While genome erosion is extensively studied in intracellular symbionts, the metabolic implications of reductive evolution in microbes subsisting extracellularly remain poorly understood. Stammera capleta-an extracellular symbiont in leaf beetles-possesses an extremely reduced genome (0.27 Mb), enabling the study of drastic reductive evolution in the absence of intracellularity. Here, we outline the genomic and transcriptomic profiles of Stammera and its host to elucidate host-symbiont metabolic interactions. Given the symbiont's substantial demands for nutrients and membrane components, the host's symbiotic organ shows repurposing of internal resources by upregulating nutrient transporters and cuticle-processing genes targeting epithelial chitin. Facilitated by this supplementation and its localization, Stammera exhibits a highly streamlined gene expression profile and a fermentation pathway for energy conversion, sharply contrasting the respiratory metabolism retained by most intracellular symbionts. Our results provide insights into a tightly regulated and metabolically integrated extracellular symbiosis, expanding our understanding of the minimal metabolism required to sustain life outside of a host cell.

中文翻译：

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最小的发酵代谢为叶甲虫的细胞外共生体提供燃料。

虽然在细胞内共生体中对基因组侵蚀进行了广泛研究，但在细胞外生存的微生物中还原进化的代谢影响仍然知之甚少。Stammera capleta - 叶甲虫中的一种细胞外共生体 - 拥有极其减少的基因组 (0.27 Mb)，能够在没有细胞内的情况下研究剧烈的还原进化。在这里，我们概述了 Stammera 及其宿主的基因组和转录组谱，以阐明宿主-共生体的代谢相互作用。鉴于共生体对营养物质和膜成分的大量需求，宿主的共生器官通过上调靶向上皮几丁质的营养转运蛋白和角质层加工基因来重新利用内部资源。在此补充及其本地化的推动下，Stammera 表现出高度流线型的基因表达谱和能量转换的发酵途径，与大多数细胞内共生体保留的呼吸代谢形成鲜明对比。我们的结果提供了对严格调节和代谢整合的细胞外共生的见解，扩大了我们对维持宿主细胞外生命所需的最小代谢的理解。