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Symbiosis

SYMBIOSIS

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 speciesthat is, at least two individuals representing two different specieslive 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 benefitwho provides a benefit to the other creatureis 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 companionshipthe 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 parasiteindeed, 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 feathersin 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 cyclessee 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 angelAmanita 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 humanssee 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 speciesin 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 hopelesslyuntil 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 benefitat 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 wholea 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.

KEY TERMS

COMMENSALISM:

A symbiotic rela tionship in which one organism, the commensal, benefits without causing any detriment to the other organism, the host.

FACULTATIVE:

A term for a symbiotic relationship in which partners are capable of living apart.

HOST:

The term for an organism that provides a benefit or benefits for another organism in a symbiotic relationship of commensalism or parasitism.

INQUILINISM:

A type of symbiosis in which one species, the inquiline, makes use of a host's nest or habitat without causing any detriment to the host. Inquilinism is considered a variety of commensalism.

OBLIGATIVE:

A term for a symbiotic relationship in which the partners, if they were separated, would be incapable of continuing to live.

PARASITISM:

A symbiotic relation ship in which one organism, the parasite, benefits at the expense of the other organism, the host.

SYMBIOSIS:

A biological relationship in which (usually) two species live in close proximity to one another and interact reg ularly in such a way as to benefit one or both of the organisms. Symbiosis may exist between two or more individuals of the same species as well as between two or more individuals representing two different species. The three principal varieties of symbiosis are mutualism, commensalism, and parasitism.

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Symbiosis

Symbiosis

A wide array of interactions among plants, animals, and microorganisms occurs in nature. Some of these relationships are characterized by a close physical association among species that persists for a significant period of the life cycle. In 1879 German botanist Heinrich Anton de Bary coined the term "symbiosis" to describe these relationships, meaning the living together of different species of organisms.

An interaction is considered a symbiosis based on the closeness of the physical association among the organisms rather than on the effect or outcome of the interaction. Symbiotic relationships span a spectrum from beneficial to detrimental effects. Many people associate symbiosis with mutualism , interactions that are beneficial to the growth, survival, and/or reproduction of both interacting species. But symbiotic interactions also include commensalism (one species receives benefit from the association and the other is unaffected), amensalism (one species is harmed, with no effect on the other), and parasitism. An example of commensalism is found in the anemone fish, which gains protection from living among the poisonous tentacles of the sea anemone, but offers no known benefit to its host.

In parasitic interactions, one species lives on or within a host organism and receives nourishment from the host, whereas the host is harmed by the interaction. In obligate interactions, the relationship is essential to at least one of the interacting species. Facultative interactions are those that are beneficial to at least one of the interacting species, but not essential.

Mutualisms in Plants

A common and widespread symbiosis occurs between terrestrial plants and fungi that colonize their roots. These associations are called "mycorrhizae," a word meaning "fungus-root." Unlike pathogenic fungi that cause disease, mycorrhizal fungi benefit the plant in several ways. These fungi germinate from spores in the soil to form thin threadlike structures called hyphae, which grow into the roots of plants. Once the roots are colonized, the fungal hyphae grow out from the root in an extensive network to explore the soil beyond the reach of the roots, gathering essential mineral nutrients and transporting them into the plant, increasing its growth. In return, the plant provides carbohydrates as a food source for the fungus.

Mycorrhizal symbiosis occurs in about 80 percent of all plant species. It is essential to many plants in low-nutrient environments because their roots alone are incapable of absorbing adequate amounts of some essential minerals such as phosphorus. The symbiosis is essential to the fungus because, unlike plants, fungi cannot make their own food via photosynthesis.

Mycorrhizal fungi provide other benefits to plants including improved resistance to drought and disease. The additional mineral nutrients acquired by these fungi have been shown to aid plants in coping with competitors and herbivores. This symbiosis plays a large role in the growth and functioning of plants in both natural and agricultural ecosystems .

