The players in a mutualistic symbiosis: Insects, bacteria, viruses, and virulence genes
The players in a mutualistic symbiosis: Insects, bacteria, viruses, and virulence genes
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DOI：
10.1073/pnas.0507029102
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发表时间：
2005-11-22
影响因子：
11.1
通讯作者：
Ochman, H
中科院分区：
文献类型：
--
作者：
Moran, NA;Degnan, PH;Ochman, H
蚜虫维持着由共同遗传的有机体组成的联合体之间的互惠共生。它们都有一种主要的内生共生体Buchnera，可以弥补饮食不足；许多还含有次生共生体，如防御汉密尔顿氏菌，它可以防御天敌。由于难以分离足够的DNA样本，无法培养的次生共生体的基因组序列一直难以分析。通过从受感染的豌豆蚜血淋巴中扩增DNA，我们获得了一组防御性棉铃虫的基因组序列和一个相关的噬菌体。防御性嗜血杆菌有两个III型分泌系统，与肠道病原体介导宿主细胞进入的分泌系统有关。该噬菌体被称为APSE-2，是先前测序的APSE-1的近亲，但包含编码细胞致死性膨胀毒素(CDtB)基因的完整同源物，该基因中断真核细胞周期，并从多种哺乳动物病原体中已知。CDtB同源物高度表达，其基因组位置对应于APSE-1中STX(编码志贺毒素)的同源物。APSE-2基因组在受感染的豌豆蚜虫中持续丰富，在来自许多昆虫物种的所有防御性粘虫分离物中都发现了相关的噬菌体。根据它们的普遍存在和丰富程度，这些噬菌体似乎是防御性血吸虫生活史中的一个必不可少的组成部分。我们认为，在这些互惠共生的共生体中，噬菌体携带的毒素基因为蚜虫宿主提供防御，是观察到的对真核寄生虫的保护作用的基础。
Aphids maintain mutualistic symbioses involving consortia of coinherited organisms. All possess a primary endosymbiont, Buchnera, which compensates for dietary deficiencies; many also contain secondary symbionts, such as Hamiltonella defensa, which confers defense against natural enemies. Genome sequences of uncultivable secondary symbionts have been refractory to analysis due to the difficulties of isolating adequate DNA samples. By amplifying DNA from hemolymph of infected pea aphids, we obtained a set of genomic sequences of H. defensa and an associated bacteriophage. H. defensa harbors two type III secretion systems, related to those that mediate host cell entry by enteric pathogens. The phage, called APSE-2, is a close relative of the previously sequenced APSE-1 but contains intact homologs of the gene encoding cytolethal distending toxin (cdtB), which interrupts the eukaryotic cell cycle and which is known from a variety of mammalian pathogens. The cdtB homolog is highly expressed, and its genomic position corresponds to that of a homolog of stx (encoding Shiga-toxin) within APSE-1. APSE-2 genomes were consistently abundant in infected pea aphids, and related phages were found in all tested isolates of H. defensa, from numerous insect species. Based on their ubiquity and abundance, these phages appear to be an obligate component of the H. defensa life cycle. We propose that, in these mutualistic symbionts, phage-borne toxin genes provide defense to the aphid host and are a basis for the observed protection against eukaryotic parasites.