Endosymbiosis played key roles in the development ofeukaryotes and plants. Roughly 2.2 billion years ago anarchaeon absorbed abacterium throughphagocytosis, that eventually became themitochondria that provide energy to almost all livingeukaryotic cells. Approximately 1 billion years ago, some of those cells absorbedcyanobacteria that eventually becamechloroplasts,organelles that produce energy from sunlight.[4] Approximately 100 million years ago, a lineage of amoeba in the genusPaulinella independently engulfed a cyanobacterium that evolved to be functionally synonymous with traditional chloroplasts, called chromatophores.[5]
Some 100 million years ago,UCYN-A, a nitrogen-fixing bacterium, became an endosymbiont of the marine algaBraarudosphaera bigelowii, eventually evolving into anitroplast, which fixes nitrogen.[6] Similarly,diatoms in the familyRhopalodiaceae have cyanobacterial endosymbionts, called spheroid bodies or diazoplasts, which have been proposed to be in the early stages of organelle evolution.[7][8]
Symbionts are either obligate (require their host to survive) or facultative (can survive independently).[9] The most common examples of obligate endosymbiosis aremitochondria andchloroplasts; however, they do not reproduce viamitosis in tandem with their host cells. Instead, they replicate viabinary fission, a replication process uncoupled from the host cells in which they reside.[10][11] Some human parasites, e.g.Wuchereria bancrofti andMansonella perstans, thrive in their intermediate insect hosts because of an obligate endosymbiosis withWolbachia spp.[12] They can both be eliminated by treatments that target their bacterial host.[13]
An overview of the endosymbiosis theory of eukaryote origin (symbiogenesis).
Symbiogenesis theory holds that eukaryotes evolved via absorbingprokaryotes. Typically, one organism envelopes a bacterium and the two evolve a mutualistic relationship. The absorbed bacterium (the endosymbiont) eventually lives exclusively within the host cells. This fits the concept of observed organelle development.[14][15][16][17][18]
Typically the endosymbiont's genome shrinks, discarding genes whose roles are displaced by the host.[19] For example, theHodgkinia genome ofMagicicadacicadas is much different from that of the prior freestanding bacteria. The cicada life cycle involves years of stasis underground. The symbiont produces many generations during this phase, experiencing littleselection pressure, allowing their genomes to diversify. Selection is episodic (when the cicadas reproduce). The originalHodgkinia genome split into three much simpler endosymbionts, each encoding only a few genes—an instance ofpunctuated equilibrium producing distinct lineages. The host requires all three symbionts.[20]
Symbiont transmission is the process where the host acquires its symbiont. Since symbionts are not produced by host cells, they must find their own way to reproduce and populate daughter cells as host cells divide. Horizontal, vertical, and mixed-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 another host.[9] The Rhizobia-Legume symbiosis (bacteria-plant endosymbiosis) is a prime example of this modality.[21] The Rhizobia-legume symbiotic relationship is important for processes such as the formation of root nodules. It starts with flavonoids released by the legume host, which causes the rhizobia species (endosymbiont) to activate itsNod genes.[21] TheseNod genes generatelipooligosaccharide signals that the legume detects, leading to root nodule formation.[22] This process bleeds into other processes such as nitrogen fixation in plants.[21] The evolutionary advantage of such an interaction allows genetic exchange between both organisms involved to increase the propensity for novel functions as seen in the plant-bacterium interaction (holobiont formation).[23]
Vertical transmission takes place when the symbiont moves directly from parent to offspring.[24][25] In horizontal transmission each generation acquires symbionts from the environment. An example is nitrogen-fixing bacteria in certain plant roots, such aspea aphid symbionts. A third type is mixed-mode transmission, where symbionts move horizontally for some generations, after which they are acquired vertically.[26][27][28]
Wigglesworthia, a tsetse fly symbiont,[28] is vertically transmitted (via mother's milk).[28] Invertical transmission, the symbionts do not need to survive independently, often leading them to have a reduced genome. For instance,pea aphid symbionts have lost genes for essential molecules and rely on the host to supply them. In return, the symbionts synthesize essentialamino acids for the aphid host.[22] When a symbiont reaches this stage, it begins to resemble a cellularorganelle, similar tomitochondria orchloroplasts. Such dependent hosts and symbionts form aholobiont. In the event of a bottleneck, a decrease in symbiont diversity could compromise host-symbiont interactions, as deleterious mutations accumulate.[29]
The best-studied examples of endosymbiosis are ininvertebrates. These symbioses affect organisms with global impact, includingSymbiodinium (corals), orWolbachia (insects). Many insect agricultural pests and human disease vectors have intimate relationships with primary endosymbionts.[30]
