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category: Biology

Black Queen Tells Microbes to Be Lazy

Post: March 27, 2012 11:47 am
Author: Michael Marshall         Source: New Scientist

Confronted by a deadly threat, most bacteria let someone else handle it.

According to the Black Queen hypothesis, evolution pushes microorganisms to lose essential functions when there is another species around to perform them. The idea could explain why microbes are so dependent on each other.

Jeffrey Morris of Michigan State University in East Lansing got the idea from an ocean-dwelling bacterium called Prochlorococcus. "It is the most common photosynthetic organism on Earth," he says, but for decades no one could grow it.

That's because Prochlorococcus relies on other bacteria to break down toxic hydrogen peroxide. This led Morris to the Black Queen hypothesis, named after the card game Hearts, in which players try to discard the costly queen of spades.

For microorganisms every ability is costly – carrying genes and making proteins uses up energy – so they benefit from losing genes if possible.

As long as one microbe breaks down hydrogen peroxide all bacteria in the area benefit, so it is in the microbes' interests to discard the associated genes quickly.

Morris acknowledges that this kind of outsourcing is a dangerous game to play, though. In theory, all of the microbes may lose the genes at the same time, leaving none to deal with hydrogen peroxide.

Unpublished experiments support the Black Queen hypothesis. Morgan created Escherichia coli vulnerable to hydrogen peroxide, then gave them a resistance gene. Many, but not all, of the bacteria promptly lost the gene.

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