Novel experiment documents evolution of genome in near-real time

November 20th, 2006 · 2 Comments

Now that’s something!

A team led by bioengineering researchers at UC San Diego report in the November issue of Nature Genetics rapid evolutionary changes in a bacterial genome, observed in near-real time over a few days. Scientists have previously published static “snapshots” of the genome sequences of more than 100 bacterial species, from the harmless to those that cause plague, but this new report shows how these genomes are moving targets.

“Paleontologists look at the fossil record to study how evolution of dinosaurs and other animals occurred over millions of years, but in the case of the E. coli bacterium, new technology has given us the ability to observe evolution as it is occurring over a matter of days,” said Bernhard. Palsson, the senior author of the study and professor of bioengineering at UCSD. “The published genomic sequences of bacteria are like a fossil record and our experiments confirm that these genomes can change quickly as bacteria adapt to new conditions.”

Because of past technical limitations, biologists have historically made inferences about rapid bacterial evolution by carefully observing changes in a handful of genes at a time or by monitoring the visible characteristics, or phenotypes, as the organisms adapt. Palsson’s team used comparative genome sequencing technology developed by NimbleGen Systems Inc., a Madison, WI-based biotech firm, to identify changes that occurred during the experiment in the bacterium’s complete set of genes.

In a paper scheduled for online publication Nov. 5 on Nature Genetics’s Website, the researchers report that they grew E. coli in an environment that favored the emergence of mutants: the organism was fed a poorly metabolized carbon and energy source called glycerol. The researchers removed samples of cells from the culture and sequenced their entire genomes as a way to find mutations that enabled faster growth.

After six days of growth, mutations appeared in the gene for an enzyme that initiates the process of enzymatically breaking down glycerol. Cells with mutations in the so-called glycerol kinase gene grew 20 to 60 percent faster than those without the mutation.

Mutations also appeared in a second, unrelated gene for an enzyme called RNA polymerase. “That was a surprise to almost everybody because RNA polymerase is involved in one of the core processes of any cell,” said Palsson. “You wouldn’t expect that gene to change because a wide variety of cellular process would be affected; it’s like replacing the wiring system in a building when a light bulb burns out. But we repeated the experiment more than 50 times and mutations in the RNA polymerase gene appeared again and again.”

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“Opinion surveys indicate that many people don’t believe that evolution occurs,” said Herring, “but if the skeptics could witness evolution actually occurring, as we did, I think they’d be more likely to believe that it’s not just a theory.”