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 as Primary or Secondary. Primary endosymbionts (P-endosymbionts) have been associated with theirinsect hosts for millions of years (from ten to several hundred million years). They form obligate associations and displaycospeciation with their insect hosts. Secondary endosymbionts more recently associated with their hosts, may be horizontally transferred, live in thehemolymph of the insects (not specialized bacteriocytes, see below), and are not obligate.[31]
Among primary endosymbionts of insects, the best-studied are the peaaphid (Acyrthosiphon pisum) and its endosymbiontBuchnera sp. APS,[32][22] thetsetse flyGlossina morsitans morsitans and its endosymbiontWigglesworthia glossinidia brevipalpis and the endosymbioticprotists in lowertermites. As with endosymbiosis in other insects, the symbiosis is obligate. Nutritionally enhanced diets allow symbiont-free specimens to survive, but they are unhealthy, and at best survive only a few generations.[33]
In some insect groups, these endosymbionts live in specialized insect cells calledbacteriocytes (also calledmycetocytes), and are maternally transmitted, i.e. the mother transmits her endosymbionts to her offspring. In some cases, the bacteria are transmitted in theegg, as inBuchnera; in others likeWigglesworthia, they are transmitted via milk to the embryo. In termites, the endosymbionts reside within the hindguts and are transmitted throughtrophallaxis among colony members.[34]
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 ofBuchnera is to synthesizeessential amino acids that the aphid cannot acquire from its diet of plant sap. The primary role ofWigglesworthia is to synthesizevitamins that the tsetse fly does not get from theblood 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 topredators 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 havelost 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 parallelphylogeny of bacteria and insects was inferred supports the assumption hat primary endosymbionts are transferred only vertically.[35][36]
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 organismTrypanosoma brucei that causes Africansleeping sickness.[37] 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 areBlochmannia bacteria, which are the primary endosymbiont ofCamponotus ants. In 2018 a new ant-associated symbiont,Candidatus Westeberhardia Cardiocondylae, was discovered inCardiocondyla. It is reported to be a primary symbiont.[38]
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.
One of the best-understood defensive symbionts is the spiral bacteriaSpiroplasma poulsonii.Spiroplasma sp. can be reproductive manipulators, but also defensive symbionts ofDrosophila flies. InDrosophila neotestacea,S. poulsonii has spread across North America owing to its ability to defend its fly host againstnematode parasites.[41] This defence is mediated by toxins called "ribosome-inactivatingproteins" that attack the molecular machinery of invading parasites.[42][43] 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.[44]
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 ofSodalis and tsetse.[45] UnlikeWigglesworthia,Sodalis has been culturedin vitro.[46]
Cardinium and many other insects have secondary endosymbionts.[47][19]
Extracellular endosymbionts are represented in all four extant classes ofEchinodermata (Crinoidea,Ophiuroidea,Echinoidea, andHolothuroidea). Little is known of the nature of the association (mode of infection, transmission, metabolic requirements, etc.) butphylogenetic analysis indicates that these symbionts belong to the classAlphaproteobacteria, relating them toRhizobium andThiobacillus. Other studies indicate that these subcuticular bacteria may be both abundant within their hosts and widely distributed among the Echinoderms.[48]
Some marineoligochaeta (e.g.,Olavius algarvensis andInanidrillus spp.) have obligate extracellular endosymbionts that fill the entire body of their host. These marine worms are nutritionally dependent on their symbioticchemoautotrophic bacteria lacking any digestive or excretory system (no gut, mouth, ornephridia).[49]
The sea slugElysia chlorotica's endosymbiont is thealgaeVaucheria litorea. ThejellyfishMastigias 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.[50]
Trichoplax have two bacterial endosymbionts. Ruthmannia lives inside the animal's digestive cells. Grellia lives permanently inside theendoplasmic reticulum (ER), the first known symbiont to do so.[51]
Paracatenula is aflatworm 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.[52]
Previously thought to be a single species, molecularphylogenetic evidence reported diversity inSymbiodinium. In some cases, the host requires a specificSymbiodiniumclade. 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 ofcoral bleaching and recovery. Thus, the distribution ofSymbiodinium on coral reefs and its role in coral bleaching is an important in coral reef ecology.[53]
