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

Adaptation to heterogeneous environmental conditions plays an important role in the process of speciation. Two important components of ecological differentiation that are often associated with speciation are 1) habitat segregation, i.e. the use of different spatially distinct habitat niches, and 2) dietary specialisation, i.e. exploitation of different food resources within the same or between different spatial niches. Both of these may contribute to the evolution and maintenance of reproductive isolation between diverging populations (Schluter, 2001; Maan and Seehausen, 2011).

Shifts in diet and habitat often coincide with exposure to different natural enemies. In particular, parasites are often associated with specific habitats and foraging strategies. For example, parasitic worms typically infest their host through the ingestion of specific foods. Given that parasites exert strong selection on host resistance and immune defense, successful niche expansion and establishment in a new niche requires rapid evolution of defences against locally abundant parasites. In this way, parasites may play important roles in adaptation to divergent niches, speciation and perhaps adaptive radiation (Karvonen and Seehausen, 2012).

In this project, we aim to investigate the role of parasites in speciation and adaptive radiation of cichlid fish. Cichlid fish constitute one of the most species-rich families of vertebrates (Kocher, 2004), with more than 2000 species that inhabit tropical rivers and lakes. Different species experience different environmental conditions, and lineages vary in species richness. These properties provide a natural laboratory for testing evolutionary hypotheses.

Recent studies on African lake cichlids show that different populations and species, within the same lake, exhibit marked differences in parasite infestation patterns (Maan et al., 2008; Raeymaekers et al., 2013; Karvonen et al., in prep) as well as rapid evolution of genes involved in parasite resistance (MHC; Blais et al., 2007). Moreover, our own work in Lake Victoria cichlids suggests that parasites affect the expression of sexually selected characters (Maan et al., 2006; Maan et al., 2008). These findings indicate that parasites exert strong selection on host populations. It is therefore possible that parasite infections initiate co-evolutionary arms races that may facilitate ecological speciation. Here, we will address this hypothesis, by a combination of observational and experimental approaches, in Lake Victoria cichlids.