This article needs additional citations for verification. (July 2018) (Learn how and when to remove this template message)
An endosymbiont or endobiont is any organism that lives within the body or cells of another organism in a mutualistic (formerly called symbiotic) relationship with the host body or cell, often but not always to mutual benefit. The term endosymbiosis is from the Greek: ἔνδον endon "within", σύν syn "together" and βίωσις biosis "living"). Examples are nitrogen-fixing bacteria (called rhizobia), which live in root nodules on legume roots, single-cell algae 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 of the genus Riftia, which get nutrition from their endosymbiotic bacteria. The most common examples of obligate endosymbioses are mitochondria and chloroplasts. Some human parasites, e.g. Wuchereria bancrofti and Mansonella perstans, thrive in their intermediate insect hosts because of an obligate endosymbiosis with Wolbachia spp. They can both be eliminated from said hosts by treatments that target this bacterium. However, not all endosymbioses are obligate and some endosymbioses can be harmful to either of the organisms involved.
In marine environments, bacterial endosymbionts have more recently been discovered. These endosymbiotic relationships are especially prevalent in oligotrophic or nutrient-poor regions of the ocean like that of the North Atlantic. In these oligotrophic waters, cell growth of larger phytoplankton like that of diatoms is limited by low nitrate concentrations. Endosymbiotic bacteria fix nitrogen for their diatom hosts and in turn receive organic carbon from photosynthesis. These symbioses play an important role in global carbon cycling in oligotrophic regions (See Microbial Loop).
One known symbiosis between Hemiaulus spp. and the cyanobacterium Richelia intracellularis has been found in the North Atlantic, Mediterranean, and Pacific Ocean. The Richelia endosymbiont is found within the diatom frustule of Hemiaulus spp., and has a reduced genome likely losing genes related to pathways the host now provides. Research by Foster et al. (2011) measured nitrogen fixation by the cyanobacterial host Richelia intracellularis well above intracellular requirements, and found the cyanobacterium was likely fixing excess nitrogen for Hemiaulus host cells. Additionally, both host and symbiont cell growth were much greater than free-living Richelia intracellularis or symbiont-free Hemiaulus spp. The Hemaiulus-Richelia symbiosis is not obligatory especially in areas with excess nitrogen (nitrogen replete).
Richelia intracellularis is also found in Rhizosolenia spp., a diatom found in oligotrophic oceans. Compared to the Hemaiulus host, the endosymbiosis with Rhizosolenia is much more consistent, and Richelia intracellularis is generally found in Rhizosolenia. There are some asymbiotic (occurs without an endosymbiont) Rhizosolenia, however there appears to be mechanisms limiting growth of these organisms in low nutrient conditions. 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.
Other endosymbiosis with nitrogen fixers in open oceans include Calothrix in Chaetocerous spp. and UNCY-A in prymnesiophyte microalga. The Chaetocerous-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. 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. This endosymbiont reduction in genome size, might be a step that occurred in the evolution of organelles (above).
In marine invertebrates
Extracellular endosymbionts are also represented in all four extant classes of Echinodermata (Crinoidea, Ophiuroidea, Echinoidea, and Holothuroidea). Little is known of the nature of the association (mode of infection, transmission, metabolic requirements, etc.) but phylogenetic analysis indicates that these symbionts belong to the alpha group of the class Proteobacteria, relating them to Rhizobium and Thiobacillus. Other studies indicate that these subcuticular bacteria may be both abundant within their hosts and widely distributed among the Echinoderms in general.
Some marine oligochaeta (e.g., Olavius and 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).
Mixotricha paradoxa 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.
Scientists classify insect endosymbionts in two broad categories, 'Primary' and 'Secondary'. Primary endosymbionts (sometimes referred to as P-endosymbionts) have been associated with their insect hosts for many millions of years (from 10 to several hundred million years in some cases). They form obligate associations (see below), and display cospeciation with their insect hosts. Secondary endosymbionts exhibit a more recently developed association, are sometimes horizontally transferred between hosts, live in the hemolymph of the insects (not specialized bacteriocytes, see below), and are not obligate.
