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

平衡选择的概念是指在种群中的一个基因座上保持两个或两个以上等位基因的生态和进化机制。这种机制的重要例子包括杂合子优势、选择压力在时间和空间上的变化、基因类型竞争资源和对竞争做出反应的方式的差异，以及宿主-寄生虫的相互作用。这些机制不一定是相互排斥的；例如，一些时间上不同的情景导致杂合子具有比纯合子更高的几何平均适合度。根据生态情景和遗传结构，等位基因可以通过新的突变(等位基因周转)定期再生，或者在极长的时间段内稳定保持，甚至跨物种或属。最近在许多分类群中发现了这种反式特异的多态，包括果蝇属和拟南芥属植物。然而，目前还不清楚如何区分基于遗传数据的各种可能的多样性维持机制。在这个项目中，我们将结合进化生态学(适应动力学)和种群遗传学(合并理论)的建模方法，推导出时空异质性、繁荣-萧条竞争场景和寄主-寄生虫共同进化下平衡选择的预期遗传足迹-这些机制已被用于维持果蝇和拟南芥的多样性。基于这些结果，我们将开发一个生态情景的分类框架，该框架将突出从遗传数据中推断长期平衡选择的潜在机制和局限性。使用这个框架，我们将推断或缩小对果蝇和拟南芥中反式特异性多态的可能解释。

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.