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Hosts and parasites : many ways of interactions ; an approach with two model organisms, the three-spined stickleback (Gasterosteus aculeatus) and its cestode parasite (Schistocephalus solidus)


Jäger,  Ilonka
Department Evolutionary Ecology, Max Planck Institute for Evolutionary Biology, Max Planck Society;


Milinski,  Manfred
Department Evolutionary Ecology, Max Planck Institute for Evolutionary Biology, Max Planck Society;

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Jäger, I. (2006). Hosts and parasites: many ways of interactions; an approach with two model organisms, the three-spined stickleback (Gasterosteus aculeatus) and its cestode parasite (Schistocephalus solidus). PhD Thesis, Christian-Albrechts-Universität, Kiel.

Parasites are considered to be one of the major driving forces in the evolution of organisms. Host-parasite interactions have thus evoked great interest in evolutionary biology. However, the outcome of an infection might not only be determined by hostparasite interactions but also by within-host dynamics. Using the three-spined stickleback (Gasterosteus aculeatus) and the tapeworm Schistocephalus solidus as model organisms, various aspects of such potential interactions were investigated. Multiple infections of the stickleback with S. solidus occur frequently in nature and the specific within-host dynamics might have important consequences for the outcome of the infection. Interactions between co-infecting S. solidus individuals could be influenced by the fact that they are likely to be future mating partners in the final bird host. In a mate choice experiment, we let cestodes choose between a related (full-sibling) and an unrelated mating partner. To investigate whether the behavioural preference is reflected in the number of eggs produced with the respective mating partner, parentage analyses of the offspring were conducted. A second experiment was carried out to calculate the difference in hatching success between related and unrelated matings. Despite the 3.5-fold lower hatching rate in incestuous matings, the cestodes showed a significant preference for their sibling. This was confirmed by the molecular data: the cestodes produced significantly more eggs with the related than with the unrelated partner (Chapter I). To investigate potential fitness consequences for S. solidus in multiple infections, we double infected sticklebacks. Thereby, relatedness of the parasites and the time between infections were taken into account. Double infections were established more often when the two parasites were related. In sequential infections, the secondly acquired parasite was more likely to survive and grew to a larger size compared to the first one, despite being one week younger. This difference was not apparent when both parasites were acquired simultaneously (Chapter II). An earlier in vitro-study suggests that S. solidus is capable of suppressing innate immune responses of the stickleback. However, little is known about the adaptive immune response with respect to S. solidus infections. By repeatedly injecting antigens in vivo, we challenged the adaptive part of the immune system of S. solidus-exposed and non-exposed sticklebacks. Parameters of innate and adaptive immunity were measured 9, 13 and 23 weeks post-infection. No adaptive immune reaction towards S. solidus or the applied antigens was detected. S. solidus was, however, found to elicit an innate immune response confirming previous studies (Chapter III). The major histocompatibility complex (MHC) plays a key role in the specific parasite resistance of vertebrates. In the three-spined stickleback, there is mounting evidence that an intermediate rather than a maximal individual MHC allele diversity is favoured. In a correlative study, individual allele diversity of MHC class IIB as well as MHC class I was related to important fitness traits of the stickleback. While for male nest quality an optimum concerning MHC class IIB diversity was found, male breeding colouration was most intense at a maximal MHC class I diversity. Two MHC class I alleles were identified to be associated with a higher intensity of red colouration. Moreover, the probability of carrying these alleles increased with an increasing individual allele diversity, possibly explaining the positive correlation mentioned above (Chapter IV).