The role of parasites in host speciation: Testing for parasite-mediated divergent selection at different stages of speciation in cichlid fish

Parasites represent one of the most successful modes of life in nature and are an important component of ecosystems. By imposing fitness costs on their hosts, parasites constitute a major agent of ecological selection. The host-parasite coevolutionary dynamics of adaptation and counter-adaptation may promote host diversification. The cichlid fish of Lake Victoria have rapidly diversified into hundreds of species, making them a good model system for studying the early stages of speciation.
I investigate whether parasites drive or contribute to host speciation by analysing infection patterns in closely related cichlid populations. I found support for two prerequisites for parasite-mediated selection: host species differed in their infection profiles and these infection differences were maintained over multiple sampling years. Different parasites were located in different microhabitats in the fish gills, also depending on the host species. Since microhabitat segregation constitutes another axis of infection heterogeneity, I suggest to include it in future studies.
Host species variation in infection was not fully explained by exposure to parasites, suggesting that other intrinsic host properties (i.e. immunity) also play a role. Incipient cichlid species reared in aquaria with uniform parasite exposure did not differ in infection, indicating they have not (yet) evolved differences in immunity. This is inconsistent with a role of parasites in driving host divergence. Infection differences accumulate as hosts become more genetically differentiated, but only reproductive isolated host species display a significant difference in infection. This suggests that parasites may contribute to host divergence but they are not initiating it.