Evolution outside the lab
Bacteria and their parasitic phages evolve just as quickly in a natural soil community as they do in a test tube, but other selective pressures can influence the changes
Bacteria and the viruses that infect them are widely used as models of coevolution, but are primarily studied in the lab. In a more natural soil environment, these organisms adapt in response to the evolution of the other just as rapidly, but in a slightly different way, according to new research published today in Science
|A scanning electron micrograph of bacteria of the same genus (Pseudonomas) as those used in this experiment|
Image: CDC, Janice Haney Carr
"The novelty here is that instead of trying to measure the dynamics of phage versus bacteria in the test tube, they actually did this in more realistic environment," said linkurl:Rotem Sorek,;http://www.weizmann.ac.il/molgen/Sorek/ a microbial genomicist at the Weizmann Institute of Science who was not involved in the research. "And their results challenge the common notion of the 'arms race'" -- a typical model of the coevolution of hosts and their parasites, with hosts evolving defenses to fight off parasites, and parasites evolving new weapons in response.
The study of evolution of the bacteria-phage parasitism has been studied extensively in the past decade, but always in isolation. The organisms are typically cultured in test tubes in high-nutrient media, with no other competing species or parasites, explained paper author linkurl:Angus Buckling,;http://www.zoo.ox.ac.uk/staff/academics/buckling_agj.htm an evolutionary biologist at the University of Oxford. "We don't know if it's relevant in any natural context really."
So Buckling and his post-doc Pedro Gomez grew bacteria and their parasitic phages in sterilized soil seeded with other microbes from their natural communities, measuring every few days the density of bacteria and phages, their resistance and infectivity, respectively, and the density and diversity of the surrounding microbial community. Surprisingly, the team found that the organisms were evolving just as rapidly in the soil community as they did in the simplified test tube experiments done previously.
However, there was something different going on. When an "arms race" occurs in a test tube, the bacteria and phages are just as effective at using past adaptations to compete with one another -- as if they're hauling around their entire weapons arsenal. But in the soil, the bacteria "lost resistance to the first phage, but were resistant to the contemporary phage," said Sorek, suggesting that in a more natural setting, selection forces the bacteria to leave their old weapons behind as they evolve new ones required to survive.
The selective pressure to streamline their arsenal may stem from having to cope with local environmental conditions, find food, and compete with other organisms, Sorek suggested. "It may gain resistance, but may lose in other fronts," he said. "This is a very important observation that might explain why phages and bacteria can coexist in nature," and don't simply drive each other to extinction.
Plus, Buckling added, the "natural" environment used in these experiments isn't even completely natural, as the researchers only added a subset of common soil bacteria to their cultures. Bacteria and phages are likely to contend with even more forces in a truly natural setting, he said, including predatory protists, other phages, and different species of bacteria. "It's a battlefield out there."
P. Gomez and A. Buckling, "Bacteria-Phage Antagonistic
Coevolution in Soil," Science, 332: 106-9, 2011.
**__Related stories:__*** linkurl:The war against war metaphors;http://www.the-scientist.com/news/display/52851/
[16th February 2007]*linkurl:Evolvability, observed;http://www.the-scientist.com/news/display/58057/
[17th March 2011]
**__Related F1000 Evaluations:__** *linkurl:When does coevolution promote diversification?;http://f1000.com/9003956?key=n98fg9ptpdk5nny
J.B. Yoder and S.L. Nuismer, American Naturalist
, 176(6):802-17, 2010. Evaluated by David Hembry and Rosemary Gillespie, University of California.*linkurl:Antagonistic coevolution accelerates molecular evolution;http://f1000.com/3023967?key=ht9cphm8wff9rgz
S. Paterson et al., Nature
, 464:275-8, 2010. Evaluated by Jordi Bascompte, Estacion Biologica de Donana, CSIC.