When it comes to evolution, sometimes there's only one right way to do things. Biologists have discovered that, when forced to adapt to hotter and hotter conditions in the lab, a heat-loving bacteria evolves the same survival strategies time and time again. The findings indicate that genetic changes that allow organisms to cope with environmental challenges are not as random as researchers have thought.
In most organisms, evolution proceeds too slowly to be observed directly. But microbes, with generation times measured in minutes, adapt in a matter of days. By coming up with a way to home in on just one gene, Yousif Shamoo, a structural biologist at Rice University in Houston, Texas, hoped to nail down the genetic and protein changes that underlie evolutionary advances.
Shamoo and his colleagues modified the genome of the heat-loving Geobacillus stearothermophilus by substituting a key energy metabolism gene with the same gene from a close relative that thrives in lower temperatures (55 degrees C versus up to 73 degrees C). The researchers grew the altered G. stearothermophilus in a large vat, complete with a perpetual supply of nourishing media, upping the temperature 0.5 degrees per day for a month. Throughout the experiment, the researchers kept track of how the gene changed and determined the mutations underlying the evolution of new strains. They also measured how well each strain was doing relative to the rest.
A particular mutation allowed one strain to take over the vat right away. But at 62 degrees C, it disappeared, pushed out by five descendent strains--each with a unique mutation in that gene. These strains jostled for control as the temperature continued to rise, but by 70 degrees C, only one remained, Shamoo and his colleagues report in the current issue of Molecular Cell. Further experiments revealed that each mutation produced a more stable version of the energy metabolism protein, with the winner making the most stable one.
The researchers repeated the experiments, expecting they might see a different set of mutations on the second go-around. Instead, they observed the same series of mutations--and the same winning strain.
Other researchers have shown that evolving viruses tends to get in this kind of rut, but this result "is in the context of a much larger, bacterial genome," says Vaughn Cooper, an evolutionary microbiologist at the University of New Hampshire, Durham. And, says Harvard evolutionary biologist Christopher Marx, the gene-switching approach may have broad application. "They were able to find new proteins that functioned in a way they would predict. It's potentially an avenue worth pursuing for protein evolution."