An experiment repeated 600 times uncovers clues to the secrets of evolution

In a lab in Atlanta, thousands of yeast cells fight for their lives every day. Those that live another day grow fast, reproduce quickly, and form huge clusters. Over about a decade, the cells evolved to hang on top of each other, forming branched snowflake shapes.

These strange snowflakes are at the center of experiments investigating what might have happened millions of years ago when single-celled organisms first converged and became multicellular. That process, however decelerated, eventually produced extraordinary, strange 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 a single cell goes from having multiple cells that share a fate. But, inside A paper published Wednesday in the journal Nature, researchers reveal a clue to how cells can begin to form themselves into a body. The team that produced the snowflake yeast found that over 3,000 generations, the yeast clumps grew large enough to be seen with the naked eye. Along the way, they evolved from a soft, thin material to something with the hardness of wood.

Will Radcliffe, a professor at Georgia Tech, began experimenting with yeast while in graduate school. He was inspired by biologist Richard Lenski and his colleagues at the University of Michigan, who had studied E. Coley’s 12 vials have been grown for more than 75,000 generations, documenting how the population has changed since 1988. Could an evolutionary study of what encourages cells to stick together shed light on the origins of multicellularity? Radcliffe was surprised.

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“All the lineages we know of about multicellularity took this step hundreds of millions of years ago,” he said. “We don’t have a lot of information about how single cells form groups.”

So he set up a simple experiment. Each day, he swirled yeast cells in a test tube, rapidly scooping up those that sank to the bottom and using them to increase the yeast population the next day. He reasoned that if he selected very heavy individuals or clusters of cells, the yeast would have an incentive to develop a way to stick together.

It worked: Within 60 days, snowflake yeast appeared. When these yeasts divide, due to a mutation, they do not completely separate from each other. Instead they form branching structures of genetically identical cells. Yeast became multicellular.

But Snowflakes, Dr. Radcliffe found as he continued his investigation that it didn’t seem large, but stubbornly microscopic. He credits Ozan Bostock, a postdoctoral researcher in his group, with a breakthrough involving oxygen, or the lack thereof.

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

Dr. In Bostock’s experiment, some yeasts were given oxygen, and some had a mutation that prevented them from using it. He found that during 600 transfers, the oxygen-free yeast levels exploded. Their snowflakes grow and grow, 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.

The scientists think that density may explain why oxygen was a barrier to the yeast’s ability to grow large. For yeast that can use oxygen, being large has significant disadvantages.

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As long as snowflakes are small, cells generally have equal access to oxygen. But in large, dense vats the cells within each cluster were cut off from oxygen.

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

The clusters formed are also hard.

“The amount of energy needed to break these things up has increased by more than a million,” said Peter Yunger, a professor at Georgia Tech and co-author of the paper.

That strength may be another step in the evolution of multicellularity, Dr. Radcliffe says – the development of something like a circulatory system. If the cells inside a large tumor need help accessing nutrients, a body strong enough to direct the flow of fluid is critical.

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

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

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