Among primary endosymbionts of insects, the best-studied are the pea aphid (Acyrthosiphon pisum) and its endosymbiont Buchnera sp. APS, the tsetse fly Glossina morsitans morsitans and its endosymbiont Wigglesworthia glossinidia brevipalpis and the endosymbiotic protists in lower termites. As with endosymbiosis in other insects, the symbiosis is obligate in that neither the bacteria nor the insect is viable without the other. Scientists have been unable to cultivate the bacteria in lab conditions outside of the insect. With special nutritionally-enhanced diets, the insects can survive, but are unhealthy, and at best survive only a few generations.
In some insect groups, these endosymbionts live in specialized insect cells called bacteriocytes (also called mycetocytes), and are maternally-transmitted, i.e. the mother transmits her endosymbionts to her offspring. In some cases, the bacteria are transmitted in the egg, as in Buchnera; in others like Wigglesworthia, they are transmitted via milk to the developing insect embryo. In termites, the endosymbionts reside within the hindguts and are transmitted through trophallaxis among colony members.
The primary endosymbionts are thought to help the host either by providing nutrients that the host cannot obtain itself or by metabolizing insect waste products into safer forms. For example, the putative primary role of Buchnera is to synthesize essential amino acids that the aphid cannot acquire from its natural diet of plant sap. Likewise, the primary role of Wigglesworthia, it is presumed, is to synthesize vitamins that the tsetse fly does not get from the blood that it eats. In lower termites, the endosymbiotic protists play a major role in the digestion of lignocellulosic materials that constitute a bulk of the termites' diet.
Bacteria benefit from the reduced exposure to predators and competition from other bacterial species, the ample supply of nutrients and relative environmental stability inside the host.
Genome sequencing reveals that obligate bacterial endosymbionts of insects have among the smallest of known bacterial genomes and have lost many genes that are commonly found in closely related bacteria. Several theories have been put forth to explain the loss of genes. It is presumed that some of these genes are not needed in the environment of the host insect cell. A complementary theory suggests that the relatively small numbers of bacteria inside each insect decrease the efficiency of natural selection in 'purging' deleterious mutations and small mutations from the population, resulting in a loss of genes over many millions of years. Research in which a parallel phylogeny of bacteria and insects was inferred supports the belief that the primary endosymbionts are transferred only vertically (i.e., from the mother), and not horizontally (i.e., by escaping the host and entering a new host).
Attacking obligate bacterial endosymbionts may present a way to control their insect hosts, many of which pests or carriers of human disease. For example, aphids are crop pests and the tsetse fly carries the organism Trypanosoma brucei that causes African sleeping sickness. Other motivations for their study is to understand symbiosis, and to understand how bacteria with severely depleted genomes are able to survive, thus improving our knowledge of genetics and molecular biology.
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 of tsetse flies that lives inter- and intracellularly in various host tissues, including the midgut and hemolymph. Phylogenetic studies have not indicated a correlation between evolution of Sodalis and tsetse. Unlike tsetse's P-symbiont Wigglesworthia, though, Sodalis has been cultured in vitro.
Dinoflagellate endosymbionts of the genus Symbiodinium, commonly known as zooxanthellae, are found in corals, mollusks (esp. giant clams, the Tridacna), sponges, and foraminifera. These endosymbionts drive the formation of coral reefs by capturing sunlight and providing their hosts with energy for carbonate deposition.
Previously thought to be a single species, molecular phylogenetic evidence over the past couple decades has shown there to be great diversity in Symbiodinium. In some cases, there is specificity between host and Symbiodinium clade. More often, however, there is an ecological distribution of Symbiodinium, the symbionts switching between hosts with apparent ease. When reefs become environmentally stressed, this distribution of symbionts is related to the observed pattern of coral bleaching and recovery. Thus, the distribution of Symbiodinium on coral reefs and its role in coral bleaching presents one of the most complex and interesting current problems in reef ecology.
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.
