Wolbachia strengthens the match between pre-mating and early post-mating isolation in spider mites

Endosymbiotic reproductive manipulators are widely studied as sources of post-zygotic isolation in arthropods, but their effect on pre-zygotic isolation between genetically differentiated populations has garnered less attention. We tested this using two partially isolated populations of the red and green colour forms of Tetranychus urticae, either uninfected or infected with different Wolbachia strains, one inducing cytoplasmic incompatibility and the other not. We first investigated male and female preferences, and found that, in absence of infection, females were not choosy but all males preferred red-form females. Wolbachia effects were more subtle, with only the CI-inducing strain slightly strengthening colour-form based preferences. We then performed a double-mating experiment to test how incompatible matings affect subsequent mating behaviour and offspring production, as compared to compatible matings. Females mated with an incompatible male (infected and/or heterotypic) were more attractive and/or receptive to subsequent (compatible) matings, although analyses of offspring production revealed no clear benefit for this re-mating behaviour (i.e., apparently unaltered first male sperm precedence). Finally, by computing the relative contributions of each reproductive barrier to total isolation, we showed that pre-mating isolation matches both host-associated and Wolbachia-induced post-mating isolation, suggesting that Wolbachia could contribute to reproductive isolation in this system.