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Concept

Symbiosis is a biological relationship in which two species live in close proximity to each other and interact regularly in such a way as to benefit one or both of the organisms. When both partners benefit, this variety of symbiosis is known as mutualism. The name for a situation in which only one of the partners benefits is far more well known. Such an arrangement is known as parasitism, and a parasite is an organism that obtains nourishment or other life support from a host, usually without killing it. By their very nature, parasites are never beneficial, and sometimes they can be downright deadly. In addition to the extremes of mutualism and parasitism, there is a third variety of symbiosis, called commensalism. As with parasitism, in a relationship characterized by commensalism only one of the two organisms or species derives benefit, but in this case it manages to do so without causing harm to the host.

How It Works

Varieties of Symbiosis

When two species—that is, at least two individuals representing two different species—live and interact closely in such a way that either or both species benefit, it is symbiosis. It is also possible for a symbiotic relationship to exist between two organisms of the same species. Organisms engaging in symbiotic relationships are called symbionts.

There are three basic types of symbiosis, differentiated as to how the benefits (and the detriments, if any) are distributed. These are commensalism, parasitism, and mutualism. In the first two varieties, only one of the two creatures benefits from the symbiotic relationship, and in both instances the creature who does not benefit—who provides a benefit to the other creature—is called the host. In commensalism the organism known as the commensal benefits from the host without the host's suffering any detriment. By contrast, in parasitism the parasite benefits at the expense of the host.

Mutualism: Human and Dog

Mutualism is distinguished from the other two types of symbiosis, because in this variety both creatures benefit. Thus, there is no host, and theoretically the partners are equal, though in practice one usually holds dominance over the other. An example of this inequality is the relationship between humans and dogs. In this relationship, both human and dog clearly benefit: the dog by receiving food, shelter, and care and the human by receiving protection and loving companionship—the last two being benefits the dog also receives from the human. Additionally, some dogs perform specific tasks, such as fetching slippers, assisting blind or disabled persons, or tracking prey for hunting or crime-solving purposes.

For all this exchange of benefits, one of the two animals, the human, clearly holds the upper hand. There might be exceptions in a few unusual circumstances, such as dog lovers who are so obsessive that they would buy food for their dogs before feeding themselves. Such exceptions, however, are rare indeed, and it can be said that in almost all cases the human is dominant.

Obligate and Facultative Relationships

Most forms of mutualism are facultative, meaning that the partners can live apart successfully. Some relationships of mutualism are so close that the interacting species are unable to live without each other. A symbiotic relationship in which the partners, if separated, would be unable to continue living is known as an obligate relationship. In commensalism or parasitism, the relationship is usually obligate for the commensal or the parasite, since by definition they depend on the host. At the same time, and also by definition, the host is in a facultative relationship, since it does not need the commensal or parasite—indeed, in the case of the parasite, would be much better off without it. It is possible, however, for an organism to become so adjusted to the parasite attached to its body that the sudden removal of the parasite could cause at least a short-term shock to the system.

Inquilinism

A special variety of commensalism is inquilinism, in which the commensal species makes use of the host's nest or habitat, without causing any inconvenience or detriment to the host. Inquilinism (the beneficiary is known as an inquiline) often occurs in an aquatic environment, though not always. In your own yard, which is your habitat or nest, there may be a bird nesting in a tree. Supposing you benefit from the bird, through the aesthetic enjoyment of its song or the pretty colors of its feathers—in this case the relationship could be said to be a mutualism. In any case, the bird still benefits more, inasmuch as it uses your habitat as a place of shelter.

The bird example is an extremely nonintrusive case of inquilinism; more often than not, however, a creature actually uses the literal nest of another species, which would be analogous to a bird nesting in your attic or even the inside of your house. This is where the analogy breaks down, of course, because such an arrangement would no longer be one of commensalism, since you would be suffering a number of deleterious effects, not the least of which would be bird droppings on the carpet.

Inquilinism sometimes is referred to as a cross between commensalism and parasitism and might be regarded as existing on a continuum between the two. Certainly, there are cases of a creature making use of another's habitat in a parasitic way. Such is the case with the North American cowbird and the European cuckoo, both of which leave their offspring in the nests of other birds to be raised by them. (See Instinct and Learning for a discussion of how these species exploit other birds' instinctive tendency to care for their young.)

Real-Life Applications

Mycorrhizae

One of the best examples of mutualism is known by the unusual name mycorrhiza, which is a "fungus root," or a fungus living in symbiosis with the roots of a vascular plant. (A vascular plant is any plant species containing a vascular system, which is a network of vessels for moving fluid through the body of the organism.) The relationship is a form of mutualism because, while the fungus benefits from access to carbohydrates, proteins, and other organic nutrients excreted by or contained in the roots of the host plant, the host plant benefits from an enhanced supply of inorganic nutrients, especially phosphorus, that come from the fungus.

The fungus carries out this function primarily by increasing the rate at which organic matter in the immediate vicinity of the plant root decomposes and by efficiently absorbing the inorganic nutrients that are liberated by this process-nutrients it shares with the plant. (The term organic refers to the presence of carbon and hydrogen together, which is characteristic not only of all living things but of many nonliving things as well.) The most important mineral nutrients that the fungus supplies to the plant are compounds containing either phosphorus or, to a lesser degree, nitrogen. (These elements are present in biogeochemical cycles—see The Biosphere.) So beneficial is the mycorrhizal mutualism that about 90% of all vascular plant families, including mustards and knotweeds (family Brassicaceae and Polygonaceae, respectively), enjoy some such relationship with fungi.

