Symbiosis Plasmids Bring Their Own Mutagen to the Wedding Party

Entering into a symbiotic relationship is not something to be taken lightly. Previously free-living organisms become beholden to each other and have to tolerate invasion of their personal space to accommodate varying degrees of intimacy, especially when one partner lives within the body or even the cells of the other. Symbiosis brings with it so many potential mutual benefits that it has arisen independently many times in evolution, but overcoming these barriers requires some encouragement. Among bacteria this independent emergence of symbiosis can be encouraged by the horizontal transfer of ‘‘symbiosis plasmids’’—DNA-encoded toolkits that help the recipient bacterium acquire the crucial attributes needed to abandon the single life and settle down with a symbiotic host. A classic example of this transition is the establishment of mutualistic relationships between ‘‘rhizobial’’ bacteria and the roots of leguminous plants. Here a taxonomically diverse range of free-living bacteria have acquired the ability to infect plant root cells, to trigger the formation of nodule organs, and to fix nitrogen from the atmosphere that can then be used for biosynthesis by bacteria and plants alike. As a quid pro quo, the plant provides other nutrients and shelter to the bacteria. Although the symbiosis plasmids might be expected to magically confer symbiotic properties on any bacteria that happens to encounter them, in practice, the horizontal transfer of symbiosis is quite hard to achieve in the lab between distantly related species. This perhaps suggests that the recipient bacteria need to undergo further adaptions, beyond the acquisition of a plasmid, in order to go down the symbiotic route. Catherine Masson-Boivin’s group has previously published a study in PLOS Biology in which they identified some of the adaptations needed. They took a nonsymbiotic soil bacterium Ralstonia solanacearum (a pathogen that causes wilt in a wide range of plants) and gave it the symbiosis plasmid from the symbiotic bacterium Cupriavidus taiwanensis, which forms nodules on the leguminous plant Mimosa pudica (more poetically known as touch-me-not). Despite carrying the genes needed to fix nitrogen and to send seductive signals to the host plant, the plasmid wasn’t sufficient to allow the new bacteria to set up a symbiotic relationship with Mimosa. Instead, this depended on the rare acquisition of further mutations that disrupted Ralstonia genes normally involved in virulence, giving rise to spontaneously arising nodulating strains. In subsequent work, these authors followed up their findings, setting up an elegant experimental evolution study in which they set out to reproduce the continuing adaptive process and watch it in action. They passed three replicates of each of three mutant nodulating chimeric Ralstonia bacteria through 17 cycles of alternating culture in liquid plant medium (ex planta) and within plant roots (in planta). At the end of this process, which involved about 400 generations of bacterial evolution, they found substantial further improvements in the ability of all nine bacterial lines to nodulate Mimosa. Now, in a new PLOS Biology article, Philippe Remigi, Masson-Boivin, and colleagues look at the mutation process that gave rise to this increased performance and find some surprises. They sequenced the genomes of the nine evolved bacteria strains and discovered an unexpectedly high number of mutations, given the modest amount of evolutionary time