Editing Endosymbiont (section)

== Symbiogenesis ==
[[File:Endosymbiotic theory.svg|thumb|An overview of the endosymbiosis theory of eukaryote origin (symbiogenesis).]][[Symbiogenesis]] theory holds that eukaryotes evolved via absorbing [[prokaryotes]]. Typically, one organism envelopes a bacterium and the two evolve a mutualistic relationship. The absorbed bacterium (the endosymbiont) eventually lives exclusively within the host cells. This fits the concept of observed organelle development.<ref>{{cite journal |vauthors=Moore KR, Magnabosco C, Momper L, Gold DA, Bosak T, Fournier GP |date=2019 |title=An Expanded Ribosomal Phylogeny of Cyanobacteria Supports a Deep Placement of Plastids |journal=Frontiers in Microbiology |volume=10 |pages=1612 |doi=10.3389/fmicb.2019.01612 |pmc=6640209 |pmid=31354692 |doi-access=free}}</ref><ref name="McCutcheon">{{cite journal |vauthors=McCutcheon JP |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 |date=October 2021 |pmid=34242059 |doi=10.1146/annurev-cellbio-120219-024122 |s2cid=235786110 |doi-access=free }}</ref><ref name="Callier">{{cite journal |vauthors=Callier V |title=Mitochondria and the origin of eukaryotes |journal=Knowable Magazine |date=8 June 2022 |doi=10.1146/knowable-060822-2 |doi-access=free |url=https://knowablemagazine.org/article/living-world/2022/mitochondria-origin-eukaryotes |access-date=18 August 2022}}</ref><ref>{{cite journal |vauthors=Sagan L |title=On the origin of mitosing cells |journal=Journal of Theoretical Biology |volume=14 |issue=3 |pages=255–274 |date=March 1967 |pmid=11541392 |doi=10.1016/0022-5193(67)90079-3 |author-link=Lynn Margulis |bibcode=1967JThBi..14..225S }}</ref><ref name="Gabaldón">{{cite journal |vauthors=Gabaldón T |title=Origin and Early Evolution of the Eukaryotic Cell |journal=Annual Review of Microbiology |volume=75 |issue=1 |pages=631–647 |date=October 2021 |pmid=34343017 |doi=10.1146/annurev-micro-090817-062213 |s2cid=236916203 }}</ref>

Typically the endosymbiont's genome shrinks, discarding genes whose roles are displaced by the host.<ref name="pmid12415315"/> For example, the ''Hodgkinia'' genome of ''[[Magicicada]]'' [[cicadas]] is much different from that of the prior freestanding bacteria. The cicada life cycle involves years of stasis underground. The symbiont produces many generations during this phase, experiencing little [[natural selection|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.<ref name="pmid29129532">{{cite journal |vauthors=Campbell MA, Łukasik P, Simon C, McCutcheon JP |title=Idiosyncratic Genome Degradation in a Bacterial Endosymbiont of Periodical Cicadas |journal=Current Biology |volume=27 |issue=22 |pages=3568–3575.e3 |date=November 2017 |pmid=29129532 |pmc=8879801 |doi=10.1016/j.cub.2017.10.008 |doi-access=free |bibcode=2017CBio...27E3568C }}</ref>