Bacterial adaptation to diet is a key evolutionary force shaping Drosophila-Lactobacillus symbiosis

Animal-microbe facultative symbioses play a fundamental role in ecosystem and organismal health (1–3). Yet, due to the flexible nature of their association, the selection pressures acting on animals and their facultative symbionts remain elusive (4, 5). Here, by applying experimental evolution to a well-established model of facultative symbiosis: Drosophila melanogaster associated with Lactobacillus plantarum , one of its growth promoting symbiont (6, 7), we show that the diet, instead of the host, is a predominant driving force in the evolution of this symbiosis and identify the mechanism resulting from the bacterial adaptation to the diet, which confers host growth benefits. Our study reveals that adaptation to the diet can be the foremost step in the determination of the evolutionary course of a facultative symbiosis.