In a lab in Atlanta, thousands of yeast cells struggle to survive every day. Organisms that live another day grow faster, reproduce faster and form the largest agglomerations. For about a decade, cells have evolved to attach themselves to each other, forming branching snowflake shapes.
These strange snowflakes are at the center of experiments that explore what might have happened millions of years ago when single-celled organisms got together to become multicellular. This process, however, eventually led to such fantastically impractical and bizarre creatures as octopuses, ostriches, hamsters, and humans.
Although multicellularity is believed to have evolved at least 20 times in the history of life on Earth, it is not clear how organisms transitioned from a single cell to many organisms sharing a fate. But in A research paper published Wednesday in the journal NatureResearchers reveal one clue to how cells begin to build themselves in the body. The team that produced Snowflake yeast found that over 3,000 generations, clumps of yeast had grown so large they could be seen with the naked eye. Along the way, it’s evolved from a soft, squishy material to something with the hardness of wood.
Will Ratcliffe, a professor at Georgia Tech, started experimenting with yeast when he was in graduate school. He was inspired by Richard Lenski, a University of Michigan biologist, and colleagues who grew 12 strains of E. coli across more than 75,000 generations and documented since 1988 how populations changed. Dr. Ratcliffe wondered if studying evolution that encourages cells to stick together could shed light on the origins of multicellularity.
“All the lineages that we know of that have evolved multicellularity,” he said, “made this step hundreds of millions of years ago.” “And we don’t have much information about how single cells form clusters.”
So he did a simple experiment. Each day, he would rotate the yeast cells in a test tube, sucking up the ones that sank to the bottom the fastest, and then use them to grow the yeast population the next day. He reasoned that if it selected for the heaviest individuals or groups of cells, there would be an incentive for the yeast to evolve a way to stick together.
And it worked: Within 60 days, snowflake yeast has emerged. When these yeasts divide, thanks to the mutation, they don’t completely separate from each other. Instead, they form branching structures of genetically identical cells. Yeast has become multicellular.
But Ratcliffe found that the snowflakes, as he continued to investigate, did not appear to have become very large, and remained stubbornly microscopic. He credits Ozan Bozdag, a postdoctoral researcher in his group, with the breakthrough in oxygen, or hypoxia.
For many living things, oxygen serves as a kind of rocket fuel. The energy stored in sugars is easily accessible.
Dr. Bozdag gave oxygen to some of the yeast in the experiment and transplanted others that had a mutation that prevented them from using it. He found that over the course of 600 transfers, the oxygen-deficient yeast exploded in volume. Their snowflakes grew and grew, eventually becoming visible to the naked eye. Close examination of the formulations revealed that the yeast cells were much longer than normal. The branches had grown intertwined into a dense clump.
Scientists believe that this density may explain why oxygen is such a barrier to yeast growth. For yeast that could use oxygen, increasing their volume had major downsides.
As long as the snowflakes remained small, cells generally had equal access to oxygen. But the large, dense fillings mean that the cells within each lump are cut off from oxygen.
Yeast that can’t use oxygen, by contrast, has nothing to lose, and so it went big. The results indicate that feeding all the cells in a cluster is an important part of the trade-offs that an organism faces as it becomes multicellular.
The groups formed are also difficult.
“The amount of energy needed to break these things has increased by over a factor of a million,” said Peter Junker, a professor at Georgia Tech and a co-author of the paper.
This power may be the key to another step in the development of multicellularity, says Dr. Ratcliffe – the development of something like a circulatory system. If cells within a large mass need help accessing nutrients, an object strong enough to direct the flow of fluids is key.
“It’s like firing a fire hose into a yeast mass,” said Dr. Juncker. If the cellular mass is poor, this nutrient influx will destroy it before every cell can feed.
The team is now exploring whether dense clumps of snowflake yeast might evolve ways to deliver nutrients to their innermost organs. If they do, then these yeasts in their test tubes in Atlanta may tell us something about what it was like, eons ago, when your ancestors and many organisms around you began building bodies out of cells.
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