Induced symbiosis: distinctive Escherichia coli-Dictyostelium discoideum transferable co-cultures on agar.

Despite the near ubiquity of symbiosis, only a few new symbiotic associations have been reported. The establishment of the unique amoeba-bacterial symbiosis observed by Jeon and his colleagues has been difficult to retrace experimentally mainly because of the failure to grow both partners in pure culture. The details of symbiosis origin and especially laboratory induction are unknown in all cases. Here, we present an experiment in which specific strains of four-year subculture Escherichia coli and Dictyostelium discoideum evolved interdependently to produce a new morphological entity on agar plates. The cocultured organisms lost their pure culture identities under the conditions in which both control organisms retained their independent culturability. Between days 32 and 101 of culturing of E. coli and between days 259 and 645 in D. discoideum pure culture identity was lost. Yet through the four years both organisms could always be cocultured and stored frozen. We traced the emergence of characteristic changes toward a repeatedly inducible symbiotic relationship in pure cultures of both cocultured organisms. Since both the enteric bacterium and the cellular slime mold are free-living and culturable in pure culture, genetically well-characterized. We provide a useful model for the laboratory study of symbiotic evolution.