A Novel Human-Infection-Derived Bacterium Provides Insights into the Evolutionary Origins of Mutualistic Insect–Bacterial Symbioses

Despite extensive study, little is known about the origins of the mutualistic bacterial endosymbionts that inhabit approximately 10% of the world's insects. In this study, we characterized a novel opportunistic human pathogen, designated “strain HS,” and found that it is a close relative of the insect endosymbiont Sodalis glossinidius. Our results indicate that ancestral relatives of strain HS have served as progenitors for the independent descent of Sodalis-allied endosymbionts found in several insect hosts. Comparative analyses indicate that the gene inventories of the insect endosymbionts were independently derived from a common ancestral template through a combination of irreversible degenerative changes. Our results provide compelling support for the notion that mutualists evolve from pathogenic progenitors. They also elucidate the role of degenerative evolutionary processes in shaping the gene inventories of symbiotic bacteria at a very early stage in these mutualistic associations.尽管已有大量研究，但对于栖息于全球约10%昆虫体内的互利共生细菌内共生体（mutualistic bacterial endosymbionts）的起源，学界仍知之甚少。本研究对一株命名为‘HS菌株’的新型机会致病性人类病原体进行了系统表征，发现其与昆虫内共生体格氏 Sodalis菌（Sodalis glossinidius）亲缘关系密切。研究结果证实，HS菌株的祖先类群曾作为若干昆虫宿主体内格氏 Sodalis菌类内共生体独立演化的祖先来源。比较基因组分析显示，这些昆虫内共生体的基因组成均通过一系列不可逆的退化性改变，从某一共同的祖先模板独立演化而来。本研究结果为‘互利共生体由致病性祖先进化而来’这一观点提供了强有力的支撑。同时，本研究还阐明了退化性演化过程在这类互利共生关系建立的极早期阶段，对共生细菌基因组成塑造所发挥的作用。