Endosymbiont

Source: Wikipedia, the free encyclopedia.

endosymbiotic theory

An endosymbiont or endobiont

nitrogen-fixing bacteria (called rhizobia), which live in the root nodules of legumes, single-cell algae inside reef-building corals and bacterial endosymbionts that provide essential nutrients to insects.[2][3]

Endosymbiosis played key roles in the development of

organelles that produce energy from sunlight.[4] Some 100 million years ago, UCYN-A, a nitrogen-fixing bacteria became an endosymbiont of marine algae Braarudosphaera bigelowii, eventually evolving into a nitroplast.[5] Similarly, Diatoms in the family Rhopalodiaceae have evolved a diazoplast, a nitrogen-fixing organelle.[6]

Symbionts are either obligate (require their host to survive) or facultative (can survive independently).

mitochondria and chloroplasts, which reproduce via mitosis in tandem with their host cells. Some human parasites, e.g. Wuchereria bancrofti and Mansonella perstans, thrive in their intermediate insect hosts because of an obligate endosymbiosis with Wolbachia spp.[8] They can both be eliminated by treatments that target their bacterial host.[9]

Etymology

Endosymbiosis comes from the Greek: ἔνδον endon "within", σύν syn "together" and βίωσις biosis "living".

Symbiogenesis

prokaryotes. Typically, one organism envelopes a bacteria and the two evolve a mutualistic relationship. The absorbed bacteria (the endosymbiont) eventually lives exclusively within the host cells. This fits the concept of observed organelle development.[10][11][12][13][14]

Typically the endosymbiont's genome shrinks, discarding genes whose roles are displaced by the host.

cicadas is much different from the prior freestanding bacteria. The cicada life cycle involves years of stasis underground. The symbiont produces many generations during this phase, experiencing little selection pressure, allowing their genomes to diversify. Selection is episodic (when the cicadas reproduce). The original Hodgkinia genome split into three much simpler endosymbionts, each encoding only a few genes—an instance of punctuated equilibrium producing distinct lineages. The host requires all three symbionts.[16]

Transmission

Main articles: Horizontal transmission and Vertical transmission

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

Horizontal symbiont transfer (horizontal transmission) is a process where a host acquires a facultative symbiont from the environment or another host.[7] The Rhizobia-Legume symbiosis (bacteria-plant endosymbiosis) is a prime example of this modality.[17] 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 its Nod genes.[17] These Nod genes generate lipooligosaccharide signals that the legume detects, leading to root nodule formation.[18] This process bleeds into other processes such as nitrogen fixation in plants.[17] 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).[19]

Vertical

Vertical transmission takes place when the symbiont moves directly from parent to offspring.[20][21] In horizontal transmission each generation acquires symbionts from the environment. An example is nitrogen-fixing bacteria in certain plant roots, such as pea aphid symbionts. A third type is mixed-mode transmission, where symbionts move horizontally for some generations, after which they are acquired vertically.[22][23][24]

chloroplasts. Such dependent hosts and symbionts form a holobiont. In the event of a bottleneck, a decrease in symbiont diversity could compromise host-symbiont interactions, as deleterious mutations accumulate.[26]

Hosts

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.[27]

Insects

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 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.[28]

Primary

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

protists in lower termites. 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.[citation needed
]

In some insect groups, these endosymbionts live in specialized insect cells called

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.[30]

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

phylogeny of bacteria and insects was inferred supports the belief that primary endosymbionts are transferred only vertically.[31][32]

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.[33] 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

Camponotus ants. In 2018 a new ant-associated symbiont, Candidatus Westeberhardia Cardiocondylae, was discovered in Cardiocondyla. It is reported to be a primary symbiont.[34]

Secondary
Pea aphids are commonly infested by parasitic wasps. Their secondary endosymbionts attack the infesting parasitoid wasp larvae promoting the survival of both the aphid host and its endosymbionts.

