Host-range Evolution in Aphidius Parasitiods: Ecological and Evolutionary Perspectives

A fundamental objective in biology is to understand the evolutionary and ecological processes that lead to patterns of diversity. Specialized insect-host relationships have long been suspected of facilitating evolutionary diversification due to the intimate relationship parasites share with their hosts and the potential for disruptive selection when utilizing different host species resulting in genetic divergence. However, the vast majority of research investigating insect-host specialization has focused on herbivores, while the mechanisms of population and species divergence in insects that parasitize and kill other insects (parasitoids) have been largely ignored. Parasitoids have great potential for research in host-specialization and speciation due to their unique biological properties and the immense diversity that is characteristic of this ecologically important guild. The goal of my Ph.D. thesis was to contribute to the greater understanding of host specialization and speciation in parasitoid wasps, with a particular focus on the processes that drive adaptive diversification and the behavioural mechanisms that maintain genetic variation. These topics were addressed through a combination of experimental biology, fieldwork and theoretical modelling using a host-parasite system consisting of a single parasitoid (Aphidius ervi) and two host species (the pea aphid, Acyrthosiphon pisum and the foxglove aphid, Aulacorthum solani). Specifically, the physiological mechanism involved in overcoming host defences was investigated to determine if parasitoids adaptively evolve to overcome host defences and if this process is under directional selection. Host fidelity, host plant preference, mating preference, and host-instar selection were investigated to determine the importance of these behaviours in mediating gene flow between parasitoid populations utilizing different host species. In combination, these studies further contribute to our basic understanding ofthe mechanisms that lead to and maintain genetic diversity in insect parasitoids.