Host-associated microbiota play a fundamental role in the training and induction of different forms of immunity, including inducible as well as constitutive components. However, direct experiments analysing the relative importance of microbiota on diverse forms of evolved immune functions are missing. We addressed this gap by using experimentally evolved lines of Tribolium castaneum that either produced inducible immune memory-like responses (immune priming) or constitutively expressed basal resistance (without priming), as divergent counterstrategies against Bacillus thuringiensis infection. We altered the microbial communities present in the diet (i.e. wheat flour) of these evolved lines using UV irradiation and estimated the impact on the beetle's ability to mount a priming response versus basal resistance. Populations that had evolved immune priming lost the ability to mount a priming response upon alteration of diet microbiota. Microbiota manipulation also caused a drastic reduction in their reproductive output and post-infection longevity. In contrast, in pathogen-resistant beetles, microbiota manipulation did not affect post-infection survival or reproduction. The divergent evolution of immune responses across beetle lines was thus associated with divergent reliance on the microbiome. Whether the latter is a direct outcome of differential pathogen exposure during selection or reflects evolved immune functions remains unclear. We hope that our results will motivate further experiments to understand the mechanistic basis of these complex evolutionary associations between microbiota, host immune strategies and fitness outcomes.

中文翻译：

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饮食微生物群的改变限制了面粉甲虫实验进化的免疫启动反应，但不限制病原体抵抗力。

宿主相关微生物群在不同形式免疫的训练和诱导中发挥着重要作用，包括诱导型和组成型免疫。然而，缺乏分析微生物群对多种形式的进化免疫功能的相对重要性的直接实验。我们通过使用实验进化的赤拟谷盗品系来解决这一差距，这些品系要么产生诱导性免疫记忆样反应（免疫启动），要么组成性表达基础抗性（无启动），作为对抗苏云金芽孢杆菌感染的不同对策。我们使用紫外线照射改变了这些进化品系的饮食（即小麦粉）中存在的微生物群落，并估计了对甲虫启动反应与基础抗性的能力的影响。已经进化出免疫启动的人群失去了在饮食微生物群改变时启动启动反应的能力。微生物群控制还导致其繁殖能力和感染后寿命急剧下降。相比之下，在病原体抗性甲虫中，微生物群的控制并不影响感染后的生存或繁殖。因此，不同甲虫品系的免疫反应的不同进化与对微生物组的不同依赖有关。后者是选择过程中不同病原体暴露的直接结果还是反映了进化的免疫功能仍不清楚。我们希望我们的结果将激发进一步的实验，以了解微生物群、宿主免疫策略和健身结果之间这些复杂的进化关联的机制基础。