organisms. The definition of symbiosis has been applied to a wide range of biological relationships, which can be classified mutualism, commensalism, or parasitism A more narrow definition of symbiosis limits the usage to only interactions from which both organisms benefit, in which case it would be equivalent to mutualism.Symbiosis describes close and often long-term interactions between two or more different
Symbiotic interactions may be either obligate, i.e., necessary for the survival of at least one of the organisms involved, or facultative, where the relationship is beneficial but not essential for survival of the organisms. The term symbiosis was first used in 1879 by the mycologist Heinrich Anton de Bary, who termed it "the living together of unlike organisms".
There are two fundamental geometric relationships between the two bionts involved in any given symbiosis: endosymbiosis and exosymbiosis. Endosymbiosis is a relationship in which one symbiont lives within the tissues of the other partner, either within the cells or outside cells. Endosymbiosis is particularly notable in the cladistics of algae. In particular, primary endosymbiosis has involved the capture of a cyanobacterium by a heterotrophic ancestor of the plantae; secondary endosymbiosis likely involved the entrapment of a red or green algae. Tertiary endosymbiosis involves replacement of a plastid from a secondary endosymbiosis with another plastid which was also derived from a secondary endosymbiosis. Other common instance of endosymbiosis include nitrogen-fixing bacteria that live inside root nodules of legume roots; actinomycete nitrogen-fixing bacteria, which reside in a number of tree and grass root nodules (a widely studied species being sugarcane); and bacterial endosymbionts that provide essential nutrients for large numbers of insect species.
Exosymbiosis, is a counterpart relationship in which the symbiont lives upon the host's body surface, including inner surfaces of the alimentary tract or exocrine gland ducts. Examples include ectoparasites such as ticks, commensal exosymbionts such as light emitting organisms attached to teleost fish, and mutualist ectosymbionts such as cattle egrets.
Mutualism is an interaction between two species in which both benefit. One of the species may be so dependent that it cannot live without its mutualistic partner (obligate mutualism). In some cases, a species can interact mutualistically with more than one partner (diffuse mutualism) or even live without its partner(s) under certain conditions (facultative mutualism). Although all species involved in a mutualistic relationship contribute to the partnership, there is the expectation that each of the symbionts will exhibit a self-interest and thus evolve traits that provide the maximum possible fitness benefit while minimizing biological costs.
Commensalism is a type of symbiosis where one organism benefits, but the other is unaffected. This classification is frequently debated since it is sometimes difficult for researchers to perceive the benefits to one of the commensal partners. The largest class of commensal examples are numerous ordhids, mosses and ferns which grow atop higher plants. In many of these cases the fern or moss recieves benefits of structural support, greater access to sunlight and nutrient supply (e.g. dissolved nutrients dripping down a host tree's bark).
Parasitism is a relationship of two different species where one organism termed the parasite, benefits at the expense of the host species. Originally the term parasite referred to organisms with lifestages that utilized multiple host species in succession (e.g. Taenia solium), which are now called macroparasites (typically protozoa and helminths). Parasites also include microorganisms such as viruses and bacteria, which can be directly transmitted between hosts of one species to another.
A parasite is typically much smaller than its host, showing a high degree of specialization for its mode of life, and reproducing at a more rapid rate than the host. Classic examples of parasitism include interactions between vertebrate hosts and diverse animals such as tapeworms, trematodes, the Plasmodia species, and fleas. Parasitism is differentiated from parasitoidism, the latter being a relationship in which the host necessarily dies.
Symbiosis has exerted a prime influence on co-evolution processes, by allowing preferential survival of phenotypes that have superior adaptive response to their symbiont partners. For example angiosperms and the insects that pollinate them have co-evolved to their mutual benefit in adaptation. In particular numerous flowering plants have very specialized inflorescences modified to optimize pollination efficiency. Early flowering plants in the fossil record had simple flowers. Adaptive speciation led to diverse groups of plants, and speciation occurred in parallel within certain insect groups. Some flora developed nectar and large sticky pollen, while insects evolved more specialized features to access and collect these rich food sources. In some cases, co-evolution may lead to a high degree of specialization whereby species are interdependent for surivival.
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