Some Examples of Mycorrhizae

Many mycorrhizal fungi in the Basiodiomycete group develop edible mushrooms, which are gathered by many people for use in gourmet cooking. Mushroom collectors have to be careful, of course, because some mycorrhizal fungi are deadly poisonous, as is the case with the death angel, or destroying angel—Amanita virosa.

Perhaps the most famous of the edible mushrooms produced by mycorrhizae are the many varieties known by the name truffle. Among these mushrooms is Tuber melanosporum, which is commonly mycorrhizal on various species of oak tree. The spore-bearing bodies of the truffle fungi develop underground and are usually brown or black and covered with warts. Truffle hunters require the help of truffle-sniffing pigs or dogs, but their work is definitely worth the trouble: good truffles command a handsome price, and particularly in France the truffle industry is big business. Given the lucrative nature of the undertaking, one might ask why people do not cultivate truffles rather than hunting for them. To create the necessary conditions for cultivation, however, so much effort is required that it is difficult to make a profit, even at the high prices charged for truffles. The soil composition must be just right, and under conditions of cultivation this takes about five years.

Orchids are an example of a plant in an obligate mutualism: they can thrive only in a mycorrhizal relationship. Tiny and dustlike, orchid seeds have virtually no stored energy to support the seedling when it germinates, or begins to grow. Only with the assistance of an appropriate mycorrhizal fungus can these seedlings begin developing. Until horticulturists discovered this fact, orchids were extremely difficult to propagate and grow in greenhouses; today, they are relatively easy to breed and cultivate.

The Importance of Mycorrhizae

Some species of vascular plants do not contain chlorophyll, the chemical necessary for photosynthesis, or the conversion of light energy from the Sun into usable chemical energy in a plant. Such a plant is like a person missing a vital organ, and under normal circumstances, it would be impossible for the plant to survive. Yet the Indian pipe, or Monotropa uniflora, has managed to thrive despite the fact that it produces no chlorophyll; instead, it depends entirely on mycorrhizal fungus to supply it with the organic nutrients it needs. This obligate relationship is just one example of the critical role mycorrhizae perform in the lives of plants throughout the world.

Mycorrhizae are vital to plant nutrition, especially in places where the soil is poor in nutrients. Whereas many plant roots develop root hairs as a means of facilitating the extraction of water and nutrients from the soil, plant roots that have a mycorrhizal fungus usually do not. Instead, these plants rely heavily on the fungus itself to absorb moisture and vital chemical elements from the ground. This means that it may be difficult or impossible for plants to survive if they are removed from an environment containing mycorrhizal fungus, a fact that indicates an obligate relationship.

Often, when species of trees and shrubs grown in a greenhouse are transplanted to a non-forested outdoor habitat, they exhibit signs of nutritional distress. This happens because the soils in such habitats do not have populations of appropriate species of mycorrhizal fungi to colonize the roots of the tree seedlings. If, however, seedlings are transplanted into a clear-cut area that was once a forest dominated by the same or closely related species of trees, the plants generally will do well. This happens because the clear-cut former forest land typically still has a population of suitable mycorrhizal fungi.

Plants' dependence on mycorrhizal fungi may be so acute that the plants do not do well in the absence of such fungi, even when growing in soil that is apparently abundant in nutrients. Although most mycorrhizal relationships are not so obligate, it is still of critical important to consider mycorrhizal fungi on a site before a natural ecosystem is converted into some sort of anthropogenic habitat (that is, an area dominated by humans—see Biomes). For example, almost all the tree species in tropical forests depend on mycorrhizae to supply them with nutrients from the soils, which are typically infertile. (See The Biosphere for more about the soil in rain forests.) If people clear and burn the forest to develop new agricultural lands, they leave the soil bereft of a key component. Even though some fungi will survive, they may not necessarily be the appropriate symbionts for the species of grasses and other crops that farmers will attempt to grow on the cleared land.

Interkingdom and Intrakingdom Partnerships

Mycorrhizae are just one example of the ways that mutualism brings into play interactions between widely separated species—in that particular case, between members of two entirely different kingdoms, those of plant and fungi. In some cases, mutualism may bring together an organism of a kingdom whose members are incapable of moving on their own (plants, fungi, or algae) with one whose members are mobile (animals or bacteria). An excellent example is the relationship between angiosperm plants and bees, which facilitate pollination for the plants (see Ecosystems and Ecology.)

Another plant-insect mutualism exists between a tropical ant (Pseudomyrmex ferruginea) and a shrub known as the bull's horn acacia (Acacia cornigera). The latter has evolved hollow thorns, which the ants use as protected nesting sites. The bull's horn acacia has the added benefit, from the ant's perspective, of exuding proteins at the tips of its leaflets, thus providing a handy source of nutrition. In return, the ants protect the acacia both from competition with other plants (by removing any encroaching foliage from the area) and from defoliating insects (by killing herbivorous, or plant-eating, insects and attacking larger herbivores, such as grazing mammals).

A much less dramatic, though biologically quite significant, example of interkingdom mutualism is the lichen. Lichen is the name for about 15,000 varieties, including some that are incorrectly called mosses (e.g., reindeer moss). Before the era of microscopy, botanists considered lichens to be single organisms, but they constitute an obligate mutualism between a fungus and an alga or a blue-green bacterium. The fungus benefits from access to photosynthetic products, while the alga or bacterium benefits from the relatively moist habitat that fungus provides as well as from enhanced access to inorganic nutrients.

