Article: On Symbiosis
- From: "Robert Karl Stonjek" <rstonjek@xxxxxxxxxxxxxx>
- Date: Thu, 2 Nov 2006 17:02:28 -0500 (EST)
On Symbiosis
The following points are made by Nancy A. Moran (Current Biology 2006
16:R866):
1) Microorganisms arose and diversified before the appearance of large
multicellular organisms. The bodies of the latter provided new potential
habitats for microbes -- habitats that were persistent, stable (from a
microbial perspective), and nutrient-rich. As a result, large organisms,
from oaks to humans, have been continuously enmeshed in complex interactions
with microorganisms during their evolution. Biologists have paid most
attention to associations with microbes that are pathogenic; typically,
microbial infection has been viewed as deleterious, or at best irrelevant,
to vigor and reproduction. But the last 15 years have witnessed increased
appreciation of interactions that benefit the host as well as the microbe.
These interactions are loosely grouped under the term "symbiosis" and the
microbial partners called "symbionts". Although the term "symbiont" is
typically applied to mutualistic microorganisms, it is often used to include
associates for which the full spectrum of effects on hosts is not known.
2) Symbiosis is ubiquitous in terrestrial, freshwater, and marine
communities. It has played a key role in the emergence of major life forms
on Earth and in the generation of biological diversity. Numerous authors
have pointed out that Darwin did not emphasize symbiosis as an evolutionary
mechanism; its role is, of course, not inconsistent with Darwin's main
ideas, but appreciation of symbiosis as a source of evolutionary novelty has
developed relatively recently. Its importance is perhaps most strikingly
illustrated by the symbiotic events that led to the evolution of eukaryotic
organelles -- plastids and mitochondria -- from cyanobacterial and
alpha-Proteobacterial ancestors, respectively.
3) For plants, associations with fungi and bacteria were key innovations in
the colonization of land and of specific habitats. Eukaryote-associated
microbes act as metabolic partners for accessing limiting nutrients and also
as protectors, producing toxins that ward off herbivores or pathogens.
Similar associations have arisen with animals, allowing colonization of
diverse niches, such as the specialized feeding on plant or animal tissues,
and the use of deep ocean hydrothermal vent habitats. Often, the
associations are persistent for the hosts, frequently being transmitted
vertically across generations, from mother to progeny. The organisms
involved in a symbiosis may be sufficiently fused that they cannot live
apart or be recognized as distinct entities without close scrutiny.
4) Before biological research became focused on genetic model organisms,
greater attention was paid to the study of symbiosis. Microscopy was used
extensively to discover and elucidate symbioses before the middle of the
20th century. The reasons for a period of relative neglect, until about
1990, are complex. One contributing factor has been the unsuitability of
most symbiotic partners as models for laboratory studies of genetics and
development. The organisms that are easiest to grow and study in the
laboratory, such as Escherichia coli, Arabidopsis thaliana, Drosophila
melanogaster and Mus domesticus, are weedy species adapted to show fast
growth in temporary niches. These species have lifestyles relatively free of
complex interdependencies with other species. But most microorganisms in
natural communities are likely to have obligate dependencies on other
species (often other microbes), explaining why 99% of microorganisms are
difficult or impossible to culture. Similarly, most symbionts of plants and
animals cannot be readily cultured independently of hosts, precluding most
conventional microbiological analyses.(1-5)
References (abridged):
1. Arnold, A.E., Mejia, L.C., Kyllo, D., Rojas, E.I., Maynard, Z., Robbins,
N., and Herre, E.A. (2003). Fungal endophytes limit pathogen damage in a
tropical tree. Proc. Natl. Acad. Sci. USA 100, 15649-15654.
2. Baumann, P. (2005). Biology of bacteriocyte-associated endosymbionts of
plant sap-sucking insects. Annu. Rev. Microbiol. 59, 155-189.
3. Brownlie, J.C., and O'Neill, S.L. (2005). Wolbachia genomes: insights
into an intracellular lifestyle. Curr. Biol. 15, 507-509.
4. Ley, R.E., Peterson, D.A., and Gordon, J.I. (2006). Ecological and
evolutionary forces shaping microbial diversity in the human intestine. Cell
124, 837-848.
5. McFall-Ngai, M.J., and Gordon, J.I. (2006). Experimental models of
symbiotic host-microbial relationships: understanding the underpinnings of
beneficence and the origins of pathogenesis. In: Seifert, H.S., Dirita, V.J.
(Eds.), Evolution of Microbial Pathogens. (2006). ASM Press, Washington,
D.C.
Source: ScienceWeek
http://scienceweek.com/2006/sw061103.htm
Posted by
Robert Karl Stonjek
.
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