Phenotypic parallelism during experimental adaptation of a free-living bacterium to the zebrafish gut

Despite the fact that animals encounter a plethora of bacterial species throughout their lives, only a subset are capable of colonizing vertebrate digestive tracts, and these bacteria can profoundly influence the health and development of their animal hosts. However, it is still unknown how bacteria evolve symbioses with animal hosts, and this process is central to both the assembly and function of gut bacterial communities. Therefore, we used experimental evolution to study a free-living bacterium as it adapts to a novel vertebrate host. We serially passaged replicate populations of Shewanella oneidensis , through the digestive tracts of larval zebrafish ( Danio rerio ). After only 20 passages, representing approximately 200 bacterial generations, isolates from replicate evolved populations displayed an improved ability to colonize larval zebrafish digestive tracts during competition against their unpassaged ancestor. Upon sequencing the genomes of these evolved isolates, we discovered that the two isolates with the highest mean competitive fitness accumulated unique sets of mutations. We characterized the swimming motility and aggregation behavior of these isolates, as these phenotypes have previously been shown to alter host-microbe interactions. Despite exhibiting different biofilm characteristics, both isolates evolved augmented swimming motility. These enhancements are consistent with expectations based on the behavior of a closely related Shewanella strain previously isolated from the zebrafish digestive tract and suggest that our evolved isolates are pursuing a convergent adaptive trajectory with this zebrafish isolate. In addition, parallel enhancements in swimming motility among isolates from independently adapted populations implicates increased dispersal as an important factor in facilitating the onset of host association. Our results demonstrate that free-living bacteria can rapidly improve their associations with vertebrate hosts.