Symbiosis

From EvoWiki

Symbiosis is an interaction between individuals of different species. Symbiosis includes those relationships where one species lives inside another, or two species merge, but also includes relationships were the organisms remain separate.

Contents 1 Types of symbiotic relationship 1.1 Ammensalism 1.2 Commensalism 1.3 Synergism 1.4 Mutualism 1.5 Parasitism 2 Symbionts and their "Extended Phenotype" 3 Endosymbiosis 4 Further reading

Types of symbiotic relationship

Ammensalism

Non-obligatory relationship where the actions of one organism inadvertently or purposely harms or otherwise inhibits other species.

e.g.

• Australian mistletoe birds planting voided mistletoe berries onto the trunks of eucalyptus trees.
• Cyanobacteria bloom rendering a lake toxic to animal life.
• Lactobacillus bacteria secreting lactic acid to inhibit the growth of other microbes.
• Bellminer birds causing eucalyptus tree die-offs due to protecting bell lerp psyllids (a parasite of eucalyptus trees) from predators.

Commensalism

The "commensal" symbiont benefits, with no harm to the host.

e.g.

• Staphylococcus, a bacterium living on skin.
• Mites living in hair follicles of mammals, and feeding on sebaceous secretions.
• Remora attach to many species (ie. turtles, sharks, whales) for protection and scavenging loose fragments of a meal.

Synergism

Non-obligatory relationship where both populations benefit.

e.g.:

• Birds clean rotting food from between the teeth of crocodiles.
• Cleaner fish eating ectoparasites on the skin of larger fish.
• Escherichia coli, a bacterium living in mammal guts which produces vitamin K.

Mutualism

Obligatory relationship where both symbionts are metabolically dependent upon each other.

e.g.:

• Lichens are algae or cyanobacteria and a species of ascomycote fungus. The fungus produces organic acids which dissolve rocks to provide mineral nutrients, as well as carbon dioxide, water and nitrogenous waste produces, while the algae/cyanobacteria provides oxygen and sugars through photosynthesis.
• Some nitrogen fixing bacteria, such as Rhizobium only fix N₂ when in a legume plant's root nodule. The Rhizobium gain organic nutrients from the plant.
• Bioluminescence in fish and squid is produced by Vibrio bacteria. The fish provide the bacteria with protection and nutrients, while the bacteria produce light for hunting or signaling.
• Sulfur-metabolizing bacteria living in riftworms and deep sea clams.

Parasitism

One symbiont (the parasite) benefits at the expense of the other (the host).

e.g.

• Pathogenic bacteria.
• Bacteriophages - viruses which reproduce in bacterial cells and then destroy the cell.
• Fleas drinking blood from their bird or mammalian hosts.

Symbionts and their "Extended Phenotype"

Symbionts are often thought of as having effects on their host's bodies, and would therefore traditionally be thought of as being part of the host's environment. However, when taking a gene centered look at evolution it is often useful to think of the host's body as being that of both the symbionts, since the genes of both species contribute to the "host" body. The genes in the body could be split up into two groups -- those of the two different symbionts. In the case of a parasite-host relationship the two groups may have a different "aim" (here using the Selfish gene metaphor of a conscious Natural Selection), as the two species reproduce in different ways. For the host to reproduce it wants to be finding food and mate(s), while it may be a necessary part of the parasite's lifecycle that its host must be eaten by a different species (the next host species in its cycle).

Endosymbiosis

In a mutual relationship, however, the two species usually have very similar aims. The DNA of the two symbionts may be kept in separate cells or organelles, but some of its phenotypic effects may be on the cells which contain the other species' DNA, and may be positively selected because of this. Here the boundary between the two species becomes blured to the point where it may be irrelevant. This is Endosymbiosis.

The endosymbiotic theory of the origin of mitochondria, chloroplasts and possibly some other eukaryotic cell organelles are evolved from bacteria which lived mutualistically with proto-eukaryotic cells. In eukaryotes the cell's DNA is separated into mitochondria, chloroplasts (in plants) and the nucleus. The three organelles have mutualistically evolved and their "goals" are so similar that to view them as separate species serves little purpose, even though they have different histories.

Further reading

Some popular science books:

• On parasite genes having phenotypic effects on hosts:
  • Dawkins, R., The Extended Phenotype. OUP 1982. Ch 12. ( ISBN 0192880519 )
• Zimmer, C., Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures. Arrow, 2003. ( ISBN 074320011X )