Legumes and certain other plants are colonized by Rhizobium bacteria that form small swellings or nodules on their roots. These symbiotic bacteria carry out the process of nitrogen fixation, the conversion of nitrogen gas into ammonia. Nitrogen is an essential element required by all organisms. Although nitrogen gas is abundant in the air, plants are unable to use nitrogen in this form, but they can readily use the ammonia formed by these bacteria and thus benefit from this symbiosis. As with mycorrhizal associations, the host plant benefits its symbiont by providing a carbohydrate energy source.

Mutualisms in Animals

In animals, a common mutualistic symbiosis occurs between many herbivores and microorganisms of their digestive tracts. Ungulates (hoofed animals) and some other animals eat plant material that is high in cellulose , even though they lack enzymes capable of breaking down cellulose molecules. They obtain energy from cellulose with the help of symbiotic bacteria and protozoa living within their digestive tracts. These microbes produce enzymes called cellulase that break down cellulose into smaller molecules that the host animal can then utilize. Similarly, wood-consuming termites depend upon symbiotic protozoans living within their intestines to digest cellulose. These are obligate symbioses. The termites cannot survive without their intestinal inhabitants, and the microorganisms cannot live without the host. In each of these symbioses, the host animal benefits from the food provided by the microorganism and the microorganism benefits from the suitable environment and nourishment provided by the host.

A variety of animals engage in a mutualistic relationship referred to as cleaning symbioses. Birds such as oxpeckers benefit their large ungulate hosts by removing their external parasites , benefiting in return from the food source the host provides. In the marine environment, certain species of fish and shrimp similarly specialize in cleaning parasites from the outside of fishes. This mutualistic relationship promotes the well-being of the host fishes and provides food for those that do the cleaning. Unlike herbivores and their gut microorganisms, these interactions do not involve a close association of one organism living exclusively within another. These and other mutualistic but not clearly symbiotic relationships, such as those between plants and their pollinators, are sometimes referred to as proto-cooperation.

Parasitism

Perhaps the most common type of symbiotic interaction in nature is parasitism. Many kinds of worms, protozoa, bacteria, and viruses are important animal parasites. Some, such as fleas or ticks, are ectoparasites, living on the outside of their host. Others, such as tapeworms or hookworms, are endoparasites that live inside their host.

A variety of parasitic symbionts also occur in plants. In some plants, insects deposit their eggs within the growing shoot tips or other plant part, at the same time producing chemicals that cause the development of a large swelling or tumorlike growth called a gall. The insect larvae then develop within the gall, feeding on the plant tissue as they grow. When its development is completed, the adult insect emerges from the gall to mate and then initiate the gall-forming cycle again. This is an obligate symbiosis because the insect larvae lives inside the plant and cannot complete its life cycle without its host plant. It is also a parasitic association because the insect living within the plant consumes plant tissue and causes harm to its host plant, while benefiting from the food resources and shelter provided by the plant. In addition to insects, other gall-forming symbionts include viruses, bacteria, and fungi.

Symbioses are widespread and important in the life of many organisms and ecologically important in the functioning of natural ecosystems. The patterns of adaptations of mutualists, parasites, and hosts suggest that these interactions are the product of coevolution, leading to increasingly specialized, and often increasingly beneficial, associations. In many mutualistic symbioses such as lichens (symbioses of algae and fungi) and corals (cnidarians and endosymbiotic algae), the adaptive value of the association is that one organism acquires from its partner some new metabolic capability (for example, photosynthesis) that it does not itself possess.

see also Cnidarian; Coral Reef; Mycorrhizae; Population Dynamics

David C. Hartnett

Bibliography

Abrahamson, Warren G. Plant-Animal Interactions. New York: McGraw-Hill, 1989.

Begon, Michael, John L. Harper, and Colin R. Townsend. "Symbiosis and Mutualism." In Ecology, 3rd ed. Oxford: Blackwell Sciences Ltd., 1996.

Douglas, Angela E. Symbiotic Interactions. New York: Oxford University Press, 1994.

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Symbiosis

Symbiosis


The term symbiosis, from the Greek words syn (together with) and bios (life), refers to different kinds of organisms living together in ongoing physical association. Although symbiosis is a fundamental biological relationship, it was a disputed concept until the late 1800s, and the term was only first used in 1878. Its role in ecology and evolutionary theory is still developing.

