Effects of host phenotypic and genotypic variations on parasitic infections in sticklebacks

Host phenotype and genotype have a direct effect on the outcome of host-parasite interactions, and therefore have the potential to alter the consequences of the disease processes. This thesis examines how host-parasite interactions might be affected by host factors in both naturally and experimentally infected three-spined sticklebacks using the Schistocephalus solidus and Diplostomum spathaceum models. In the River Soar, Leicestershire, 12 different parasite species belonging to various taxa have been recorded in the course of eight months survey. The prevalence and intensity of D. spathaceum infection was high throughout the year. S. solidus plerocercoid masses varied between the sexes, and infected fish had a poorer body condition than non-infected fish. In contrast to wild study results, fish sex was not found to be a strong predictor of fish susceptibility to experimental S. solidus infection. The establishment of S. solidus plerocercoids was not related to host size or mass. Neither host sex nor fish size or mass at exposure had a significant influence on subsequent parasitic growth rates. However, the parasite index was higher in female than male fish. Reproductive reduction often arise in infected males and it does not rely on gonad development alone, but also on the glue spiggin production for nest building as well. The effect of parental genotype and phenotypes on offspring susceptibility to S. solidus infections and spiggin gene expression have been investigated. I found that the prevalence of infected offspring sired by infected males that subsequently became infected following exposure was significantly higher than those sired by non-infected fathers. S. solidus-infected sticklebacks showed significantly increased spiggin C1 and spiggin C2 expression compared to non-infected fish, expression of all gene investigated did not affected by paternal infection treatment. A significant effect of provenance (host population) and host sex on susceptibility to D. spathaceum infection was found. This variation reflected the genetic differences in the coevolutionary processes between the host and parasite. Sticklebacks invade freshwater, they typically evolved by reduce lateral plate count. Therefore, in the association between parasite infection and Eda genotype, plate number, age and sex were examined as possible factors determining fish susceptibility to D. spathaceum infection. Fish with a greater number of lateral plates were found to show increased parasitic loads, and older fish had higher loads than juveniles. The overall conclusion of the thesis is that the outcome of host-parasite interactions is significantly affected by host phenotype and genotype factors. Pre-existing host variation effects have the potential to directly alter parasitic development and shift the consequences of the host-parasite interaction. In addition, host variation has helped to shed some light on the ecology and evolution of stickleback-parasite interactions.