肉足鞭毛类原生动物中宿主—共生体系统的研究

目前已在20多种变形虫和70多种鞭毛虫中发现细菌内共生体。大部分细菌内共生体位于宿主细胞质共生泡中,仅少数鞭毛虫的内共生体位于核质中。变形虫-细菌共生系统形成后,共生体影响宿主细胞基因,对其基因缺陷产生互补作用。灰胞藻类鞭毛虫-蓝绿藻共生体系统的研究表明,叶绿体起源于一种原始的共生蓝细菌。锥体亚目鞭毛虫细胞质内普遍含有双心体,该共生体可能是由来自波豆亚目的锥体类鞭毛虫遗传的。作者推测,继续研究鞭毛虫和原校生物共生关系起源的基本阶段,可阐明原生动物的共生系统起源的基本原则,并为真核细胞起源的理论提供进一步的证据;深入研究变形虫-细菌共生系统,可在遗传精细结构和代谢调节的进化方面为真核细胞内共生起源的理论提供分子水平上的证据。     

纤毛类原生动物中宿主－共生体系统的研究

目前已经在100多种纤毛虫中观察到细菌、藻类和其他微生物等共生体。对纤毛虫中宿主－共生体系统的研究表明,双小核草履虫中卡巴粒的遗传为细胞质遗传理论提供了例证;含细菌共生体的许多厌氧纤毛虫无线粒体,共生体对宿主代谢有重要作用;尾草履虫－钝状全孢螺菌共生作用中,共生菌感染形式的39kDa、15kDa周质蛋白可分别与IF-3-1、IF-3-2两种单抗反应,其共生体早期感染过程中两种抗原的量发生显著变化,并且共生体生殖形式选择性地合成63kDa蛋白质,该蛋白质可能是与共生作用有联系的关键分子;绿草履虫－小球藻共生系统中,共生藻中存在葡糖胺硬性壁是其与草履虫发生共生关系的基本条件,其中,共生藻参与宿主代谢,与宿主形成相互受益的专一性关系,并且藻类共生体的作用可能影响了宿主草履虫基因组有关结构,改变了其基因表达。作者推测,探索共生体对宿主基因结构及其表达产物的影响可能是对纤毛虫中共生作用研究的主要趋势,这对于深入了解真核细胞中宿主－共生体双方的相互作用、物质交流在分子水平上的调控机理、细胞结构与功能的关系等细胞生命活动规律是有意义的。     

纤毛类原生动物中宿主—共生体系统的研究

目前已经在100多种纤毛虫中观察到细菌、藻类和其他微生物等共生体。对纤毛虫中宿主－共生体系统的研究表明,双小核草履虫中卡巴粒的遗传为细胞质遗传理论提供了例证;含细菌共生体的许多厌氧纤毛虫无线粒体,共生体对宿主代谢有重要作用;尾草履虫－钝状全孢螺菌共生作用中,共生菌感染形式的39kDa、15kDa周质蛋白可分别与IF－3－1、IF－3－2两种单抗反应,其共生体早期感染过程中两种抗原的量发生显著变化,并且共生体生殖形式选择性地合成63kDa蛋白质,该蛋白质可能是与共生作有联系的关键分子;绿草履虫－小球藻共生系统中,共生藻中存在葡糖胺硬性壁是其与草履虫发生共生关系的基本条件,其中,共生藻参与宿主代谢,与宿主形成相互受益的专一性关系,并且藻类共生体的作用可能影响了宿主草履虫基因组有关结构,改变了其基因表达。作者推测,探索共生体对宿主基因结构及其表达产物的影响可能是对纤毛虫中共生作用研究的主要趋势,这对于深入了解真核细胞中宿主－共生体双方的相互作用、物质交流在分子水平上的调控机理、细胞结构与功能的关系等细胞生命活动规律是有意义的。     

草履虫中的内共生现象

双小核草履虫细胞质和细胞核内含有9种颗粒状细菌共生体,尾草履虫细胞核内含有3种全孢螺菌共生体,绿草履虫细胞质中含有小球藻共生体.双小核草屐虫中的某些细菌共生体能感染无共生体草履虫,具有感染能力的全孢螺菌和小球藻也能成功地进入到尾草履虫或绿草履虫细胞内,成为相应草履虫的内共生体.并报告了这几种草履虫中发生的互惠共生现象,以及绿草履虫中宿主与共生体间的"生态调节"现象.     