Biologists recognize several variations of symbiotic association. Obligate symbiosis, such as the tropical reef relationship between Zooxanthellae algae and the coral they inhabit, is necessary for the survival of one or more partners. Facultative symbionts are optional; in tidepools, some sea anemones have green flecks of algae growing inside them, while neighboring anemones do not. Endosymbiosis occurs when one species lives inside another, as cellulose-digesting bacteria inhabit the gut of herbivores. Ectosymbiosis, which does not involve internalization, occurs when, for example, birds or fish clean larger species. Finally, there is a range of interactive impacts. In mutualism, both species benefit; all the above and what is perhaps the first-described case, the algae-fungus association that forms lichens, are examples of mutualism. Commensalism involves advantage to one species and neutral impact on another. Parasitic symbiosis benefits one species at a cost to another. Some biologists use the term symbiosis only for mutualistic associations, although scholarly literature and popular textbooks are ambiguous on this point.

Symbiosis was catapulted to prominence in evolutionary theory by the notion that mitochondria and chloroplasts (internal organelles within cells) originated through the endosymbiotic internalization of simpler prokaryotic cells. This theory has been championed by Lynn Margulis, who developed the serial endosymbiosis theory, which attempts to account for the successive development of all eukaryotic cells (cells with nuclei), through a sequence of unions between various prokaryotic bacteria (non-nucleated cells). While some details of serial endosymbiosis theory are still debated, the endosymbiotic origin of eukaryotes is found in virtually all textbooks.

Symbiosis theory has been extended in several profound but controversial ways. The notion of symbiogenesis suggests that symbiosis contributes significantly to the origin of novel traits and new species. Traditional Darwinian theory argues that speciation occurs by natural selection operating on random genetic mutations. Symbiogenesis posits that the symbiotic union of diverse genetic information is a source of creative novelty on which selection acts. Some symbioses, such as lichens, result in an altogether different kind of organism. Moreover, instead of the win-lose scenarios of competitive individual selection, symbiogenesis may more readily create win-win cooperative scenarios that entail new capabilities and resources. Symbiosis as a major evolutionary mechanism has significant though still debated implications, especially for notions of cooperation and complexity in evolutionary history.

Another provocative extension of symbiosis theory entails the scale at which symbiotic associations are conceived to exist. Traditional examples of symbiosis involve individual organisms in physical association with other individuals: for example, a plant and the nitrogen-fixing fungi in its roots. However, one could think of symbioses as involving groups of organisms, such as oxygen-breathing animals and oxygen-generating plants in a pond community. In principle, this could be extended to communities interacting in an ecosystem, or global ecosystems interacting with each other on a planetary scale. James Lovelock's notion of Gaia holds that the entire living world, or biosphere, interacts to regulate water, atmospheric gasses, pH, and temperature. Margulis and others suggest that this reflects the symbiotic integration of life into a global superorganism.

See also Competition; Evolution, Biological


Bibliography

margulis, lynn. symbiotic planet: a new look at evolution. new york: basic books, 1998.

paracer, surindar, and ahmadjian, vernon. symbiosis: an introduction to biological associations. new york: oxford university press, 2000.

sapp, jan. evolution by association: a history of symbiosis. new york: oxford university press, 1994.

seckbach, joseph, ed. symbiosis: mechanisms and model systems. dordrecht, netherlands: kluwer, 2002.

jeffrey p. schloss

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symbiosis

symbiosis (sĬmbēō´sĬs), the habitual living together of organisms of different species. The term is usually restricted to a dependent relationship that is beneficial to both participants (also called mutualism) but may be extended to include parasitism, in which the parasite depends upon and is injurious to its host; commensalism, an independent and mutually beneficial relationship; and helotism, a master-slave relationship found among social animals (e.g., the ant and the aphid). True symbiosis is illustrated by the relationship of herbivorous animals (e.g., cockroaches, termites, cows, and rabbits) to the cellulose-digesting protozoans or bacteria that live in their intestines; neither organism could survive without the other. Other symbiotic relationships include the interdependence of the alga and the fungus that form a lichen and the relationship between leguminous plants and the nitrogen-fixing bacteria, which is important in agriculture (see nitrogen cycle). Two obvious examples of a plant-to-animal relationship are yucca and yucca moth, fig and fig wasp; in both cases the insect fertilizes the plant, and the plant supplies food for the larvae of the insect.

