Editing Endosymbiont (section)

==== Insects ====
[[File:Cospeciation (5 processes) - with key.png|thumb|right|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.<ref>{{cite book |vauthors=Baumann P, Moran NA, Baumann L |chapter=Bacteriocyte-associated endosymbionts of insects |veditors=Dworkin M |title=The prokaryotes |publisher=Springer |location=New York |date=2000 |chapter-url=http://link.springer.de/link/service/books/10125/ }}</ref>

===== Primary =====
Among primary endosymbionts of insects, the best-studied are the pea [[aphid]] (''[[Acyrthosiphon pisum]]'') and its endosymbiont ''[[Buchnera (proteobacteria)|Buchnera]] sp.'' APS,<ref>{{cite journal |vauthors=Douglas AE |title=Nutritional interactions in insect-microbial symbioses: aphids and their symbiotic bacteria Buchnera |journal=Annual Review of Entomology |volume=43 |pages=17–37 |date=January 1998 |pmid=15012383 |doi=10.1146/annurev.ento.43.1.17 |s2cid=29594533 }}</ref><ref name="pmid109930772"/> the [[tsetse fly]] ''Glossina morsitans morsitans'' and its endosymbiont ''[[Wigglesworthia glossinidia brevipalpis]]'' and the endosymbiotic [[protists]] in lower [[termite]]s. 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.<ref>{{Cite web |title=Figure 5—figure supplement 2. KEGG metabolic reconstructions based on the intact genes present in the Acromyrmex, Solenopsis, Apis mellifera and Anopheles gambiae genomes, together constituting the urea cycle. |url=https://elifesciences.org/articles/39209/figures#fig5s2 |doi=10.7554/elife.39209.022 |doi-access=free }}</ref>

In some insect groups, these endosymbionts live in specialized insect cells called [[bacteriocyte]]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 (biology)|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.<ref>{{Cite journal |last=Nalepa |first=Christine A. |date=2020 |title=Origin of Mutualism Between Termites and Flagellated Gut Protists: Transition From Horizontal to Vertical Transmission |journal=Frontiers in Ecology and Evolution |volume=8 |doi=10.3389/fevo.2020.00014 |issn=2296-701X |doi-access=free }}</ref>

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 acid]]s that the aphid cannot acquire from its diet of plant sap. The primary role of ''Wigglesworthia'' is to synthesize [[vitamin]]s 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 [[predator]]s 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 [[genome reduction|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 hat primary endosymbionts are transferred only vertically.<ref>{{cite journal |vauthors=Wernegreen JJ |title=Endosymbiosis: lessons in conflict resolution |journal=PLOS Biology |volume=2 |issue=3 |pages=E68 |date=March 2004 |pmid=15024418 |pmc=368163 |doi=10.1371/journal.pbio.0020068 |df=dmy |doi-access=free }}</ref><ref>{{cite journal |vauthors=Moran NA |title=Accelerated evolution and Muller's rachet in endosymbiotic bacteria |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=93 |issue=7 |pages=2873–2878 |date=April 1996 |pmid=8610134 |pmc=39726 |doi=10.1073/pnas.93.7.2873 |doi-access=free |bibcode=1996PNAS...93.2873M }}</ref>

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 [[African trypanosomiasis|sleeping sickness]].<ref>{{cite journal |vauthors=Aksoy S, Maudlin I, Dale C, Robinson AS, O'Neill SL |title=Prospects for control of African trypanosomiasis by tsetse vector manipulation |journal=Trends in Parasitology |volume=17 |issue=1 |pages=29–35 |date=January 2001 |pmid=11137738 |doi=10.1016/S1471-4922(00)01850-X }}</ref> 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.<ref>{{cite journal |display-authors=6 |vauthors=Klein A, Schrader L, Gil R, Manzano-Marín A, Flórez L, Wheeler D, Werren JH, Latorre A, Heinze J, Kaltenpoth M, Moya A, Oettler J |date=February 2016 |title=A novel intracellular mutualistic bacterium in the invasive ant Cardiocondyla obscurior |journal=The ISME Journal |volume=10 |issue=2 |pages=376–388 |bibcode=2016ISMEJ..10..376K |doi=10.1038/ismej.2015.119 |pmc=4737929 |pmid=26172209 |doi-access=free}}</ref>

