More than 500 million years ago, single-celled organisms on Earth's surface began forming multicellular clusters that ultimately became plants and animals. Just how that happened is a question that has eluded evolutionary biologists.
But scientists in the University of Minnesota's College of Biological Sciences have replicated that key step in the laboratory using natural selection and common brewer's yeast, which are single-celled organisms. The yeast "evolved" into multicellular clusters that work together cooperatively, reproduce and adapt to their environment -- in essence, precursors to life on Earth as it is today.
Their achievement is published in the January 16 issue of Proceedings of the National Academy of Sciences.
It all started about two years ago with a casual comment over coffee that bridging the famous multi-cellularity gap would be "just about the coolest thing we could do," recall postdoctoral researcher Will Ratcliff and associate professor Michael Travisano, both from the Department of Ecology, Evolution and Behavior.
So they decided to give it a try. Then came the big surprise. It wasn't actually that difficult. Using yeast cells, culture media and a centrifuge, it only took them one experiment conducted over about 60 days, says Travisano, who is senior author on the PNAS paper.
"I don't think anyone had ever tried it before," says lead author Ratcliff. "There aren't many scientists doing experimental evolution, and they're trying to answer questions about evolution, not recreate it."
Despite their modesty, the achievement has earned praise and admiration from evolutionary biologists around the world.
"To understand why the world is full of plants and animals, including humans, we need to know how one-celled organisms made the switch to living as a group, as multicelled organisms," said Sam Scheiner, program director in the National Science Foundation (NSF)'s Division of Environmental Biology. "This study is the first to experimentally observe that transition, providing a look at an event that took place hundreds of millions of years ago."
Funding for the research was obtained in February 2011, with coauthors R. Ford Denison and Mark Borrello, adjunct and associate professors, respectively, in the Department of Ecology, Evolution and Behavior.
Ratcliff and Travisano gave the scientific community a glimpse of their discovery at a conference last summer and have subsequently been invited to talk about it at other meetings. The PNAS article represents the first time full details about the research have been disclosed. "The article provides us with the first opportunity to show the breadth of evolutionary change that we've observed," Travisano says.
In essence, here's how the experiments worked. The two chose brewer's yeast or Saccharomyces cerevisiae, a species of yeast used since ancient times to make bread and beer, because it is abundant in nature and grows easily. They added it to a nutrient-rich culture media and allowed the cells to grow for a day in test tubes. Then they used a centrifuge to stratify the contents by weight. As the mixture settled, cell clusters landed on the bottom of the tubes faster because they are heavier. They removed the clusters, transferred them to fresh media, and grew them up again. Sixty cycles later, the clusters -- now hundreds of cells -- looked roughly like spherical snowflakes.
Analysis showed that the clusters were not just groups of random cells that adhered to each other, but related cells that remained attached following cell division. That was significant because it meant they were genetically similar, which promotes cooperation. When the clusters reached a critical size, some cells essentially committed suicide (apoptosis) to allow offspring to separate. The offspring reproduced only after they attained the size of their parents.
"A cluster alone isn't multiellular," Ratcliff said. "But when cells in a cluster cooperate, make sacrifices for the common good, and adapt to change, that's an evolutionary transition to multicellularity."
In order for multicellular organisms to form, most cells need to sacrifice their ability to reproduce, an altruistic action that favors the whole but not the individual, Ratcliff said. For example, all cells in the human body are essentially a support system that allows sperm and eggs to pass DNA along to the next generation. Thus, multicellularity is by its nature extremely cooperative. "Some of the best competitors in nature are those that engage in cooperation, and our experiment bears that out," said Travisano.
Evolutionary biologists have estimated that multicellularity evolved independently in about 25 groups. Travisano and Ratcliff wonder why it didn't evolve more often in nature, since it's not that difficult to recreate it in a lab. Considering that trillions of one-celled organisms lived on Earth for millions of years, it seems as if it should have, Ratcliff said.
Maybe that's a question they will answer in the future, using the fossil record for thousands of generations of their multicellular clusters, which is stored in a freezer in Travisano's lab. Since the frozen samples contain multiple lines that independently became multicellular, they can compare them to learn if similar or different mechanisms and genes were responsible in each case, Travisano said.
The research duo's next steps will be to look at the role of multicellularity in cancer, aging and other critical areas of biology.
"Our multicellular yeast are a valuable resource for investigating a wide variety of medically and biologically important topics," Travisano said. "Cancer was recently described as a fossil from the origin of multicellularity, which can be directly investigated with the yeast system. Similarly the origins of aging, development, and the evolution of complex morphologies are open to direct experimental investigation that would otherwise be difficult or impossible."
- William C. Ratcliff, R. Ford Denison, Mark Borrello, and Michael Travisano. Experimental evolution of multicellularity. Proceedings of the National Academy of Sciences, January 17, 2012 DOI: 10.1073/pnas.1115323109
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