What started out as a single cell in 2018, invisible to the human eye, has now morphed into a multicellular, flea-sized beast.
An ongoing study on a brewer’s yeast (Saccharomycess cerevisiae) mutated to stay attached in clusters like “snowflake” yeast shows what can happen to microscopic single-celled organisms after thousands of generations of careful selection.
When researchers at the Georgia Institute of Technology selected the largest and fastest growing groups of yeast from five populations, generation after generation, they cultivated an organism containing more than half a million clonal cells – 20 000 times larger than its ancestor.
The results are an unparalleled example of sustained multicellular evolution.
“By putting our finger on the evolutionary ladder of a single-celled organism, we can understand how it evolved into increasingly complex and integrated multicellular organisms, and we can study this process along the way,” he added. explain evolutionary biologist William Ratcliff of Georgia Tech.
Today, the evidence suggests that life on Earth began with single-celled organisms around 3.5 billion years ago.
Yet little is known about how isolated cells that all looked and behaved alike evolved into multicellular lifeforms with specialized tissues capable of coordinated activity about two to two years ago. three billion years.
Snowflake yeast experiments are now helping experts try to tell that story.
The study is called the Multicellularity Long-Term Evolution Experiment (MuLTEE), and the researchers hope to run it for decades. The first major discoveries took place after 3,000 generations of evolution.
Already, researchers say, individual yeast populations changed from substances “weaker than gelatin” to those “with the strength and tenacity of wood”.
“We discovered that there was a totally new physical mechanism that allowed groups to grow to this very, very large size,” explain evolutionary biologist Ozan Bozdag.
First, the yeast cells in the experiments developed larger branches which reduced the overall density of the organism.
Then the branches tangled together, forming a clump that resembles the consistency of modern gels.
Ultimately, this new structure made the organism 10,000 times stronger than its single-celled ancestor. More such a snowflake.
“The branches of the yeast had become entangled”, explain Bozdag, “the cluster cells developed a vine-like behavior, wrapping around each other and strengthening the whole structure.”
Another important finding from the experiments concerned the role of oxygen in setting limits to evolutionary progress.
In the youth of the Earth, oxygen was in limited quantity. Only when a special type of bacteria ‘breathed life‘ in the atmosphere a few billion years ago that multicellular life forms are thought to have really taken off.
The evolution of snowflake yeast in the lab supports the idea that oxygen was an important constraint on early multicellular life on Earth. In the experiments, only yeast populations that did not rely on oxygen to produce energy were able to grow to such large sizes.
Yeast clusters that required oxygen, on the other hand, were forced to distribute supplies among all of its cells, creating an additional cost to grow.
These findings, scientists sayemphasize “the critical role of oxygen levels in the evolution of multicellular size”.
“I’m really excited to have a model system where we can evolve early multicellular life over thousands of generations, harnessing the awesome power of modern science,” said Ratcliff.
“In principle, we can understand everything from evolutionary cell biology to biophysical traits that are directly under selection.”
It will be fascinating to see what happens to this yeast in the years to come.
The study was published in Nature.