Are multicellular organisms identical

The fixed evolution

It took only 60 days or just 350 generations in the laboratory for yeast cells, which are typical unicellulars, to become a simple multicellular organism.

"We were surprised by the results. Many evolutionary biologists are working to explain the process of evolution from single to multicellular. One of the central questions is how long this development took. Our research shows that multicellularity can develop much faster than previously thought. "

Will Ratcliff is an evolutionary biologist at the University of Minnesota. In search of the possible beginnings of multicellularity, he and colleagues started a simple experiment.

"The first step on the way to becoming a multicellular organism is the evolution of simple cell assemblies. To do this, the unicellular ancestor has to be placed in an environment in which it is advantageous to join forces with others."

Will Ratcliff used the unicellular brewer's yeast in the laboratory Saccharomyces cerevisiae an artificially created selection pressure. He created an environment in which cells or cell clusters have a greater chance of survival if they are heavier: To do this, he first cultivated the yeast cells in a liquid nutrient medium for 24 hours. He then hurled the solution in a centrifuge to separate the cells according to their weight. Just as sand sinks faster in water than fine clay particles, heavier components migrate to the bottom faster than light ones in the centrifuge. Will Ratcliff specifically removed only the sediment from the centrifuge tube and multiplied the cells in it again for 24 hours in a fresh nutrient medium. He repeated this procedure over and over again, in the expectation that the selection pressure created in this way would encourage the formation of multicellular and therefore heavier structures.

"Originally we expected that flake formation would occur: Individual cells attach themselves to others and thus form composite clusters. But we have observed a different form of cluster formation. Newly formed cells simply remain connected to their mother cell after cell division. This is how typical cells develop Patterns of branching cells. Each cell in the compound is genetically identical to the others. "

The formed clusters of cells look like branched crystals of a snowflake. They can even multiply as a composite in a surprising way: large flakes divide into two. Instead, a connecting cell simply dies. It is a programmed cell death called apoptosis. It is unusual for single-cell organisms to sacrifice themselves for the good of the community. Will Ratcliff sees the development of this ability as one of the foundations for the transition to the multicellular organism.

"They only become multicellular cells through further evolution. An important step is the shifting of the point of attack of the selection. At first it only works between individual cells, then mainly between whole cell clusters."

Before Will Ratcliff published his study in the specialist magazine "PNAS", the results were already causing a sensation at congresses among specialist colleagues. But he also met with criticism. It is known of yeasts that they were already multicellular in the course of evolution before they developed into unicellular cells again. So it could be that the experiments just evoked an old genetic program in the yeast cells. Then the rapid evolution in the laboratory would no longer be so surprising.

Will Ratcliff disagrees:

"Yeast was actually multicellular in the past. But that was a long, long time ago. We don't believe that yeast still carries genes for multicellularity with it today - after hundreds of millions of years."

To be on the safe side, Will Ratcliff has started a repetition of his experiment. However, he no longer uses yeast cells, but single-cell green algae of the genus Chlamydomonas. These are not known to have ever been multicellular. He expects the first results in a few months.

Link to the research group's website