How mutants save a species

Posted by Emma Woollacott

There's one obvious reason why it's easier for a species to survive a gradual environmental change than a sudden one: more individuals stay alive when change is gradual or moderate, meaning there are more chances for a winning mutation to emerge.

But now University of Washington biologists have found a second reason, and one that could have implications for both disease resistance and how species react to global warming.

They've found that the mutation that wins the race in the harshest environment is often dependent on a 'relay team' of other, earlier, mutations, that emerge only as conditions worsen slowly.

Without these earlier mutations, says the team, it's unlikely there will even be a runner in the anchor position when conditions become extreme.

"That's a problem, given the number of factors on the planet being changed with unprecedented rapidity under the banner of climate change and other human-caused changes," says Benjamin Kerr, UW assistant professor of biology.

The researchers evolved hundreds of populations of E. coli under environments made increasingly stressful with an antibiotic that cripples and kills them. The antibiotic was ramped up at gradual, moderate and rapid rates.

Mutations at known genes confer protection to the drug; and when researchers examined these genes in surviving populations from gradual- and moderate-rate environments, they found multiple mutations.

Some of these were only advantageous at the lower concentration of the drug and unable to save the population at the highest concentrations. But those mutations predispose the lineage to gain others that allow it to escape extinction at high stress, says the team.

"That two-step path leading to the double mutant is not available if a population is immersed abruptly into the high-concentration environment," says Kerr. For populations in that situation, there were only single mutations that gave protection against the antibiotic.

The findings have implications for those concerned about antibiotic-resistant organisms - for instance, antibiotics found at very low concentrations in industrial and agricultural waste run-off might be evolutionarily priming bacteria to become drug resistant, even at high doses.

As for populations threatened by human-caused climate change, "our study does suggest that there is genuine reason to worry about unusually high rates of environmental change," the authors write. "As the rate of environmental deterioration increases, there can be pronounced increases in the rate of extinction."