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

Wolbachiaare 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 addressingWolbachia-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 miteTetranychus urticaeto dissect the interaction betweenWolbachia-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 thanWolbachia-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 (byca. 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 thatWolbachiadid not contribute to this outcome. This study identifies the mechanistic independence and additive nature of host-intrinsic andWolbachia-induced sources of isolation. It suggests thatWolbachiacould drive reproductive isolation in this system, thereby potentially affecting host differentiation and distribution in the field.