Wolbachia are intracellular α-proteobacteria that have a wide distribution among various arthropods and nematodes. They affect the host reproduction favoring their maternal transmission, which sets up a potential conflict in inbreeding situations when the host avoids sexual reproduction preventing inbreeding depression, while Wolbachia pushes it. In this study, we used the wasp Habrobracon hebetor to test the hypothesis that Wolbachia modulate inbreeding avoidance behaviour and promote sib mating. To test this, we first cured wasps of Wolbachia using tetracycline treatment and produced infected and uninfected isolines. Then, we paired the uninfected and infected females with sibling (inbred) and non-sibling (outbred) males in choice and non-choice experiments. Our results showed no obvious precopulatory inbreeding avoidance in this wasp as brother-sister mating rates (in both choice and nonchoice experiments) were not significantly different form non-sibling pairs, regardless of Wolbachia infection. However, our results indicated that H. hebetor shows a strong postcopulatory inbreeding avoidance behaviour that results in a low fertilization rate of uninfected siblings and therefore high rate of production of male progeny was obtained. We observed higher rates of fertilization success in the Wolbachia-infected lines that resulted in significantly higher female progeny production compared to the uninfected sib mates. Since diploid females are the result of successful fertilization due to haplodiploidy sex determination system in this insect, our results indicate that Wolbachia promoted fertile sib mating in H. hebetor. Interestingly, the rate of adult emergence in the progeny of Wolbachia-infected sib mates were almost similar to the non-sib mate crosses and significantly more than those observed in the uninfected sib mate crosses. We support the idea that Wolbachia modulate inbreeding avoidance and promote sib mating and also mitigate inbreeding depression. The wasp Habrobracon hebetor siblings infected with Wolbachia show higher rates of fertilization success and higher adult emergence rates compared to the uninfected sib mates. By promoting successful sex with siblings and increasing the probability of female progeny, Wolbachia enhance their transmission to the next generation and also mitigate inbreeding depression. This is an undescribed effect of Wolbachia (symbiont) on the host reproduction.

Wolbachia promote successful sex with siblings