The potential for coevolution in an aquatic host-parasite system : it takes all the swimming you can do to stay in the same place

摘要

The objective of this PhD project was to obtain a better insight in the evolutionary ecology of the host-parasite relationship between the freshwater diatom Asterionella formosa and its obligatory host-specific parasite Zygorhizidium planktonicum. In Chapter 2, we review the use of molecular techniques in phytoplankton studies. Background information and the rationale behind the application of the various molecular techniques used in this thesis can be found in this chapter. Chapter 3, contains a review on phytoplankton parasites, with special emphasise on previous work on the host-parasite relationship between A. formosa and its fungal parasites. The review concludes with a paragraph on coevolution between phytoplankton parasites and their hosts. The required preconditions for coevolution, especially the first precondition (the clear reciprocal fitness costs between A. formosa and Z. planktonicum), are discussed for our host-parasite system. Chapter 4, primarily focused on the second precondition for the potential of coevolution, which is the existence of genetic variation for parasite infectivity and host resistance, on which selection can act. In Chapter 5, we investigate if the level of genetic variation within A. formosa populations can be attributed to recombination via sexual reproduction. Chapter 6 focused on the potential of the fungal parasite Z. planktonicum to adapt to new host genotypes of A. formosa. The aim of this study was to investigate if parasite adaptation is hindered by genetic variation for susceptibility and resistance within host populations. In Chapter 7, we investigate if the parasite Z. planktonicum is locally adapted to host populations of A. formosa. We tested the fitness of the parasite on sympatric (local) and allopatric (novel) host populations. In the final part of this thesis, Chapter 8, we combine our findings of the subsequent chapters and try to give an overview of the insights obtained from our studies on this host-parasite relationship.