A new study has created a model of what scientists think the first multicellular cooperation might have looked like, showing that yeast cells – in an environment that requires them to work for their food – grow and reproduce better in multicellular clumps than singly.
Researchers found that cells of brewer’s yeast that clumped together were able more effectively to manipulate and absorb sugars in their environment than were similar cells that lived singly. The experiments showed that in environments where the yeast’s sugar food source is dilute and the number of cells is small, the ability to clump together allowed cells that otherwise would have remained hungry and static to grow and divide.
The work used the yeast Saccharomyces cerevisiae, which is commonly used in brewing and bread-making and has long been used by scientists as a model organism for understanding single-celled life. The researchers devised a series of experiments that presented two problems for the yeast cells to solve if they were to take in enough food to grow and divide: the first was how to change their food from an unusable form to a usable form; the second was how to actually take in this food. The researchers put the yeast in a solution of sucrose – table sugar – which is composed of two simpler sugars, glucose and fructose. Yeast lives on sugar, but the sucrose can’t get through the membrane that surrounds the cell. So the yeast makes an enzyme called invertase to chop the sucrose into glucose and fructose, each of which can enter the cell using gate-keeping molecules, called transporters, that form part of the membrane.
Working alone, a single yeast cell in a dilute solution of sucrose would never take in enough glucose and fructose to be able to grow and divide. But by cooperating, clumps of yeast in that same solution might have a chance. With several cells in proximity, all releasing invertase to create smaller sugars, these cooperating yeast cells would increase the density of those sugars near the clump, increasing the chances that each cell could take in enough to grow and divide. Sure enough, when the researchers tested these hypotheses on two strains of yeast, they found that the strain which clumped cells together was growing and dividing, while the yeast cells living alone were not.
Sucrose Utilization in Budding Yeast as a Model for the Origin of Undifferentiated Multicellularity. (2011) PLoS Biol 9(8): e1001122. doi:10.1371/journal.pbio.1001122
We use the budding yeast, Saccharomyces cerevisiae, to investigate one model for the initial emergence of multicellularity: the formation of multicellular aggregates as a result of incomplete cell separation. We combine simulations with experiments to show how the use of secreted public goods favors the formation of multicellular aggregates. Yeast cells can cooperate by secreting invertase, an enzyme that digests sucrose into monosaccharides, and many wild isolates are multicellular because cell walls remain attached to each other after the cells divide. We manipulate invertase secretion and cell attachment, and show that multicellular clumps have two advantages over single cells: they grow under conditions where single cells cannot and they compete better against cheaters, cells that do not make invertase. We propose that the prior use of public goods led to selection for the incomplete cell separation that first produced multicellularity.