禾草内生真菌的遗传多样性及其共生关系

 近百年的禾草内生真菌研究历经了由浅入深的过程,从最初的家畜中毒事件认识到是一种共生内生真菌存在的缘故,到如今利用分子生物学技术揭示其共生机制,人类发现这类植物内生真菌并非想象中的对生态系统无足轻重。Epichloë及其无性型Neotyphodium与禾本科植物是系统发生的互利共生关系,尤其是Neotyphodium可提高宿主抵抗环境胁迫的能力和抵御动物的取食,增强植物的竞争力。禾草内生真菌有3种生活史：有性生活史、无性生活史和兼性生活史,后者表明真菌在不同的宿主及环境下既能营有性生殖也可营无性繁殖,是一种更灵活而有效的生活史对策。对内生真菌分子系统学、生活史以及与宿主禾草协同进化的研究发现,Neotyphodium起源于禾草致病真菌Epichloë的某些种,或是Epichloë与Neotyphodium的种间杂交后代。植物和内生真菌各异的生活史策略,真菌的种间杂交,两者的协同进化亦或种群间基因流的差异,都促成了共生体多样化的基因组合（Genetic combination ）,也是其共生关系多样化的根源。内生真菌对宿主的有益作用只在特定基因型真菌、宿主和一定环境条件下才起作用,自然生态系统的共生关系要比农业系统复杂得多,是一个从互利共生至寄生关系的连续系统。未来对于更多共生体的遗传背景和基因与环境相互作用的阐明将有助于对禾草内生真菌共生关系本质更加深入的认识。     

禾草内生真菌作为生防因子的潜力分析

早熟禾亚科多种禾草可与Neotyphodium内生真菌形成禾草-内生真菌共生体, 这种植物-微生物共生体性状较为稳定, 且在自然界中广泛存在。禾草-内生真菌共生体稳定的互利共生关系不但保证了内生真菌所需的全部营养物质, 而且共生体产生的次生代谢物又可显著提高宿主禾草对生物胁迫的抗逆性。众多研究表明, 内生真菌的侵染可显著提高宿主禾草对虫害、病害及伴生植物等多种生物胁迫的抗性。据不完全统计, 禾草内生真菌对蛛形纲、线虫纲、昆虫纲3个纲至少79个种的害虫表现出较明显的抗性, 对至少22个种的病原真菌表现出明显的抗性。尽管利用内生真菌进行禾草品种选育及其品质改良的技术日趋成熟, 但是内生真菌在不同宿主禾草之间高效的替代转化技术, 及其在宿主体内遗传的稳定性仍有待于进一步深入探索。研究者把禾草内生真菌作为生防手段, 在未来的应用过程中不应只考虑其与宿主禾草之间的共生特异性, 而应更全面地分析禾草-内生真菌-生态环境之间的相互关系, 让内生真菌更好地为人类服务。     

昆虫专性内共生细菌及其基因组研究进展

昆虫专性内共生细菌是一类与宿主昆虫长期协同进化的共生微生物,在许多昆虫体内均有发现,主要存在于昆虫特化的器官(含菌体)内,以垂直传播的方式由母系遗传。专性内共生细菌与昆虫的生存、繁殖以及进化等方面息息相关,其主要功能是为宿主提供必需氨基酸等营养物质。因其长期生活在宿主细胞内处于封闭的高营养的环境中,其基因组的特征与普通细菌基因组有很大区别,包括基因组大小、GC含量、基因缺失等方面。通过对共生细菌基因水平上的深入研究,有助于理解专性内共生细菌在宿主昆虫协同进化过程中的作用。目前,昆虫内共生细菌基因的生物学功能、内共生细菌之间以及内共生细菌与宿主之间的互作机制还不是很清楚,有待进一步的研究和探索。     

