The evolution of yeast globules can unravel the mysteries of multicellular life
Life has several paths towards the evolution of simple multicellularity, as seen with snowflake yeast. But the real question is how to get stable clusters of cells numbering in the hundreds of thousands or more, says Harvard University paleontologist Andy Knoll, an expert on the origins of multicellularity who was not involved in the news. study. Putting together so many cells would have been a crucial step in the evolution of complex multicellular life. So Ratcliff looked for ways to grow bigger clumps of yeast.
Natural selection in a test tube
To make evolution work in the lab, Ratcliff and his colleagues grew the yeast in constantly shaking incubators to keep it mixed and moving. Each day, the researchers randomly removed a tenth of the yeast fluid from a test tube and placed it in a new tube. The yeast clusters in this smaller sample were then allowed to settle to the bottom of the tube for five minutes. The larger the cluster of yeast, the faster it settles.
Scientists only used the bottom of the deposited yeast, i.e. the largest clumps, within each generation to seed the next generation. This procedure put enormous evolutionary pressure on the yeast to create the largest possible clusters.
But from 2012 and 2016, Ratcliff continued to hit a wall. During the first two months of an experimental trial, the yeast clusters would grow larger, but would then peak at just 300 to 400 cells. Ratcliff began to suspect that the system was self-limiting, one way or another.
The key to providing the right evolutionary pressures for the yeast to grow the clusters came when postdoctoral researcher G. Ozan Bozdag joined Ratcliff’s lab and suggested growing the yeast at different oxygen levels. Throughout the history of life, oxygen levels in Earth’s atmosphere have varied widely, with potentially significant effects on how and when multicellular life evolved. Bozdag and Ratcliff therefore conducted the experiment with zero, partial and full oxygen levels.
The three-way experiment, which began in late 2016, used five replica lines in each oxygen treatment. The size of the clusters gradually increased at the start and stagnated until about 200 days into the experiment, when one of the oxygen-free lines began to show a few clumps large enough to be seen with the naked eye. Then the other four oxygen-free lines also developed visible clumps.
Bozdag thought the clusters were “just an accident, a fortuitous event” at first. But after carrying out the experiment several times, he began to realize that “it was not an accident, it was the result of natural selection.”
After 600 days, the yeast clusters that have developed without the oxygen has expanded to contain an average of 450,000 cells each. The surprising result suggests that in the early days of multicellular life, oxygen may have been a barrier to some developing organisms.
As the experiment continued, Bozdag and Ratcliff observed that the cells of the larger clusters began to appear more elongated, very different from their almost spherical ancestors. The cells of the evolved clusters also had larger points of contact with the cells that sprouted them, probably strengthening the branches of the clusters.