Yeast snowflakes shed light on evolution

Elizabeth Polsdofer

The journey from stardust to opposable thumbs may have taken 4.5 billion years, but in Minnesota, evolution moves a little faster.

In a matter of weeks, two researchers at the BioTechnology Institute at the University of Minnesota — Mike Travisano and Will Ratcliff — have observed the evolution of the simple yeast cell “saccharomyces cerevisiae” from a single cell to a multicellular entity.

Multicellular entities contain two or more cells that are connected and have a vested interest in the welfare of each other. The evolution from a single-celled state to a multicellular state is vital for everything that grows and can be seen with the naked eye. Without multicellular critters, the earth would be bare — devoid of land plants and animal life.

Despite the controversy surrounding the word “evolution,” scientists study the change in genetics and behaviors of life every day. What makes Travisano and Ratcliff’s work so special is the rare look into an evolutionary process that happened at least 500 million years ago and the speed at which the evolution occurred.

“One thing that our [research] shows is that dramatic biological changes can occur very quickly given the proper selection,” Ratcliff said. “Now that that’s known, I’m sure that other people will do creative and interesting things that previously would have been written off as impossible.”

The multicellular yeast research has received large attention from the press and scientific community since it was previously believed the evolution into multicellularity could not be observed in a timely manner.

“Often when we don’t know much about something, we might assume that something is difficult or we might assume something is easy,” said John Nason, professor of ecology, evolution and organismal biology. “Then we get there under the hood and learn more about it, we learn our initial assumptions weren’t necessarily correct.”

In order for the yeast cells to evolve from single cells into multicellular clusters, Travisano and Ratcliff looked for candidates that showed the most potential for multicellular evolution. The researchers chose yeast cells in which the parent and daughter cells remained together after reproduction.

Most of the time when yeast cells reproduce, the daughter cell grows on the parent until the daughter cell is developed enough to detach. The next generation of yeast cells in Travisano and Ratcliff’s experiment also remained attached to the parent so that cluster grew in size with each generation.

The successive generations form what the research call a “snowflake,” named for the circular shape of the clusters and the vague resemble to the winter snowflake.

In addition to the attached growth of yeast cell generations, Travisano and Ratcliff observed what appeared to be some yeast cells committing suicide. Like a mountaineer on the end of the rope cutting himself off so his companions are able to survive, these yeast cells are actively choosing death to benefit the snowflake as a whole. The sacrifice of these yeast cells shows the cells are thinking not as individuals, but as a group, a hallmark of multicellular beings.

Travisano and Ratcliff are currently working on studying the evolution of cancer and aging.

“Now that we have a model system spanning the transition from single-celled to simple multicellular organisms, we’re able to study how the transition to multicellularity affects the subsequent evolution of aging,” Ratcliff said. “… We’re using our yeast as a model system for how tumors grow, reproduce and evolve.”