Wolbachia manipulate fitness benefits of olfactory associative learning in a parasitoid wasp.

Upon encountering a host, a female parasitoid wasp has to decide whether to learn positive or negative cues related to the host. The optimal female decision will depend on the fitness costs and benefits of learned stimuli. Reward quality is positively related to the rate of behavioral acquisition in processes such as associative learning. Wolbachia, an endosymbiotic bacterium, often plays an impressive role in the manipulation of its arthropod host's biology. Here, we studied the responses of two natural Wolbachia infected/uninfected Trichogramma brassicae wasp populations to theoretically high- and low-reward values during a conditioning process and the consequences of their responses in terms of memory duration. According to our results, uninfected wasps showed an attraction response to high-value rewards, but showed aversive learning in response to low-value rewards. The memory span of uninfected wasps after conditioning by low-value rewards was significantly shorter than that for high-value rewards. As our results revealed, responses to high-quality hosts will bring more benefits (bigger size, increased fecundity and enhanced survival) than those to low-quality hosts for uninfected wasps. Infected wasps were attracted to conditioned stimuli with the same memory duration after conditioning by both types of hosts. This was linked to the fact that parasitoids emerging from both types of hosts present the same life-history traits. Therefore, these hosts represent the same quality reward for infected wasps. According to the obtained results, it can be concluded that Wolbachia manipulates the learning ability of its host, resulting in the wasp responding to all reward values similarly.