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|Title:||Effects of host variation and environmental conditions on Schistocephalus solidus infections in sticklebacks|
|Authors:||Simmonds, Natalie Evelyn|
|Presented at:||University of Leicester|
|Abstract:||The outcomes of host-parasite interactions are potentially affected by both the genotype and phenotype of the hosts and parasites involved, and modulated by the environmental conditions under which they interact. Anthropogenic environmental changes therefore have the potential to shift the balance in host-parasite interactions, with consequences for disease processes. This thesis examines how host factors and environmental conditions influence the outcome of host-parasite interactions in the experimentally amenable three-spined stickleback-Schistocephalus solidus model. When sticklebacks invade freshwaters, their lateral plate count typically reduces and becomes more variable. In a freshwater population with unusually high diversity in plate morphology, fish with fewer numbers of lateral plates were found to show increased susceptibility to experimental S. solidus challenge. Hypoxic conditions often arise in degraded environments and have the potential to interact with infection status. Schistocephalus solidus infected sticklebacks showed significantly reduced expression of genes (including GADD45 and LDHA) usually associated with the normal cellular hypoxic response of fish. These results suggest that S. solidus infections impair the normal cellular response to hypoxia, and may contribute to observed behavioural changes observed in infected individuals under hypoxic conditions. The results of a study investigating the effects of salinity on the development of S. solidus showed embryonic development was prevented at salinity greater than 20 ppt, indicating that the threat of infection is confined to brackish and freshwaters. Schistocephalus solidus therefore represents a novel selection pressure for sticklebacks colonising freshwaters from ancestral marine populations. Both the level of host food intake and host body size significantly affected the outcome of experimental parasite challenge. Experimentally manipulated host ration generated faster plerocercoid growth under greater levels of alimentation. The establishment of S. solidus plerocercoids was related to host size, with infections developing more frequently in smaller than larger sticklebacks. However, plerocercoids grew more quickly in larger hosts. Hence, environmental changes that affect host size, the timing of infections or the availability of food have the potential to influence parasite growth and life cycle completion rates. The overarching conclusion of the thesis is that the biology of host-parasite interactions is highly susceptible to environmental changes, which can exert their effects through direct impacts on hosts, on parasites or on the interaction between them.|
|Rights:||Copyright © the author. All rights reserved.|
|Appears in Collections:||Theses, Dept. of Biology|
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