 Symbiosis
in animals
Intracellular symbioses
in insects
Insects feeding on plant phloem
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Symbiosis in animals
Animals
live in a microbial world
- Microorganisms colonise the surfaces of most animals,
and the tissues and even cells of many animals. Most do the animal no
harm, and many are beneficial for the animal.
- The beneficial associations may involve many different
microorganisms, e.g. in the digestive tract, or single microbial species
which are often restricted to specialised organs or cells.
- Associations that persist for long periods, often
the full lifespan of the animal, are called symbioses; the animals and
microorganisms in many symbioses are never found apart and cannot grow
separate from their partners.
Animal-microbial symbioses include Intracellular symbioses in insects
and Coral symbioses with zooxanthellae.
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Aiptasia pallida with zooxanthellae
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The pea aphid Acyrthosiphon pisum with bacteria Buchnera
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Symbiosis as a source of novel capabilities
- Animals are metabolically impoverished, lacking the
capacity to fix inorganic carbon and nitrogen, and to synthesize ‘essential
amino acids’ and ‘vitamins’. Insects and other arthropods
cannot make sterols, and vertebrates cannot degrade cellulose and other
plant polysaccharides.
- Various animals have circumvented these metabolic
limitations by forming symbioses with microorganisms possessing these
metabolic capabilities: cellulose-degrading bacteria in herbivorous
mammals, nitrogen-fixing bacteria in some termites, essential amino
acid synthesising bacteria in aphids, photosynthetic micro-algae in
some corals, luminescent bacteria in some fish and squid, and so on.
Some general references on symbiosis
Douglas AE. 2008. Conflict, cheats and persistence of symbioses. New Phytologist 177, 849.
Douglas AE 2004. Strategies in antagonistic and
cooperative interactions. In: Microbial Evolution: Gene Establishment,
Survival and Exchange (ed. RV Miller & MJ Day), pp. 275-289.
Douglas AE & Raven JA. 2003. Genomes at the interface between bacteria
and organelles. Philosophical Transactions of the Royal Society of London
B 358, 5-17.
Douglas AE, 2001. Symbiosis. In Encyclopedia of Evolution, ed. M Pagel,
pp.1093-1099. Oxford University Press, New York.
Douglas AE 1998. Host benefit and the evolution of specialization in
symbiosis. Heredity 80: 599-603.
Douglas AE l995. The ecology of symbiotic microorganisms. Advances in
Ecological Research 26: 69-103.
Douglas AE l994. Symbiotic Interactions. Oxford University Press.
Douglas AE l992. Symbiosis in evolution. Oxford Surveys in Evolutionary
Biology 8, 347-382 . Oxford University Press.
Smith DC and Douglas AE l987. The Biology of Symbiosis. Edward Arnold
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