Long-term balancing selection: From ecological mechanisms to genetic footprints

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The concept of balancing selection refers to ecological and evolutionary mechanisms that can maintain two or more alleles at a locus in a population. Important examples for such mechanisms are heterozygote advantage, selection pressures varying across time and space, differences in the way genotypes compete for resources and respond to competition, and host-parasite interactions. These mechanisms are not necessarily mutually exclusive; for example some of the temporally heterogeneous scenarios cause heterozygotes to have a higher geometric mean fitness than homozygotes. Depending on ecological scenario and genetic architecture, alleles can be regularly regenerated by new mutations (allelic turnover) or stably maintained over extremely long time periods, even across species or genera. Such trans-specific polymorphisms have recently been found in many taxonomic groups, including the fruit fly genus Drosophila and the plant genus Arabidopsis. However, it is currently unclear how to distinguish between the various possible mechanisms of diversity maintenance based on genetic data. In this project, we will combine modeling approaches from evolutionary ecology (adaptive dynamics) and from population genetics (coalescent theory) to derive the expected genetic footprints of balancing selection under spatio-temporal heterogeneity, boom-bust competition scenarios, and host-parasite coevolution - mechanisms that have been implicated in diversity maintenance in Drosophila and Arabidopsis. Based on these results, we will develop a classification framework of ecological scenarios, which will highlight the potential and the limitations for inferring mechanisms underlying long-term balancing selection from genetic data. Using this framework, we will then infer or narrow down possible explanations for trans-specific polymorphism in Drosophila and Arabidopsis.