Lab yeast makes evolutionary leap to multicellularity

New Scientist : A research group at the University of Minnesota in St. Paul caused a single-celled yeast to evolve into a multicelled organism. Evolution to multicellularity has occurred at least 20 times since life began, but the last time was about 200 million years ago, and it left few clues to the precise sequence of events, writes Bob Holmes for New Scientist. William Ratcliff and colleagues grew the yeast in a liquid, gently centrifuged each culture, and inoculated the next batch with the yeast that settled to the bottom of each tube. Because groups of cells settle faster than single cells, the team was able to select for yeast that clumped together. Within 60 daysabout 350 generationsall their culture lines had evolved a clumped, snowflake form. “The key step in the evolution of multicellularity is a shift in the level of selection from unicells to groups. Once that occurs, you can consider the clumps to be primitive multicellular organisms,” said Ratcliff. Skeptics say, however, that because some yeast strains were multicellular tens or hundreds of millions of years ago, the yeast may have retained some evolved mechanisms for cell adhesion and programmed cell death, effectively stacking the deck in favor of Ratcliff’s experiment. Ratcliff and his colleagues are planning to address that objection by doing similar experiments with Chlamydomonas, a single-celled alga that has no multicellular ancestors.