Experimental evolution of Escherichia coli under inefficient selection reveals the evolutionary trajectories of symbiotic bacteria

Author(s): Fares MA, Toft C, Ruiz-González MX, Alvarez-Ponce D

Summary:

Many bacteria that establish mutualistic associations with insects are transmitted from mother to offspring under a vertical regime. This transmission mode imposes strong bottlenecks on the effective population sizes of bacteria, resulting in a high genetic drift effect and, consequently, increased genetic variability among populations and decreased polymorphism within populations. Inactivation and degradation of genes that are unnecessary in an intracellular environment is a common symptom of symbiotic lifestyle. Two main questions remain unanswered with regards to the evolution of these bacteria: (a) how do essential genes for symbiosis counteract the effect of mutations fixed under inefficient selection in the genome? And (b) what mechanisms increase the mutational robustness of symbiotic genes? Here we have conducted an evolution experiment using the bacterium Escherichia coli to simulate the mutation dynamics under inefficient natural selection occurring in symbiotic bacteria. Evolution of E. coli for more than 5000 generations reveals the network of interactions between mutations, the set of useful, although not essential, genes for symbiosis and the role of GroEL in mutational robustness. Our results unveil a more complex scenario for the evolution of symbiosis than anticipated by previous studies.

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