Abstract
Facultative animal-bacteria symbioses, which are critical determinants of animal fitness, are largely assumed to be mutualistic. However, whether commensal bacteria benefit from the association has not been rigorously assessed. Using a simple and tractable gnotobiotic model- Drosophila mono-associated with one of its dominant commensals, Lactobacillus plantarum-we reveal that in addition to benefiting animal growth, this facultative symbiosis has a positive impact on commensal bacteria fitness. We find that bacteria encounter a strong cost during gut transit, yet larvae-derived maintenance factors override this cost and increase bacterial population fitness, thus perpetuating symbiosis. In addition, we demonstrate that the maintenance of the association is required for achieving maximum animal growth benefits upon chronic undernutrition. Taken together, our study establishes a prototypical case of facultative nutritional mutualism, whereby a farming mechanism perpetuates animal-bacteria symbiosis, which bolsters fitness gains for both partners upon poor nutritional conditions.