Geographical isolation, habitat variation and trophic specialization have contributed to a large extent to the astonishing diversity of cichlid fishes in the Great East African lakes. Because parasite communities often vary across space and environments, parasites can accompany and potentially enhance cichlid species diversification. However, host dispersal may reduce opportunities for parasite-driven evolution by homogenizing parasite communities and allele frequencies of immunity genes. To test for the relationships between parasite community variation, host dispersal and parasite-induced host evolution, we studied two sympatric cichlid species with contrasting dispersal capacities along the shores of southern Lake Tanganyika. Whereas the philopatric Tropheus moorii evolved into several genetically differentiated colour morphs, Simochromis diagramma is phenotypically rather uniform across its distribution range and shows only weak population structure. Populations of both species were infected with divergent parasite communities and harbour differentiated variant pools of an important set of immune genes, the major histocompatibility complex (MHC). The overall extent of geographical variation of parasites and MHC genes was similar between host species. This indicates that immunogenetic divergence among populations of Lake Tanganyika cichlids can occur even in species that are strongly dispersing. However, because this also includes species that are phenotypically uniform, parasite-induced evolution may not represent a key factor underlying species diversification in this system.

Laboratory of Aquatic Ecology Evolution and Conservation University of Leuven Ch de Bériotstraat 32 B 3000 Leuven Belgium

Laboratory of Biodiversity and Evolutionary Genomics University of Leuven Ch de Bériotstraat 32 B 3000 Leuven Belgium

Laboratory of Biodiversity and Evolutionary Genomics University of Leuven Ch de Bériotstraat 32 B 3000 Leuven Belgium Centre for Biodiversity Dynamics Norwegian University of Science and Technology N 7491 Trondheim Norway

Laboratory of Biodiversity and Evolutionary Genomics University of Leuven Ch de Bériotstraat 32 B 3000 Leuven Belgium Department of Botany and Zoology Faculty of Science Masaryk University Kotlářská 2 611 37 Brno Czech Republic

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