Download We can also cause evolution to happen in the laboratory, particularly

http://www.nytimes.com/2013/08/15/science/watching-bacteria-evolve-with-predictableresults.html
August 15, 2013
Watching Bacteria Evolve, With Predictable Results
By CARL ZIMMER
If we could somehow rewind the history of life to the dawn of the animal
kingdom, it would be unlikely that we humans would ever evolve, the
evolutionary biologist Stephen Jay Gould argued. The history of life was shaped
by too many flukes and contingencies to repeat its course.
Scientists can’t turn back the clock 700 million years, so we can’t know for sure
whether Dr. Gould was right on that particular point. But in experiments using
bacteria and other fast-breeding organisms, scientists can replay evolution many
times over in their labs. And the results of a new experiment published
Thursday in the journal Cell Reports demonstrate — with movies — that
evolution can be astoundingly predictable.
The experiment was carried out by Joao Xavier of Memorial Sloan-Kettering
Cancer Center and his colleagues. They studied a common species of bacteria
called Pseudomonas aeruginosa. These microbes live pretty much everywhere
— in dirt, in water, on our skin. Under certain conditions, they also invade our
bodies and cause dangerous infections. People with cystic fibrosis, for example,
can get P. aeruginosa infections in their lungs, which are often impossible to
eradicate.
To better understand the biology of this pathogen, Dr. Xavier began to study
how it searches for food. In a process called swarming, the bacteria spray out
gooey molecules that form a slippery carpet; they can then slither over it by
whipping their tails, devouring food they encounter along the way.
“I just wondered why nobody had filmed them before, because the pattern is so
striking,” said Dr. Xavier. He dropped a few hundred microbes in the middle of
a petri dish laced with sugar and switched on a camera overhead.
Video: A common species of bacteria called Pseudomonas aeruginosa sprays
out gooey molecules that form a slippery carpet that it can slither over.
To better understand how the bacteria swarm, Dr. Xavier and his colleagues
allowed them to evolve. They seeded petri dishes with a few hundred microbes
and gave them a day to swarm and reproduce. The next day, they drew a small
sample of the bacteria from the dishes and used them to seed new ones.
The scientists reasoned that, with each generation, new mutations would arise
from time to time. If a mutation helped bacteria thrive in this new environment,
it might become more common because of natural selection.
And so it did.
Within a few days, the evolution of the bacteria took a dramatic turn. The
bacteria became 25 percent faster than their ancestors — Dr. Xavier dubbed
them “hyperswarmers.” A movie of hyperswarmers starkly illustrates how
different they had become, able to fill up the entire dish.
Video: After a few generations, a common species of bacteria called
Pseudomonas aeruginosa evolved into a much faster swarmer.
“We thought, ‘Something weird has happened,'” said Dr. Xavier.
The hyperswarmers emerged in three lines of bacteria overseen by Dr. Xavier’s
post-doctoral researcher Dave van Ditmarsch. Dr. Xavier and another lab
member, Jen Oyler, each ran the experiment again. “I wanted to make sure this
wasn’t just due to Dave’s magic fingers,” said Dr. Xavier.
But no matter who applied their fingers to the task, the result was the same. Out
of 27 lines of bacteria, 27 evolved into hyperswarmers.
When the scientists put the hyperswarmers under a microscope, they could see
what had changed. An ordinary P. aeruginosa sports a single tail. The
hyperswarmers had evolved so that they had as many as half a dozen tails.
Those extra tails gave the bacteria more speed.
To determine how the bacteria had gained their tails, Dr. Xavier and his
colleagues sequenced the DNA of 24 lines of hyperswarmers. In 24 out of 24
cases, they discovered that they have gained a mutation in the same gene, called
FleN.
FleN encodes a protein that controls other genes involved in building tails.
Somehow — Dr. Xavier doesn’t yet know how — the mutations cause FleN to
produce a multitude of tails, all of which are fully functional.
Using their many tails, the hyperswarmers were able to get out in front of
ordinary bacteria and reach fresh food first. They could then reproduce faster,
leaving behind more offspring. As a result, each population of the bacteria
rapidly turned into pure hyperswarmers.
Hyperswarmers evolved so reliably in Dr. Xavier’s experiments that he began to
wonder why they had never been seen before. He speculated that, in his lab, the
bacteria gained an ability to swim fast at the expense of some other trait that
they need in nature.
Swarming, after all, is not the only essential task that P. aeruginosa must carry
out. When the bacteria find a place that’s good for settling down, they anchor
themselves to a surface — on a leaf, for example, or inside a human lung. They
form a rubber sheet known as a biofilm.
Dr. Xavier and his colleagues found that the hyperswarmers are bad at making
biofilms on their own. They then mixed hyperswarmers with normal bacteria
and allowed the two types of microbes to make biofilms together. When the
biofilm formed, the scientists tallied up how many bacteria in it were ordinary
microbes and how many were hyperswarmers.
In a video showing the 3-D structure of one of these biofilms, the ordinary
bacteria win, and the hyperswarmers have practically gone extinct —
confirming that the ability to make microfilms is more important to the
bacteria’s survival than being speedier consumers of food.
Video: A 3-D sheet of bacteria called a biofilm, with normal bacteria in red and
the few remaining hyperswarmers in green.
Dr. Xavier’s discovery could help doctors who are struggling to fight P.
aeruginosa. In hospitals around the world, the bacteria are evolving resistance to
many antibiotics, and biofilms provide some of their protection by acting like a
shield. If scientists could find a way to coax ordinary P. aeruginosa to behave
more like hyperswarmers, they might lose their ability to make biofilms.
But Dr. Xavier’s research also provides a scientific thrill in itself: the chance to
see evolution in action — over and over again.
And if there’s one thing Dr. Xavier can now be sure of, it’s that his bacteria will
end up as hyperswarmers, thanks to mutations to the same gene.
“In this case, it could be that there are only a few solutions in the evolutionary
space,” he said.