基因芯片在细菌基因组学和细胞微生物学研究中的应用

利用基因芯片可以鉴定病原菌在不同宿主微环境中受到差异调节的基因、直接或间接由转录因子控制的基因,以及编码多步骤代谢和生物合成途径中各种组分的基因;通过比较基因组学研究,评价相关菌种和菌株的自然群体内遗传多样性的范围和特性,并在ORF水平描述病原体和共生体之间的差异;同时也可进行感染组织细胞基因表达分析,研究病原体和宿主之间复杂的相互作用关系。     

源于微生物的海鞘天然产物

海洋动物是具有生物活性海洋天然产物的重要来源。海鞘中含有丰富的微生物类群,如细菌、放线菌、真菌和蓝细菌。越来越多的直接或间接证据表明,一些从海鞘中分离的天然产物并不是海鞘本身产生的,而是由其共生微生物产生的。本文对近些年来的海鞘天然产物的微生物来源的研究方法进行综述,包括可培养细菌的分离、不可培养细菌的粗提物检测、宏基因组学、全基因组测序等直接方法,以及化合物结构比对的间接方法。通过对海鞘-微生物共生体中天然产物生物合成来源的研究,不仅可以从根本上解决动物药源的问题,而且可为研究海鞘与微生物共生关系提供有力证据。     

珊瑚共生体中“细菌-虫黄藻-宿主”三角关系的通讯交流

“细菌-虫黄藻-珊瑚”是生态系统中一对经典的三角关系,其中包含着复杂的物质流、信息流和能量流,三者的平衡与稳定是维护珊瑚礁生态系统健康的重要保障。过去20年里针对共生体交互关系进行了大量研究,并取得了一些重要成果,明确了“细菌-虫黄藻-宿主”三者之间的物质代谢、营养交换以及与环境的交互关系。然而,基于共生系统的复杂性,一些现象背后的机制仍然未被充分揭示,尤其是共生体之间的通讯交流。信号分子介导的相互作用是珊瑚共生体稳态维持和高效运转的内在驱动力。本文以珊瑚共生体系中化学信号为重点,尝试梳理最新的研究进展,包括细菌与细菌、细菌与珊瑚、细菌与虫黄藻以及虫黄藻与珊瑚之间的通讯方式,重点关注了群体感应信号(QS)、二甲基巯基丙酸盐(DMSP)、糖类信号、脂类信号以及非编码RNA。选择性例举了QS信号介导的微生物协作和竞争、DMSP调节下的细菌和宿主的相互作用,以及环境胁迫下珊瑚和虫黄藻对非编码RNA的响应过程,强调了它们在共生体中的作用模式和生态意义。并对今后的研究重点和可能方向进行了提炼,包括研究维度的扩充、新技术-新方法的应用以及生态模型的构建等,旨在提升对三角关系互作方式的认识,增进对珊瑚共生体的理解,探索基于通讯语言的操纵方式为珊瑚礁生态系统的恢复和保护提供新思路。     

Endosymbiosis of Aphids with Microorganisms: a Model Case of Dynamic Endosymbiotic Evolution

Abstract Almost all aphids harbor prokaryotic intracellular symbionts in the cytoplasm of mycetocytes, huge cells in the abdomen specialized for this purpose. The aphids and their intracellular symbionts are in close mutualistic association and unable to live without their partner. The intracellular symbionts of various aphids are of a single origin; they are descendants of a prokaryote that was acquired by the common ancestor of the present aphids. The date of establishment of the symbiotic association is estimated to be 160–280 million years ago using 16S rRNA molecular clock calibrated by aphid fossils. Molecular phylogeny indicates that the intracellular symbiont belongs to a group of gut bacteria, suggesting the possibility that it was derived from a gut microbe of aphids. While the in-tracellular symbionts are universal and highly conserved amongst aphids, other types of symbiotic microorganisms are also present. In various aphids, bacterial “secondary” intracellular symbionts are found in addition to the standard symbionts. They are thought to be acquired many times in various lineages independently. Some Cerataphidini aphids do not have intracellular symbiotic system but harbor yeast-like extracellular symbionts in the hemocoel. In a lineage of this group, symbiont replacement from intracellular prokaryote to extracellular yeast must have occurred. The diversity of the endosymbiotic system of aphids illuminates a dynamic aspect of endosymbiotic evolution.     

