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Abstract
Insects, including important pest species, are often attacked and eventually killed by internally-developing parasitoid wasps; a process that leads to strong selection for host resistance. The pea aphid, Acyrthosiphon pisum, is a model for studying variation in resistance to its most important parasitoid, Aphidius ervi, its variation in survival ranging from nearly 0 100% following parasitism. 1) Originally, it was assumed that this variation was due to host genotype, but the discovery of the defensive bacterial symbiont, Hamiltonella defensa and its bacteriophage APSE, which show tremendous strain variation in levels of protection conferred, led to the view that this aphid relies primarily on H. defensa for protection. By considering a large number of aphid genotypes uninfected with H. defensa or other symbionts, however, I discovered that aphids also encode strong intrinsic protection against this wasp, with no apparent trade-offs between intrinsic protection and aphid fitness. 2) I also found that some aphids harbor both symbiont-based and intrinsic resistance. In most cases tested, H. defensa is at least somewhat beneficial to aphid survival after parasitism but, depending on aphid genotype, can be costly on fecundity and longevity compared to uninfected aphids and is ultimately not beneficial to carry the symbiont in some cases. 3) I then examined how parasitism by A. ervi influences aphid nutritional and defensive symbioses. While effects were highly variable, parasitism generally led to increase in abundance of APSE, decrease of H. defensa, and resistance-dependent changes in the nutritional symbiont, Buchnera. These are consistent with wasp manipulation of the nutritional symbiont and may signify changes in the defensive symbiosis related to resistance. 4) Finally, Aphidius ervi has largely displaced competing parasitoids in North America, but the related aphidiine braconid Praon pequodorum persists on this host. Surprisingly, I found that both symbiont-based and intrinsic aphid defenses against A. ervi are completely ineffective against P. pequodorum, which does not suffer sub-lethal fitness costs from developing in even the most resistant hosts. Moreover I found differences in embryonic development between both wasp species that may influence their parasitism success. In summary, my work shows that pea-aphid parasitoid interactions are much more complex than previously thought, a finding that likely generalizes to other systems as defensive symbiosis is increasingly recognized as a common phenomenon.