Experimental evolution of virulence and associated traits in a Drosophila melanogaster–Wolbachia symbiosis

Evolutionary theory predicts that vertically transmitted symbionts are selected for low virulence, as their fitness is directly correlated to that of their host. In contrast with this prediction, the Wolbachia strain w MelPop drastically reduces its Drosophila melanogaster host lifespan at high rearing temperatures. It is generally assumed that this feature is maintained because the D. melanogaster - w MelPop symbiosis is usually not exposed to environmental conditions in which the symbiont is virulent. To test this hypothesis, we submitted w MelPop-infected D. melanogaster lines to 17 generations of experimental evolution at a high temperature, while enforcing late reproduction by artificial selection. The fly survival was measured at different time points, as well as two traits that have been proposed to be causally responsible for w MelPop virulence: its relative density and the mean number of copies of octomom, an 8-genes region of the Wolbachia genome. We hypothesised that these conditions (high temperature and late reproduction) would select for a reduced w MelPop virulence, a reduced w MelPop density, and a reduced octomom copy number. Our results indicate that density, octomom copy number and virulence are correlated to each other. However, contrary to our expectations, we could not detect any reduction in virulence during the course of evolution. We discuss the significance of our results with respect to the evolutionary causes of w MelPop virulence.