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Symbiosis

Symbiosis

Symbiosis is simply defined as living together. Scientists use this term to describe intimate relationships between members of different species. By definition there are at least two species in a symbiotic relationship; it is unknown the maximum number of species that a symbiosis can sustain. This number may be very great; fungal partners (mycorrhizae) of plant roots link many photosynthetic plants of different species in one continuous networked symbiosis. Partners may belong to the same kingdom (for example, plants in symbiosis with other plant species) or may include partners from different kingdoms. A lichen symbiosis consists of partners from two or three kingdomsa fungus, a protist (algae), and often a cyanobacterium (eubacteria). The smaller partner(s) are usually called the symbiont(s) and the larger partner the host. The host's cells, body, body surface, or even its home may be shared with its symbionts.

To what extent must two species live together be considered a symbiosis? A general rule is that the partners must spend a significant amount of time together (part or all of their life cycles). This sustained contact enables a relationship to develop that affects how both species adapt and evolve. The symbiotic relationship is usually classified as belonging to one of three types: mutualism (benefiting both partners), parasitism (one partner, the parasite, benefits at the expense of the host), or commensalism (one partner benefits while the other is unaffected). However, it is too simplistic to place symbioses into such restrictive categories since the environment and ecological interactions with other species may affect the nature of the relationship. Under one set of conditions a relationship may be characterized as mutualistic, while under different conditions it may be parasitic. For example, the relative benefit for plants to host ants as a way to defend them against herbivores depends on the degree of herbivory and must be weighed against the cost of synthesizing the nutritional compounds needed to support resident ants. Both of these are subject to external influences.

Symbiosis provides an important source of evolutionary novelty. Special symbiont capabilities include photosynthesis and the transfer of photo-synthetic products from cyanobacteria and algae to animal and fungal hosts, and the supply of nutrients (nitrogen fixation by bacteria in legumes and

SYMBIOTIC ASSOCIATIONS INVOLVING PLANTS OR PHOTOSYNTHETIC ALGAE PARTNERS
Type of Symbiotic Relationship Partners Nature of Interaction
Symbionts Living in Host Lichen Symbionts: Algae, cyanobacteria (Rhizobium, actinomycetes, cyanobacteria) Algae provide photosynthetic sugars and cyanobacteria provide nutrients (nitrogen )
Host: Fungus Fungus provides protection against environmental extremes
Coral (and anemones) Symbionts: Algae Algae provide photosynthetic sugars
Host: Cnidarian (animal) Animal host provides recycled nutrients (nitrogen and phosphorus) and protection
Bacteria-plant Symbionts: Nitrogen-fixing bacteria Bacteria provide nutrients (nitrogen)
Host: Plant (legumes, alders, cycads, Azolla ferns) Plant host provides photosynthetic sugars
Symbionts Living on or in Intimate Contact With Host Ant-plant mutualisms Symbionts: Ant colonies Ants provide defense against herbivores, nutrients from colony wastes, protection
Host: Plant (e.g., acacia trees) Plant host provides nutrition (nectar, food bodies) and shelter (hollow thorns)
Plant-plant Symbionts: Parasitic plant (mistletoes, Rafflesia ) Gains photosynthetic sugars, water, and other nutrition from host
Host: Plant Harmed only
Mycorrhizae Symbionts: Fungus (ectomycorrhizae include many basidiomycete fungi; endomycorrhizae include many ascomycete fungi) Absorption of nutrients and water from soil; transfer of photosynthetic sugars among different plant species
Host: Plant (many partners) Plant host provides photosynthetic sugars