The pea aphid (Acyrthosiphon pisum) contains at least three secondary endosymbionts, Hamiltonella defensa, Regiella insecticola, and Serratia symbiotica. Hamiltonella defensa defends its aphid host from parasitoid wasps.[35] This symbiosis replaces lost elements of the insect's immune response.[36]

One of the best-understood defensive symbionts is the spiral bacteria

proteins" that attack the molecular machinery of invading parasites.[38][39] 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.[40]

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.[41] Unlike Wigglesworthia, Sodalis has been cultured in vitro.[42]

Cardinium and many other insects have secondary endosymbionts.[43][15]

Marine

Extracellular endosymbionts are represented in all four extant classes of

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.[44]

Some marine

nephridia).[45]

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.[46]

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.[47]

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.[48]

Dinoflagellates

sponges, and the unicellular foraminifera. These endosymbionts capture sunlight and provide their hosts with energy via carbonate deposition.[49]

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.[49]

Phytoplankton

Further information: Microbial loop

In marine environments,[50][51][52][53] endosymbiont relationships are especially prevalent in oligotrophic or nutrient-poor regions of the ocean like that of the North Atlantic.[50][54][51][52] In such waters, cell growth of larger phytoplankton such as diatoms is limited by (insufficient) nitrate concentrations.[55]  Endosymbiotic bacteria fix nitrogen for their hosts and in turn receive organic carbon from photosynthesis.[54] These symbioses play an important role in global carbon cycling.[56][51][52]

One known symbiosis between the diatom

diatom frustule of Hemiaulus spp., and has a reduced genome.[58] 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.[55] Additionally, both host and symbiont cell growth were much greater than free-living Richelia intracellularis or symbiont-free Hemiaulus spp.[55] The Hemaiulus-Richelia symbiosis is not obligatory, especially in nitrogen-replete areas.[50]

Richelia intracellularis is also found in Rhizosolenia spp., a diatom found in oligotrophic oceans.[54][55][52] Compared to the Hemaiulus host, the endosymbiosis with Rhizosolenia is much more consistent, and Richelia intracellularis is generally found in Rhizosolenia.[50] There are some asymbiotic (occurs without an endosymbiont) Rhizosolenia, however there appears to be mechanisms limiting growth of these organisms in low nutrient conditions.[59] 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.[59]

Other endosymbiosis with nitrogen fixers in open oceans include

prymnesiophyte microalga.[60]  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.[58] 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.[58] This endosymbiont reduction in genome size, might be a step that occurred in the evolution of organelles (above).[60]

Protists

protozoan 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.[61]

Paramecium bursaria, a species of ciliate, has a mutualistic symbiotic relationship with green alga called Zoochlorella. The algae live in its cytoplasm.[62]

Platyophrya chlorelligera is a freshwater ciliate that harbors Chlorella that perform photosynthesis.[63][64]

Strombidium purpureum is a marine ciliate that uses endosymbiotic, purple, non-sulphur bacteria for anoxygenic photosynthesis.[65][66]

cyanobacterium
endosymbiont.

Many foraminifera are hosts to several types of algae, such as red algae, diatoms, dinoflagellates and chlorophyta.[67] 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.[68]

Several species of radiolaria have photosynthetic symbionts. In some species the host digests algae to keep the population at a constant level.[69]

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

prokaryotes and protists occurred.[71][72][73]

Vertebrates

The spotted salamander (

Oophila amblystomatis, which grows in its egg cases.[74]

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.[75]

Plant symbionts can be categorized into epiphytic, endophytic, and mycorrhizal. These relations can also be categorized as beneficial, mutualistic, neutral, and pathogenic.[76][77] Microorganisms living as endosymbionts in plants can enhance their host's primary productivity either by producing or capturing important resources.[78] These endosymbionts can also enhance plant productivity by producing toxic metabolites that aid plant defenses against herbivores.[79][80]

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.[81]  

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.[82]

Fungal endophytes

Fungal endophytes can be found in all plant tissues. Fungi living below the ground amidst plant roots are known as

hyphae into the outer rhizosphere are known as ectendosymbionts.[83][84]

Arbuscular Mycorrhizal Fungi (AMF)

flavonoids and strigolactones that act as chemical signals and attracts the AMF.[85] AMF Gigaspora margarita lives as a plant endosymbiont and also harbors further endosymbiont intracytoplasmic bacterium-like organisms.[86] AMF generally promote plant health and growth and alleviate abiotic stresses such as salinity, drought, heat, poor nutrition, and metal toxicity.[87] Individual AMF species have different effects in different hosts – introducing the AMF of one plant to another plant can reduce the latter's growth.[88]

