Department of Aquatic Ecology, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland.
D-USYS, Department of Environmental Systems Science, ETH Zürich, Zürich, Switzerland.
Mol Ecol. 2023 Jul;32(14):4063-4077. doi: 10.1111/mec.16976. Epub 2023 May 9.
Parasite-mediated selection can rapidly drive up resistance levels in host populations, but fixation of resistance traits may be prevented by costs of resistance. Black bean aphids (Aphis fabae) benefit from increased resistance to parasitoids when carrying the defensive bacterial endosymbiont Hamiltonella defensa. However, due to fitness costs that come with symbiont infection, symbiont-conferred resistance may result in either a net benefit or a net cost to the aphid host, depending on parasitoid presence as well as on the general ecological context. Balancing selection may therefore explain why in natural aphid populations, H. defensa is often found at intermediate frequencies. Here we present a 2-year field study where we set out to look for signatures of balancing selection in natural aphid populations. We collected temporally well-resolved data on the prevalence of H. defensa in A. f. fabae and estimated the risk imposed by parasitoids using sentinel hosts. Despite a marked and consistent early-summer peak in parasitism risk, and significant changes in symbiont prevalence over time, we found just a weak correlation between parasitism risk and H. defensa frequency dynamics. H. defensa prevalence in the populations under study was, in fact, better explained by the number of heat days that previous aphid generations were exposed to. Our study grants an unprecedentedly well-resolved insight into the dynamics of endosymbiont and parasitoid communities of A. f. fabae populations, and it adds to a growing body of empirical evidence suggesting that not only parasitism risk, but rather multifarious selection is shaping H. defensa prevalence in the wild.
寄生虫介导的选择可以迅速提高宿主种群的抗药性水平，但抗药性特征的固定可能会被抗药性的代价所阻止。携带防御性细菌共生体汉密尔顿氏菌（Hamiltonella defensa）的黑腹果蝇（Aphis fabae）从寄生蜂抗性增加中受益。然而，由于共生体感染带来的适应代价，共生体赋予的抗性可能对蚜虫宿主产生净收益或净成本，这取决于寄生蜂的存在以及一般的生态背景。因此，平衡选择可以解释为什么在自然蚜虫种群中，H. defensa 通常以中等频率存在。在这里，我们进行了一项为期两年的野外研究，旨在寻找自然蚜虫种群中平衡选择的特征。我们收集了关于 H. defensa 在 A. f. fabae 中的流行率的时间分辨率良好的数据，并使用哨兵宿主估计了寄生蜂带来的风险。尽管寄生蜂风险在初夏有明显且一致的高峰，而且共生体的流行率随时间发生了显著变化，但我们发现寄生蜂风险与 H. defensa 频率动态之间只有微弱的相关性。实际上，研究种群中 H. defensa 的流行率更多地取决于前几代蚜虫所经历的炎热天数。我们的研究前所未有地深入了解了 A. f. fabae 种群中内共生体和寄生蜂群落的动态，并且增加了越来越多的经验证据表明，不仅是寄生蜂风险，而是多种选择正在塑造野生 H. defensa 的流行率。
