Bioproduction Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8566, Japan;
Computational Bio Big-Data Open Innovation Laboratory, National Institute of Advanced Industrial Science and Technology, Tokyo 169-8555, Japan.
Proc Natl Acad Sci U S A. 2017 Oct 3;114(40):E8382-E8391. doi: 10.1073/pnas.1712857114. Epub 2017 Sep 18.
Beetles, representing the majority of the insect species diversity, are characterized by thick and hard cuticle, which plays important roles for their environmental adaptation and underpins their inordinate diversity and prosperity. Here, we report a bacterial endosymbiont extremely specialized for sustaining beetle's cuticle formation. Many weevils are associated with a γ-proteobacterial endosymbiont lineage , whose evolutionary origin is estimated as older than 100 million years, but its functional aspect has been elusive. Sequencing of genomes from diverse weevils unveiled drastic size reduction to 0.2 Mb, in which minimal complete gene sets for bacterial replication, transcription, and translation were present but almost all of the other metabolic pathway genes were missing. Notably, the only metabolic pathway retained in the genomes was the tyrosine synthesis pathway, identifying tyrosine provisioning as 's sole biological role. Weevils are armored with hard cuticle, tyrosine is the principal precursor for cuticle formation, and experimental suppression of resulted in emergence of reddish and soft weevils with low tyrosine titer, confirming the importance of -mediated tyrosine production for host's cuticle formation and hardening. Notably, 's tyrosine synthesis pathway was incomplete, lacking the final step transaminase gene. RNA sequencing identified host's aminotransferase genes up-regulated in the bacteriome. RNA interference targeting the aminotransferase genes induced reddish and soft weevils with low tyrosine titer, verifying host's final step regulation of the tyrosine synthesis pathway. Our finding highlights an impressively intimate and focused aspect of the host-symbiont metabolic integrity via streamlined evolution for a single biological function of ecological relevance.
甲虫是昆虫物种多样性的主要代表，其特征是表皮厚实坚硬，这对它们的环境适应至关重要，并支撑着它们异常的多样性和繁荣。在这里，我们报告了一种细菌内共生体，它非常专门地维持甲虫的表皮形成。许多象鼻虫与一种γ-变形菌内共生体有关，其进化起源估计早于 1 亿年，但它的功能方面一直难以捉摸。对来自不同象鼻虫的基因组进行测序揭示了其大小急剧缩小到 0.2Mb，其中存在细菌复制、转录和翻译的最小完整基因集，但几乎所有其他代谢途径基因都缺失了。值得注意的是，基因组中保留的唯一代谢途径是酪氨酸合成途径，这表明酪氨酸供应是其唯一的生物学作用。象鼻虫身上覆盖着坚硬的表皮，酪氨酸是表皮形成的主要前体，而实验抑制会导致表皮呈红色和柔软，酪氨酸含量低，证实了 介导的酪氨酸产生对宿主表皮形成和硬化的重要性。值得注意的是， 的酪氨酸合成途径是不完整的，缺乏最后一步转氨酶基因。RNA 测序鉴定了细菌体中宿主的转氨酶基因上调。针对这些转氨酶基因的 RNA 干扰诱导了红色和柔软的象鼻虫，酪氨酸含量低，验证了宿主对酪氨酸合成途径的最后一步调控。我们的发现突出了一个令人印象深刻的方面，即通过简化进化来实现单一的生态相关生物学功能，宿主-共生体的代谢完整性达到了一种紧密而集中的程度。
