic and abiotic stresses. However, few studies have investigated the role of archaea in plant health and its symbiotic relationships. Most plant endosymbiosis studies focus on fungal or bacteria using
metagenomic approaches. The characterization of archaea includes crop plants such as
rice and
maize, but also aquatic plants. This archaeal abundance is associated with plant species type, environment and the plant's developmental stage. In a study on plant
genotype-specific archaeal and bacterial endophytes, 35% of archaeal sequences were detected in overall sequences (achieved using
amplicon sequencing and verified by
real time-PCR). The archaeal sequences belong to the phyla
Thaumarchaeota,
Crenarchaeota, and
Euryarchaeota.
Bacteria Some
Betaproteobacteria have
Gammaproteobacteria endosymbionts.
Fungi Fungi host endohyphal bacteria; the effects of the bacteria are not well studied. Many such fungi in turn live within plants. These interactions may impact the way that fungi interact with the environment by modulating their
phenotypes. In 2024, researchers injected individual cells of the bacterium
Mycetohabitans rhizoxinica into cells of the fungus
Rhizopus microsporus and were able to propagate the pair of cells for ten rounds using
fluorescence-activated cell sorting to select fungal cells containing the bacterium. They found that the fungus's DNA changed during the rounds of propagation. This was claimed to be the first time that endosymbiosis was artificially induced in a laboratory. == Virus endosymbionts ==