Endophytic fungi

Endophytic fungi in

transgenic dwarf rice plants.[93] Similarly, Aureobasidium and preussia species of endophytic fungi isolated from Boswellia sacra produce indole acetic acid hormone to promote plant health and development.[94]

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.[78]

Endophytic bacteria

Endophytic bacteria belong to a diverse group of plant endosymbionts characterized by systematic colonization of plant tissues. The most common genera include

stomata.[96] Generally, the endophytic bacteria are isolated from the plant tissues by surface sterilization of the plant tissue in a sterile environment.[97] 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.[98] 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.[99] 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.[100]

Archaea endosymbionts

metagenomic approaches.[102]

The characterization of archaea includes crop plants such as

Crenarchaeota, and Euryarchaeota.[106]

Bacteria

Some Betaproteobacteria have Gammaproteobacteria endosymbionts.[107]

Fungi

Fungi host endohyphal bacteria;[108] the effects of the bacteria are not well studied. Many such fungi in turn live within plants.[108] These fungi are otherwise known as fungal endophytes. It is hypothesized that the fungi offers a safe haven for the bacteria, and the diverse bacteria that they attract create a micro-ecosystem.[109]

These interactions may impact the way that fungi interact with the environment by modulating their

phenotypes.[108] The bacteria do this by altering the fungi's gene expression.[108] For example, Luteibacter sp. has been shown to naturally infect the ascomycetous endophyte Pestalotiopsis sp. isolated from Platycladus orientalis.[108] The Luteibacter sp. influences the auxin and enzyme production within its host, which, in turn, may influence the effect the fungus has on its plant host.[108] Another interesting example of a bacteria living in symbiosis with a fungus is the fungus Mortierella. This soil-dwelling fungus lives in close association with a toxin-producing bacteria, Mycoavidus, which helps the fungus defend against nematodes.[110]

Virus endosymbionts

Main article: Endogenous retrovirus

The

human genome project found several thousand endogenous retroviruses, endogenous viral elements in the genome that closely resemble and can be derived from retroviruses, organized into 24 families.[111][citation needed][112]

