Lawrence Berkeley National Laboratory

Mutualistic interactions are taxonomically and functionally diverse. Despite their ubiquity, the basic ecological and evolutionary processes underlying their origin and maintenance are poorly understood. A major reason for this has been the lack of an experimentally tractable model system. We examine the evolution of an experimentally imposed obligate mutualism between sulfate-reducing and methanogenic microorganisms that have no known history of prior interaction. Twenty-four independent pairings (cocultures) of the bacterium Desulfovibrio vulgaris and the archaeon Methanococcus maripaludis were established and followed for 300 community doublings in two environments, one allowing for the development of a heterogeneous distribution of resources and the other not. Evolved cocultures grew up to 80percent faster and were up to 30percent more productive (biomass yield per mole substrate) than the ancestors. The evolutionary process was marked by periods of significant instability leading to extinction of two of the cocultures, but resulted in more stable, efficient, and productive mutualisms for most replicated pairings. Comparisons of evolved cocultures with those assembled from one evolved and one ancestral mutualist showed that evolution of both species contributed to improved productivity. Surprisingly, however, overall improvements in growth rate and yield were less than the sum of individual contributions, suggesting antagonistic interactions between mutations from the coevolved populations. Physical constraints on the transfer of metabolites in the evolution environment affected the evolution of M. maripaludis but not D. vulgaris. Together, these results show that challenges can imperil nascent obligate mutualisms and demonstrate the evolutionary responses that enable their persistence and future evolution.