Editing Symbiosis (section)

=== Ectosymbiosis versus endosymbiosis ===

[[File:An alder root nodule gall.JPG|thumb|Alder tree root nodule houses endosymbiotic [[nitrogen-fixing bacteria]].]]

{{Main|Ectosymbiosis}}{{Further|Endosymbiont}}

[[Ectosymbiosis]] is any symbiotic relationship in which the symbiont lives on the body surface of the [[Host (biology)|host]], including the inner surface of the [[digestion|digestive]] tract or the ducts of [[exocrine gland]]s.<ref name="Paracer-2000-2"/><ref>{{Harvnb|Nardon|Charles|2001}}</ref> Examples of this include [[ectoparasites]] such as [[lice]]; [[commensalism|commensal]] ectosymbionts such as the [[barnacles]], which attach themselves to the jaw of [[baleen whales]]; and mutualist ectosymbionts such as [[cleaner fish]].<!--REWRITE-->

Contrastingly, [[endosymbiosis]] is any symbiotic relationship in which one symbiont lives within the tissues of the other, either within the cells or extracellularly.<ref name="Paracer-2000-2" /><ref>{{Harvnb|Sapp|1994|p=142}}</ref> Examples include diverse [[microbiome]]s: [[rhizobia]], [[nitrogen-fixing bacteria]] that live in [[root nodules]] on [[legume]] roots; [[actinomycete]]s, nitrogen-fixing bacteria such as ''[[Frankia]]'', which live in [[alder]] root nodules; single-celled [[algae]] inside reef-building [[coral]]s; and bacterial [[endosymbiont]]s that provide essential nutrients to about 10%–15% of insects.<ref>{{Cite journal |last1=Mus |first1=Florence |last2=Crook |first2=Matthew B. |last3=Garcia |first3=Kevin |last4=Garcia Costas |first4=Amaya |last5=Geddes |first5=Barney A. |last6=Kouri |first6=Evangelia D. |last7=Paramasivan |first7=Ponraj |last8=Ryu |first8=Min-Hyung |last9=Oldroyd |first9=Giles E. D. |last10=Poole |first10=Philip S. |last11=Udvardi |first11=Michael K. |last12=Voigt |first12=Christopher A. |last13=Ané |first13=Jean-Michel |last14=Peters |first14=John W. |date=1 July 2016 |editor-last=Kelly |editor-first=R. M. |title=Symbiotic Nitrogen Fixation and the Challenges to Its Extension to Nonlegumes |journal=Applied and Environmental Microbiology |volume=82 |issue=13 |pages=3698–3710 |doi=10.1128/AEM.01055-16 |pmc=4907175 |pmid=27084023 |bibcode=2016ApEnM..82.3698M }}</ref>

In endosymbiosis, the host cell lacks some of the nutrients which the [[endosymbiont]] provides. As a result, the host favors endosymbiont's growth processes within itself by producing some specialized cells. These cells affect the genetic composition of the host in order to regulate the increasing population of the endosymbionts and ensure that these genetic changes are passed onto the offspring via [[Vertical transmission (symbiont)|vertical transmission]] ([[heredity]]).<ref>{{cite book |last1=Latorre |first1=A. |author2=Durban, A. |author3=Moya, A. |author4=Pereto, J. |title=The role of symbiosis in eukaryotic evolution. Origins and evolution of life – An astrobiological perspective |year=2011 |pages=326–339 }}</ref>

As the endosymbiont adapts to the host's lifestyle, the endosymbiont changes dramatically. There is a drastic reduction in its [[genome]] size, as many genes are lost during the process of [[metabolism]], and [[DNA]] repair and recombination, while important genes participating in the DNA-to-RNA [[Transcription (genetics)|transcription]], protein [[Translation (biology)|translation]] and DNA/RNA replication are retained. The decrease in genome size is due to loss of protein coding genes and not due to lessening of inter-genic regions or [[open reading frame]] (ORF) size. Species that are naturally evolving and contain reduced sizes of genes can be accounted for an increased number of noticeable differences between them, thereby leading to changes in their evolutionary rates. When endosymbiotic bacteria related with insects are passed on to the offspring strictly via vertical genetic transmission, intracellular bacteria go across many hurdles during the process, resulting in the decrease in effective population sizes, as compared to the free-living bacteria. The incapability of the endosymbiotic bacteria to reinstate their wild type [[phenotype]] via a recombination process is called ''[[Muller's ratchet]]'' phenomenon. Muller's ratchet phenomenon, together with less effective population sizes, leads to an accretion of deleterious [[mutation]]s in the non-essential genes of the intracellular bacteria.<ref>{{cite journal |last=Moran |first=N. A. |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> This can be due to lack of [[Selection (biology)|selection]] mechanisms prevailing in the relatively "rich" host environment.<ref>{{cite journal |last1=Andersson |first1=Siv G.E |author2-link=Charles Kurland |last2=Kurland |first2=Charles G. |title=Reductive evolution of resident genomes |journal=Trends in Microbiology |volume=6 |issue=7 |pages=263–268 |date=July 1998 |pmid=9717214 |doi=10.1016/S0966-842X(98)01312-2 |author1-link=Siv G. E. Andersson }}</ref><ref>{{cite journal |last=Wernegreen |first=J.J. |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 }}</ref>