Biologists Replicate Key Evolutionary Step
ScienceDaily (Jan. 17, 2012) — 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.
Yeast. (Credit: © Dmitry Knorre / Fotolia)
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.
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Experimental evolution of multicellularity
William C. Ratcliff a,1, R. Ford Denison a, Mark Borrello a, and Michael Travisano a,b
aDepartment of Ecology, Evolution and Behavior and
bBioTechnology Institute, University of Minnesota, Minneapolis, MN 55108
Edited by Richard E. Lenski, Michigan State University, East Lansing, MI, and approved December 14, 2011 (received for review September 19, 2011)
Multicellularity was one of the most significant innovations in the history of life, but its initial evolution remains poorly understood. Using experimental evolution, we show that key steps in this transition could have occurred quickly. We subjected the unicellular yeast Saccharomyces cerevisiae to an environment in which we expected multicellularity to be adaptive. We observed the rapid evolution of clustering genotypes that display a novel multicellular life history characterized by reproduction via multicellular propagules, a juvenile phase, and determinate growth. The multicellular clusters are uniclonal, minimizing within-cluster genetic conflicts of interest. Simple among-cell division of labor rapidly evolved. Early multicellular strains were composed of physiologically similar cells, but these subsequently evolved higher rates of programmed cell death (apoptosis), an adaptation that increases propagule production. These results show that key aspects of multicellular complexity, a subject of central importance to biology, can readily evolve from unicellular eukaryotes.
complexity, cooperation, major transitions, individuality, macro evolution
1To whom correspondence should be addressed. E-mail:firstname.lastname@example.org.
Author contributions: W.C.R., R.F.D., and M.T. designed research; W.C.R. performed research; W.C.R., R.F.D., M.B., and M.T. analyzed data; and W.C.R., R.F.D., M.B., and M.T. wrote the paper.
The authors declare no conflict of interest.
*This Direct Submission article had a prearranged editor.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1115323109/-/DCSupplemental.
Freely available online through the PNAS open access option.