Faculty of Agriculture, University of Miyazaki, 1-1 Gakuenkibanadai-Nishi, Miyazaki, Miyazaki, 889-2192, Japan.
Biodiversity Research Center, Academia Sinica, No.128, Sec 2, Academia Rd, Nangang, Taipei, 11529, Taiwan.
Microbiome. 2020 Aug 24;8(1):123. doi: 10.1186/s40168-020-00880-3.
Population outbreaks of the crown-of-thorns starfish (Acanthaster planci sensu lato; COTS), a primary predator of reef-building corals in the Indo-Pacific Ocean, are a major threat to coral reefs. While biological and ecological knowledge of COTS has been accumulating since the 1960s, little is known about its associated bacteria. The aim of this study was to provide fundamental information on the dominant COTS-associated bacteria through a multifaceted molecular approach.
A total of 205 COTS individuals from 17 locations throughout the Indo-Pacific Ocean were examined for the presence of COTS-associated bacteria. We conducted 16S rRNA metabarcoding of COTS to determine the bacterial profiles of different parts of the body and generated a full-length 16S rRNA gene sequence from a single dominant bacterium, which we designated COTS27. We performed phylogenetic analysis to determine the taxonomy, screening of COTS27 across the Indo-Pacific, FISH to visualize it within the COTS tissues, and reconstruction of the bacterial genome from the hologenome sequence data.
We discovered that a single bacterium exists at high densities in the subcuticular space in COTS forming a biofilm-like structure between the cuticle and the epidermis. COTS27 belongs to a clade that presumably represents a distinct order (so-called marine spirochetes) in the phylum Spirochaetes and is universally present in COTS throughout the Indo-Pacific Ocean. The reconstructed genome of COTS27 includes some genetic traits that are probably linked to adaptation to marine environments and evolution as an extracellular endosymbiont in subcuticular spaces.
COTS27 can be found in three allopatric COTS species, ranging from the northern Red Sea to the Pacific, implying that the symbiotic relationship arose before the speciation events (approximately 2 million years ago). The universal association of COTS27 with COTS and nearly mono-specific association at least with the Indo-Pacific COTS provides a useful model system for studying symbiont-host interactions in marine invertebrates and may have applications for coral reef conservation. Video Abstract.
棘冠海星（Acanthaster planci sensu lato；COTS）是印度洋-太平洋海域造礁石珊瑚的主要捕食者，其种群爆发对珊瑚礁构成了重大威胁。虽然自 20 世纪 60 年代以来，人们对 COTS 的生物学和生态学知识有了一定的积累，但对其相关细菌知之甚少。本研究旨在通过多方面的分子方法为主要的 COTS 相关细菌提供基础信息。
对来自印度洋-太平洋地区 17 个地点的 205 只 COTS 个体进行了 COTS 相关细菌的存在情况检查。我们对 COTS 的 16S rRNA 进行了 metabarcoding，以确定不同身体部位的细菌分布，并从单一优势细菌（我们命名为 COTS27）中生成全长 16S rRNA 基因序列。我们进行了系统发育分析以确定其分类，在整个印度洋-太平洋范围内筛选 COTS27，通过 FISH 技术在 COTS 组织内对其进行可视化，并根据全基因组序列数据重建细菌基因组。
我们发现，有一种单一的细菌在 COTS 的表皮下空间中以高密度存在，在表皮和角质层之间形成了一种类似生物膜的结构。COTS27 属于一个假定代表螺旋体门中的一个独特目（所谓的海洋旋体）的进化枝，在整个印度洋-太平洋地区的 COTS 中普遍存在。COTS27 的重建基因组包括一些可能与适应海洋环境和作为表皮下空间的细胞外共生体进化相关的遗传特征。
COTS27 可以在三种分布范围不同的 COTS 物种中找到，从红海北部到太平洋，这表明共生关系是在物种形成事件（大约 200 万年前）之前出现的。COTS27 与 COTS 的普遍关联以及至少与印度洋-太平洋 COTS 的近单物种关联为研究海洋无脊椎动物共生体-宿主相互作用提供了一个有用的模型系统，并可能为珊瑚礁保护应用提供参考。视频摘要。
