By Brandon Keim
An evolutionary transition that took several billion years to occur in nature has happened in a laboratory, and it needed just 60 days.
Under artificial pressure to become larger, single-celled yeast became multicellular creatures. That crucial step is responsible for life’s progression beyond algae and bacteria, and while the latest work doesn’t duplicate prehistoric transitions, it could help reveal the principles guiding them.
“This is actually simple. It doesn’t need mystical complexity or a lot of the things that people have hypothesized — special genes, a huge genome, very unnatural conditions,” said evolutionary biologist Michael Travisano of the University of Minnesota, co-author of a study in the Proceedings of the National Academy of Sciences.
In the new study, researchers led by Travisano and William Ratcliff grew brewer’s yeast, a common single-celled organism, in flasks of nutrient-rich broth.
Once per day they shook the flasks, removed yeast that most rapidly settled to the bottom, and used it to start new cultures. Free-floating yeast were left behind, while yeast that gathered in heavy, fast-falling clumps survived to reproduce.
Within just a few weeks, individual yeast cells still retained their singular identities, but clumped together easily. At the end of two months, the clumps were a permanent arrangement. Each strain had evolved to be truly multicellular, displaying all the tendencies associated with “higher” forms of life: a division of labor between specialized cells, juvenile and adult life stages, and multicellular offspring.
Multicellular yeast reproduces itself; the offspring will not reproduce until it has grown. Photo: Ratcliff et al./PNAS
“Multicellularity is the ultimate in cooperation,” said Travisano, who wants to understand how cooperation emerges in selfishly competing organisms. “Multiple cells make make up an individual that cooperates for the benefit of the whole. Sometimes cells give up their ability to reproduce for the benefit of close kin.”
Image: At left, an original strain of brewer’s yeast. At right, the multicellular form. (Ratcliff et al./PNAS)
Citation: “Experimental evolution of multicellularity.” By William C. Ratcliff, R. Ford Denison, Mark Borrello, and Michael Travisano. Proceedings of the National Academy of Sciences, Jan. 17, 2012.
