Expression and Putative Function of Innate Immunity Genes under <em>in situ</em> Conditions in the Symbiotic Hydrothermal Vent Tubeworm <em>Ridgeia piscesae</em>

The relationships between hydrothermal vent tubeworms and sulfide-oxidizing bacteria have served as model associations for understanding chemoautotrophy and endosymbiosis. Numerous studies have focused on the physiological and biochemical adaptations that enable these symbioses to sustain some of the highest recorded carbon fixation rates ever measured. However, far fewer studies have explored the molecular mechanisms underlying the regulation of host and symbiont interactions, specifically those mediated by the innate immune system of the host. To that end, we conducted a series of studies where we maintained the tubeworm, Ridgeia piscesae, in high-pressure aquaria and examined global and quantitative changes in gene expression via high-throughput transcriptomics and quantitative real-time PCR (qPCR). We analyzed over 32,000 full-length expressed sequence tags as well as 26 Mb of transcript sequences from the trophosome (the organ that houses the endosymbiotic bacteria) and the plume (the gas exchange organ in contact with the free-living microbial community). R. piscesae maintained under conditions that promote chemoautotrophy expressed a number of putative cell signaling and innate immunity genes, including pattern recognition receptors (PRRs), often associated with recognizing microbe-associated molecular patterns (MAMPs). Eighteen genes involved with innate immunity, cell signaling, cell stress and metabolite exchange were further analyzed using qPCR. PRRs, including five peptidoglycan recognition proteins and a Toll-like receptor, were expressed significantly higher in the trophosome compared to the plume. Although PRRs are often associated with mediating host responses to infection by pathogens, the differences in expression between the plume and trophosome also implicate similar mechanisms of microbial recognition in interactions between the host and symbiont. We posit that regulation of this association involves a molecular “dialogue” between the partners that includes interactions between the host’s innate immune system and the symbiont.热液喷口管蠕虫与氧化硫化物细菌之间的共生关系，一直是阐释化能自养（chemoautotrophy）与内共生（endosymbiosis）的经典模式共生体系。既往诸多研究聚焦于这类共生体维持迄今已记录的最高固碳速率之一所依赖的生理与生化适应性机制。然而，针对宿主与共生体互作调控的分子机制——尤其是由宿主先天免疫系统介导的互作调控——的探索却寥寥无几。为此，我们开展了一系列研究：将双脊管蠕虫（Ridgeia piscesae）饲养于高压水族箱中，并通过高通量转录组学与实时荧光定量PCR（qPCR）分析基因表达的全局与定量变化。我们分析了来自营养体（即内共生细菌的定植器官）与羽枝（即与自由生活微生物群落接触的气体交换器官）的超过32000条全长表达序列标签，以及26 Mb的转录组序列。在处于化能自养促进条件下饲养的R. piscesae中，我们检测到多种推定的细胞信号通路与先天免疫相关基因，包括常参与识别微生物相关分子模式（MAMPs）的模式识别受体（PRRs）。我们进一步通过qPCR分析了18个与先天免疫、细胞信号通路、细胞应激及代谢物交换相关的基因。相较于羽枝，营养体中多种模式识别受体的表达量显著更高，其中包括5个肽聚糖识别蛋白与1个Toll样受体（Toll-like receptor）。尽管模式识别受体通常被认为介导宿主对病原体感染的免疫应答，但羽枝与营养体之间的表达差异也表明，宿主与共生体的互作中也存在类似的微生物识别机制。我们推测，这类共生关系的调控涉及共生双方的分子“对话”，其中包含宿主先天免疫系统与共生体之间的相互作用。