Big and Small

In contrast to these cross-kingdom or interkingdom types of mutualism, there may be intrakingdom (within the same kingdom) symbiotic relationships between two very different types of animal. Often, mutualism joins forces in such a way that humans, observing these interactions, see in them object lessons, or stories illustrating the concept that the meek sometimes provide vital assistance to the mighty. One example of this is purely fictional, and it is a very old story indeed: Aesop's fable about the mouse and the lion.

In this tale a lion catches a mouse and is about to eat the little creature for a snack when the mouse pleads for its life; the lion, feeling particularly charitable that day, decides to spare it. Before leaving, the mouse promises one day to return the favor, and the lion chuckles at this offer, thinking that there is no way that a lowly mouse could ever save a fierce lion. Then one day the lion steps on a thorn and cannot extract it from his paw. He is in serious pain, yet the thorn is too small for him to remove with his teeth, and he suffers hopelessly—until the mouse arrives and ably extracts the thorn.

Many real-life examples of this strong-weak or big-small symbiosis exist, one of the more well-known versions being that between the African black rhinoceros (Diceros bicornis) and the oxpecker, or tickbird. The oxpecker, of the genus Buphagus, appears in two species, B. africanus and B. erythrorhynchus. It feeds off ticks, flies, and maggots that cling to the rhino's hide. Thus, this oddly matched pair often can be seen on the African savannas, the rhino benefiting from the pest-removal services of the oxpecker and the oxpecker enjoying the smorgasbord that the rhino's hide offers.

Humans and Other Species

Humans engage in a wide variety of symbiotic relationships with plants, animals, and bacteria. Bacteria may be parasitic on humans, but far from all microorganisms are parasites: without the functioning of "good" bacteria in our intestines, we would not be able to process and eliminate food wastes properly. The relationship of humans to animals that provide a source of meat might be characterized as predation (i.e., the relationship of predator to prey), which is technically a form of symbiosis, though usually it is not considered in the same context. In any case, our relationship to the animals we have domesticated, which are raised on farms to provide food, is a mixture of predation and mutualism. For example, cows (Bos taurus) benefit by receiving food, veterinary services, and other forms of care and by protection from other predators, which might end the cows' lives in a much more unpleasant way than a rancher will.

All important agricultural plants exist in tight bonds of mutualism with humans, because human farmers have bred species so selectively that they require assistance in reproducing. For example, over time, agricultural corn, or maize (Zea mays), has been selected in such a way as to favor those varieties whose fruiting structure is enclosed in a leafy sheath that does not open and whose seeds do not separate easily from the supporting tissue. In other words, thanks to selective breeding, the corn that grows on farms is enclosed in a husk, and the kernels do not come off of the cob readily. Such corn may be desirable as a crop, but because of these characteristics, it is incapable of spreading its own seeds and thereby reproducing on its own. Obviously, agricultural corn is not on any endangered species list, the reason being that farmers continue to propagate the species through breeding and planting.

Another example of human-animal mutualism, to which we alluded earlier, is the relationship between people and their pets, most notably dogs (Canis familiaris) and house cats (Felis catus). Fed and kept safe in domestication, these animals benefit tremendously from their interaction with humans. Humans, in turn, gain from their pets' companionship, which might be regarded as a mutual benefit—at least in the case of dogs. (And even cats, though they pretend not to care much for their humans, have been known to indulge in at least a touch of sentimentality.) In addition, humans receive other services from pets: dogs protect against burglars, and cats eradicate rodents.

Symbiosis Among Insects

Where insects and symbiosis are involved, perhaps the ideas that most readily come to mind are images of parasitism. Indeed, many parasites are insects, but insects often interact with other species in relationships of mutualism, such as those examples mentioned earlier (bees and angiosperms, ants and bull's horn acacia plants). Additionally, there are numerous cases of mutualism between insect species. One of the most intriguing is the arrangements that exists between ants and aphids, insects of the order Homoptera, which also are known as plant lice.

In discussing the ant-aphid mutualism, scientists often compare the aphids to cattle, with the ants acting as protectors and "ranchers." What aphids have that ants want is something called honeydew, a sweet substance containing surplus sugar from the aphid's diet that the aphid excretes through its anus. In return, ants protect aphid eggs during the winter and carry the newly hatched aphids to new host plants. The aphids feed on the leaves, and the ants receive a supply of honeydew.

In another mutualism involving a particular ant species, Formica fusca, two organisms appear to have evolved together in such a way that each benefits from the other, a phenomenon known as coadaptation. This particular mutualism involves the butterfly Glaucopsyche lygdamus when it is still a caterpillar, meaning that it is in the larval, or not yet fully developed, stage. Like the aphid, this creature, too, produces a sweet "honeydew" solution that the ants harvest as food. In return, the ants defend the caterpillar against parasitic wasps and flies.

When Mutualism Also Can Be Parasitism

As the old saying goes, "One man's meat is another man's poison"—in other words, what is beneficial to one person may be harmful to another. So it is with symbiotic relationships, and often a creature that plays a helpful, mutualistic role in one relationship may be a harmful parasite in another interaction. Aphids, for instance, are parasitic to many a host plant, which experiences yellowing, stunting, mottling, browning, and curling of leaves as well as inhibiting of its ability to produce crops.

