Editing Endosymbiont (section)

== Transmission ==
{{Main|Horizontal transmission|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.<ref name="Bright-2010" /> The Rhizobia-Legume symbiosis (bacteria-plant endosymbiosis) is a prime example of this modality.<ref name="Gage-2004" /> 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.<ref name="Gage-2004">{{Cite journal |last=Gage |first=Daniel J. |date=June 2004 |title=Infection and Invasion of Roots by Symbiotic, Nitrogen-Fixing Rhizobia during Nodulation of Temperate Legumes |journal=Microbiology and Molecular Biology Reviews |language=en |volume=68 |issue=2 |pages=280–300 |doi=10.1128/MMBR.68.2.280-300.2004 |issn=1092-2172 |pmc=419923 |pmid=15187185}}</ref> These ''Nod'' genes generate [[Lipopolysaccharide|lipooligosaccharide]] signals that the legume detects, leading to root nodule formation.<ref name="pmid109930772">{{cite journal |vauthors=Shigenobu S, Watanabe H, Hattori M, Sakaki Y, Ishikawa H |date=September 2000 |title=Genome sequence of the endocellular bacterial symbiont of aphids Buchnera sp. APS |journal=Nature |volume=407 |issue=6800 |pages=81–86 |bibcode=2000Natur.407...81S |doi=10.1038/35024074 |pmid=10993077 |doi-access=free}}</ref> This process bleeds into other processes such as nitrogen fixation in plants.<ref name="Gage-2004" /> 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).<ref>{{Cite journal |last1=Chrostek |first1=Ewa |last2=Pelz-Stelinski |first2=Kirsten |last3=Hurst |first3=Gregory D. D. |last4=Hughes |first4=Grant L. |date=2017 |title=Horizontal Transmission of Intracellular Insect Symbionts via Plants |journal=Frontiers in Microbiology |volume=8 |page=2237 |doi=10.3389/fmicb.2017.02237 |issn=1664-302X |pmc=5712413 |pmid=29234308 |doi-access=free }}</ref>

=== Vertical ===
Vertical transmission takes place when the symbiont moves directly from parent to offspring.<ref name="McCutcheon2">{{cite journal |vauthors=McCutcheon JP |date=October 2021 |title=The Genomics and Cell Biology of Host-Beneficial Intracellular Infections |journal=Annual Review of Cell and Developmental Biology |volume=37 |issue=1 |pages=115–142 |doi=10.1146/annurev-cellbio-120219-024122 |pmid=34242059 |s2cid=235786110 |doi-access=free}}</ref><ref name="Callier2">{{cite journal |vauthors=Callier V |date=8 June 2022 |title=Mitochondria and the origin of eukaryotes |url=https://knowablemagazine.org/article/living-world/2022/mitochondria-origin-eukaryotes |journal=Knowable Magazine |doi=10.1146/knowable-060822-2 |access-date=18 August 2022 |doi-access=free}}</ref> In horizontal transmission each generation acquires symbionts from the environment. An example is nitrogen-fixing bacteria in certain plant roots, such as [[Acyrthosiphon pisum|pea aphid]] symbionts. A third type is mixed-mode transmission, where symbionts move horizontally for some generations, after which they are acquired vertically.<ref name="Wierz2">{{cite journal |vauthors=Wierz JC, Gaube P, Klebsch D, Kaltenpoth M, Flórez LV |date=2021 |title=Transmission of Bacterial Symbionts With and Without Genome Erosion Between a Beetle Host and the Plant Environment |journal=Frontiers in Microbiology |volume=12 |pages=715601 |doi=10.3389/fmicb.2021.715601 |pmc=8493222 |pmid=34630349 |doi-access=free}}</ref><ref name="Ebert2">{{cite journal |vauthors=Ebert D |date=23 November 2013 |title=The Epidemiology and Evolution of Symbionts with Mixed-Mode Transmission |url=https://doi.org/10.1146/annurev-ecolsys-032513-100555 |journal=Annual Review of Ecology, Evolution, and Systematics |language=en |volume=44 |issue=1 |pages=623–643 |doi=10.1146/annurev-ecolsys-032513-100555 |issn=1543-592X |access-date=19 August 2022|url-access=subscription }}</ref><ref name="pmid201573403">{{cite journal |vauthors=Bright M, Bulgheresi S |date=March 2010 |title=A complex journey: transmission of microbial symbionts |journal=Nature Reviews. Microbiology |volume=8 |issue=3 |pages=218–230 |doi=10.1038/nrmicro2262 |pmc=2967712 |pmid=20157340}}</ref>

''[[Wigglesworthia]],'' a tsetse fly symbiont,<ref name="pmid201573403" /> is vertically transmitted (via mother's milk).<ref name="pmid201573403"/> In [[Vertical transmission (symbiont)|vertical transmission]], the symbionts do not need to survive independently, often leading them to have a reduced genome. For instance, [[Acyrthosiphon pisum|pea aphid]] symbionts have lost genes for essential molecules and rely on the host to supply them. In return, the symbionts synthesize essential [[amino acids]] for the aphid host.<ref name="pmid109930772" /> When a symbiont reaches this stage, it begins to resemble a cellular [[organelle]], similar to [[mitochondria]] or [[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.<ref>{{Cite journal |last1=Smith |first1=Noel H. |last2=Gordon |first2=Stephen V. |last3=de la Rua-Domenech |first3=Ricardo |last4=Clifton-Hadley |first4=Richard S. |last5=Hewinson |first5=R. Glyn |date=September 2006 |title=Bottlenecks and broomsticks: the molecular evolution of Mycobacterium bovis |url=https://www.nature.com/articles/nrmicro1472 |journal=Nature Reviews Microbiology |language=en |volume=4 |issue=9 |pages=670–681 |doi=10.1038/nrmicro1472 |pmid=16912712 |s2cid=2015074 |issn=1740-1534|url-access=subscription }}</ref>