Abstract
To understand how two organisms that have not previously been in contact can establish mutualism, it is first necessary to examine temporal changes in their phenotypes during the establishment of mutualism. Instead of tracing back the history of known, well-established, natural mutualisms, we experimentally simulated the development of mutualism using two genetically-engineered auxotrophic strains of Escherichia coli, which mimic two organisms that have never met before but later establish mutualism. In the development of this synthetic mutualism, one strain, approximately 10 hours after meeting the partner strain, started oversupplying a metabolite essential for the partner's growth, eventually leading to the successive growth of both strains. This cooperative phenotype adaptively appeared only after encountering the partner strain but before the growth of the strain itself. By transcriptome analysis, we found that the cooperative phenotype of the strain was not accompanied by the local activation of the biosynthesis and transport of the oversupplied metabolite but rather by the global activation of anabolic metabolism. This study demonstrates that an organism has the potential to adapt its phenotype after the first encounter with another organism to establish mutualism before its extinction. As diverse organisms inevitably encounter each other in nature, this potential would play an important role in the establishment of a nascent mutualism in nature.
Highlights
Mutualism is based on a mutually beneficial interaction between two organisms and is ubiquitous in nature [1,2,3,4,5,6]
We found that the coculture-specific changes in gene expression in L– cells were not related to the local activation of the biosynthesis and transport of isoleucine, but were related to the global activation of anabolic metabolism
In our synthetic model of obligate mutualism comprising two auxotrophs of E. coli, strains I– and L, the increase in the Ile supply from L– cells occurred before the population growth of L
Summary
Mutualism is based on a mutually beneficial interaction between two organisms and is ubiquitous in nature [1,2,3,4,5,6]. The findings of that study clearly demonstrated the importance of spatially structured environments for the establishment of mutualism, providing proof of principle of natural selection of cooperative behavior that has been proposed by the theoretical studies [10,11,12,13] These types of experimental studies using microbial ecosystems to test the theories of cooperative systems have recently been reported [14,15,16,17,18]. Most of these studies focused not on the adaptation of the organisms but on the environmental conditions required for the persistence of cooperative behavior in natural selection
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