Abstract
BackgroundWhile mutualistic interactions between different genotypes are pervasive in nature, their evolutionary origin is not clear. The dilemma is that, for mutualistic interactions to emerge and persist, an investment into the partner genotype must pay off: individuals of a first genotype that invest resources to promote the growth of a second genotype must receive a benefit that is not equally accessible to individuals that do not invest. One way for exclusive benefits to emerge is through spatial structure (i.e., physical barriers to the movement of individuals and resources).ResultsHere we propose that organisms can evolve their own spatial structure based on physical attachment between individuals, and we hypothesize that attachment evolves when spatial proximity to members of another species is advantageous. We tested this hypothesis using experimental evolution with combinations of E. coli strains that depend on each other to grow. We found that attachment between cells repeatedly evolved within 8 weeks of evolution and observed that many different types of mutations potentially contributed to increased attachment.ConclusionsWe postulate a general principle by which passive beneficial interactions between organisms select for attachment, and attachment then provides spatial structure that could be conducive for the evolution of active mutualistic interactions.
Highlights
While mutualistic interactions between different genotypes are pervasive in nature, their evolutionary origin is not clear
We found that the mutualistic partners could grow when they were inoculated together but could not grow when they were inoculated in isolation, demonstrating that we could create the expected passive mutualistic interaction
Evolution of cell aggregation We examined whether the created passive mutualistic interaction promotes the subsequent evolution of cell aggregation
Summary
While mutualistic interactions between different genotypes are pervasive in nature, their evolutionary origin is not clear. The dilemma is that, for mutualistic interactions to emerge and persist, an investment into the partner genotype must pay off: individuals of a first genotype that invest resources to promote the growth of a second genotype must receive a benefit that is not accessible to individuals that do not invest. How mutualistic interactions emerge and persist is not clear [9,10,11]. Starting from a situation where two microbial genotypes exist in the same environment but do not affect each other’s growth, how could a mutant of one genotype emerge that excretes a metabolite that positively affects the growth of another genotype and increases in frequency, laying the foundation for a mutualistic interaction to evolve? The mutant might not be able to increase in frequency relative to its ancestor and a mutualistic interaction could not establish
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