One particular butterfly group, Heliconiinae (a member of the Nymphalidae, largest of the butterfly families) furnishes another example of the fact that a mutualistic symbiont, in separate interaction, can serve as a parasite. Moreover, in this particular case the heliconius butterfly can be a mutualistic symbiont and parasite for the very same plant. Heliconius butterflies scatter the pollen from the flowers of passionflower vines (genus Passiflora), thus benefiting the plant, but their females also lay eggs on young Passiflora shoots, and the developing larva may eat the entire shoot. As an apparent adaptive response, several Passiflora species produce new shoots featuring a small structure that closely resembles a heliconius egg. A female butterfly that sees this "egg" will avoid laying her own egg there, and the shoot will be spared.

Commensalism

Years ago a National Geographic article on the Indian city of Calcutta included a photograph that aptly illustrated the idea of commensalism, though in this case not between animals or plants but between people. The photograph showed a street vendor in a tiny wooden stall with a window, through which he sold his wares to passers-by. It was a rainy day, and huddled beneath the window ledge (which also served as a counter-top) was another vendor, protecting himself and his own tray of goods from the rain.

The photograph provided a stunning example, in microcosm, of the overpopulation problem both in Calcutta and in India as a whole—a level of crowding and of poverty far beyond the comprehension of the average American. At the same time it also offered a beautiful illustration of commensalism (though this was certainly not the purpose of including the picture with the article). The vendor sitting on the ground acted in the role of commensal to the relatively more fortunate vendor with the booth, who would be analogous to the host.

The relationship was apparently commensal, because the vendor on the ground received shelter from the other vendor's counter without the other vendor's suffering any detriment. If the vendor in the booth wanted to move elsewhere, and the vendor on the ground somehow prevented him from doing so, then the relationship would be one of parasitism. And, of course, if the vendor with the booth charged his less-fortunate neighbor rent, then the relationship would not be truly commensal, because the vendor on the ground would be paying for his shelter. To all appearances, however, the interaction between the two men was perfectly commensal.

Commensalism in Nature

Plants that grow on the sides of other plants without being parasitic are known as epiphytic plants.

Among these plants are certain species of orchids, ferns, and moss. By "standing on the shoulders of giants," these plants receive enormous ecological benefits: the height of their hosts gives them an opportunity to reach a higher level in the canopy (the upper layer of trees in the forest) than they would normally attain, and this provides them with much greater access to sunlight. At the same time, the hosts are not affected either negatively or positively by this relationship.

Another commensal relationship, known as phoresy, is a type of biological hitchhiking in which one organism receives access to transportation on the body of another animal, without the transporting animal being adversely affected by this arrangement. The burdock (Arctium lappa) is one of several North American plant species that produce fruit that adheres to fur and therefore is dispersed easily by the movement of mammals. The burdock is special from a human standpoint, however, inasmuch as the anatomical adaptation that makes possible its adhesion to fur provided designers with the model for that extremely useful innovation, Velcro.

As with the illustration of the street vendors in Calcutta, it is always possible that commensalism, through a slight alteration, may yield a relationship in which the host is affected negatively. There are instances in which individual animals may become loaded heavily with sticky fruit from the burdock (or other plants that employ a similar mechanism), thus causing their fur to mat excessively and perhaps resulting in significant detriment. This is not common, however, and usually this biological relationship is truly commensal. Furthermore, phoresy should not be confused with parasitic relationships in which a creature such as a tick attaches itself to the body of another organism for transport or other purposes. (For much more about parasitism, see Parasites and Parasitology.)

Where to Learn More

"Biology 160, Animal Behavior: Symbiosis and Social Parasitism." Department of Biology, University of California at Riverside (Web site). <http://www.biology.ucr.edu/Bio160/lecture25.html>.

Knutson, Roger M. Furtive Fauna: A Field Guide to the Creatures Who Live on You. New York: Penguin Books, 1992.

Lanner, Ronald M. Made for Each Other: A Symbiosis of Birds and Pines. New York: Oxford University Press, 1996.

Lembke, Janet. Despicable Species: On Cowbirds, Kudzu, Hornworms, and Other Scourges. New York: Lyons Press, 1999.

Margulis, Lynn. Symbiotic Planet: A New Look at Evolu tion. New York: Basic Books, 1998.

Mutualism and Commensalism. Neartica: The Natural World of North America (Web site). <http://www.nearctica.com/ecology/pops/symbiote.htm>.

"Parasites and Parasitism." University of Wales, Aberystwyth (Web site). <http://www.aber.ac.uk/parasitology/Edu/Para_ism/PaIsmTxt.html>.

Sapp, Jan. Evolution by Association: A History of Symbiosis. New York: Oxford University Press, 1994.

Symbiosis and Commensalism. The Sea Slug Forum (Web site). <http://www.seaslugforum.net/symbio.htm>.

Trager, William. Living Together: The Biology of Animal Parasitism. New York: Plenum Press, 1986.

An association of two participants whereby both partners benefit. Thus, flowering plants rely on insects for pollination and the insects feed on their nectar. Lichens are an amalgamation of fungus and algae so close that it is difficult to separate them. Such an interdependence may be termed mutualism. Measures of interdependence vary from total to slight.

A relationship between two different organisms, to the benefit of each.

