Mutualisms between symbiotic microbes and animals have been well documented, and nutritional relationships provide the foundation for maintaining beneficial associations. The well-studied mutualism between bark beetles and their fungi has become a classic model system in the study of symbioses. Despite the nutritional competition between bark beetles and beneficial fungi in the same niche due to poor nutritional feeding substrates, bark beetles still maintain mutualistic associations with beneficial fungi over time. The mechanism behind this phenomenon, however, remains largely unknown. Here, we demonstrated the bark beetle Dendroctonus valens LeConte relies on the symbiotic bacterial volatile ammonia, as a nitrogen source, to regulate carbohydrate metabolism of its mutualistic fungus Leptographium procerum to alleviate nutritional competition, thereby maintaining the stability of the bark beetle–fungus mutualism. Ammonia significantly reduces competition of L. procerum for carbon resources for D. valens larval growth and increases fungal growth. Using stable isotope analysis, we show the fungus breakdown of phloem starch into d-glucose by switching on amylase genes only in the presence of ammonia. Deletion of amylase genes interferes with the conversion of starch to glucose. The acceleration of carbohydrate consumption and the conversion of starch into glucose benefit this invasive beetle–fungus complex. The nutrient consumption–compensation strategy mediated by tripartite beetle–fungus–bacterium aids the maintenance of this invasive mutualism under limited nutritional conditions, exacerbating its invasiveness with this competitive nutritional edge.

共生微生物与动物之间的相互关系已得到充分证明，营养关系为维持有益的联系提供了基础。树皮甲虫与其真菌之间经过深入研究的共生关系已成为共生研究中的经典模型系统。尽管由于营养喂养基质差，树皮甲虫和有益真菌之间的营养竞争处于同一生态位，但随着时间的流逝，树皮甲虫仍与有益真菌保持相互联系。但是，这种现象背后的机制仍然未知。在这里，我们证明了树皮甲虫Dendroctonus valens LeConte依靠共生细菌挥发性氨作为氮源来调节其共生真菌的碳水化合物代谢淡水ograph可以减轻营养竞争，从而保持树皮甲虫-真菌共生的稳定性。氨显着减少了角果劳氏菌对缬草幼虫生长所需碳资源的竞争，并增加了真菌的生长。使用稳定同位素分析，我们显示了韧皮部淀粉的真菌分解为d-葡萄糖仅在氨存在下才通过打开淀粉酶基因来实现。淀粉酶基因的缺失会干扰淀粉向葡萄糖的转化。碳水化合物消耗的加速和淀粉向葡萄糖的转化有益于这种侵入性的甲虫－真菌复合物。由三方甲虫-真菌-细菌介导的营养消耗-补偿策略有助于在有限的营养条件下维持这种侵入性共生，并以竞争性营养优势加剧其侵入性。