Symbiogenesis and organelles
Symbiogenesis explains the origins of eukaryotes, whose cells contain two major kinds of organelle, mitochondria and chloroplasts. The theory proposes that these organelles evolved from certain types of bacteria that eukaryotic cells engulfed through endophagocytosis. These cells and the bacteria trapped inside them entered an endosymbiotic relationship, meaning that the bacteria lived within the eukaryotic cells.
- Margulis, Lynn; Chapman, Michael J. (2009). Kingdoms & domains an illustrated guide to the phyla of life on Earth (4th ed.). Amsterdam: Academic Press/Elsevier. p. 493. ISBN 978-0-08-092014-6. Retrieved 2 August 2016.
- Villareal, Tracy (1994). "Widespread occurrence of the Hemiaulus-cyanobacterial symbiosis in the southwest North Atlantic Ocean". Bulletin of Marine Science. 54: 1–7.
- Carpenter, Edward J.; Montoya, Joseph P.; Burns, James; Mulholland, Margaret R.; Subramaniam, Ajit; Capone, Douglas G. (1999-08-20). "Extensive bloom of a N2-fixing diatom/cyanobacterial association in the tropical Atlantic Ocean". Marine Ecology Progress Series. 185: 273–283. doi:10.3354/meps185273.
- Foster, R. A.; Subramaniam, A.; Mahaffey, C.; Carpenter, E. J.; Capone, D. G.; Zehr, J. P. (March 2007). "Influence of the Amazon River plume on distributions of free-living and symbiotic cyanobacteria in the western tropical north Atlantic Ocean". Limnology and Oceanography. 52 (2): 517–532. doi:10.4319/lo.2007.52.2.0517.
- Subramaniam, A.; Yager, P. L.; Carpenter, E. J.; Mahaffey, C.; Björkman, K.; Cooley, S.; Kustka, A. B.; Montoya, J. P.; Sañudo-Wilhelmy, S. A. (2008-07-29). "Amazon River enhances diazotrophy and carbon sequestration in the tropical North Atlantic Ocean". Proceedings of the National Academy of Sciences. 105 (30): 10460–1046 5. doi:10.1073/pnas.0710279105. PMC 2480616. PMID 18647838.
- Goebel, Nicole L.; Turk, Kendra A.; Achilles, Katherine M.; Paerl, Ryan; Hewson, Ian; Morrison, Amanda E.; Montoya, Joseph P.; Edwards, Christopher A.; Zehr, Jonathan P. (December 2010). "Abundance and distribution of major groups of diazotrophic cyanobacteria and their potential contribution to N2 fixation in the tropical Atlantic Ocean". Environmental Microbiology. 12 (12): 3272–3289. doi:10.1111/j.1462-2920.2010.02303.x.
- Foster, Rachel A; Kuypers, Marcel M M; Vagner, Tomas; Paerl, Ryan W.; Musat, Niculina; Zehr, Jonathan P (2011-03-31). "Nitrogen fixation and transfer in open ocean diatom–cyanobacterial symbioses". The ISME Journal. 5 (9): 1484–1493. doi:10.1038/ismej.2011.26. PMC 3160684. PMID 21451586.
- Scharek, Renate; Tupas, Luis M.; Karl, David M. (1999-06-11). "Diatom fluxes to the deep sea in the oligotrophic North Pacific gyre at Station ALOHA". Marine Ecology Progress Series. 182: 55–67. doi:10.3354/meps182055.
- Zeev, Edo Bar; Yogev, Tali; Man-Aharonovich, Dikla; Kress, Nurit; Herut, Barak; Béjà, Oded; Berman-Frank, Ilana (2008-06-26). "Seasonal dynamics of the endosymbiotic, nitrogen-fixing cyanobacterium Richelia intracellularis in the eastern Mediterranean Sea". The ISME Journal. 2 (9): 911–923. doi:10.1038/ismej.2008.56.
