Editing Symbiogenesis (section)

== Secondary endosymbiosis ==
{{see also|Plastid evolution#Secondary endosymbiosis}}
Primary endosymbiosis involves the engulfment of a cell by another free living organism. Secondary endosymbiosis occurs when the product of primary endosymbiosis is itself engulfed and retained by another free living eukaryote. Secondary endosymbiosis has occurred several times and has given rise to extremely diverse groups of algae and other eukaryotes. Some organisms can take opportunistic advantage of a similar process, where they engulf an alga and use the products of its photosynthesis, but once the prey item dies (or is lost) the host returns to a free living state. Obligate secondary endosymbionts become dependent on their organelles and are unable to survive in their absence. A secondary endosymbiosis event involving an ancestral [[Red algae|red alga]] and a [[heterotrophic]] eukaryote resulted in the evolution and diversification of several other photosynthetic lineages including [[Cryptophyta]], [[Haptophyta]], [[Heterokont|Stramenopiles (or Heterokontophyta)]], and [[Alveolata]].<ref name="McFadden 2001">{{cite journal |last=McFadden |first=G. I. |title=Primary and secondary endosymbiosis and the origin of plastids |journal=Journal of Phycology |year=2001 |volume=37 |issue=6 |pages=951–959 |doi=10.1046/j.1529-8817.2001.01126.x |bibcode=2001JPcgy..37..951M |s2cid=51945442 }}</ref> 

A possible secondary endosymbiosis has been observed in process in the heterotrophic protist ''[[Hatena arenicola|Hatena]]''. This organism behaves like a predator until it ingests a [[green alga]], which loses its flagella and cytoskeleton but continues to live as a symbiont. ''Hatena'' meanwhile, now a host, switches to photosynthetic nutrition, gains the ability to move towards light, and loses its feeding apparatus.<ref>{{cite journal |last1=Okamoto |first1=N. |last2=Inouye |first2=I. |title=A secondary symbiosis in progress? |journal=Science |volume=310 |issue=5746 |pages=287 |date=October 2005 |pmid=16224014 |doi=10.1126/science.1116125 |s2cid=22081618 |url=https://www.researchgate.net/publication/7542363 }}</ref>

Despite the diversity of organisms containing plastids, the morphology, biochemistry, genomic organisation, and molecular phylogeny of plastid RNAs and proteins suggest a single origin of all extant plastids – although this theory was still being debated in 2008.<ref>{{cite journal |last1=McFadden |first1=G. I. |last2=van Dooren |first2=G. G. |title=Evolution: red algal genome affirms a common origin of all plastids |journal=Current Biology |volume=14 |issue=13 |pages=R514-6 |date=July 2004 |pmid=15242632 |doi=10.1016/j.cub.2004.06.041 |s2cid=18131616 |doi-access=free |bibcode=2004CBio...14.R514M }}</ref><ref>{{cite journal |last1=Gould |first1=Sven B. |last2=Waller |first2=Ross F. |last3=McFadden |first3=Geoffrey I. |title=Plastid evolution |journal=Annual Review of Plant Biology |volume=59 |issue=1 |pages=491–517 |year=2008 |pmid=18315522 |doi=10.1146/annurev.arplant.59.032607.092915 |bibcode=2008AnRPB..59..491G |s2cid=30458113 }}</ref><!--this para is misplaced, should be in another section-->