VanDieren Anthony J, Barrick Jeffrey E
Department of Molecular Biosciences, Center for Systems and Synthetic Biology, The University of Texas at Austin, Austin, Texas, USA.
mBio. 2025 Sep 8:e0204125. doi: 10.1128/mbio.02041-25.
bacteria exhibit a range of relationships with aphids. They may be co-obligate mutualists, commensals, or even pathogens depending on the strain, aphid host species, and environment. CWBI-2.3 (CWBI), a culturable member of this group, is transmitted to embryos transovarially when it is injected into pea aphids (), the same route used by strains that are vertically inherited endosymbionts. Yet, aphids colonized with CWBI die before they give birth to infected offspring. We evolved laboratory populations of CWBI through 15-30 serial passages at two different temperatures in rich media. These nutrient-replete conditions mimic aspects of environments within aphid hosts that lead to the evolution of reduced endosymbiont genomes. Unexpectedly, all populations propagated at one temperature appeared to evolve slower growth after only a few days due to reactivation of a lytic prophage from the CWBI genome. Though these populations continued to reach saturating cell densities slower than cultures of the ancestor throughout the experiment, representative clones isolated from them had mutations affecting lipopolysaccharide biosynthesis and were resistant to the phage. Some evolved strains exhibited less virulence when injected into aphids, and we observed instances of gene inactivation and loss mediated by insertion elements. Our results illustrate how transposons and prophages can dominate laboratory evolution of newly cultured bacteria, particularly those that are host-associated in nature and have genomes rife with selfish DNA elements. They also suggest that bacteria-phage coevolution can catalyze evolutionary paths that contribute to converting pathogens into stably inherited endosymbionts.IMPORTANCELaboratory experiments can be used to explore evolutionary innovations in how microbes associate with animal hosts. bacteria exhibit a variety of interactions with aphids. Some strains are obligate endosymbionts. Others have facultative associations with benefits or costs depending on the environmental context. CWBI-2.3 (CWBI) resembles aphid endosymbionts in how it can be transovarially transmitted to aphid embryos. However, adults injected with CWBI do not survive long enough to give birth to infected offspring. We evolved this aphid protosymbiont in rich media to see if this would attenuate its virulence and recapitulate genome reduction observed in endosymbionts. We observed large deletions and gene inactivation, but reactivation of a prophage from the CWBI genome and then evolution of phage resistance dominated. Some evolved strains became less virulent to aphids, suggesting that evolution driven by selfish DNA elements can contribute to the emergence of new endosymbionts from pathogen ancestors.
细菌与蚜虫呈现出一系列关系。根据菌株、蚜虫宿主物种和环境的不同，它们可能是共专性互利共生体、共生菌，甚至是病原体。CWBI-2.3（CWBI）是该菌群中一种可培养的成员，当将其注入豌豆蚜时，它会通过经卵传递给胚胎，这与垂直遗传的内共生菌菌株所采用的途径相同。然而，定殖有CWBI的蚜虫在产下受感染后代之前就会死亡。我们在丰富培养基中于两种不同温度下通过15 - 30次连续传代培养了CWBI的实验室群体。这些营养丰富的条件模拟了蚜虫宿主体内环境的某些方面，这些方面会导致内共生菌基因组缩小的进化。出乎意料的是，在一个温度下繁殖的所有群体在仅仅几天后似乎生长速度就变慢了，这是由于CWBI基因组中的一个裂解性原噬菌体重新激活所致。尽管在整个实验过程中这些群体达到饱和细胞密度的速度仍然比祖先菌株的培养物慢，但从它们中分离出的代表性克隆具有影响脂多糖生物合成的突变并且对噬菌体具有抗性。一些进化菌株注入蚜虫后表现出较低的毒力，并且我们观察到由插入元件介导的基因失活和缺失的情况。我们的结果说明了转座子和原噬菌体如何能够主导新培养细菌的实验室进化，特别是那些在自然界中与宿主相关且基因组中充满自私DNA元件的细菌。它们还表明细菌 - 噬菌体的共同进化可以催化有助于将病原体转化为稳定遗传的内共生体的进化路径。
重要性
实验室实验可用于探索微生物与动物宿主相互作用方式的进化创新。细菌与蚜虫呈现出多种相互作用。一些菌株是专性内共生菌。其他菌株则根据环境背景具有兼性关联，有好处或成本。CWBI-2.3（CWBI）在其经卵传递给蚜虫胚胎的方式上类似于蚜虫内共生菌。然而，注射了CWBI的成虫存活时间不够长，无法产下受感染的后代。我们在丰富培养基中对这种蚜虫原共生菌进行进化，以观察这是否会减弱其毒力并重现在内共生菌中观察到的基因组缩小现象。我们观察到大量缺失和基因失活，但CWBI基因组中一个原噬菌体的重新激活以及随后噬菌体抗性的进化占主导地位。一些进化菌株对蚜虫的毒力降低，这表明由自私DNA元件驱动的进化可以促成从病原体祖先中出现新的内共生体。
