Variable fitness effects of Wolbachia infection in Drosophila melanogaster

Maternally inherited Wolbachia bacteria are extremely widespread among insects and their presence is usually associated with parasitic modifications of host fitness. Wolbachia pipientis infects Drosophila melanogaster populations from all continents, but their persistence in this species occurs despite any strong parasitic effects. Here, we have investigated the symbiosis between Wolbachia and D. melanogaster and found that Wolbachia infection can have significant survival and fecundity effects. Relative to uninfected flies, infected females from three fly strains showed enhanced survival or fecundity associated with Wolbachia infection, one strain showed both and one strain responded positively to Wolbachia removal. We found no difference in egg hatch rates (cytoplasmic incompatibility) for crosses between infected males and uninfected females, although there were fecundity differences. Females from this cross consistently produced fewer eggs than infected females and these fecundity differences could promote the spread of infection just like cytoplasmic incompatibility. More surprising, we found that infected females often had the greatest fecundity when mated to uninfected males. This could also promote the spread of Wolbachia infection, though here the fitness benefits would also help to spread infection when Wolbachia are rare. We suggest that variable fitness effects, in both sexes, and which interact strongly with the genetic background of the host, could increase cytoplasmic drive rates in some genotypes and help explain the widespread persistence of Wolbachia bacteria in D. melanogaster populations. These interactions may further explain why many D. melanogaster populations are polymorphic for Wolbachia infection. We discuss our results in the context of host–symbiont co-evolution.