===== Secondary =====
[[File:HEMI Aphididae Aphidius attacking pea aphid.png|thumb|right|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.<ref name="pmid18029301">{{cite journal |vauthors=Oliver KM, Campos J, Moran NA, Hunter MS |title=Population dynamics of defensive symbionts in aphids |journal=Proceedings. Biological Sciences |volume=275 |issue=1632 |pages=293–299 |date=February 2008 |pmid=18029301 |pmc=2593717 |doi=10.1098/rspb.2007.1192 }}</ref> This symbiosis replaces lost elements of the insect's immune response.<ref name="pmid20186266">{{cite journal |title=Genome sequence of the pea aphid Acyrthosiphon pisum |journal=PLOS Biology |volume=8 |issue=2 |pages=e1000313 |date=February 2010 |pmid=20186266 |pmc=2826372 |doi=10.1371/journal.pbio.1000313 |author1=International Aphid Genomics Consortium |doi-access=free }}</ref>

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.<ref>{{cite journal |vauthors=Jaenike J, Unckless R, Cockburn SN, Boelio LM, Perlman SJ |title=Adaptation via symbiosis: recent spread of a Drosophila defensive symbiont |journal=Science |volume=329 |issue=5988 |pages=212–215 |date=July 2010 |pmid=20616278 |doi=10.1126/science.1188235 |s2cid=206526012 |bibcode=2010Sci...329..212J }}</ref> This defence is mediated by toxins called "[[ribosome]]-inactivating [[proteins]]" that attack the molecular machinery of invading parasites.<ref>{{cite journal |vauthors=Hamilton PT, Peng F, Boulanger MJ, Perlman SJ |title=A ribosome-inactivating protein in a Drosophila defensive symbiont |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=113 |issue=2 |pages=350–355 |date=January 2016 |pmid=26712000 |pmc=4720295 |doi=10.1073/pnas.1518648113 |doi-access=free |bibcode=2016PNAS..113..350H }}</ref><ref>{{cite journal |vauthors=Ballinger MJ, Perlman SJ |title=Generality of toxins in defensive symbiosis: Ribosome-inactivating proteins and defense against parasitic wasps in Drosophila |journal=PLOS Pathogens |volume=13 |issue=7 |pages=e1006431 |date=July 2017 |pmid=28683136 |pmc=5500355 |doi=10.1371/journal.ppat.1006431 |doi-access=free }}</ref> 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.<ref name="Ballinger-2017">{{cite journal |vauthors=Ballinger MJ, Perlman SJ |date=July 2017 |title=Generality of toxins in defensive symbiosis: Ribosome-inactivating proteins and defense against parasitic wasps in Drosophila |journal=PLOS Pathogens |volume=13 |issue=7 |pages=e1006431 |doi=10.1371/journal.ppat.1006431 |pmc=5500355 |pmid=28683136 |doi-access=free }}</ref>

''[[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 (genus)|Sodalis]]'' and tsetse.<ref>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.</ref> Unlike ''Wigglesworthia,'' ''Sodalis'' has been cultured ''in vitro''.<ref name="pmid3662675">{{cite journal |vauthors=Welburn SC, Maudlin I, Ellis DS |title=In vitro cultivation of rickettsia-like-organisms from Glossina spp |journal=Annals of Tropical Medicine and Parasitology |volume=81 |issue=3 |pages=331–335 |date=June 1987 |pmid=3662675 |doi=10.1080/00034983.1987.11812127 }}</ref>

''[[Cardinium]]'' and many other insects have secondary endosymbionts.<ref name="pmid15189221">{{cite journal |vauthors=Zchori-Fein E, Perlman SJ |title=Distribution of the bacterial symbiont Cardinium in arthropods |journal=Molecular Ecology |volume=13 |issue=7 |pages=2009–2016 |date=July 2004 |pmid=15189221 |doi=10.1111/j.1365-294X.2004.02203.x |bibcode=2004MolEc..13.2009Z |s2cid=24361903 }}</ref><ref name="pmid12415315">{{cite journal |vauthors=Wernegreen JJ |title=Genome evolution in bacterial endosymbionts of insects |journal=Nature Reviews. Genetics |volume=3 |issue=11 |pages=850–861 |date=November 2002 |pmid=12415315 |doi=10.1038/nrg931 |s2cid=29136336 }}</ref>