Vogel, Sandra (2011) Can parasites mediate sympatric speciation through their hosts's MHC genes? Example of the three-spined stickleback from Iceland. (Diploma thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 72 pp.

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Abstract

After the last glaciations, Icelandic freshwater systems were colonised by only a small number of fish species from adjacent ancestral marine populations. Among these, the three-spined stickleback ( Gasterosteus aculeatus) could fill free niches and diversify. As in similar systems in the northern hemisphere, these fish show signs of recent genetic and morphological parallel divergence, which might be a first step towards further differentiation and might in the long run lead to establishment of reproductive isolation and therefore speciation. This is first examined at a large geographic scale, where colonisation can be shown to have occurred in one main wave and genetic isolation by distance is demonstrated. On a finer scale, recent divergence led to the formation of sympatric ecomorphs that differ in their habitat use and several morphological characteristics, and are genetically differentiated. My study supports the view that contrasting parasite communities between habitats or niches within a habitat could be a driver of this divergence. Their selection on the immune genes of the major histocompatibility complex (MHC), combined with female mate choice for sympatric morphs carrying resistance MHC alleles, could drive the evolution of the system towards diversification. Using natural replicates of sympatric three-spined stickleback ecomorphs, I found evidence for such a pattern, where suspected sympatric morphs carrying different parasite communities show low MHC allele overlap as well as signs of genetic differentiation. On the other hand, the suspected morphs that did not show contrasting parasite communities display overlapping MHC allele pools and no sign of neutral genetic differentiation. Furthermore, signatures of antagonistic evolutionary arms races between parasites and their hosts' adaptive immune defence are identified in a truly sympatric system. In a final pilot study of adaptive immune gene expression patterns, the importance of this proximal factor of parasite resistance for adaptive processes is highlighted. Here again, we show that contrasting immune defence between sympatric morphs further supports the role of parasite mediated selection for ecological host speciation.

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