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2012

Yeast Experiment Hints at a Faster Evolution From Single Cells
The New York Times, January 16, 2012
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Our ancestors were single-celled microbes for about three billion years before they evolved bodies. But in a laboratory at the University of Minnesota, brewer’s yeast cells can evolve primitive bodies in about two weeks.

The transition to multicellular life has long intrigued evolutionary biologists. The cells in our bodies have evolved to cooperate with exquisite precision. The human body has more than 200 types of cells, each dedicated to a different job. And a vast majority of the 100 trillion cells in our bodies sacrifice their own long-term legacy: Only eggs and sperm have a chance to survive our own death.

These demands for cooperation and sacrifice ought to make it hard for single-celled life to become multicellular. Yet animals, plants and other life forms have evolved bodies. “We know that multicellularity has evolved in different lineages at least 25 times in the history of life,” said William Ratcliff, a postdoctoral researcher at the University of Minnesota.

Dr. Ratcliff and his adviser, Michael Travisano, are experts in experimental evolution. They design experiments in which microbes can evolve interesting new traits within weeks.

“We were sitting in his office drinking coffee, talking about what would be the coolest thing you could do in the lab,” Dr. Ratcliff said. “O.K., the origin of life would be too hard. But other than the origin of life, what would be the coolest thing?” They decided it would be observing single-celled microbes evolving a primitive form of multicellularity.

The scientists designed an experiment with brewer’s yeast, which normally lives as single cells, feeding on sugar and budding off daughter cells to reproduce.

Dr. Ratcliff and his colleagues set up an experiment that might favor multicellularity in yeast. They reared lines of yeast, starting from a single cell, in 10 flasks of broth. They kept the flasks shaking for a day and then let the yeast settle. The scientists then took out a drop of the settled yeast cells and transferred it to a fresh flask, where the yeast could continue to grow. In this experiment, natural selection favored any new mutation that would let the yeast fall quickly. Yeast cells that were still floating high in the broth would not have a chance to be delivered to the next flask.

In a matter of weeks, Dr. Ratcliff noticed, the yeast was sinking fast, forming a cloudy layer at the bottom of the flasks. He put the yeast under a microscope and discovered that most of it was no longer growing as single cells. Instead, the broth was dominated by snowflake-shaped clusters of hundreds of cells stuck together.

These were not clumps of unrelated cells, he found. When he isolated individual cells and let them grow, they formed new snowflakes. Instead of drifting away, newly budded yeast cells remained stuck to their parents. By staying stuck together, these yeast clusters fell faster than individual cells.

A single cell needs a few hours to grow to “adult” size, Dr. Ratcliff found. After it matures, its growing branches start to press against one another until they snap apart. These broken branches are yeast versions of plant cuttings: Each one grows into a snowflake of its own, which then snaps apart in turn.

Dr. Ratcliff also found that this new form of reproduction is possible only because some of the yeast cells make the ultimate sacrifice. Once a snowflake reaches adult size, a fraction of the cells commit suicide. “The cells that kill themselves act as weak links,” he said.

The scientists describe their experiments in a paper being published this week in Proceedings of the National Academy of Sciences.

“This is a really interesting and important study,” said Richard Lenski, a biologist at Michigan State University and the editor of the paper. “It shows that a major transition in evolution -- going from unicellular to multicellular life forms -- might not be as hard to achieve as most biologists have long thought.”

Dr. Ratcliff suspects that the transformation of the yeast in his lab may offer hints about how animals and other lineages became multicellular hundreds of millions of years ago. “Forming clusters isn’t a freaky yeast thing,” he said. The closest single-celled relatives of animals, called choanoflagellates, also sometimes grow as clusters of cells.

Animals and plants did not evolve inside flasks, of course. But natural conditions could have favored clusters of cells. They might have been harder for predators to eat, for example. A cluster of cells might also be able to feed more efficiently in some cases.

Dr. Ratcliff and his colleagues are now examining 25 genomes of the evolved yeast, looking for the mutations that gave them snowflake bodies. Meanwhile, their yeast continues to evolve. Once the cells gain the ability to form snowflakes, they become better adapted to multicellular life. They snap off smaller branches, allowing them to reproduce faster.

Dr. Ratcliff would not go into detail about where the yeast evolution was going until he published the latest results. “We’re getting really interesting things happening now” was all he would say.

Copyright 2012 The New York Times Company. Reprinted with permission.
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