The process by which two organisms live together, usually to their mutual benefit. An example of a symbiotic pair are cows and the bacteria that live in their digestive tracts, enabling them to digest cellulose in grass.

The biological association of two individuals or populations of different species, classified as mutualism, commensalism, parasitism, amensalism or synnecrosis, depending on the advantage or disadvantage derived from the relationship.

• Environment, Ecology, and Animal Behavior - symbiosis: relationship in which two organisms live in close, mutually beneficial association
• Mental States, Processes, and Behavior - symbiosis: intense mutual interdependence

"Symbiology" redirects here. For use of things that represent other things by association, resemblance, or convention, see Symbology.

This article is about the biological phenomenon. For other uses, see Symbiosis (disambiguation). For the Marvel character, see Symbiote (comics).

In a symbiotic mutualism, the clownfish feeds on small invertebrates that otherwise have potential to harm the sea anemone, and the fecal matter from the clownfish provides nutrients to the sea anemone. The clownfish is additionally protected from predators by the anemone's stinging cells, to which the clownfish is immune.

Symbiosis (from Ancient Greek σύν "together" and βίωσις "living")^[1] is close and often long-term interaction between different biological species. In 1877, Bennett used the word symbiosis (which previously had been used of people living together in community) to describe the mutualistic relationship in lichens.^[2] In 1879, by the German mycologist Heinrich Anton de Bary, defined it as "the living together of unlike organisms."^[3][4]

The definition of symbiosis is controversial among scientists. Some believe symbiosis should only refer to persistent mutualisms, while others believe it should apply to any types of persistent biological interactions (i.e. mutualistic, commensalistic, or parasitic).^[5]

Some symbiotic relationships are obligate, meaning that both symbionts entirely depend on each other for survival. For example, many lichens consist of fungal and photosynthetic symbionts that cannot live on their own.^[3][6][7][8] Others are facultative, meaning that they can, but do not have to live with the other organism.

Symbiotic relationships include those associations in which one organism lives on another (ectosymbiosis, such as mistletoe), or where one partner lives inside the other (endosymbiosis, such as lactobacilli and other bacteria in humans or zooxanthelles in corals).^[9][10]

Contents 1 Physical interaction 2 Mutualism 2.1 Mutualism and endosymbiosis 3 Commensalism 4 Parasitism 5 Amensalism 6 Symbiosis and evolution 6.1 Vascular plants 6.2 Symbiogenesis 6.3 Co-evolution 7 Notes 8 See also 9 References 10 External links

Physical interaction

Alder tree root nodule

Endosymbiosis is any symbiotic relationship in which one symbiont lives within the tissues of the other, either in the intracellular space or extracellularly.^[10][11] Examples include diverse microbiomes, rhizobia, nitrogen-fixing bacteria that live in root nodules on legume roots; actinomycete nitrogen-fixing bacteria called Frankia, which live in alder tree root nodules; single-celled algae inside reef-building corals; and bacterial endosymbionts that provide essential nutrients to about 10%–15% of insects.

Ectosymbiosis, also referred to as exosymbiosis, is any symbiotic relationship in which the symbiont lives on the body surface of the host, including the inner surface of the digestive tract or the ducts of exocrine glands.^[10][12] Examples of this include ectoparasites such as lice, commensal ectosymbionts such as the barnacles that attach themselves to the jaw of baleen whales, and mutualist ectosymbionts such as cleaner fish.

Mutualism

Main article: Mutualism (biology)

Hermit crab, Calcinus laevimanus, with sea anemone.

Mutualism is any relationship between individuals of different species where both individuals derive a benefit.^[13] In general, only lifelong interactions involving close physical and biochemical contact can properly be considered symbiotic. Mutualistic relationships may be either obligate for both species, obligate for one but facultative for the other, or facultative for both. Many biologists restrict the definition of symbiosis to close mutualist relationships.

A large percentage of herbivores have mutualistic gut fauna that help them digest plant matter, which is more difficult to digest than animal prey.^[9] Coral reefs are the result of mutualisms between coral organisms and various types of algae that live inside them.^[14] Most land plants and land ecosystems rely on mutualisms between the plants, which fix carbon from the air, and mycorrhyzal fungi, which help in extracting minerals from the ground.^[15]

An example of mutual symbiosis is the relationship between the ocellaris clownfish that dwell among the tentacles of Ritteri sea anemones. The territorial fish protects the anemone from anemone-eating fish, and in turn the stinging tentacles of the anemone protect the clownfish from its predators. A special mucus on the clownfish protects it from the stinging tentacles.^[16]

Another example is the goby fish, which sometimes lives together with a shrimp. The shrimp digs and cleans up a burrow in the sand in which both the shrimp and the goby fish live. The shrimp is almost blind, leaving it vulnerable to predators when above ground. In case of danger the goby fish touches the shrimp with its tail to warn it. When that happens both the shrimp and goby fish quickly retreat into the burrow.^[17]

One of the most spectacular examples of obligate mutualism is between the siboglinid tube worms and symbiotic bacteria that live at hydrothermal vents and cold seeps. The worm has no digestive tract and is wholly reliant on its internal symbionts for nutrition. The bacteria oxidize either hydrogen sulfide or methane, which the host supplies to them. These worms were discovered in the late 1980s at the hydrothermal vents near the Galapagos Islands and have since been found at deep-sea hydrothermal vents and cold seeps in all of the world's oceans.^[18]

There are also many types of tropical and sub-tropical ants that have evolved very complex relationships with certain tree species.^[19]

