Evolution experiment turns singleton yeast into multicellular organisms

By Susan Milius

Web edition: June 22, 2011
Print edition: July 16, 2011; Vol.180 #2 (p. 11)

NORMAN, Okla. — Since humanity missed the big moment the first time around, biologists trying to understand the origins of complex life have coaxed single-celled microbes to evolve into multicellular forms capable of reproduction.

Common lab yeast normally live as single cells that bud off single-celled offspring. But challenging generations of yeast with conditions that make solo life tough led to spiky multicelled yeast forms within about two months, said Will Ratcliff of the University of Minnesota, Twin Cities. The experiment suggests going multicellular may happen more readily than previously thought, he told the Evolution 2011 conference June 18.

“It was certainly the buzz of the conference,” said Lee Dugatkin of the University of Louisville in Kentucky.

Evolutionary biologists rank the shift from one to many cells as one of the major transitions in the history of life. “To be able to examine it experimentally, in real time, in the lab, is extremely exciting,” Dugatkin said.

To provoke evolution in a test tube, Ratcliff and his colleagues put liquid suspensions of yeast cells through a daily ordeal. Every tube of cells got a mild centrifuge spin. Then the researchers saved a fraction of the sludgier part of each tube and — life is hard — tossed the rest. That regimen ensured that any change in the yeast that happened to encourage settling — such as a shift toward heavier bodies made of multiple cells — promoted survival.

Under these conditions, yeast lineages that retained budding daughter cells rather than splitting them off were more likely to make it through each daily decimation. Those buds in turn retained their own buds, creating bristly multicellular organisms the researchers call yeast snowflakes.

These snowflakes reproduced by fracturing into smaller pieces that eventually grew and fractured themselves.The researchers even saw hints of a reproductive division of labor, Ratcliff said. A scattering of snowflake cells undergo cell death, or apoptosis, and the resulting weak spots appear to serve as fault lines where babies flake off.

To see if yeast snowflakes evolve the way true multicelled creatures do, the researchers created another version of the settle-fast-or-else challenge that they varied in severity. Snowflake lineages receiving the harshest treatment responded dramatically, becoming twice as large as their ancestor snowflakes; lineages under gentler treatment changed less. That means that the yeast snowflakes respond to evolutionary pressure as whole, multicellular organisms, Ratcliff said.

Such tests take the yeast snowflakes farther than research reported in 1998 by other scientists, who coaxed a microbe to evolve a clumpy, multicellular form but didn’t describe its evolutionary dynamics.

The yeast experiment doesn’t exactly constitute starting from scratch when it comes to the evolution of multicellularity, cautions Adam Waite, who studies cooperation among yeast at the University of Washington in Seattle, because today’s single-celled yeast actually evolved from long-ago multicellular ancestors.

Dugatkin doesn’t find that a drawback, however. Whether the yeast take the same path to multicellularity today as the first multicelled organisms did billions of years ago, he says, makes a fascinating question in itself.