Interactive effects of environmental change and host-parasite co-evolution on the ecological speciation of sticklebacks

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How human-caused environmental change affects the ecological and evolutionary processes that create and maintain biodiversity is a pressing question in biology. The eutrophication of aquatic ecosystems is a widespread and ongoing problem that has manifold consequences for sustaining critical ecosystem services, including water quality and biodiversity. The loading of nutrients (e.g. phosphorus and nitrogen) to freshwater environments not only affects their chemical and physical condition, but also influences the nature of interactions between species such as host-parasite interactions which generate local adaptation patterns and may even lead to speciation. Pervasive effects of eutrophication can alter selection regimes and therefore, cause unexpected evolutionary changes in populations over short time scales. Here, using three-spined stickleback as a model system, we focus on how the interactive effects of nutrient loading and parasites might drive rapid evolutionary changes and breakdown of local adaptation. First, using an intensive field survey along a lake productivity gradient, we will test how lake productivity co-varies with i) parasite diversity and community composition, ii) host genotype diversity, and iii) host immune system function and gene expression- testing for local adaptation of host and parasite populations. Second, using a large scale experiment in mesocosms (42 X 1000 L), we will test how contrasting natural selection regimes, resulting from orthogonal combinations of parasites and nutrient levels, can affect phenotypically and genotypically stickleback adaptive abilities. In this experiment, we focus on phenotypic and genetic traits that have been implicated in mate selection, so that we can test if the contrasting selection regimes caused by manipulating nutrients and parasites (antagonistically) synergistically might contribute to population divergence, and play a role in (reverse) speciation. This work will shed light on how natural and sexual selection are mediated by parasites and environmental conditions, and how these processes jointly respond to divergent environments and affect the progress towards speciation.