ausblenden:
Schlagwörter:
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Zusammenfassung:
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).