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Abstract:
Parasites are a major evolutionary driving force. They impose a selection factor not only on individual hosts, but also on whole populations altering natural and sexual selection. The coevolution between hosts and parasites leads to an evolutionary arms race, where hosts evolve towards higher resistance and parasites towards increased exploitation. Parasites have evolved many different adaptations to increase their potential to successfully infect and exploit their hosts. A parasite with a multi-host life cycle may use the different hosts for different purposes and must adapt to different conditions at each stage. All adaptations should ultimately increase the parasite’s fitness: through higher growth and higher rates of transmission and reproduction.
The work of my thesis examines the adaptations of parasites, specifically tapeworms of the genus Schistocephalus (S. solidus and S. pungitii), to their host organisms: three-spined sticklebacks (Gasterosteus aculeatus) and nine-spined sticklebacks (Pungitius pungitius). Schistocephalus has long been known to be very specific regarding the second intermediate (stickleback) host, which indicates close coevolution of these species. In chapter I of this thesis I examined this high degree of host specificity by experimentally exposing sticklebacks to either their specific or the incompatible parasite species and monitoring the infection process histologically. This could show that the incompatible parasite species can still establish in the stickleback, but is eliminated within the first two weeks after infection. I also tested, whether the known immune manipulation by S. solidus in three-spined sticklebacks allows a superinfection with the incompatible parasite in sequential exposures, but the results indicate that this is not possible.
In the second experiment I hybridized two different Schistocephalus species (S. solidus and S. pungitii) in an in vitro breeding system and measured fitness relevant traits throughout the whole life cycle. I could show that the two species are capable of producing viable hybrid offspring, even though the outcrossing and hatching rates are lower in these pairings than in the parental species. Nevertheless, the hybrids exhibit no decreased infection rate in the first and second intermediate hosts and surprisingly show an extended host range, as they are able to infect both stickleback species, while the parental lines can only infect their specific host.
This is surprising, as natural hybrids between S. solidus and S. pungitii have not yet been observed and molecular data indicates a deep lineage divergence and no gene flow. In the
next part of this thesis I therefore tested, if prezygotic barriers prevent hybridization in natural populations. The results suggest that the species can hybridize in natural hosts, there are no barriers to hybridization in sympatric populations and the parasites even prefer parasites of the different species over conspecifics in a mate choice experiment.
In summary, these results indicate that host specificity in Schistocephalus is presumably maintained in this system due to the specific reaction of the stickleback’s immune system, even though the advantages and the mechanisms are still unclear. It is possible that the high degree of host specificity is important for successful long term interactions with the stickleback immune system, even though our results indicate no trade-off at this level.
The ability of a parasite to successfully establish and exploit a host is also determined by parasite virulence, which depends on many factors that also include intraspecific interactions among co-infecting parasites. In the last part of this thesis, I investigated the plasticity of individual parasite virulence using experimental co-infections with two different strains of S. solidus that differ in virulence within three-spined sticklebacks. This showed that intraspecific interactions alter individual virulence in S. solidus, where the less virulent parasite benefits from the presence of a high-virulent conspecific and the high-virulent parasite exhibits reduced virulence in heterologous co-infections.
This thesis demonstrates that these parasites use numerous and elaborate approaches to adapt to their host. Furthermore, the outcome of a parasitic infection is dependent on the close coevolution between parasitic exploitation strategies and host defenses, and finally, these interactions become even more complex with multiple infections.