Mutualism and endosymbiosis

During mutualistic symbioses, the host cell lacks some of the nutrients, which are provided by the endosymbiont. 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 ensuring that these genetic changes are passed onto the offspring via vertical transmission (heredity).^[20]

Adaptation of the endosymbiont to the host’s lifestyle leads to many changes in the endosymbiont – the foremost being drastic reduction in its genome size. This is due to many genes being lost during the process of metabolism, and DNA repair and recombination. While important genes participating in the DNA to RNA transcription, protein translation and DNA/RNA replication are retained. That is, a 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. Thus, 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. As the endosymbiotic bacteria related with these insects are passed on to the offspring strictly via vertical genetic transmission, intracellular bacteria goes through many hurdles during the process, resulting in the decrease in effective population sizes when compared to the free living bacteria. This incapability of the endosymbiotic bacteria to reinstate its wild type phenotype via a recombination process is called as Muller’s ratchet phenomenon. Muller’s ratchet phenomenon together with less effective population sizes has led to an accretion of deleterious mutations in the non-essential genes of the intracellular bacteria.^[21] This could have been due to lack of selection mechanisms prevailing in the rich environment of the host.^[22][23]

Commensalism

Phoretic mites on a fly (Pseudolynchia canariensis).

Main article: Commensalism

Commensalism describes a relationship between two living organisms where one benefits and the other is not significantly harmed or helped. It is derived from the English word commensal used of human social interaction. The word derives from the medieval Latin word, formed from com- and mensa, meaning "sharing a table".^[13][24]

Commensal relationships may involve one organism using another for transportation (phoresy) or for housing (inquilinism), or it may also involve one organism using something another created, after its death (metabiosis). Examples of metabiosis are hermit crabs using gastropod shells to protect their bodies and spiders building their webs on plants.

Parasitism

Flea bites on a human is an example of parasitism.

Main article: Parasitism

A parasitic relationship is one in which one member of the association benefits while the other is harmed.^[25] Parasitic symbioses take many forms, from endoparasites that live within the host's body to ectoparasites that live on its surface. In addition, parasites may be necrotrophic, which is to say they kill their host, or biotrophic, meaning they rely on their host's surviving. Biotrophic parasitism is an extremely successful mode of life. Depending on the definition used, as many as half of all animals have at least one parasitic phase in their life cycles, and it is also frequent in plants and fungi. Moreover, almost all free-living animals are host to one or more parasite taxa. An example of a biotrophic relationship would be a tick feeding on the blood of its host.

Amensalism

Amensalism is the type of symbiotic relationship that exists where one species is inhibited or completely obliterated and one is unaffected. This type of symbiosis is relatively uncommon in rudimentary reference texts, but is omnipresent in the natural world. An example is a sapling growing under the shadow of a mature tree. The mature tree can begin to rob the sapling of necessary sunlight and, if the mature tree is very large, it can take up rainwater and deplete soil nutrients. Throughout the process the mature tree is unaffected. Indeed, if the sapling dies, the mature tree gains nutrients from the decaying sapling. Note that these nutrients become available because of the sapling's decomposition, rather than from the living sapling, which would be a case of parasitism.

Symbiosis and evolution

Leafhoppers protected by an army of meat ants

While historically, symbiosis has received less attention than other interactions such as predation or competition,^[26] it is increasingly recognized as an important selective force behind evolution,^[9][27] with many species having a long history of interdependent co-evolution.^[28] In fact, the evolution of all eukaryotes (plants, animals, fungi, and protists) is believed under the endosymbiotic theory to have resulted from a symbiosis between various sorts of bacteria.^[9][29][30]

Vascular plants

About 80% of vascular plants worldwide form symbiotic relationships with fungi, for example, in arbuscular mycorrhizas.^[31]

Symbiogenesis

The biologist Lynn Margulis, famous for her work on endosymbiosis, contends that symbiosis is a major driving force behind evolution. She considers Darwin's notion of evolution, driven by competition, to be incomplete and claims that evolution is strongly based on co-operation, interaction, and mutual dependence among organisms. According to Margulis and Dorion Sagan, "Life did not take over the globe by combat, but by networking."^[32]

Co-evolution

Symbiosis played a major role in the co-evolution of flowering plants and the animals that pollinate them. Many plants that are pollinated by insects, bats, or birds have highly specialized flowers modified to promote pollination by a specific pollinator that is also correspondingly adapted. The first flowering plants in the fossil record had relatively simple flowers. Adaptive speciation quickly gave rise to many diverse groups of plants, and, at the same time, corresponding speciation occurred in certain insect groups. Some groups of plants developed nectar and large sticky pollen, while insects evolved more specialized morphologies to access and collect these rich food sources. In some taxa of plants and insects the relationship has become dependent,^[33] where the plant species can only be pollinated by one species of insect.^[34]

