Symbiosis

In the broad sense, symbiosis means simply "living together"—the union of two separately evolved organisms into a single functional unit regardless of the positive or negative influence on either species .

Symbiotic relationships fall into three categories. Mutualism describes the condition in which both organisms benefit from the relationship, commensalism exists when one organism benefits and the other is unaffected, and parasitism describes a situation in which one organism benefits while the other suffers. Symbionts may also be classified according to their mode of life. Endosymbionts carry out the relationship within the body of one species, the host. Exosymbionts are attached to the outside of the host in a variety of ways, or are unattached, seeking contact for specific purposes or at particular times. Similarly, symbionts may either be host-specific, co-evolved to one species in particular, or a generalist, able to make use of many potential partners. In either case, the relationship usually involves co-evolutionary levels of integration, where partners influence each other's fitness.

Symbiosis generally requires either behavioral or morphological adaptation . Morphological adaptation ranges from the development of special organs for attachment, color patterns that signal partners to chemical cues, and intercellular alterations to accommodate partners. Behavioral adaptations include a range of body postures and displays that indicate readiness to accommodate symbionts.

Symbiosis is fundamental to life. The various forms of relationships between species—principally symbiosis,competition , and predation—create the structure of ecosystems and produce co-evolutionary phenomena which shape species and communities. At the most fundamental level, the transition from non-nucleated prokaryotic cells to eukaryotic cells, which is the evolutionary link to multicellular organisms, is generally believed to be by symbiosis. In higher organisms, 90% of land plants use some sort of association between roots and mycorrhizal fungi to provide nutrient uptake from soil . Almost 6,000 species of fungi and about 300,000 land plants can form mycorrhizal associations. In the next step of the terrestrial food chain, almost all herbivorous species of insects and mammals use symbionts in the gut to digest cellulose in plant cell walls and allow nutrient assimilation. Thus, the energetics of life on earth, and certainly that of higher life forms, are largely dependent on symbiotic relationships.

See also Mycorrhiza; Parasites

[David A. Duffus ]

RESOURCES

BOOKS

Margulis, L., and R. Fester, eds. Symbiosis As a Source of Evolutionary Innovation. Cambridge, MA: MIT Press, 1991.