Inheritance and diversification of symbiotic trichonymphid flagellates from a common ancestor of termites and the cockroach Cryptocercus

Cryptocercus cockroaches and lower termites harbour obligate, diverse and unique symbiotic cellulolytic flagellates in their hindgut that are considered critical in the development of social behaviour in their hosts. However, there has been controversy concerning the origin of these symbiotic flagellates. Here, molecular sequences encoding small subunit rRNA and glyceraldehyde-3-phosphate dehydrogenase were identified in the symbiotic flagellates of the order Trichonymphida (phylum Parabasalia) in the gut of Cryptocercus punctulatus and compared phylogenetically to the corresponding species in termites. In each of the monophyletic lineages that represent family-level groups in Trichonymphida, the symbionts of Cryptocercus were robustly sister to those of termites. Together with the recent evidence for the sister-group relationship of the host insects, this first comprehensive study comparing symbiont molecular phylogeny strongly suggests that a set of symbiotic flagellates representative of extant diversity was already established in an ancestor common to Cryptocercus and termites, was vertically transmitted to their offspring, and subsequently became diversified to distinct levels, depending on both the host and the symbiont lineages.     

Repeated evolution of bacteriocytes in lygaeoid stinkbugs

Host–microbe symbioses often evolved highly complex developmental processes and colonization mechanisms for establishment of stable associations. It has long been recognized that many insects harbour beneficial bacteria inside specific symbiotic cells (bacteriocytes) or organs (bacteriomes). However, the evolutionary origin and mechanisms underlying bacterial colonization in bacteriocyte/bacteriome formation have been poorly understood. In order to uncover the origin of such evolutionary novelties, we studied the development of symbiotic organs in five stinkbug species representing the superfamily Lygaeoidea in which diverse bacteriocyte/bacteriome systems have evolved. We tracked the symbiont movement within the eggs during the embryonic development and determined crucial stages at which symbiont infection and bacteriocyte formation occur, using whole-mount fluorescence in situ hybridization. In summary, three distinct developmental patterns were observed: two different modes of symbiont transfer from initial symbiont cluster (symbiont ball) to presumptive bacteriocytes in the embryonic abdomen, and direct incorporation of the symbiont ball without translocation of bacterial cells. Across the host taxa, only closely related species seemed to have evolved relatively conserved types of bacteriome development, suggesting repeated evolution of host symbiotic cells and organs from multiple independent origins.     

Ethidium bromide: a fast fluorescent staining procedure for the detection of symbiotic partnership of flagellates and prokaryotes

The hindgut of 'lower' termites harbors a dense population of flagellates and bacteria. The flagellates possess ecto- and endosymbiotic prokaryotes. Most of them are hardly visible in the phase contrast microscope. Staining with the DNA-intercalating agent ethidium bromide visualizes the nuclei of the flagellates as well as the ecto- and endosymbiotic bacteria as red objects. Furthermore, it is possible to distinguish between endosymbiotic methanogens and other bacteria. Following UV excitation, the blue-green autofluorescence of the methanogenic bacteria eclipses the red fluorescence light of the intercalated ethidium bromide. The dye facilitates the observation of symbiotic bacteria and helps identify the number, shape, localization, and dividing status of the nuclei. Thus, it is a powerful tool for the examination of microorganisms in complex habitats, which are rich in strongly autofluorescent material, like wood.     

Adaptive evolution in GroEL from distantly related endosymbiotic bacteria of insects

Many symbioses between bacteria and insects resulted from ancient infections followed by strict vertical transmission within host lineages. The strong bottlenecks under which this transmission occurs promote the neutral fixation of slightly deleterious mutations by genetic drift. As predicted by Muller's ratchet, this fixation will drive endosymbiotic bacteria through an irreversible dynamics of fitness loss. The chaperonin GroEL has been proposed as a compensatory mechanism whereby endosymbiotic bacteria of aphids persist. Here, we show that endosymbiotic bacteria of insects from two phylogenetically very distant bacterial phyla have fixed amino acid substitutions by positive selection in functionally important GroEL regions involved in either GroES/peptide binding or in the en bloc movement of the GroEL apical domain. These results, together with the high levels of constitutive expression of GroEL in these endosymbionts, provide valuable insights into the evolution of a molecular mechanism responsible for the maintenance of the symbiotic lifestyle.     