Notes

1. ^ σύν, βίωσις. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at Perseus Project
2. ^ "symbiosis". Oxford English Dictionary. Oxford University Press. 3rd ed. 2001.
3. ^ ^{a b} Wilkinson 2001
4. ^ Douglas 1994, p. 1
5. ^ Douglas, Angela E. (2010), The symbiotic habit, New Jersey: Princeton University Press, pp. 5–12, ISBN 978-0-691-11341-8 
6. ^ Isaac 1992, p. 266
7. ^ Saffo 1993
8. ^ Douglas, Angela E. (2010), The symbiotic habit, New Jersey: Princeton University Press, p. 4, ISBN 978-0-691-11341-8 
9. ^ ^{a b c d} Moran 2006
10. ^ ^{a b c} Ahmadjian & Paracer 2000, p. 12
11. ^ Sapp 1994, p. 142
12. ^ Nardon & Charles 2002
13. ^ ^{a b} Ahmadjian & Paracer 2000, p. 6
14. ^ Toller, Rowan & Knowlton 2001
15. ^ Harrison 2005
16. ^ Lee 2003
17. ^ Facey, Helfman & Collette 1997
18. ^ Cordes 2005
19. ^ Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
20. ^ Latorre, A.; Durban, A., Moya, A. & Pereto, J. (2011). The role of symbiosis in eukaryotic evolution. Origins and evolution of life - An astrobiological perspective. pp. 326-339. 
21. ^ Moran, N. A. (1996). "Accelerated evolution and Muller’s ratchet in endosymbiotic bacteria.". Proceedings of the National Academy of Sciences of the United States of America 93: 2873-2878. 
22. ^ Andersson, S.G.; Kurland, C.G. (1998). "Reductive evolution of resident genomes.". Trends in Microbiology 6: 263-268. 
23. ^ Wernegreen, J.J. (2002). "Genome evolution in bacterial endosymbionts of insects.". Nature reviews, Genetics 3: 850-861. 
24. ^ Nair 2005
25. ^ Ahmadjian & Paracer 2000, p. 7
26. ^ Townsend, Begon & Harper 1996
27. ^ Wernegreen 2004
28. ^ Ahmadjian & Paracer 2000, pp. 3–4
29. ^ Brinkman 2002
30. ^ Golding & Gupta 1995
31. ^ Schüßler, A. et al. (2001), "A new fungal phylum, the Glomeromycota: phylogeny and evolution", Mycol. Res. 105 (12): 1416, doi:10.1017/S0953756201005196, http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=95091. 
32. ^ Sagan & Margulis 1986
33. ^ Harrison 2002
34. ^ Danforth & Ascher 1997

See also

• Anagenesis
• Aposymbiotic
• Aquaponics
• Cheating (biology)
• Cleaning symbiosis
• Decompiculture
• Human Microbiome Project
• List of symbiotic organisms
• List of symbiotic relationships
• Microbiome
• Multigenomic organism
• Symbiogenesis
• Symbiosis (chemical)

