Wolbachia and host intrinsic reproductive barriers contribute additively to post-mating isolation in spider mites

Wolbachia are widespread maternally-inherited bacteria suggested to play a role in arthropod host speciation through induction of cytoplasmic incompatibility, but this hypothesis remains controversial. Most studies addressing Wolbachia -induced incompatibilities concern closely-related populations, which are intrinsically compatible. Here, we used three populations of two genetically differentiated colour forms of the haplodiploid spider mite Tetranychus urticae to dissect the interaction between Wolbachia -induced and host-associated incompatibilities, and to assess their relative contribution to post-mating isolation. We found that these two sources of incompatibility act through different mechanisms in an additive fashion. Host-associated incompatibility contributes 1.5 times more than Wolbachia -induced incompatibility in reducing hybrid production, the former through an overproduction of haploid sons at the expense of diploid daugters ( ca . 75% decrease) and the latter by increasing the embryonic mortality of daughters (by ca . 49%). Furthermore, regardless of cross direction, we observed near-complete F1 hybrid sterility and complete F2 hybrid breakdown between populations of the two forms, but that Wolbachia did not contribute to this outcome. This study identifies the mechanistic independence and additive nature of host-intrinsic and Wolbachia -induced sources of isolation. It suggests that Wolbachia could drive reproductive isolation in this system, thereby potentially affecting host differentiation and distribution in the field. ### Competing Interest Statement The authors have declared no competing interest. * ### Abbreviations CI : cytoplasmic incompatibility; wCI : Wolbachia -induced cytoplasmic incompatibility; HI : Host-associated incompatibility; EM : Embryonic mortality; FM : Female mortality; MD : Male development; JM : Juvenile mortality; FP : Female proportion over total number of eggs laid; SR : Sex ratio (here ratio of females to males in the offspring).