Data from: Heritability of symbiont density reveals distinct regulatory mechanisms in a tripartite symbiosis

Beneficial eukaryotic–bacterial partnerships are integral to animal and plant evolution. Understanding the density regulation mechanisms behind bacterial symbiosis is essential to elucidating the functional balance between hosts and symbionts. Citrus mealybugs, Planococcus citri (Risso), present an excellent model system for investigating the mechanisms of symbiont density regulation. They contain two obligate nutritional symbionts, Moranella endobia, which resides inside Tremblaya princeps, which has been maternally transmitted for 100–200 million years. We investigate whether host genotype may influence symbiont density by crossing mealybugs from two inbred laboratory-reared populations that differ substantially in their symbiont density to create hybrids. The density of the M. endobia symbiont in the hybrid hosts matched that of the maternal parent population, in keeping with density being determined either by the symbiont or the maternal genotype. However, the density of the T. princeps symbiont was influenced by the paternal host genotype. The greater dependency of T. princeps on its host may be due to its highly reduced genome. The decoupling of T. princeps and M. endobia densities, in spite of their intimate association, suggests that distinct regulatory mechanisms can be at work in symbiotic partnerships, even when they are obligate and mutualistic.有益的真核-细菌共生关系是动植物演化进程中不可或缺的关键环节。解析细菌共生体系的密度调控机制，是阐明宿主与共生菌间功能平衡的核心前提。柑橘粉蚧（Planococcus citri (Risso)）是研究共生菌密度调控机制的极佳模型体系。该物种携带两种专性营养共生菌：莫兰氏菌（Moranella endobia）寄生于特伦布莱氏菌（Tremblaya princeps）体内，二者的母系传播历史已延续1至2亿年。本研究通过将两个共生菌密度存在显著差异的实验室近交饲养粉蚧种群进行杂交以构建杂交个体，探究宿主基因型是否会对共生菌密度产生影响。研究结果显示，杂交宿主体内的莫兰氏菌（Moranella endobia）密度与母本种群一致，这与共生菌密度由共生菌自身或母本基因型决定的推论相符；但特伦布莱氏菌（Tremblaya princeps）的密度则受父本宿主基因型调控。特伦布莱氏菌对宿主更强的依赖性，可能与其基因组发生大幅缩减密切相关。尽管二者存在紧密的共生关联，但两种共生菌的密度调控呈现解耦现象，这表明即便对于专性互利共生关系，不同共生菌也可能存在各自独立的密度调控机制。