Association ofAlcaligenes faecalis with wetland rice

Alcaligenes faecalis isolated from rice roots is widespread in paddy soil of China. It was found to be a close association with rice.A. faecalis accumulate on the rice root surface, and part of them could enter into the rice root. It can grow in the intercellular space, especially inside the root cells, and multiply and fix dinitrogen there.A. faecalis could synthesize nitrogenase when it was grown in the medium containing a high concentration of ammonia. The mechanisms of association are also discussed.     

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。     

Cloning and Characterization of the Ribosomal Protein Genes in the spc Operon of a Prokaryotic Endosymbiont of the Pea Aphid, Acyrthosiphon kondoi

To correlate a prokaryotic endosymbiont in the pea aphid, Acyrthosiphonkondoi, with the endosymbionts in related aphid species as wellas with free-living bacteria and subcellular organelles, andto study the mode of its gene expression within aphid cells,we have cloned and characterized the genes encoding ribosomalproteins S3, L16, L29, S17, L14, L24, L5, S14, S8, L6, L18,S5, L30, L15 and secretion protein Y (Sec Y) from the S10 andspc ribosomal protein gene operons of this endosymbiont. Theorganization of these genes is identical to that in Escherichiacoli, and their nucleotide sequences are highly similar (87%identity) to the corresponding E. coli genes. They are muchless similar to the corresponding chloroplast and mitochondrialgenes. The guanine plus cytosine G+C content of the genes ofthe A. kondoi endosymbiont is much higher than those of theendosymbionts in related aphid species reported so far. It appearseither that the A. kondoi endosymbiont is derived from an ancestralbacterium different from those in other aphids or that its G+Ccontent increased in a relatively short time after the evolutionarydivergence of its host.     

FURTHER EVIDENCE FOR A MEMBRANE-BOUND ENDOSYMBIONT WITHIN THE DINOFLAGELLATE PERIDINIUM FOLIACEUM1

The fine structure of the binucleate, fucoxanthin-containing dinoflagellate Peridinium foliaceum (Stein) Biechler was re-examined for evidence of an endosymbiout. The eucaryotic nucleus, chloroplasts and associated ribosome-dense cytoplasm were separated by a single invaginating membrane from the rest of the dinoflagellate cell. The triple membrane-enclosed eyespot, mesocaryotic nucleus, trichocysts and accumulation bodies resided in the dinoflagellate cytoplasm. These observations suggest that P. foliaceum contains a membrane-bound endosymbiont, similar to that already described for the closely related species. P. balticum (Levander) Lemmermann.     

Reduction of Tubulin Expression in Angomonas deanei by RNAi Modifies the Ultrastructure of the Trypanosomatid Protozoan and Impairs Division of Its Endosymbiotic Bacterium

In the last two decades, RNA interference pathways have been employed as a useful tool for reverse genetics in trypanosomatids. Angomonas deanei is a nonpathogenic trypanosomatid that maintains an obligatory endosymbiosis with a bacterium related to the Alcaligenaceae family. Studies of this symbiosis can help us to understand the origin of eukaryotic organelles. The recent elucidation of both the A. deanei and the bacterium symbiont genomes revealed that the host protozoan codes for the enzymes necessary for RNAi activity in trypanosomatids. Here, we tested the functionality of the RNAi machinery by transfecting cells with dsRNA to a reporter gene (green fluorescent protein), which had been previously expressed in the parasite and to α‐tubulin, an endogenous gene. In both cases, protein expression was reduced by the presence of specific dsRNA, inducing, respectively, a decreased GFP fluorescence and the formation of enlarged cells with modified arrangement of subpellicular microtubules. Furthermore, symbiont division was impaired. These results indicate that the RNAi system is active in A. deanei and can be used to further explore gene function in symbiont‐containing trypanosomatids and to clarify important aspects of symbiosis and cell evolution.     

