Slower evolving bacteria win in the end

Professor Richard Lenski of Michigan State University in East Lansing and colleagues aimed to work out if some changes in DNA affect the evolutionary potential, or "evolvability," of organisms.

In the study, the researchers investigated four genetically distinct clones of Escherichia coli clones, and sampled them periodically to look for the presence of five specific beneficial mutations.

They discovered that after 500 generations all lineages had acquired beneficial mutations but two had significantly more than the others, which should suggest they were more likely to survive in the long-term than the other line of bacteria. What they found instead was that after 1,500 generations the other two lineages had gone on to dominate.

One of the co-authors of the paper, published in the journal Science, Dr Tim Cooper of the University of Houston, Texas, said the bacterial “race” could be compared to the fable of the hare and the tortoise, saying that the hare would win a 100 meter race, but the tortoise might win a marathon.

In the “hare” bacteria at least four beneficial mutations were present at the 500th generation, but despite this after another 883 generations they were growing over two percent slower than the other lineages, and by the 1,500th generation, they were extinct in the flasks.

To try to find out why, they used frozen samples of the 500th generation of bacteria and ran the evolution experiment again a number of times. In almost, but not all, of these experiments the “tortoise” clones went on to win.

They discovered that one of the genes in which beneficial mutations were found at the 500 generation mark was topA, a gene involved in winding DNA into a twisted band, which makes it easier for genes to be turned on and off. The mutations were slightly different in the slow and fast evolving bacteria, with the mutation in the “tortoise” bacteria affecting the next link down in the protein’s amino acid chain.

After the 883 generations the “tortoise” topA mutation had interacted with a mutation in another gene called spoT, which increased its fitness. The topA mutation in the other lineages did not interact so favorably with later mutations and the spoT mutation was rendered useless, and made the “hares” less fit for long-term domination.

The bacteria used in these experiments are part of a larger evolution study that has been running since 1988, or over 50,000 generations, which makes it the longest-running evolution experiment in the world. The experiment began with a dozen strains of E.coli, bred from a single ancestor. Every 500 generations (75 days), samples of the mixed-strains are frozen and stored and the mean fitness is compared to that of the ancestor.

More information: -- http://myxo.css.msu.edu/ecoli/
-- Second-Order Selection for Evolvability in a Large Escherichia coli Population, Science 18 March 2011: Vol. 331 no. 6023 pp. 1433-1436, DOI:10.1126/science.1198914

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