In marine environments,[54][55][56][57] endosymbiont relationships are especially prevalent inoligotrophic or nutrient-poor regions of the ocean like that of the North Atlantic.[54][58][55][56] In such waters, cell growth of largerphytoplankton such asdiatoms is limited by (insufficient)nitrate concentrations.[59] Endosymbiotic bacteria fix nitrogen for their hosts and in turn receive organic carbon from photosynthesis.[58] These symbioses play an important role in globalcarbon cycling.[60][55][56]
One known symbiosis between the diatomHemialus spp. and the cyanobacteriumRichelia intracellularis has been reported in North Atlantic, Mediterranean, and Pacific waters.[54][55][61]Richelia is found within thediatom frustule ofHemiaulus spp., and has a reduced genome.[62] A 2011 study measured nitrogen fixation by thecyanobacterial hostRichelia intracellularis well above intracellular requirements, and found the cyanobacterium was likely fixing nitrogen for its host.[59] Additionally, both host and symbiont cell growth were much greater than free-livingRichelia intracellularis or symbiont-freeHemiaulus spp.[59] TheHemaiulus-Richelia symbiosis is not obligatory, especially in nitrogen-replete areas.[54]
Richelia intracellularis is also found inRhizosolenia spp., a diatom found in oligotrophic oceans.[58][59][56] Compared to theHemaiulus host, the endosymbiosis withRhizosolenia is much more consistent, andRichelia intracellularis is generally found inRhizosolenia.[54] There are some asymbiotic (occurs without an endosymbiont) Rhizosolenia, however there appears to be mechanisms limiting growth of these organisms in low nutrient conditions.[63] 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.[63]
Other endosymbiosis with nitrogen fixers in open oceans includeCalothrix inChaetoceros spp. and UNCY-A inprymnesiophyte microalga.[64] TheChaetoceros-Calothrix endosymbiosis is hypothesized to be more recent, as theCalothrix genome is generally intact. While other species like that of the UNCY-A symbiont and Richelia have reduced genomes.[62] 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.[62] This endosymbiont reduction in genome size, might be a step that occurred in the evolution of organelles (above).[64]
Mixotricha paradoxa is aprotozoan that lacks mitochondria. However, spherical bacteria live inside the cell and serve the function of the mitochondria.Mixotricha has three other species of symbionts that live on the surface of the cell.[65]
Manyforaminifera are hosts to several types of algae, such asred algae,diatoms,dinoflagellates andchlorophyta.[71] These endosymbionts can be transmitted vertically to the next generation via asexual reproduction of the host, but because the endosymbionts are larger than the foraminiferalgametes, they need to acquire algae horizontally following sexual reproduction.[72]
Several species ofradiolaria have photosynthetic symbionts. In some species the host digests algae to keep the population at a constant level.[73]
Hatena arenicola is a flagellateprotist with a complicated feeding apparatus that feeds on other microbes. When it engulfs a greenNephroselmis alga, the feeding apparatus disappears and it becomes photosynthetic. Duringmitosis 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 ofAmoeba proteus had been infected by bacteria that lived inside the cytoplasmicvacuoles.[74] 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 theprokaryotes and protists occurred.[75][76][77]
All vascular plants harbor endosymbionts or endophytes in this context. They includebacteria,fungi,viruses,protozoa and evenmicroalgae. Endophytes aid in processes such as growth and development, nutrient uptake, and defense against biotic and abiotic stresses likedrought,salinity, heat, and herbivores.[79]
Plant symbionts can be categorized intoepiphytic,endophytic, andmycorrhizal. These relations can also be categorized as beneficial,mutualistic, neutral, andpathogenic.[80][81]Microorganisms living as endosymbionts in plants can enhance their host's primary productivity either by producing or capturing important resources.[82] These endosymbionts can also enhance plant productivity by producing toxic metabolites that aid plant defenses againstherbivores.[83][84]
Plants are dependent onplastid orchloroplast organelles. The chloroplast is derived from a cyanobacterial primary endosymbiosis that began over one billion years ago. An oxygenic, photosynthetic free-livingcyanobacterium was engulfed and kept by a heterotrophicprotist and eventually evolved into the present intracellular organelle.[85]
Typically, plant endosymbiosis studies focus on a single category or species to better understand their individual biological processes and functions.[86]
Fungal endophytes can be found in all plant tissues. Fungi living below the ground amidst plant roots are known asmycorrhiza, 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 extraradicalhyphae into the outerrhizosphere are known as ectendosymbionts.[87][88]
