Tritrophic genetic interactions in Acyrthosiphon pisum

Anthropogenic climate change is putting ecosystems under threat and so it has become increasingly important to understand community structure in order to protect the integrity of remaining global ecosystems. One understudied element of community structuring is the role of intraspecific variation (ITV) and how it interacts with indirect genetic effects (IGEs). IGEs can act across multiple trophic levels and by altering these effects, ITV can have a cascading role across ecosystems. A small community across several trophic levels is therefore ideal to study the effect ITV has on IGEs and the well-studied pea aphid (Acyrthosiphon pisum) provides this. A. pisum is entirely dependent on its host plant for nutrition. The aphid also has a parasitoid wasp (Aphidius ervi) and a bacterial symbiont (Hamiltonella defensa) providing protection against the wasp. Strong coevolutionary pressures within this community would be expected. To explore the effects of intra- and interspecific variation across different trophic levels I looked at aphid fecundity and resistance to A. ervi using a variety of aphid, Hamiltonella and host plant genotypes as well as plant species. Host plant species altered innate aphid parasitoid resistance as well as symbiont mediated protection and plant genotype significantly interacted with aphid genotype to affect A. ervi susceptibility. Aphid fecundity was affected by host plant species and Hamiltonella genotype but surprisingly there was no variation in fecundity between aphid clones. Hamiltonella also offered a different protective effect across two different aphid genotypes. The observed role of host plants in parasitoid resistance was novel and raises interesting questions about the evolutionary pressures governing plant-aphid-symbiont-parasitoid interactions. Aphid host plants altering the aphid genotype effect on Hamiltonella mediated protection provides evidence for the importance of multitrophic ITV in communities. Thus, future work into understanding the role native and irregular plant hosts have on aphid-parasitoid defences would be insightful.