All together now: yeasts can evolve to form snowflake-like multicellular shapes (Image: Courtesy of Jennifer Pentz, Georgia Tech)
The leap from single-celled life to multicellular creatures is easier than we ever thought. And it seems there's more than one way it can happen.
The mutation of a single gene is enough to transform single-celled brewer's yeast into a "snowflake" that evolves as a multicellular organism.
Similarly, single-celled algae quickly evolve into spherical multicellular organisms when faced with predators that eat single cells.
These findings back the emerging idea that this leap in complexity isn't the giant evolutionary hurdle it was thought to be.
At some point after life first emerged, some cells came together to form the first multicellular organism. This happened perhaps as early as 2.1 billion years ago. Others followed – multicellularity is thought to have evolved independently at least 20 times – eventually giving rise to complex life, such as humans.
But no organism is known to have made that transition in the past 200 million years, so how and why it happened is hard to study.
Special snowflake
Back in 2011, evolutionary biologists William Ratcliff and Michael Travisano at the University of Minnesota in St Paul coaxed unicellular yeast to take on a multicellular "snowflake" form by taking the fastest-settling yeast out of a culture and using it to found new cultures. And then repeating the process. Because clumps of yeast settle faster than individual cells, this effectively selected yeast that stuck together instead of separating after cell division.
The team's latest work shows that this transformation from a single to multicellular existence can be driven by a single gene calledACE2 that controls separation of daughter cells after division, Ratcliff told the 15-19 June Astrobiology Science Conference in Chicago.
And because the snowflake grows in a branching, tree-like pattern, any later mutations are confined to single branches. When the original snowflake gets too large and breaks up, these mutant branches fend for themselves, allowing the value of their new mutation to be tested in the evolutionary arena.
"A single mutation creates groups that as a side effect are capable of Darwinian evolution at the multicellular level," says Ratcliff, who is now at Georgia Tech University in Atlanta.
Bigger is better
Ratcliff's team has previously also evolved multicellularity in single-celled algae calledChlamydomonas
, through similar selection for rapid settling. The algal cells clumped together in amorphous blobs.
Now the feat has been repeated, but with predators thrown into the mix. A team led byMatt Herron of the University of Montana in Missoula exposed Chlamydomonas to a paramecium, a single-celled protozoan that can devour single-celled algae but not multicellular ones.
Safety in even numbers (Image: Jacob Boswell)
Sure enough, two of Herron's five experimental lines became multicellular within six months, or about 600 generations, he told the conference.
This time, instead of daughter cells sticking together in an amorphous blob as they did under selection for settling, the algae formed predation-resistant, spherical units of four, eight or 16 cells that look almost identical to related species of algae that are naturally multicellular.
"It's likely that what we've seen in the predation experiments recapitulates some of the early steps of evolution," says Herron.
Neither Ratcliff's yeast nor Herron's algae has unequivocally crossed the critical threshold to multicellularity, which would require cells to divide labour between them, says Richard Michod of the University of Arizona in Tucson.
But the experiments are an important step along that road. "They're opening up new avenues for approaching this question," he says.
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