by cyanobacteria in cycads) to plant hosts. Other novel capabilities in marine animal symbioses include light production (luminous bacteria in marine fishes and invertebrates) and chemosynthesis by sulfur-reducing bacteria in hydrothermal vent host animals. In exchange for these services, the host provides the symbiont shelter and/or nutrition. These exchanges allow symbiotic relationships to thrive in marginal environments where resources such as energy and nutrients are limiting. Lichens are able to colonize bare rocks because the fungus provides shelter and protection against desiccation , its algal partners provide nutritional energy through photosynthesis, and its cyanobacteria provide nitrogen to the algae and to the fungal host. In the coral reef ecosystem , symbiotic algae called zooxanthellae photosynthesize and provide energy-rich sugars to their host corals. In nutrient-poor and sunlit tropical seawater, this symbiosis forms the base of the food web and supports the high diversity of all coral reef organisms. Other examples of mutualistic symbioses include the relationship between fungi and the roots of higher plants. Mycorrhizae (the fungal symbionts) associate with roots of higher plants and increase the water and nutrient uptake capabilities of plants. In return, they receive photosynthetic products from their host plants. In 1997, Suzanne Simard and her colleagues found that mycorrhizae connect and transport photosynthetic products between plants and trees in different environments. Other symbionts such as parasitic orchids take advantage of this association by connecting to the fungal network and withdrawing nutrients for their own use.

In parasitic symbioses, the parasite must avoid host defenses and obtain nutrients while remaining in or on the host. In so doing, the parasite often loses the ability to live independently. Plants such as dwarf mistletoes and the largest flower in the world, Rafflesia, have lost the ability to photosynthesize; they must derive nourishment from their photosynthetic host plants. These are considered to be obligate symbioses (one or more partners is dependent on another and cannot survive alone). Often these relationships include more than one partner, each with a different role in the symbiosis. For example, insect aphids specialized to suck plant juices from their hosts must rely on intracellular bacteria for essential amino acids not available in plant tissues.

The few examples of commensalistic symbioses are behavioral; one partner taking advantage of the activities of another to obtain food. It is unlikely that the unaffected partner is truly unaffected. If it is really unaffected, commensalism is difficult to define in the context of symbiosis. The definition of symbiosis usually assumes that an interaction is taking place, which means both partners must participate.

Flexibility in the type and amount of nutritional exchanges and in the roles of partners enables the symbiotic relationship to adapt and evolve over time to meet the different needs of the partners. A symbiont, host, or both may lose the ability to live independently because the partner has irrevocably assumed certain critical life functions. This concept is fundamental to the endosymbiotic theory of the origin of eukaryotic cells. Chloroplasts and mitochondria are the remnants of former symbionts that provided novel metabolic functions (photosynthesis and respiration) to their host cells.

Finally, symbiosis plays an important and often overlooked role in ecology. Nitrogen-fixing bacteria and mycorrhizal fungi provide nutrients to primary producers, and symbiotic associations like lichens are usually the first colonizers. Feeding interactions among symbiotic partners may increase the energy efficiency of food chains and promote nutrient recycling. When one thinks about saving species and biodiversity , the emphasis should be placed on understanding and preserving symbiotic relationships. If one partner is lost, all dependent partners will perish. It is rare that any species lives in isolation.

see also Coevolution; Endosymbiosis; Interactions, Plant-Fungal; Interactions, Plant-Insect; Interactions, Plant-Plant; Interactions, Plant-Vertebrate; Lichen; Mycorrhizae; Parasitic Plants.

Gisèle Muller-Parker

Bibliography

Douglas, A. E. Symbiotic Interactions. Oxford, England: Oxford University Press, 1994.

Margulis, Lynn. Symbiosis in Cell Evolution, 2nd ed. San Francisco: Freeman, 1993.

, and Rene Fester, eds. Symbiosis as a Source of Evolutionary Novelty. Cambridge, MA: M.I.T. Press, 1991.

Paracer, Surinder, and Vernon Ahmadjian. Symbiosis: An Introduction to Biological Associations, 2nd ed. Oxford: Oxford University Press, 2000.

