Fig 8. Model for an ecological advantage of a stable association between Drosophila melanogaster and beneficial gut bacteria.

(A) In the absence of stable gut bacteria, the fitness of D. melanogaster is dependent on the presence of more (red) or less (blue) beneficial bacteria in the food substrate. (B) Carrying a stable population of beneficial bacteria (green) in the gut allows constant bacterial inoculation of food substrate and consequent association with the next host generation. This leads to a higher fitness of this next generation.