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 <em>Wolbachia</em> strain <em>w</em>MelPop drastically reduces its <em>Drosophila melanogaster</em> host lifespan at high rearing temperatures. It is generally assumed that this feature is maintained because the <em>D. melanogaster</em>–<em>w</em>MelPop symbiosis is usually not exposed to environmental conditions in which the symbiont is virulent. To test this hypothesis, we submitted <em>w</em>MelPop-infected <em>D. melanogaster</em> lines to 17 generations of experimental evolution at a high temperature, while enforcing late reproduction. The fly survival was measured at different time points, as well as two traits that have been proposed to be causally responsible for <em>w</em>MelPop virulence: its relative density and the mean number of octomom copies present in its genome. We hypothesised that these conditions would select for a reduced <em>w</em>MelPop virulence, a reduced <em>w</em>MelPop density, and a reduced octomom copy number. Our results indicate that density, octomom copy number and virulence are correlated. 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 <em>w</em>MelPop virulence and propose that intra-host selection could explain this conundrum.