Wolbachia dynamics and host effects: what has (and has not) been demonstrated?

The vertically transmitted endoparasitic bacterium Wolbachia infects a wide range of invertebrates, including insects, crustaceans, mites and nematodes. These bacteria have received increasing attention over the past decade because they have been implicated in causing an array of effects in their hosts. But how convincing is the evidence that supports these conjectures and are we overemphasizing the importance of Wolbachia as an evolutionary force in host evolution? We argue that, although Wolbachia effects are widespread in arthropods, other factors, including nuclear effects and different endosymbionts might have been overlooked. In addition, factors that can influence the expression of Wolbachia effects are rarely considered in laboratory studies and this makes extrapolation to the field difficult. If we are to understand the role that Wolbachia plays in host evolution, a more thorough approach to documenting their effects is required.