Wednesday, February 8, 2012

Watching Evolution do its thing in a petri dish

Ordinarily, the lambda virus grabs onto a particular protein on E. coli’s surface with a matching receptor of its own, then uses that attachment to enter the bacterial cell. By growing E. coli under restrictive conditions, however, Meyer and his colleagues created a strain in which that surface protein was almost completely absent, making infection nearly impossible. One might expect that the lambda virus would rapidly die out in that culture.

Instead, the researchers found that in 24 out of 96 cultures, the viruses evolved an unprecedented ability to infect E. coli via a completely different surface protein — and in just 15 days. Analysis revealed that they managed this transition with just four small mutations.

Part of what makes this adaptation remarkable, as John N. Thompson of the University of California, Santa Cruz, observed in a commentary accompanying Meyer’s paper, is that it defies intuitions about how evolution works:
One of the great metaphors of evolutionary biology … is that populations evolve toward adaptive peaks separated by adaptive valleys. The peaks are combinations of genes that confer high Darwinian fitness on individuals, whereas the valleys are combinations that confer low fitness. But how can a population move from one peak to another, perhaps higher, peak, across an adaptive valley in which gene combinations are presumed to be maladaptive?

Mutations in synergy

Lenski’s group looked in detail at the specific mutations that enabled the switch to the second protein. They only conferred that ability in combination: individually, they didn’t have any affinity for the second protein. Rather, they had affinities for the original surface protein.

That was the key. Although most of the new E. coli strain couldn’t make the original protein anymore, a very few had random mutations that restored that ability (it offered no survival advantage). The viral mutations were individually advantageous because on their own, they improved or sustained the virus’s ability to infect those few precious, vulnerable cells.
In this case, the answer is in how the bacterium and the virus evolved together.

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