Simard, S. W., D. A. Perry, M. D. Jones, D. D. Myrold, D. M. Durall, and R. Molina. "Net Transfer of Carbon Between Ectomycorrhizal Tree Species in the Field." Nature 388 (1997): 579-82.

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symbiosis

sym·bi·o·sis / ˌsimbēˈōsis; -bī-/ • n. (pl. -ses / -ˌsēz/ ) Biol. interaction between two different organisms living in close physical association, typically to the advantage of both. Compare with antibiosis. ∎  a mutually beneficial relationship between different people or groups: a perfect mother and daughter symbiosis. DERIVATIVES: sym·bi·ot·ic / -ˈätik/ adj. sym·bi·ot·i·cal·ly / -ˈätik(ə)lē/ adv.

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symbiosis

symbiosis An interaction between individuals of different species (symbionts). The term symbiosis is usually restricted to interactions in which both species benefit (see cooperation; mutualism), but it may be used for other close associations, such as commensalism, inquilinism, and parasitism. Many symbioses are obligatory (i.e. the participants cannot survive without the interaction); for example, a lichen is an obligatory symbiotic relationship between an alga or a cyanobacterium and a fungus.

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symbiosis

symbiosis A general term describing the situation in which dissimilar organisms live together in close association. As originally defined, the term embraces all types of mutualistic and parasitic relationships. In modern use it is often restricted to mutually beneficial species interactions, i.e. mutualism. Compare COMMENSALISM and PARASITISM. See also CONJUNCTIVE SYMBIOSIS and DISJUNCTIVE SYMBIOSIS.

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symbiosis

symbiosis A general term describing the situation in which dissimilar organisms live together in close association. As originally defined, the term embraces all types of mutualistic and parasitic relationships. In modern use it is often restricted to mutually beneficial species interactions (i.e. mutualism). Compare commensalism and parasitism.

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symbiosis

symbiosis General term describing the situation in which dissimilar organisms live together in close association. As originally defined, the term embraces all types of mutualistic and parasitic relationships. In modern use it is often restricted to mutually beneficial species interactions, i.e. mutualism. Compare COMMENSALISM; PARASITISM.

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symbiosis

symbiosis A general term describing the situation in which dissimilar organisms live together in close association. As originally defined, the term embraces all types of mutualistic and parasitic relationships. In modern use it is often restricted to mutually beneficial species interactions.

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symbiosis

symbiosis Relationship between two or more different organisms that is generally mutually advantageous. It is more accurately referred to as mutualism. If only one partner benefits, this is called commensalism. See also parasite

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symbiosis

symbiosis (sim-by-oh-sis) n. an intimate and obligatory association between two different species of organism (symbionts) in which there is mutual aid and benefit. Compare commensal, parasite.

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symbiosis

symbiosisglacis, Onassis •abscess •anaphylaxis, axis, praxis, taxis •Chalcis • Jancis • synapsis • catharsis •Frances, Francis •thesis • Alexis • amanuensis •prolepsis, sepsis, syllepsis •basis, oasis, stasis •amniocentesis, anamnesis, ascesis, catechesis, exegesis, mimesis, prosthesis, psychokinesis, telekinesis •ellipsis, paralipsis •Lachesis •analysis, catalysis, dialysis, paralysis, psychoanalysis •electrolysis • nemesis •genesis, parthenogenesis, pathogenesis •diaeresis (US dieresis) • metathesis •parenthesis •photosynthesis, synthesis •hypothesis, prothesis •crisis, Isis •proboscis • synopsis •apotheosis, chlorosis, cirrhosis, diagnosis, halitosis, hypnosis, kenosis, meiosis, metempsychosis, misdiagnosis, mononucleosis, myxomatosis, necrosis, neurosis, osmosis, osteoporosis, prognosis, psittacosis, psychosis, sclerosis, symbiosis, thrombosis, toxoplasmosis, trichinosis, tuberculosis •archdiocese, diocese, elephantiasis, psoriasis •anabasis • apodosis •emphasis, underemphasis •anamorphosis, metamorphosis •periphrasis • entasis • protasis •hypostasis, iconostasis

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