STUART C. KILLICK
The University of Edinburgh and Department of Zoology, Oxford Unversity, Oxford OX1 3PS, UNITED KINGDOM.
The Red Queen hypothesis proposes that antagonistic coevolution between parasites and their hosts is responsible for the evolutionary maintenance of sexual reproduction. It suggests that frequency-dependent selection by parasites against common host genotypes prevents asexual clones capitalising on their two-fold reproductive advantage and out-competing their sexual counterparts. However, in order for the Red Queen to be effective parasites must be highly virulent and display genotype-specific infection patterns, and both of these requirements are known to be affected by environmental conditions. In this thesis I examine environmental influences over host-parasite interactions in Daphnia.
A survey of parasite prevalence in North American populations of Daphnia pulex represents the first attempt to examine the role of parasites in the maintenance of breeding system variation in this species. Despite evidence of over- and underparasitism in some populations, parasite prevalences were generally very low suggesting that parasites are not a major source of selection in the populations studied.
The Pluralist Approach to sex proposes that the effects of deleterious mutations and parasitism may interact. I established mutant lines of Daphnia magna using the chemical mutagen ENU and investigated the impact of the parasite Pasteuria ramosa on mutation load under different environmental conditions. I found that although parasite infection could exacerbate the effects of mutation load, this interaction was dependent on host environment and the implications of these findings for the general application of the Pluralist Approach are discussed.
The impact of mixed strain infections on genotype-specific infection was also examined. In natural populations, hosts are likely to be exposed to a range of parasite genotypes and this may potentially affect the efficiency of the immune response. I found that the ability of certain P. ramosa strains to infect their hosts is affected by prior host exposure to different strains.