Symbiont-Mediated Resistance in Insects; An Annotated Bibliography of Research Concerning Microbially Mediated Defense Against Parasites, Parasitoids, & Pathogens in the Class Insecta

In this study, researchers studied the evolutionary effects on parasitoids of symbiont-mediated resistance by Acyrthosiphon pisum to the parasitoid, Aphidis ervi. They were able to take advantage of the parthenogenic life cycle of the aphid, allowing them to effectively rule out host evolution as the factor effecting results. They performed several experiments to determine what evolutionary effects arose and the level of selection efficiency. The researchers established lineages of aphids infected or not with Hamiltonian defensa (hereafter just SS because it is the only secondary symbiont in this study), and assayed the effect of infection on parasitoid resistance. After finding a significant effect they began a ten-generation experiment in which every generation of parasitoids was allowed to parasitize the different aphid treatment groups, and after each generation, the parasitoid’s size and effective parasitism rate were measured. During this experiment, wasps were also split into groups that infected only infected or uninfected aphids for this experiment, but afterwards, a third experiment was conducted in which wasps from both groups were allowed to parasitize two clonal lineages of SS infected aphids to determine if the ability to use infected with SS aphids evolved over the course of the experiment. After only three generations, the effective parasitism rate among aphids infected with SS and those not infected became identical. And so it is not unexpected that researchers observed an increase in the ability for SS exposed wasps (exposed to SS infected aphids throughout the preceding experiment) to effectively parasitize a group of SS infected aphids that had not been subject to ten generations of parasitism. This shows that selection had a fairly high efficiency, occurring over ten generations or possibly fewer. No significant difference in parasitoid size was observed between the two treatments.