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چکیده : 

Controversy persists as to whether the acquisition of beneficial metabolic functions via endosymbiosiscan occur suddenly on an volutionary time scale. In this study, an early stage of endosymbiotic associa-tions, which evolved from previously unassociated auto (photo)- and heterotrophic unicellular organismswas analyzed using an experimental ecosystem model, called CET microcosm. This ecosystem modelwas composed of a green alga (Micractinium sp.; formerly described as Chlorella vulgaris), a bacterium(Escherichia coli), and a ciliate (Tetrahymena thermophila). Our previous study using a CET microcosm thatwas cultured 3–5 years revealed that fitness of the ciliate increased by harboring algal cells within itsown cells. This fact suggested three possibilities: (i) the ciliate evolved the ability to exploit intracellularalgal cells (“exploiter ciliate hypothesis”), (ii) the alga evolved the ability to benefit the host ciliate byproviding photosynthates (“cooperator alga hypothesis”), and (iii) a combination of (i) and (ii). To testthese hypotheses, two-by-two co-cultures were conducted between the ancestral or derived ciliate andthe ancestral or derived alga. The experimental results demonstrated that a cooperative alga evolved inthe microcosm, although the possibility remains that an exploitative genotype of the ciliate might alsoexist in the population as a polymorphism. Remarkably, an algal isolate prolonged the longevity of notonly the isolated ciliate, but also the ancestral ciliate. This result suggests that once a cooperative algalgenotype evolves in a local population, it can then be transmitted to other individuals of the prospectivehost species and spread rapidly beyond the local range due to its positive effect on the host fitness. Suchtransmission suggests the possibility of a sudden acquisition of beneficial autotrophic function by thepre-associated host.