Symbiotic

Ecto- and endosymbiosis

Contrastingly, endosymbiosis is a symbiotic relationship in which one symbiont lives within the tissues of the other, either within or between its cells. Examples include diverse microbiomes: rhizobia, nitrogen-fixing bacteria that live in root nodules on legume roots; actinomycetes, nitrogen-fixing bacteria such as Frankia, which live in alder root nodules; single-celled algae inside reef-building corals; and bacterial endosymbionts that provide essential nutrients to about 10%–15% of insects. Endosymbionts gain nutrients from their hosts, sometimes modifying the host's genome in their favour. Endosymbionts adapt to the environment inside their hosts. They often have a much reduced genome, losing protein-coding gene for metabolism and DNA repair. When insects' endosymbiotic bacteria are passed on to the insects' offspring, populations of intracellular bacteria are reduced, as compared to free-living bacteria. The endosymbiotic bacteria are unable to reinstate their wild type phenotype via a recombination process called Muller's ratchet. This, together with reduced population, leads to an accretion of deleterious mutations in the non-essential genes of the intracellular bacteria.