Editing Endosymbiont (section)

=== 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>