See also

References

  1. ISBN 978-0-08-092014-6
    .
  2. PMID 29393944
    .
  3. S2CID 10050417
    .
  4. ^ Baisas, Laura (18 April 2024). "For the first time in one billion years, two lifeforms truly merged into one organism". Popular Science. Retrieved 26 April 2024.
  5. ^ Wong, Carissa (11 April 2024). "Scientists discover first algae that can fix nitrogen — thanks to a tiny cell structure". Nature.com. Archived from the original on 14 April 2024. Retrieved 16 April 2024.
  6. PMID 29026213
    . Retrieved 25 April 2024.
  7. ^
    PMID 20157340
    .
  8. PMID 20730111
    .
  9. ISBN 978-0-19-965213-6
    .
  10. PMID 31354692
    .
  11. S2CID 235786110
    .
  12. doi:10.1146/knowable-060822-2
    . Retrieved 18 August 2022.
  13. PMID 11541392
    .
  14. S2CID 236916203
    .
  15. ^
    S2CID 29136336
    .
  16. PMID 29129532
    .
  17. ^
    PMID 15187185
    .
  18. ^
    PMID 10993077
    .
  19. PMID 29234308
    .
  20. S2CID 235786110
    .
  21. doi:10.1146/knowable-060822-2
    . Retrieved 18 August 2022.
  22. PMID 34630349
    .
  23. ISSN 1543-592X
    . Retrieved 19 August 2022.
  24. ^
    PMID 20157340
    .
  25. PMID 20157340
    .
  26. S2CID 2015074
    .
  27. PMID 23508299
    .
  28. ^ Baumann P, Moran NA, Baumann L (2000). "Bacteriocyte-associated endosymbionts of insects". In Dworkin M (ed.). The prokaryotes. New York: Springer.
  29. S2CID 29594533
    .
  30. ISSN 2296-701X
    .
  31. PMID 15024418
    .
  32. PMID 8610134
    .
  33. PMID 11137738
    .
  34. PMID 26172209
    .
  35. PMID 18029301
    .
  36. PMID 20186266
    .
  37. S2CID 206526012
    .
  38. PMID 26712000
    .
  39. PMID 28683136
    .
  40. PMID 28683136
    .
  41. ^ Aksoy, S., Pourhosseini, A. & Chow, A. 1995. Mycetome endosymbionts of tsetse flies constitute a distinct lineage related to Enterobacteriaceae. Insect Mol Biol. 4, 15–22.
  42. PMID 3662675
    .
  43. S2CID 24361903
    .
  44. PMID 9143108
    .
  45. S2CID 4420931
    .
  46. PMID 8901581
    .
  47. ^ Society, Max Planck. "Deceptively simple: Minute marine animals live in a sophisticated symbiosis with bacteria". phys.org.
  48. ^ Society, Max Planck. "How a bacterium feeds an entire flatworm". phys.org.
  49. ^
    S2CID 35278104
    .
  50. ^ a b c d e Villareal T (1994). "Widespread occurrence of the Hemiaulus-cyanobacterial symbiosis in the southwest North Atlantic Ocean". Bulletin of Marine Science. 54: 1–7.
  51. ^
    hdl:1853/43100
    .
  52. ^
    S2CID 53504106
    .
  53. PMID 18647838
    .
  54. ^
    PMID 20678117
    .
  55. ^
    PMID 21451586
    .
  56. hdl:10261/184131
    .
  57. PMID 18580972
    .
  58. ^
    PMID 23612308
    .
  59. ^
    doi:10.1080/00071618900650371
    .
  60. ^
    S2CID 206641230
    .
  61. ISSN 0932-4739
    .
  62. PMID 22891065
    .
  63. ISBN 978-3-642-79923-5
    .
  64. PMID 23196282
    .
  65. S2CID 86458030
    .
  66. ISBN 978-0-19-802788-1
    .
  67. ISBN 978-94-007-1327-7
    .
  68. S2CID 89705815
    .
  69. ISBN 978-0-19-511807-0
    .
  70. S2CID 32044949
    .
  71. ^ "Kwang W. Jeon | Biochemistry & Cellular and Molecular Biology – UTK BCMB". 28 April 2014. Archived from the original on 31 August 2018. Retrieved 14 May 2019.
  72. ISBN 978-1-118-98287-7
    .
  73. ^ K. Jeon, “Amoeba and X-bacteria: Symbiont Acquisition and Possible Species Change,” in: L. Margulis and R. Fester, eds., Symbiosis as a Source of Evolutionary Innovation (Cambridge, Mass.: MIT Press), c. 9.
  74. PMID 21464324
    .
  75. PMID 35186412
    .
  76. PMID 26136581
    .
  77. PMID 30498482
    .
  78. ^
    PMID 16720406
    .
  79. PMID 15377223
    .
  80. S2CID 54005488
    .
  81. PMID 24065973
    .
  82. PMID 19400639
    .
  83. ^
    S2CID 250548548
    .
  84. PMID 28601651
    .
  85. S2CID 206518892
    .
  86. PMID 8702293
    .
  87. PMID 31608075
    .
  88. PMID 17503581
    .
  89. PMID 30930857
    .
  90. PMID 19236579
    .
  91. S2CID 43678079
    .
  92. ISBN 9780429116407
    .
  93. PMID 27182958
    .
  94. PMID 27359330
    .
  95. ISBN 978-94-007-1599-8
    .
  96. ^ Senthilkumar et al., 2011
  97. doi:10.1139/m97-081
    .
  98. PMID 18789693
    .
  99. PMID 17537638
    .
  100. S2CID 90203067
    .
  101. ^
    PMID 33005311
    .
  102. S2CID 155746848
    .
  103. S2CID 51727014
    .
  104. PMID 22189496
    .
  105. PMID 28826642
    .
  106. PMID 25784898
    .
  107. ^ Von Dohlen, Carol D., Shawn Kohler, Skylar T. Alsop, and William R. McManus. "Mealybug β-proteobacterial endosymbionts contain γ-proteobacterial symbionts." Nature 412, no. 6845 (2001): 433-436.
  108. ^
    PMID 35293790
    .
  109. S2CID 248089525
    .
  110. PMID 34504005
    .
  111. ^ Villarreal LP (October 2001). "Persisting Viruses Could Play Role in Driving Host Evolution". ASM News. Archived from the original on 8 May 2009.
  112. PMID 15044706
    .
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