Recurrent selective sweeps versus trench warfare in host-parasitecoevolution: the influence of population size changes

该项目的目标是深入了解宿主-寄生虫系统的共同进化，特别是产生遗传多样性的过程。在第一个目标中，我们分析了一个带有宿主和寄生虫种群的等位基因频率和大小变化的相互耦合的相互变化的共同进化的流行病学模型。利用这些结果，我们展示了如何根据在宿主和寄生虫的整个基因组上观察到的遗传多态的位置频率谱来量化共同进化的特征。为了做到这一点，我们从共同进化的一般模型开始，该模型包含了在植物和动物文献中经常使用的众所周知的匹配等位基因和基因对基因的模型。然后，我们扩展了这个确定性模型，将每个寄主世代包括几个寄生虫世代。在第二个目标中，我们研究了随机性对寄主和寄生虫种群中多态的维持以及共同进化基因座上等位基因的丢失和固定的影响。建立了两个具有生物相关性的随机过程模型：(I)在假定后代产量有很大差异的情况下，寄生虫种群中允许发生遗传漂移，这是许多植物和昆虫寄生虫的共同特征；(Ii)疾病传播的随机过程被建模为影响流行病学动态的马尔可夫过程。我们具体探索了从宿主和寄生虫的三个等位基因开始的共同进化模型产生等位基因丢失或固定(从而将系统减少到两个等位基因或单态种群)的条件。我们侧重于观察两个共同进化的极端情况的条件，这两个极端情况都由我们的普遍模型描述，并且是第一个资助期工作的主题，即军备竞赛(反复的选择性扫荡)和壕沟战情景。

The goal of this project is to gain insights into the coevolution of host-parasite systems, particularly processes that generate genetic diversity. In the first objective, we analyze an epidemiological model of coevolution with coupled reciprocal changes in allele frequencies and size changes of host and parasite populations. Using these results, we show how signatures of coevolution can be quantified in terms of the site frequency spectrum of genetic polymorphisms observed over the whole genome of both hosts and parasites. To do this, we start from a general model of coevolution that encompasses the well-known matching-allele and gene-for-gene models frequently used in the plant and animal literature. Then we extend this deterministic model by including several parasite generations per host generation. In the second objective, we study the influence of stochasticicity on the maintenance of polymorphism in host and parasite populations as well as the loss and fixation of alleles at coevolving loci. Two stochastic processes of biological relevance are modeled: (i) genetic drift is allowed to occur in the parasite population assuming a large variance in offspring production, which is a common feature of many plant and insect parasites; (ii) the stochastic process of disease transmission is modeled as a Markov process affecting the epidemiological dynamics. We specifically explore the conditions under which co-evolutionary models starting from three alleles in hosts and parasites generate loss or fixation of alleles (thus reducing the system to two-allele or monomorphic populations). We focus on conditions under which we observe the two extreme cases of coevolution that are both described by our generalized models and were the subject of the work of the first funding period, the arms race (recurrent selective sweeps) and trench warfare scenarios.