Multi-cellular ‘snowflake’ yeast images with a blue cell-wall stain and red dead-cell stain. Image: Will Ratcliff and Mike Travisano |
More
than 500 million years ago, single-celled organisms on Earth’s surface
began forming multi-cellular clusters that ultimately became plants and
animals.
Just how that happened is a question that has eluded evolutionary biologists.
Now scientists have replicated that key step in the laboratory using common Brewer’s yeast, a single-celled organism.
The
yeast “evolved” into multi-cellular clusters that work together
cooperatively, reproduce and adapt to their environment–in essence,
they became precursors to life on Earth as it is today.
The results are published in this week’s issue of the journal Proceedings of the National Academy of Sciences (PNAS).
“The
finding that the division-of-labor evolves so quickly and repeatedly in
these ‘snowflake’ clusters is a big surprise,” says George Gilchrist,
acting deputy division director of the National Science Foundation’s
(NSF) Division of Environmental Biology, which funded the research.
“The
first step toward multi-cellular complexity seems to be less of an
evolutionary hurdle than theory would suggest,” says Gilchrist. “This
will stimulate a lot of important research questions.”
It
all started 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,” recalled Will Ratcliff and Michael
Travisano, scientists at the University of Minnesota (UMN) and authors
of the PNAS paper.
Other authors of the paper are Ford Denison and Mark Borrello of UMN.
Then came the big surprise: it wasn’t that difficult.
Using yeast cells, culture media and a centrifuge, it only took the biologists one experiment conducted over about 60 days.
“I
don’t think anyone had ever tried it before,” says Ratcliff. “There
aren’t many scientists doing experimental evolution, and they’re trying
to answer questions about evolution, not recreate it.”
The results have earned praise 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 multi-celled organisms,” says Sam Scheiner,
program director in NSF’s Division of Environmental Biology.
“This
study is the first to experimentally observe that transition,” says
Scheiner, “providing a look at an event that took place hundreds of
millions of years ago.”
In essence, here’s how the experiments worked:
The
scientists 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 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. The biologists removed the clusters,
transferred them to fresh media, and agitated them again.
Sixty cycles later, the clusters—now hundreds of cells—looked 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.
First steps in the transition to multi-cellularity: ‘snowflake’ yeast with dead cells stained red. Image: Will Ratcliff and Mike Travisano |
That
was significant because it meant that they were genetically similar,
which promotes cooperation. When the clusters reached a critical size,
some cells died off in a process known as apoptosis to allow offspring
to separate.
The offspring reproduced only after they attained the size of their parents.
“A
cluster alone isn’t multi-cellular,” Ratcliff says. “But when cells in a
cluster cooperate, make sacrifices for the common good, and adapt to
change, that’s an evolutionary transition to multi-cellularity.”
In
order for multi-cellular organisms to form, most cells need to
sacrifice their ability to reproduce, an altruistic action that favors
the whole but not the individual, Ratcliff says.
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 multi-cellularity is by its nature very cooperative.
“Some
of the best competitors in nature are those that engage in cooperation,
and our experiment bears that out,” says Travisano.
Evolutionary biologists have estimated that multi-cellularity evolved independently in about 25 groups.
Travisano and Ratcliff wonder why it didn’t evolve more often since it’s not that difficult to recreate in a lab.
Considering
that trillions of one-celled organisms lived on Earth for millions of
years, it seems like it should have, Ratcliff says.
That
may be a question the biologists will answer in the future using the
fossil record for thousands of generations of multi-cellular clusters,
which are stored in a freezer in Travisano’s lab.
Since
the frozen samples contain multiple cell lines that independently
became multi-cellular, the researchers can compare them to learn whether
similar or different mechanisms and genes were responsible in each
case, Travisano says.
The next steps will be to look at the role of multi-cellularity in cancer, aging and other critical areas of biology.
“Multi-cellular
yeast is a valuable resource for investigating a wide variety of
medically and biologically important topics,” Travisano says.
“Cancer
was recently described as a fossil from the origin of
multi-cellularity, 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.”
