Data from: Aphid-encoded variability in susceptibility to a parasitoid

Background: Many animals exhibit variation in resistance to specific natural enemies. Such variation may be encoded in their genomes or derived from infection with protective symbionts. The pea aphid, Acyrthosiphon pisum, for example, exhibits tremendous variation in susceptibility to a common natural enemy, the parasitic wasp Aphidius ervi. Pea aphids are often infected with the heritable bacterial symbiont, Hamiltonella defensa, which confers partial to complete resistance against this parasitoid depending on bacterial strain and associated bacteriophages. That previous studies found that pea aphids without H. defensa (or other symbionts) were generally susceptible to parasitism, together with observations of a limited encapsulation response, suggested that pea aphids largely rely on infection with H. defensa for protection against parasitoids. However, the limited number of uninfected clones previously examined, and our recent report of two symbiont-free resistant clones, led us to explicitly examine aphid-encoded variability in resistance to parasitoids. Results: After rigorous screening for known and unknown symbionts, and microsatellite genotyping to confirm clonal identity, we conducted parasitism assays using fifteen clonal pea aphid lines. We recovered significant variability in aphid-encoded resistance, with variation levels comparable to that contributed by H. defensa. Because resistance can be costly, we also measured aphid longevity and cumulative fecundity of the most and least resistant aphid lines under permissive conditions, but found no trade-offs between higher resistance and these fitness parameters. Conclusions: These results indicate that pea aphid resistance to A. ervi is more complex than previously appreciated, and that aphids employ multiple tactics to aid in their defense. While we did not detect a tradeoff, these may become apparent under stressful conditions or when resistant and susceptible aphids are in direct competition. Understanding sources and amounts of variation in resistance to natural enemies is necessary to understand the ecological and evolutionary dynamics of antagonistic interactions, such as the potential for coevolution, but also for the successful management of pest populations through biological control.

研究背景：诸多动物对特定天敌的抗性存在变异，此类变异既可由宿主基因组编码决定，也可通过感染保护性共生菌（protective symbionts）获得。以豌豆蚜（Acyrthosiphon pisum）为例，其对常见天敌寄生蜂（parasitic wasp）Aphidius ervi的易感性存在显著差异。豌豆蚜常携带可垂直传播的细菌共生菌汉氏防御杆菌（Hamiltonella defensa），该共生菌可根据自身菌株及伴随的噬菌体（bacteriophages）类型，为宿主提供针对该寄生蜂的部分至完全抗性。既往研究发现，未感染H. defensa（或其他共生菌）的豌豆蚜通常易被寄生蜂寄生，结合此前观察到的有限包囊反应（encapsulation response），提示豌豆蚜主要依赖感染H. defensa来抵御寄生蜂。然而，既往研究所检测的无共生菌克隆株数量有限，且我们近期报道了两株无共生菌却具抗性的克隆株，这促使我们专门探究豌豆蚜自身基因组编码的寄生蜂抗性变异。

研究结果：在对已知及未知共生菌进行严格筛选，并通过微卫星基因分型（microsatellite genotyping）确认克隆身份后，我们使用15个豌豆蚜克隆株系开展了寄生试验。我们检测到豌豆蚜自身编码的抗性存在显著变异，其变异幅度与H. defensa介导的抗性变异相当。鉴于抗性通常存在适合度代价，我们还在适宜条件下测定了抗性最强和最弱的豌豆蚜克隆株系的寿命及总繁殖力，但未发现更高抗性与这些适合度参数（fitness parameters）之间存在适合度权衡（trade-offs）。

研究结论：本研究结果表明，豌豆蚜对A. ervi的抗性机制比此前认知更为复杂，豌豆蚜可通过多种策略实现免疫防御。尽管本研究未检测到适合度权衡，但这类权衡可能在胁迫条件下，或抗性蚜与易感蚜直接竞争时显现。明确天敌抗性变异的来源与程度，有助于理解拮抗互作（如协同进化潜力）的生态与进化动力学，同时也可为通过生物防治（biological control）实现害虫种群的有效管理提供理论支撑。