Is Evolution Predictable?

17 August 2011 1:01 pm

Image courtesy of Manuel Zimmer

Tough choices. Developing nematodes must choose to mature quickly into reproductive adults or spend extra time as a nonfeeding larva (smaller worm, top).

If one could rewind the history of life, would the same species appear with the same sets of traits? Many biologists have argued that evolution depends on too many chance events to be repeatable. But a new study investigating evolution in three groups of microscopic worms, including the strain that survived the 2003 Columbia space shuttle crash, indicates otherwise. When raised in a lab under crowded conditions, all three underwent the same shift in their development by losing basically the same gene. The work suggests that, to some degree, evolution is predictable.

More than 50 years ago, researchers studying basic cell biology began raising a tiny soil worm, the nematode Caenorhabditis elegans. A young worm takes one of two life paths: Either it matures in 3 days, reproduces, and dies within 2 weeks, or it goes into a state of suspended animation, remaining what's called a dauer larva. Dauer larvae don't eat, and they can survive stressful environmental conditions for months before turning into adults. Typically, too little food, the wrong temperature, or crowded conditions prompt young worms to become dauer larvae.

The animals know their numbers are too high because they can sense odor chemicals called pheromones emitted by their peers. When there's too much pheromone, they choose the dauer route.

Two years ago, researchers noticed that one 50-year-old lab strain of C. elegans no longer followed that pattern. Larvae matured quickly despite crowding, rarely if ever entering the dauer state. In the wild, crowded conditions generally mean food is short, so it makes sense for larvae to hunker down and wait for better times. In the lab, however, where there's always plenty of food to go around, the most successful worms are those that evolve to ignore the "too crowded" signal and quickly reproduce.

Geneticist Cornelia Bargmann of Rockefeller University in New York City and her postdoc Patrick McGrath tracked a key change to a mutation that got rid of two genes for a particular type of pheromone receptor. The loss of these genes delayed reproduction, and restoring either gene back to worms hurried them into adulthood. They found that the presence or absence of the genes determined how quickly the worms matured and reproduced.

Satisfied that they had fingered the genetic cause of this evolutionary change, Bargmann's group wanted to see if the same deletion was involved in other cases of dauer loss under crowded lab conditions. The Columbia space shuttle strain fit the bill: In preparation for the space shuttle experiments, biologists had grown these worms in high densities. Within 4 years, the space shuttle worms had ceased to form dauer larvae. As Bargmann and her colleagues report today in Nature, this strain is missing the same pheromone receptor genes.

Digging through the scientific literature, the researchers found a different lab-tamed nematode species that had also bypassed the dauer stage. This species, C. briggsae, split off from C. elegans 20 million years ago, long enough ago for genes to diverge so much that the worms didn't have the exact same pheromone receptor genes. Nonetheless, the group reported, the gene that was the closest equivalent to the missing genes in the two nematode strains was also deleted in these C. briggsae.

"What's surprising to me is that the strategies that they evolved are so similar, while the organisms themselves have been separated for a long time," says Jon Clardy, a chemist at Harvard Medical School in Boston, who was not involved in the work.

"It's an amazing study," adds Patrick Phillips, an evolutionary geneticist at the University of Oregon, Eugene, who also was not involved in the work. "One would have predicted there are many ways to break a system," he explains. But only a few can do it without affecting other parts of the organism.

Fifty to 100 genes affect whether a worm enters the dauer state. In theory, deletions on any of them could keep worms from becoming dauer larvae. But many of these genes affect several aspects of the animal's development and physiology, whereas the pheromone receptors simply sense the environment and thus can be lost harmlessly, Bargmann suggests. The study may point to "a general rule," adds Phillips: that evolution tends to delete genes whose loss will not have widespread effects, an idea that is very slowly gaining ground.

Other researchers have uncovered instances in fruit flies, cavefish, and stickleback fish wherein evolution has taken the same path more than once. This work is "another excellent example," says Princeton University evolutionary biologist David Stern. These cases, Bargmann says, are "opening our eyes to a new way of thinking about how evolution happens."

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