A key step in the evolution of multi-cellularity has been reproduced in the lab, and it involves the death of cells. Will Ratcliff, Micheal Travisano, Ford Denison and Mark Borrello of University of Minnesota (UMN) recreated the process of cell death in multi-cellular organisms. The subject of the study – the humble Brewer’s yeast! Their work figures in the week’s issue of the prestigious journal Proceedings of the National Academy of Sciences (PNAS).
The Division of Labor and Cell Death!
The Brewer’s yeast is a single-celled organism and it was seen to evolve into multi-cellular families or clusters that worked as a unit, employing the principle of division of labor. This is a major step from single-celled to multi-celled organisms. A single cell needs to do all it has to sustain itself on its own. What multi-cellularity introduces is the concept of labor division and, thereby, specialization. Certain cells can specialize in a certain way, and other cells in other ways and they can then sustain each other. An important process in this organizational structure is the faculty of cell death.
Cells have an inherent program designed to ensure that they have a tendency to die. Apoptosis is the term used to describe this kind of programmed cell death. It occurs in multi-cellular organisms and is to the advantage of the living organism. How this program evolved in single-celled organisms before they clump up to form multi-cellular organisms is a major question. As Ratcliff puts it nicely, a clump is not really multi-cellular:
A cluster alone isn’t multi-cellular. But when cells in a cluster cooperate, make sacrifices for the common good, and adapt to change, that’s an evolutionary transition to multi-cellularity.
Sacrifice for a greater good – what’s the evolutionary background to that? And how long does it take?
This is how the experiment was conducted.
Saccharomyces cerevisiae or Brewer’s yeast was chosen and grown on a nutrient rich substrate in a test-tube for a day. Then they centrifuged the culture. Clusters of cells landed in the bottom of the test-tube. This was repeated every day for sixty days. What they found was that the balls of cells were not just balls of cells anymore, but had become fully-cooperative and acted as a unit. Cells remained attached after cell division and did not simply branch off. After a certain size of the cluster, cells started to die off – as if the program for apoptosis had been switched on! This was the onset of true multi-cellularity.
What about reproduction? Reproduction happened asexually as is typical for yeasts. Budding happens along ‘fault lines’ created by rows of dead cells. However, the budding process occurred only after the yeast achieved adulthood, which was ascertained by the size of the clump.
Just to point out, the greatest wonder of this is that the road from unicellularity to multi-cellularity took so little time! This hardly seems like a bottleneck evolutionary development, which occurs after a long standstill. This process took just 60 days! Two months is utterly insignificant in evolutionary timescale.
Cancer… And more
Current medical research into cancer leads one to believe that cancer is just a vestigial process of apoptosis, which can be directly traced to the origin of multicellularity. However, little is known and the researchers hope that this will shed some critical light on the whole issue. Travisano says:
Multi-cellular yeast is a valuable resource for investigating a wide variety of medically and biologically important topics. Cancer was recently described as a fossil from the origin of multi-cellularity, which can be directly investigated with the yeast system.
A lot of hope and a great opportunity all due to a humble microbe.