February 21, 2024

Scientists are conducting a long-term experiment in evolution in the lab to study how single-celled organisms evolve into multicellular life forms. Over thousands of generations, their yeast grew 20,000 times larger and 10,000 times stronger.

The idea of ​​a “missing link” in evolution often conjures up images of furry hominids, but there are actually deeper missing links in the chain. One of the largest gaps exists between single-celled and multicellular organisms, marking a crucial step in the development of complex life on Earth.

Now, scientists at the Georgia Institute of Technology report the initial results of an experiment they hope will continue to run for decades with a lofty goal — to evolve single-celled life forms into entirely new multicellular ones. Directed evolution experiments have been going on for decades, and even won the 2018 Nobel Prize in Chemistry, but these experiments are usually focused on making new drugs or solving other problems, rather than filling holes in our distant family tree.

In the first phase of this multicellular long-term evolution experiment (MuLTEE), the researchers started with a species called snowflake yeast. The microbes were grown in a shaking incubator, and each day the team underwent artificial natural selection — the colonies that grew the fastest and reached the largest size were selected for further cultivation. Repeat this process thousands of times, and you can get a pretty good approximation of the environmental forces that favor certain traits in natural evolution.

Sure enough, after about 3,000 generations, the yeast evolved to form clusters of more than 500,000 cells—more than 20,000 times larger than the original strain. In the process, they become visible to the naked eye and become about 10,000 times stronger, comparable to wood.

Snowflake yeast cells grow from clusters of about 100 cells (left) to 500,000 cells (right) through the team's evolution experiments
Snowflake yeast cells grow from clusters of about 100 cells (left) to 500,000 cells (right) through the team’s evolution experiments

Georgia Institute of Technology

The scientists dug deeper to find out what was happening at the cellular level that allowed them to make such impressive progress. They found that individual yeast cells all stretched out, which reduced the density of each group of cells and therefore reduced the pressure they exerted on each other. This prevents star clusters from breaking apart as they usually do at a certain density, making them larger.

But this mechanism alone should not have led to such a dramatic increase, the team said. So they took a closer look using a scanning electron microscope.

“We discovered that there is an entirely new physical mechanism that allows these populations to grow to such enormous sizes,” said Ozan Bozdag, the study’s first author. “The branches of yeast have become entangled — tufts of cells have evolved vine-like behavior. , intertwine and strengthen the entire structure.”

While “evolved” yeast still lack many of the biological hallmarks of true multicellular organisms, this cellular entanglement appears to be a milestone on the way to that goal. The experiments are also far from over, so further developments are likely.

The study was published in the journal nature.

source: Georgia Institute of Technology