Symbiosis between hydra and chlorella: Molecular phylogenetic analysis and experimental study provide insight into its origin and evolution

Although many physiological studies have been reported on the symbiosis between hydra and green algae, very little information from a molecular phylogenetic aspect of symbiosis is available. In order to understand the origin and evolution of symbiosis between the two organisms, we compared the phylogenetic relationships among symbiotic green algae with the phylogenetic relationships among host hydra strains. To do so, we reconstructed molecular phylogenetic trees of several strains of symbiotic chlorella harbored in the endodermal epithelial cells of viridissima group hydra strains and investigated their congruence with the molecular phylogenetic trees of the host hydra strains. To examine the species specificity between the host and the symbiont with respect to the genetic distance, we also tried to introduce chlorella strains into two aposymbiotic strains of viridissima group hydra in which symbiotic chlorella had been eliminated in advance. We discussed the origin and history of symbiosis between hydra and green algae based on the analysis.     

Coupling of bacterial endosymbiont and host mitochondrial genomes in the hydrothermal vent clam Calyptogena magnifica

The hydrothermal vent clam Calyptogena magnifica (Bivalvia: Vesicomyidae) depends for its nutrition on sulfur-oxidizing symbiotic bacteria housed in its gill tissues. This symbiont is transmitted vertically between generations via the clam's eggs; however, it remains uncertain whether occasionally symbionts are horizontally transmitted or acquired from the environment. If symbionts are transmitted strictly vertically through the egg cytoplasm, inheritance of symbiont lineages should behave as if coupled to the host's maternally inherited mitochondrial DNA. This coupling would be obscured, however, with low rates of horizontal or environmental transfers, the equivalent of recombination between host lineages. Population genetic analyses of C. magnifica clams and associated symbionts from eastern Pacific hydrothermal vents clearly supported the hypothesis of strictly maternal cotransmission. Host mitochondrial and symbiont DNA sequences were coupled in a clam population that was polymorphic for both genetic markers. These markers were not similarly coupled with sequence variation at a nuclear gene locus, as expected for a randomly mating sexual population. Phylogenetic analysis of the two cytoplasmic genes also revealed no evidence for recombination. The tight association between vesicomyid clams and their vertically transmitted bacterial endosymbionts is phylogenetically very young (<50 million years) and may serve as a model for the origin and evolution of eukaryotic organelles.     

Diversity and host specificity of the Verminephrobacter-earthworm symbiosis

Symbiotic bacteria of the genus Verminephrobacter (Betaproteobacteria) were detected in the nephridia of 19 out of 23 investigated earthworm species (Oligochaeta: Lumbricidae) by 16S rRNA gene sequence analysis and fluorescence in situ hybridization (FISH). While all four Lumbricus species and three out of five Aporrectodea species were densely colonized by a mono-species culture of Verminephrobacter, other earthworm species contained mixed bacterial populations with varying proportions of Verminephrobacter; four species did not contain Verminephrobacter at all. The Verminephrobacter symbionts could be grouped into earthworm species-specific sequence clusters based on their 16S rRNA and RNA polymerase subunit B (rpoB) genes. Closely related host species harboured more closely related symbionts than did distantly related hosts. Co-diversification of the symbiotic partners could not be demonstrated unambiguously due to the poor resolution of the host phylogeny [based on histone H3 and cytochrome c oxidase subunit I (COI) gene sequence analyses]. However, there was a pattern of symbiont diversification within four groups of closely related hosts. The mean rate of symbiont 16S rRNA gene evolution was determined using a relaxed clock model, and the rate was calibrated with paleogeographical estimates of the time of origin of Lumbricid earthworms. The calibrated rates of symbiont 16S rRNA gene evolution are 0.012-0.026 substitutions per site per 50 million years and thus similar to rates reported from other symbiotic bacteria.