ABSTRACT
Symbiosis is a term that is widely used in multiple elds. In the social sciences, the term “symbiosis” may apply to any bene cial association between distinct units (e.g., parts of the self in Jungian philosophy; see Jung, 1959); or symbiosis may refer to an unhealthy dependence between two people that detracts from individuals realizing their full independent potentials (Horner, 1985). In recent years, the term has been used to refer to working or living in harmony with the biotic and abiotic environment (see Chertow, 2000). In popular biology, the term is sometimes used to denote any association between two living organisms of different species (Margulis and Sagan, 1986). But this popular view is an over simpli cation of a more complex and variable relationship (Margulis and Fester, 1992). Most organisms that engage in symbiotic interactions may also exist independently and a range of outcomes of the interaction is possible depending on many variables. Further, in an evolutionary sense, symbiosis is not a phenomenon of individual organisms but rather of populations. In this sense the term symbiosis refers to any short-or long-term association between populations of different species where the survival or “evolutionary tness” of one or more population partners is enhanced by the association. Symbioses may be diffuse associations as in some generalist pathogen-host associations, where the pathogen population may associate with multiple host species. Symbioses may also be very speci c as is seen in the case of many biotrophic fungal plant pathogens, e.g., powdery mildews, where the pathogens are closely adapted for growth on a single host species or a very narrow range of closely related host species. Symbioses may be transitory as in some facultative plant pathogens like Pythium spp., where the pathogen may exist for part of its life as a saprotroph and may irregularly exist as a plant pathogen; or they may be continuous as in obligate endosymbionts of plants like Epichloë/Neotyphodium species that cannot exist outside the host plant.
