Bacterial Adaptation to the Host's Diet Is a Key Evolutionary Force Shaping Drosophila-Lactobacillus Symbiosis

Animal-microbe facultative symbioses play a fundamental role in ecosystem and organismal health. Yet, due to the flexible nature of their association, the selection pressures that act on animals and their facultative symbionts remain elusive. Here we apply experimental evolution to Drosophila melanogaster associated with its growth-promoting symbiont Lactobacillus plantarum, representing a well-established model of facultative symbiosis. We find that the diet of the host, rather than the host itself, is a predominant driving force in the evolution of this symbiosis. Furthermore, we identify a mechanism resulting from the bacterium's adaptation to the diet, which confers growth benefits to the colonized host. Our study reveals that bacterial adaptation to the host's diet may be the foremost step in determining the evolutionary course of a facultative animal-microbe symbiosis. [Display omitted] •L. plantarum experimental evolution leads to the improvement of its symbiotic benefit•L. plantarum increases its growth-promotion ability by adapting to Drosophila diet•Mutation of ackA gene enhances both L. plantarum fitness and benefit to the host•N-acetyl-glutamine production is sufficient to improve L. plantarum growth promotion Martino et al. demonstrate that, in the symbiosis between Drosophila and Lactobacillus plantarum, the host diet represents the driving force in the evolution of L. plantarum symbiotic effect. This is a clear example of by-product mutualism, where the host capitalizes on the by-products of the self-serving traits of their symbionts.