Explaining Interspecific Variation in Susceptibility and Resistance to Parasitism in Damselflies

Past research has focused on species-specific characteristics of parasites to explain why different, but phylogenetically closely related host species, are under different selection regimes imposed by parasites. However, the evolutionary ecology of hosts is also expected to have important influence in their associations with parasite species. This thesis explores host factors principally that are expected to influence host susceptibility and resistance, in damselflies parasitized internally by gregarines and externally by water mites. When comparing species grouped into sibling species pairs, gregarine and water mite parasitism was explained in part by geographic range size of host species, but in most of the cases where there was difference in parasitism between the two closely related host species, it was the host species with the smaller range that had higher levels of parasitism. At a higher taxonomic level where the host species were not grouped into species pairs, but where host phylogeny was controlled for through comparative methods, host phenology, regional representation and geographic range size were the best predictors of parasite host interactions. Interspecific variation in resistance to parasitism and levels of constitutive measures of innate immunity (PO activity) was also documented. However, the species that had highest levels of PO activity were not the ones that showed highest resistance. The most resistant hosts also tended to be the ones with the smaller geographic ranges. In a case study, the ecologically restricted host species (Nehalennia gracilis) resists all its mites in a novel way; the more widespread host species (Nehalennia irene) does not resist any individuals of the same species of parasite, determined by genetic barocding. Resistance evolution in N. gracilis could be because this species was able to evolve parasite recognition for a species from a closed population. The findings of this thesis research further our understanding of host-parasite associations across phylogenetic and geographic scales. I suggest researchers need to be conscious of the complexity of the host-parasite associations and need to consider multiple host-multiple parasite systems to be norm.