References

• Ahmadjian, Vernon; Paracer, Surindar (2000), Symbiosis: an introduction to biological associations, Oxford [Oxfordshire]: Oxford University Press, ISBN 0-19-511806-5 
• Burgess, Jeremy (1994), Forum: What's in it for me, New Scientist, http://media.newscientist.com/article/mg14119115.200-forum-whats-in-it-for-me--jeremy-burgess-examines-therole-of-cooperation-within-natures-competitive-ways-.html 
• Boucher, Douglas H (1988), The Biology of Mutualism: Ecology and Evolution, New York: Oxford University Press, ISBN 0-19-505392-3 
• Cordes, E.E.; Arthur, M.A.; Shea, K.; Arvidson, R.S.; Fisher, C.R. (2005), "Modeling the mutualistic interactions between tubeworms and microbial consortia", PLoS Biol 3 (3): 1–10, doi:10.1371/journal.pbio.0030077, PMC 1044833, PMID 15736979 
• Brinkman, F.S.L.; Blanchard, J.L.; Cherkasov, A.; Av-gay, Y.; Brunham, R.C.; Fernandez, R.C.; Finlay, B.B.; Otto, S.P.; Ouellette, B.F.F.; Keeling, P.J.; Others, (2002), "Evidence That Plant-Like Genes in Chlamydia Species Reflect an Ancestral Relationship between Chlamydiaceae, Cyanobacteria, and the Chloroplast", Genome Research 12 (8): 1159–1167, doi:10.1101/gr.341802, PMC 186644, PMID 12176923, http://www.csa.com/partners/viewrecord.php?requester=gs&collection=ENV&recid=5449063, retrieved 2007-09-30 
• Danforth, B.N.; Ascher, J. (1997), "Flowers and Insect Evolution", Science 283 (5399): 143, doi:10.1126/science.283.5399.143a, http://www.sciencemag.org/cgi/reprint/283/5399/143a.pdf, retrieved 2007-09-25 
• Douglas, Angela (2010), The Symbiotic Habit, Princeton University Press, ISBN 0-19-854294-1 
• Douglas, Angela (1994), Symbiotic interactions, Oxford [Oxfordshire]: Oxford University Press, ISBN 0-19-854294-1 
• Facey, Douglas E.; Helfman, Gene S.; Collette, Bruce B. (1997), The diversity of fishes, Oxford: Blackwell Science, ISBN 0-86542-256-7 
• Golding, RS; Gupta, RS (1995), "Protein-based phylogenies support a chimeric origin for the eukaryotic genome", Mol. Biol. Evol. 12 (1): 1–6, PMID 7877484 
• Harrison, Rhett (2002), "Balanced mutual use (symbiosis)", Quarterly journal Biohistory 10 (2), https://www.brh.co.jp/en/experience/journal/32/ss_3.html, retrieved 2007-09-23 
• Harrison, Maria J. (2005), "Signaling in the arbuscular mycorrhizal symbiosis", Annu. Rev. Microbiol. 59: 19–42, doi:10.1146/annurev.micro.58.030603.123749, PMID 16153162 
• Lee, J. (2003), "Amphiprion percula" (On-line), Animal Diversity Web, http://animaldiversity.ummz.umich.edu/site/accounts/information/Amphiprion_percula.html, retrieved 2007-09-29 
• Isaac, Susan (1992), Fungal-plant interactions, London: Chapman & Hall, ISBN 0-412-36470-0 
• Isaak, Mark (2004), CB630: Evolution of obligate mutualism, TalkOrigins Archive, http://www.talkorigins.org/indexcc/CB/CB630.html, retrieved 2007-09-25 
• Moran, N.A. (2006), "Symbiosis", Current Biology 16 (20): 866–871, doi:10.1016/j.cub.2006.09.019, PMID 17055966, http://linkinghub.elsevier.com/retrieve/pii/S0960982206022123, retrieved 2007-09-23 
• Nardon, P.; Charles, H. (2002), "Morphological aspects of symbiosis", Symbiosis: Mechanisms and Systems. Dordercht/boson/London, Kluwer Academic Publishers 4: 15–44, doi:10.1007/0-306-48173-1_2 
• Powell, Jerry (1992), "Interrelationships of yuccas and yucca moths", Trends in Ecology and Evolution 7 (1): 10–15, doi:10.1016/0169-5347(92)90191-D, PMID 21235936 
• Nair, S. (2005), "Bacterial Associations: Antagonism to Symbiosis", in Ramaiah, N, Marine Microbiology: Facets & Opportunities;, National Institute of Oceanography, Goa, pp. 83–89, http://drs.nio.org/drs/handle/2264/74, retrieved 2007-10-12 
• Roughgarden, J.; Gomulkiewicz; Holt; Thompson (1975), "Evolution of Marine Symbiosis--A Simple Cost-Benefit Model", Ecology 56 (5): 1201–1208, doi:10.1046/j.1420-9101.2000.00157.x, JSTOR 1936160 
• Saffo, M.B. (1993), "Coming to terms with a field: Words and concepts in symbiosis", Symbiosis. 14 (1-3), http://www.csa.com/partners/viewrecord.php?requester=gs&collection=ENV&recid=3004656, retrieved 2007-10-05 
• Sagan, Dorion; Margulis, Lynn (1986), Origins of sex: three billion years of genetic recombination, New Haven, Conn: Yale University Press, ISBN 0-300-03340-0 
• Sagan, Dorion; Margulis, Lynn (1997), Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors, Berkeley: University of California Press, ISBN 0-520-21064-6 
• Sapp, Jan (1994), Evolution by association: a history of symbiosis, Oxford [Oxfordshire]: Oxford University Press, ISBN 0-19-508821-2 
• Sapp, Jan (2009), The New Foundations of Evolution. On the Tree of Life, New York: Oxford University Press 
• Toller, W. W.; Rowan, R.; Knowlton, N. (2001), "Repopulation of Zooxanthellae in the Caribbean Corals Montastraea annularis and M. faveolata following Experimental and Disease-Associated Bleaching", The Biological Bulletin (Marine Biological Laboratory) 201 (3): 360–373, doi:10.2307/1543614, JSTOR 1543614, PMID 11751248, http://www.biolbull.org/cgi/content/full/201/3/360 
• Townsend, Colin R; Begon, Michael; Harper, John D. (1996), Ecology: individuals, populations and communities, Oxford: Blackwell Science, ISBN 0-632-03801-2 
• Weiblen, G.D. (2002), "How to be a fig wasp", Annual Review of Entomology 47 (1): 299–330, doi:10.1146/annurev.ento.47.091201.145213, PMID 11729077 
• Wernegreen, J.J. (2004), "Endosymbiosis: lessons in conflict resolution", PLoS Biology 2 (3): e68, doi:10.1371/journal.pbio.0020068, PMC 368163, PMID 15024418 

External links

• TED-Education video - Symbiosis: a surprising tale of species cooperation.

Wikimedia Commons has media related to: Symbiosis

Look up symbiosis in Wiktionary, the free dictionary.

v t e Inter-species biological interactions in ecology Amensalism • Commensalism • Inquilinism • Mutualism • Neutralism • Synnecrosis • Predation (Carnivory • Herbivory • Intraguild • Parasitism • Parasitoidism • Cheating) • Symbiosis • Competition • Mimicry

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Dansk (Danish)
n. - symbiose

Nederlands (Dutch)
symbiose, wederzijds voordelige samenleving

Français (French)
n. - symbiose

Deutsch (German)
n. - Symbiose

Ελληνική (Greek)
n. - (βιολ.) συμβίωση

Italiano (Italian)
simbiosi

Português (Portuguese)
n. - simbiose (f)

Русский (Russian)
симбиоз

Español (Spanish)
n. - simbiosis

Svenska (Swedish)
n. - symbios

中文（简体）(Chinese (Simplified))
共生, 合作关系

中文（繁體）(Chinese (Traditional))
n. - 共生, 合作關係

한국어 (Korean)
n. - 공생, 공동 생활

日本語 (Japanese)
n. - 共生, 共同生活, 共働, 協力

العربيه (Arabic)
‏(الاسم) التكافل : تعايش كائنين يعتمدان على بعضهما البعض‏

עברית (Hebrew)
n. - ‮חיי שיתוף, חיים הרמוניים, סימביוזה‬

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