Data from: Are aphid parasitoids locally adapted to the prevalence of defensive symbionts in their hosts?

Background: Insect parasitoids are under strong selection to overcome their hosts’ defences. In aphids, resistance to parasitoids is largely determined by the presence or absence of protective endosymbionts such as Hamiltonella defensa. Hence, parasitoids may become locally adapted to the prevalence of this endosymbiont in their host populations. To address this, we collected isofemale lines of the aphid parasitoid Lysiphlebus fabarum from 17 sites in Switzerland and France, at which we also estimated the frequency of infection with H. defensa as well as other bacterial endosymbionts in five important aphid host species. The parasitoids’ ability to overcome H. defensa-mediated resistance was then quantified by estimating their parasitism success on a single aphid clone (Aphis fabae fabae) that was either uninfected or experimentally infected with one of three different isolates of H. defensa. Results: The five aphid species (Aphis fabae fabae, A. f. cirsiiacanthoides, A. hederae, A. ruborum, A. urticata) differed strongly in the relative frequencies of infection with different bacterial endosymbionts, but there was also geographic variation in symbiont prevalence. Specifically, the frequency of infection with H. defensa ranged from 22 to 47 % when averaged across species. Parasitoids from sites with a high prevalence of H. defensa tended to be more infective on aphids possessing H. defensa, but this relationship was not significant, thus providing no conclusive evidence that L. fabarum is locally adapted to the occurrence of H. defensa. On the other hand, we observed a strong interaction between parasitoid line and H. defensa isolate on parasitism success, indicative of a high specificity of symbiont-conferred resistance. Conclusions: This study is the first, to our knowledge, to test for local adaptation of parasitoids to the frequency of defensive symbionts in their hosts. While it yielded useful information on the occurrence of facultative endosymbionts in several important host species of L. fabarum, it provided no clear evidence that parasitoids from sites with a high prevalence of H. defensa are better able to overcome H. defensa-conferred resistance. The strong genetic specificity in their interaction suggests that it may be more important for parasitoids to adapt to the particular strains of H. defensa in their host populations than to the general prevalence of this symbiont, and it highlights the important role symbionts can play in mediating host-parasitoid coevolution.

研究背景：昆虫寄生蜂面临着克服宿主防御的强烈选择压力。在蚜虫中，对寄生蜂的抗性主要由保护性内共生菌（endosymbionts）的存在与否决定，例如汉密尔顿菌（Hamiltonella defensa）。因此，寄生蜂可能会对宿主种群中该内共生菌的流行率产生局部适应。为探究这一问题，我们从瑞士与法国的17个采样点收集了蚜茧蜂（Lysiphlebus fabarum）的单雌系（isofemale lines），同时估算了5种重要蚜虫宿主中感染H. defensa及其他细菌内共生菌的频率。随后，我们通过测定寄生蜂在单个豆蚜指名亚种（Aphis fabae fabae）克隆种群上的寄生成功率，量化其克服H. defensa介导的抗性的能力；该克隆种群要么未被感染，要么经实验感染了3种不同分离株的H. defensa之一。
研究结果：5种蚜虫（豆蚜指名亚种Aphis fabae fabae、蓟罂粟蚜A. f. cirsiiacanthoides、常春藤蚜A. hederae、悬钩子蚜A. ruborum、荨麻蚜A. urticata）在不同细菌内共生菌的感染相对频率上差异显著，且共生菌的流行率存在地理变异。具体而言，跨物种平均后，H. defensa的感染频率介于22%至47%之间。来自H. defensa高流行率采样点的寄生蜂，在携带H. defensa的蚜虫上的侵染能力往往更强，但该相关性未达到显著水平，因此尚无确凿证据表明L. fabarum会对H. defensa的存在产生局部适应。另一方面，我们观察到寄生蜂品系与H. defensa分离株之间对寄生成功率存在强烈的交互作用，这暗示共生菌介导的抗性具有高度特异性。
研究结论：据我们所知，本研究首次检验了寄生蜂是否会对宿主的防御性共生菌流行率产生局部适应。尽管本研究获取了关于L. fabarum的多种重要宿主种群中兼性内共生菌分布的有效信息，但并未找到明确证据证明，来自H. defensa高流行率采样点的寄生蜂更能克服H. defensa介导的抗性。二者互作中存在的强遗传特异性表明，相较于该共生菌的整体流行率，寄生蜂更需要适应宿主种群中特定菌株的H. defensa；这也凸显了共生菌在介导宿主-寄生蜂协同进化过程中可发挥的重要作用。