The impact of spatial structure and sexual selection on a host/parasite interaction

Abstract

It is only relatively recently that we have begun to focus on the impact of ecological and population level effects on the outcome of host-parasite interactions. This dissertation uses the Plodia interpunctella moth/granulosis virus system to examine two distinct questions (1) the effect of spatial structure on parasite transmission and (2) the effect of varying sexual selection on the evolution of resistance. I manipulate the host environment to influence the speed of larval movement in a virus-naive host population in order to produce rare empirical data on the impact of spatial structure on transmission (Chapter 2). Contrary to the simple prediction that more movement would lead to more infection, increasing food viscosity (0-35%) was shown to significantly increase the infection prevalence. I argue that this is due to the extra effort of the host needed to move through higher viscosity foods that is traded off with immunity although it may also result from higher local densities promoting higher transmission rates. Next I explored how changes in the level of sexual selection in a population can influence the evolution of host defence. I found that the resistance to viral infection in the populations evolving under female bias mating sex ratio was significantly lower than that of the male bias populations. I argue that either there is a trade off between investment in reproduction (male mating frequency is higher in female biased populations) and immunity, or that sexual selection may have been more effective in the male bias treatments resulting in offspring with an improved immune system. Since the populations evolved in the absence of disease, my work emphasises that viral resistance can be influenced indirectly by behavioural traits such as different mating strategies.