Endosymbiosis: infection and cure.

Symbiosis is a term met by beginning biology students well before they reach college. A dictionary definition includes, "the living together in more or less intimate association or even close union of two dissimilar organisms."' There are two classes of symbiotes-ectosymbiotes and endosymbiotes. Ectosymbiosis is a less controversial topic because the partners are fairly easily separable and identified as individuals. Studies of endosymbiosis, on the other hand, are made lively by arguments about the very nature of the structure under study, i.e., is it an organelle or a symbiote? Is any organelle a previously captured symbiote that has become integrated into the host cell ? We are here restricting our coverage to infection and cure of endosymbiosis because successful demonstration of these two biological processes reveals the nature of the endosymbiotic relationship. Infection implies the taking in and successful perpetuation of one organism within another through many cell divisions and therefore bears on questions of mode of entrance of the potential symbiote, escape from digestion by the potential host (initial compatibility), and metabolic accommodation and synchrony between members of the symbiotic pair. Cure usually implies dissociation of the symbiotic partners followed by independent existence of one or both. Thus, release and independent perpetuation of the symbiote also must be considered under cure if one agrees that the symbiote is, by definition, also an organism. Neither studies of infection nor cure demand that methods must mimic those which are at least candidates for the natural process. Because of the ubiquity of symbiotic relationships, we are selecting a few areas in which either a modus operandi has been established or where there are lively controversies. Relationships in which symbiotes become at least