- Hilton, Jason A.; Foster, Rachel A.; James Tripp, H.; Carter, Brandon J.; Zehr, Jonathan P.; Villareal, Tracy A. (2013-04-23). "Genomic deletions disrupt nitrogen metabolism pathways of a cyanobacterial diatom symbiont". Nature Communications. 4 (1). doi:10.1038/ncomms2748. PMC 3667715. PMID 23612308.
- Villareal, Tracy A. (December 1989). "Division cycles in the nitrogen-fixingRhizosolenia(Bacillariophyceae)-Richelia(Nostocaceae) symbiosis". British Phycological Journal. 24 (4): 357–365. doi:10.1080/00071618900650371.
- Zehr, Jonathan P. (2015-09-11). "How single cells work together". Science. 349 (6253): 1163–1164. doi:10.1126/science.aac9752. PMID 26359387.
- Burnett WJ, McKenzie JD (1 May 1997). "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". Appl. Environ. Microbiol. 63 (5): 1721–4. PMC 168468. PMID 9143108.
- Dubilier, N.; Mülders, C.; Ferdelman, T.; et al. (May 2001). "Endosymbiotic sulphate-reducing and sulphide-oxidizing bacteria in an oligochaete worm". Nature. 411 (6835): 298–302. doi:10.1038/35077067. PMID 11357130.
- 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/
- Douglas, A E (1998). "Nutritional interactions in insect-microbial symbioses: Aphids and their symbiotic bacteria Buchnera". Annual Review of Entomology. 43: 17–38. doi:10.1146/annurev.ento.43.1.17. ISSN 0066-4170. PMID 15012383.
- Douglas AE (January 1998). "Nutritional interactions in insect-microbial symbioses: Aphids and Their Symbiotic Bacteria Buchnera". Annual Review of Entomology. 43: 17–37. doi:10.1146/annurev.ento.43.1.17. PMID 15012383.
- Shigenobu S, Watanabe H, Hattori M, Sakaki Y, Ishikawa H (September 2000). "Genome sequence of the endocellular bacterial symbiont of aphids Buchnera sp. APS". Nature. 407 (6800): 81–6. doi:10.1038/35024074. PMID 10993077.
- Wernegreen JJ (2004). "Primer – Endosymbiosis: Lessons in Conflict Resolution". PLoS Biol. 2 (3): e68. doi:10.1371/journal.pbio.0020068. PMC 368163. PMID 15024418. Archived from the original on 14 April 2004.
- Wernegreen JJ (November 2002). "Genome evolution in bacterial endosymbionts of insects". Nat. Rev. Genet. 3 (11): 850–61. doi:10.1038/nrg931. PMID 12415315.
- Moran NA (April 1996). "Accelerated evolution and Muller's rachet in endosymbiotic bacteria". PNAS. 93 (7): 2873–8. doi:10.1073/pnas.93.7.2873. PMC 39726. PMID 8610134.
- Aksoy S, Maudlin I, Dale C, Robinson AS, O'Neill SL (January 2001). "Prospects for control of African trypanosomiasis by tsetse vector manipulation". Trends Parasitol. 17 (1): 29–35. doi:10.1016/S1471-4922(00)01850-X. PMID 11137738.
- 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.
- Welburn, S.C., Maudlin, I. & Ellis, D.S. 1987. In vitro cultivation of rickettsia-like-organisms from Glossina spp. Ann. Trop. Med. Parasitol. 81, 331-335.
- Baker AC (November 2003). "FLEXIBILITY AND SPECIFICITY IN CORAL-ALGAL SYMBIOSIS: Diversity, Ecology, and Biogeography of Symbiodinium". Annual Review of Ecology, Evolution, and Systematics. 34: 661–89. doi:10.1146/annurev.ecolsys.34.011802.132417.
- Villarreal LP (October 2001). "Persisting Viruses Could Play Role in Driving Host Evolution". ASM News. American Society for Microbiology. Archived from the original on 8 May 2009.
- Sagan, Lynn (1967). "On the origin of mitosing cells". J Theor Biol. 14 (3): 255–274. doi:10.1016/0022-5193(67)90079-3. PMID 11541392.