School of Biological Sciences, University of Bristol, Bristol BS8 1TQ, UK.
Department of Zoology, University of Oxford, Oxford OX1 3PS, UK.
Proc Biol Sci. 2021 Nov 24;288(1963):20211993. doi: 10.1098/rspb.2021.1993.
Many insects rely on intracellular bacterial symbionts to supplement their specialized diets with micronutrients. Using data from diverse and well-studied insect systems, we propose three lines of evidence suggesting that hosts have tight control over the density of their obligate, intracellular bacterial partners. First, empirical studies have demonstrated that the within-host symbiont density varies depending on the nutritional and developmental requirements of the host. Second, symbiont genomes are highly reduced and have limited capacity for self-replication or transcriptional regulation. Third, several mechanisms exist for hosts to tolerate, regulate and remove symbionts including physical compartmentalization and autophagy. We then consider whether such regulation is adaptive, by discussing the relationship between symbiont density and host fitness. We discuss current limitations of empirical studies for exploring fitness effects in host-symbiont relationships, and emphasize the potential for using mathematical models to formalize evolutionary hypotheses and to generate testable predictions for future work.
许多昆虫依靠细胞内共生细菌来补充其专门的饮食中的微量营养素。利用来自不同且研究充分的昆虫系统的数据，我们提出了三条证据表明，宿主对其必需的、细胞内共生细菌伙伴的密度有严格的控制。首先，实证研究表明，宿主体内共生体的密度取决于宿主的营养和发育需求。其次，共生体基因组高度简化，自我复制或转录调控的能力有限。第三，宿主存在几种耐受、调节和去除共生体的机制，包括物理分隔和自噬。然后，我们通过讨论共生体密度与宿主适应性之间的关系，来探讨这种调节是否具有适应性。我们讨论了当前实证研究在探索宿主-共生体关系中适应性影响的局限性，并强调了使用数学模型来形式化进化假说和为未来的工作生成可测试的预测的潜力。
