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Symbiotic associations involving microorganisms are widespread in nature. For a long time, however, symbiosis research
has been dominated by studies of four major topics about which a vast body of information is available: mitochondria,
chloroplasts, and the plant-associated prokaryotes of the genera Rhizobium and Agrobacterium. During the past decade,
there has been a quiet change in research into symbioses involving microorganisms; this change can perhaps best be
characterized as a lateral spread of interest to encompass many different symbiotic associations. As a result, there has been
a shift in the way in which these less well known symbiotic interactions are approached: Purely descriptive or speculative
work has given way to investigations in which data are generated that allow meaningful interpretation and limit speculation.
Although the symbiotic associations are diverse, the acquisition of new knowledge has generally been a consequence of
applying similar modern techniques, usually involving nucleic acid manipulation.
This issue brings together overviews of five very different symbiotic associations by authors who have made major
contributions in their respective research areas. Table 1 summarizes some of the major features of these associations. The
symbioses included span a gradient of interdependence between the symbiont and the host. In the case of the luminous
bacteria-squid (McFall-Ngai and Ruby 1998) and cyanobacteria-plant (Meeks 1998) associations, the host and the symbiont
can be cultured separately and the system reconstitued. Consequently, the currently available arsenal of molecular, genetic,
and physiological techniques can be used to elucidate both the mechanisms by which these symbioses are established and
maintained and the benefits they bring to both partners.
The associations between sulfur-oxidizing bacteria and invertebrates (Distel 1998), Wolbachia and arthropods (Bourtzis
and O'Neill 1998), and bacteriocyte-associated prokaryotes and insects (Moran and Telang 1998) are less readily amenable
to study because in all of these associations the symbiont has not been grown in culture. Moreover, with the exception of the
host in the Wolbachia-insect association, the host also cannot be grown in the absence of the symbiont. Most of the
information available for these associations results from analyses of their nucleic acids.
A sixth article (Beckage 1998) provides an overview of a remarkable association involving parasitoid wasps, viruses, and
insect hosts. This association falls into a novel category in that it appears to involve the packaging of wasp genome-
associated nucleic acids into viral particles that interfere with the wasp-infected insect host's immune response.
Although these symbiotic associations are of major interest in their own right, their understanding has potentially
significant practical applications. These applications are summarized in Table 1 and are also discussed in individual articles.
A survey of this sort is bound to omit additional well-studied associations. In particular, the association of termites and
their gut microbes and nematodes and their associated bacteria of the genera Xenorhabdus and Photorhabdus. (For more
information, see Breznak and Brune 1994, Frost and Nealson 1996).
Added material
Paul Baumann (e-mail: pabaumann@ ucdavis.edu) is a professor in the Microbiology Section, University of California,
Davis, CA 95616-8665. © 1998 American Institute of Biological Sciences.
Table 1. Some characteristics of the associations considered in this special issue of BioScience.
Type of Location of Symbiont
Partners symbiosis symbionts function Cultivation
Luminous bacteria/ Mutualistic Extracellular Ventral camouflage Both partners can
squid be cultivated
separately
Cyanobacteria/plants Mutualistic Extracellular Nitrogen fixation Both partners can
be cultivated
separately
Sulfur-oxidizing Mutualistic Intracellular, Provision of organic Symbionts have not
bacteria/invertebrates extracellular carbon been cultivated
outside hosts, in
most cases hosts
probably cannot be
cultivated without
symbionts
Wolbachial Parasitic Intracellular Reproductive Symbionts have not
The H.W. Wilson Company/WilsonWeb
AUTHOR:Paul Baumann
TITLE:Symbiotic Associations Involving Microorganisms
SOURCE:BioScience 48 no4 254-5 Ap '98
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arthropods alterations been cultivated
outside hosts, hosts
can be cultivated
without symbionts
Bacteriocyte- Mutualistic Intracellular Provision of Both partners have
associated essential nutrients not been cultivated
prokaryotes/insects separately
Parasitoid wasps/ Mutualistic/ Not Interference with the Some may grow in
viruses/insect hosts Parasitic applicable immune response of tissue culture
the parasitoid host
Potential
practical
Partners applications Reference
Luminous bacteria/ Model for studies McFall-Ngai and
squid of animal-bacteria Ruby 1998
interactions
Cyanobacteria/plants Increased plant Meeks 1998
productivity; model
for studies of plant-
bacteria interactions
Sulfur-oxidizing Removal of Distel 1998
bacteria/invertebrates pollution-
generated hydrogen
sulfide
Wolbachial Control of insect Bourtzis and O'Neill
pests; modification 1998
of insect disease
vectors
Bacteriocyte- Modification of Moran and Telang
associated insect disease 1998
prokaryotes/insects vectors
Parasitoid wasps/ Control of insect Beckage 1998
viruses/insect hosts pests; modifications
of parasitoids
REFERENCES CITED

Beckage NE. 1998. Parasitoids and polydnaviruses. BioScience 48: 305-311.
Bourtzis K, O'Neill S. 1998. Wolbachia infections and arthropod reproduction. BioScience 48: 287-293.
Breznak JA, Brune A. 1994. Role of microorganisms in the digestion of lignocellulase by termites. Annual Review of
Entomology 39: 453-487.
Distel DL. 1998. Evolution of chemoautotrophic endosymbioses in bivalves. BioScience 48: 277-286.
Frost S, Nealson K. 1996. Molecular biology of the symbiotic-pathogenic bacteria Xenorhabdus spp. and Photorhabdus
spp. Microbiological Reviews 60: 21-43.
McFall-Ngai MJ, Ruby EG. 1998. Sepiolids and vibrios: When first they meet. BioScience 48: 257-265.
Meeks JC. 1998. Symbiosis between nitrogen-fixing cyanobacteria and plants. BioScience 48: 266-276.
Moran NA, Telang A. 1998. Bacteriocyte-associated symbionts of insects. BioScience 48: 295-304.
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