Sorting out the effects of Wolbachia, genotype and inbreeding on life-history traits of a spider mite.

Wolbachia bacteria manipulate host reproduction by inducing cytoplasmic incompatibility (CI) and sex ratio distortion. Wolbachia are transmitted from mother to offspring through the cytoplasm of the egg. Therefore, reproduction of Wolbachia is tightly coupled to reproduction of its host. Mathematical analysis predicts that in the course of evolution, traits that reduce the physiological costs of the infection will be selectively favored. For a Wolbachia-host system to evolve, traits under selection must have some genetic component and variation must be present in the population. We have previously established that highly inbred isofemale lines of the two-spotted spider mite Tetranychus urticae may differ regarding the effects of infection by Wolbachia, and that at least some of the traits affected had a genetic component. However, the effects measured could have been affected by the fact that the lines were severely inbred prior to the experiments. In this paper we attempt to distinguish between the effects of Wolbachia, isofemale line, and inbreeding. We show that Wolbachia did not affect longevity but infected females produced smaller clutch sizes, more daughter-biased sex ratios and had decreased F1 mortality; between-line variation was found for clutch size, F1 mortality and sex ratio; finally, inbreeding resulted in an overall reduction of clutch sizes, and a change in survival curves and mean longevity.