Arbuscular mycorrhizal fungi or AMF are the most diverse plant microbial endosymbionts. With exceptions such as theEricaceae family, almost all vascular plants harborAMF endosymbionts as endo and ecto as well. AMF plant endosymbionts systematically colonizeplant roots and help the plant host acquire soilnutrients such as nitrogen. In return it absorbs plant organic carbon products.[87]Plant root exudates contain diverse secondary metabolites, especiallyflavonoids andstrigolactones that act aschemical signals and attracts the AMF.[89] AMFGigaspora margarita lives as a plant endosymbiont and also harbors further endosymbiont intracytoplasmic bacterium-like organisms.[90] AMF generally promote plant health and growth and alleviateabiotic stresses such as salinity, drought, heat, poor nutrition, andmetal toxicity.[91] Individual AMF species have different effects in different hosts – introducing the AMF of one plant to another plant can reduce the latter's growth.[92]
Endophytic fungi inmutualistic 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.[93] Fungalendophytes are taxonomically diverse and are divided into categories based on mode of transmission,biodiversity, in planta colonization and host plant type.[94][95] Clavicipitaceous fungi systematically colonize temperate season grasses. Non-clavicipitaceous fungi colonize higher plants and even roots and divide into subcategories.[96]Aureobasidiumandpreussia species of endophytic fungi isolated fromBoswellia sacra produceindole acetic acidhormone to promote plant health and development.[97]
Endophytic bacteria belong to a diverse group of plant endosymbionts characterized by systematic colonization of plant tissues. The most common genera includePseudomonas,Bacillus,Acinetobacter,Actinobacteria,Sphingomonas. Some endophytic bacteria, such asBacillus amyloliquefaciens, a seed-born endophytic bacteria, produce plant growth by producinggibberellins, which are potent plant growth hormones.Bacillus amyloliquefaciens promotes the taller height oftransgenic dwarf rice plants.[98] Some endophytic bacteria genera additionally belong to theEnterobacteriaceae family.[99] Endophytic bacteria typically colonize the leaf tissues from plant roots, but can also enter the plant through the leaves through leafstomata.[100] Generally, the endophytic bacteria are isolated from the plant tissues by surfacesterilization of the plant tissue in a sterile environment.[101] Passenger endophytic bacteria eventually colonize inner tissue of plant bystochastic events while True endophytes possess adaptive traits because of which they live strictly in association with plants.[102] Thein vitro-cultivated endophyticbacteria 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.[103] This endosymbiotic relation is important in terms ofecology,evolution and diversity. Endophytic bacteria such asSphingomonas sp. andSerratia sp. that are isolated from arid land plants regulate endogenoushormone content and promote growth.[104]
Archaea are members of mostmicrobiomes. 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.[105] Most plant endosymbiosis studies focus on fungal or bacteria usingmetagenomic approaches.[106]
The characterization of archaea includes crop plants such asrice[107] andmaize, but also aquatic plants.[105] The abundance of archaea varies by tissue type; for example archaea are more abundant in therhizosphere than thephyllosphere andendosphere.[108] This archaeal abundance is associated with plant species type, environment and the plant's developmental stage.[109] In a study on plantgenotype-specific archaeal and bacterial endophytes, 35% of archaeal sequences were detected in overall sequences (achieved usingamplicon sequencing and verified byreal time-PCR). The archaeal sequences belong to the phylaThaumarchaeota,Crenarchaeota, andEuryarchaeota.[110]
Fungi host endohyphal bacteria;[112] the effects of the bacteria are not well studied. Many such fungi in turn live within plants.[112] These fungi are otherwise known as fungalendophytes. It is hypothesized that the fungi offers a safe haven for thebacteria, and the diverse bacteria that they attract create a micro-ecosystem.[113]
These interactions may impact the way that fungi interact with the environment by modulating theirphenotypes.[112] The bacteria do this by altering the fungi'sgene expression.[112] For example,Luteibacter sp. has been shown to naturally infect theascomycetousendophytePestalotiopsis sp. isolated fromPlatycladus orientalis.[112] TheLuteibacter sp. influences theauxin and enzyme production within its host, which, in turn, may influence the effect the fungus has on its plant host.[112] Another interesting example of a bacterium living in symbiosis with a fungus is the fungusMortierella. This soil-dwelling fungus lives in close association with a toxin-producing bacteria,Mycoavidus, which helps the fungus defend againstnematodes.[114]
In 2024, researchers injected individual cells of the bacteriumMycetohabitans rhizoxinica into cells of the fungusRhizopus microsporus and were able to propagate the pair of cells for ten rounds usingfluorescence-activated cell sorting to select fungal cells containing the bacterium. They found that the fungus's DNA changed during the rounds of propagation.[115] This was claimed to be the first time that endosymbiosis was artifically induced in a laboratory.[116]
List of symbiotic organisms – Close, long-term biological interaction between distinct organisms (usually species)Pages displaying short descriptions of redirect targets
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