Plant green-island phenotype induced by leaf-miners is mediated by bacterial symbionts

The life cycles of many organisms are constrained by the seasonality of resources. This is particularly true for leaf-mining herbivorous insects that use deciduous leaves to fuel growth and reproduction even beyond leaf fall. Our results suggest that an intimate association with bacterial endosymbionts might be their way of coping with nutritional constraints to ensure successful development in an otherwise senescent environment. We show that the phytophagous leaf-mining moth Phyllonorycter blancardella (Lepidoptera) relies on bacterial endosymbionts, most likely Wolbachia, to manipulate the physiology of its host plant resulting in the ‘green-island’ phenotype—photosynthetically active green patches in otherwise senescent leaves—and to increase its fitness. Curing leaf-miners of their symbiotic partner resulted in the absence of green-island formation on leaves, increased compensatory larval feeding and higher insect mortality. Our results suggest that bacteria impact green-island induction through manipulation of cytokinin levels. This is the first time, to our knowledge, that insect bacterial endosymbionts have been associated with plant physiology.     

Strong specificity in the interaction between parasitoids and symbiont‐protected hosts

Coevolution between hosts and parasites may promote the maintenance of genetic variation in both antagonists by negative frequency‐dependence if the host–parasite interaction is genotype‐specific. Here we tested for specificity in the interaction between parasitoids (Lysiphlebus fabarum) and aphid hosts (Aphis fabae) that are protected by a heritable defensive endosymbiont, the γ‐proteobacterium Hamiltonella defensa. Previous studies reported a lack of genotype specificity between unprotected aphids and parasitoids, but suggested that symbiont‐conferred resistance might exhibit a higher degree of specificity. Indeed, in addition to ample variation in host resistance as well as parasitoid infectivity, we found a strong aphid clone‐by‐parasitoid line interaction on the rates of successful parasitism. This genotype specificity appears to be mediated by H. defensa, highlighting the important role that endosymbionts can play in host–parasite coevolution.     

DIVERSITY OF PLASTID DNA CONFIGURATION AMONG CLASSES OF EUKARYOTE ALGAE1

Information is presented concerning the overall arrangement of plastid DNA (ptDNA) in plastids of approximately 100 spp. of eukaryote algae, representing all classes. The three-dimensional arrangement of the ptDNA was assessed by study of both living and fixed material, stained with the DNA fluorochrome 4′,6-diamidino-2-phenylindole (DAPI), using both phase and fluorescence microscopy. The widespread occurrence of two major types of ptDNA configuration known from prior electron microscopy studies was confirmed. These are (1) DNA densities (nucleoids) of variable size and morphology, scattered throughout the plastid, and (2) a ring nucleoid, beaded or unbeaded, lying just within the girdle lamella. Type 1 is characteristic of Rhodophyta, Dinophyta, Chlorophyta, Cryptophyta, Prymnesiophyceae and Eustigmatophyceae (with one exception). Type 2 is characteristic of Phaeophyceae, Bacillariophyceae, Raphidophyceae, Chrysophyceae (except silicoflagellates and organisms such as Synura and Dinobryon), and Xanthophyceae (with the exception of Vaucheria and three genera known to lack girdle lamellae, Bumilleria, Bumilleriopsis, and Pseudobumilleriopsis). Some of these exceptional forms, as well as Euglenophyta, have configurations of ptDNA not previously recognized. In all the configurations observed, the DNA of a single plastid could be interpreted as being in continuity. This character of plastids appears to be stable under varied conditions of growth and at differing stages of the life cycle, where examined, and has confirmed the reclassification made on other grounds of several taxonomic entities. It has also revealed new questionable classifications. Since DAPI staining is far simpler than serial sectioning for electron microscopy in revealing ptDNA architecture, use of the technique may be valuable for future studies of numerous organisms, both to help in their identification and as an aid to unravelling major taxonomic affinities. In light of the endosymbiont hypothesis, plastid characters may require as great attention as those of the remainder of the cell.