An experiment repeated 3,000 times finds clues to the secrets of evolution

In a laboratory in Atlanta, thousands of yeast cells fight for their lives every day. Those that live another day grow faster, reproduce faster and form larger clumps. For about a decade, cells have evolved to hang on top of each other, forming branching snowflake shapes.

These strange snowflakes are at the heart of experiments exploring what might have happened millions of years ago when single-celled organisms joined together to become multicellular. That process, however short it was, eventually led to extraordinary, extraordinarily exotic creatures like octopuses and ostriches and hamsters and humans.

Although multicellularity is thought to have evolved at least 20 times in the history of life on Earth, it is not clear how organisms go from a single cell to many sharing a fate. But, in a paper published Wednesday in the journal Nature, researchers have uncovered a clue about how cells begin to build themselves into bodies. The team that made the snowflake yeast found that after 3,000 generations, the yeast clumps grew so large that they could be seen with the naked eye. Along the way, they evolved from a soft, squishy material with the hardness of wood.

Will Ratcliffe, a professor at Georgia Tech, began experimenting with yeast while in graduate school. They were inspired by Richard Lenski, a biologist at the University of Michigan, and his colleagues, who grew 12 flasks of E. coli through more than 75,000 generations, documenting how the population had changed since 1988. The study of evolution that encourages cells to stick together could shed light on the origins of multicellularity, Dr. Ratcliffe wondered.

“All the lineages we know have evolved multicellularity, they made this step hundreds of millions of years ago,” he said. “And we don’t know much about how single cells form groups.”

So he set up a simple experiment. Each day, he swirled yeast cells in a test tube, quickly scooping up those that sank to the bottom, then used them to grow the next day’s yeast population. They reasoned that if they picked heavier individuals or groups of cells, the yeast would have an incentive to evolve a way to stick together.

And it worked: within 60 days, snowflake yeast appeared. When these yeasts divide, thanks to a mutation, they do not completely separate from each other. Instead they form branching structures of genetically identical cells. Yeast has become multicellular.

But snowflakes, Dr. Ratcliffe found, as he continued to investigate, never looked too big, remaining obstinately subtle. They include in their group postdoctoral researcher Ozan Bozdag with breakthroughs involving oxygen, or lack thereof.

For many organisms, oxygen acts as a kind of rocket fuel. This makes it easier to access the energy stored in sugar.

Dr. Bozdog experimented with giving some yeast oxygen and growing others with a mutation that prevented them from using it. They found that the yeast, deprived of oxygen, exploded in size. Their snowflakes grew and grew, eventually becoming visible to the naked eye. A closer examination of the structures revealed that the yeast cells were much longer than normal. The branches grew tangled and formed a dense clump.

That concentration may explain why oxygen prevents yeast from growing larger, scientists think. For yeast that can use oxygen, growing larger has significant disadvantages.

As long as the snowflakes remained small, the cells generally had equal access to oxygen. But large, dense wads mean the cells within each clump are cut off from oxygen.

Yeast that can’t use oxygen, in contrast, have nothing to lose, and so they get bigger. The research suggests that feeding all the cells in a cluster is a critical part of the trade-offs an organism faces when going multicellular.

The clusters formed are also tough.

“The amount of energy required to break these things is greater than a factor of a million,” said Peter Yunker, a professor at Georgia Tech and co-author of the paper.

That energy may be key to another step in the development of multicellularity, Dr. Ratcliffe says – such development as the circulatory system. A body strong enough to channel the flow of fluid is important if it needs help getting nutrients into the cells inside a large group.

“It’s like shooting a fire hose into a yeast cluster,” Dr. Yunker said. If the cellular clump is weak, the flow of nutrients destroys each cell before it can receive nourishment.

The team is now exploring whether dense clumps of snowflake yeast can develop ways to get nutrients to their inner members. If they do, this yeast in their test tubes in Atlanta might tell us something about what it was like eons ago, when your ancestors and many of the creatures around you first started building bodies from cells.

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